“Behavior of Temperature-Responsive Microgels at
Interfaces:
Deformability and Interfacial Activity”
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen
University zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften
genehmigte Dissertation
vorgelegt von
Master of Science
Yaodong Wu
aus Anhui, China
Berichter: Universitätsprofessor Dr. Andrij Pich
Universitätsprofessor Dr. Alexander Böker
Tag der mündlichen Prüfung: 26.09.2014
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.
i
Contents
Kurzfassung .................................................................................................................. I
Abstract ....................................................................................................................... III
List of Abbreviations and Symbols ........................................................................... V
Chapter 1. Introduction .............................................................................................. 1
1.1. Microgels ........................................................................................................ 1
1.1.1. Synthesis of microgels ............................................................................. 1
1.1.2. Internal structure of microgels ................................................................. 6
1.2. Nanoparticles at interfaces ........................................................................... 8
1.2.1. Conventional solid particles ..................................................................... 8
1.2.2. Soft microgel particles ........................................................................... 10
1.3. Challenges in the microgel research .......................................................... 12
1.4. References .................................................................................................... 14
Chapter 2. Motivation and Aims .............................................................................. 21
Chapter 3. Synthesis of Microgels with Controlled Cross-linking Density and
Copolymer Composition ............................................................................................ 23
3.1. Introduction ................................................................................................. 23
3.2.1. Materials ................................................................................................ 25
3.2.2. Synthesis of microgels ........................................................................... 26
3.2.3. Characterization methods....................................................................... 28
3.3. Results and discussion ................................................................................. 29
3.3.1. PVCL/acrylamides-based neutral microgels.......................................... 29
3.3.1.1. PVCL/NIPAm and PVCL/NIPMAm microgels............................. 29
3.3.1.2. PVCL/NIPAm microgel system with different cross-linking
densities…......................................................................................................... 31
3.3.1.3. Microgels synthesized with different amounts of surfactant .......... 36
3.3.2. PVCL-based microgels with functional groups ..................................... 37
3.3.2.1. PVCL/AAEM/VIm microgels ........................................................ 38
3.3.2.2. PVCL/AAEM/AAc microgels ........................................................ 41
3.4. Conclusions .................................................................................................. 42
3.5. References .................................................................................................... 43
Chapter 4. Internal Structure of Microgels by Light Scattering and Microscopy
...................................................................................................................................... 49
ii
4.1. Introduction ................................................................................................. 49
4.2. Experimental section ................................................................................... 51
4.2.1. Sample preparation ................................................................................ 51
4.2.2. Characterization methods....................................................................... 52
4.3. Results and discussion ................................................................................. 53
4.3.1. Internal structure of microgels by light scattering ................................. 53
4.3.2. Microscopy observation of microgels with different cross-linking
degrees… .............................................................................................................. 55
4.3.3. Polymer network domains in microgels observed by AFM .................. 61
4.3.4. Microscopy observation of PVCL-based microgels with functional
groups… ............................................................................................................... 62
4.4. Conclusions .................................................................................................. 67
4.5. References .................................................................................................... 68
Chapter 5. Deformation and Spreading of Copolymer Microgels at the
Air/Water and Air/Solid Interfaces .......................................................................... 73
5.1. Introduction ................................................................................................. 73
5.2. Experimental section ................................................................................... 76
5.2.1. Sample preparation ................................................................................ 76
5.2.2. Characterization methods....................................................................... 76
5.2.3. Computer simulation of microgels at the interface ................................ 77
5.3. Results and discussion ................................................................................. 82
5.3.1. Dependence of microgel modulus on the cross-linking density ............ 82
5.3.2. Interfacial activity of microgels at the air/water interface ..................... 84
5.3.3. Conformation of microgels at the air/water interface ............................ 86
5.3.4. Dimensions of microgels varied in the cross-linking degree ................. 87
5.3.5. Deformation of microgels ...................................................................... 90
5.4. Conclusions .................................................................................................. 92
5.5. References .................................................................................................... 93
Chapter 6. Behavior of Temperature-Responsive Copolymer Microgels at the
Oil/Water Interface .................................................................................................... 97
6.1. Introduction ................................................................................................. 97
6.2. Experimental section ................................................................................. 100
6.2.1. Microgel synthesis ............................................................................... 100
6.2.2. Interfacial tension................................................................................. 101
iii
6.2.3. Interfacial dilatational rheology ........................................................... 101
6.2.4. Preparation of microgel-stabilized emulsions ...................................... 102
6.2.5. Preparation of emulsion-templated materials ...................................... 102
6.2.6. Fluorescence microscopy ..................................................................... 103
6.2.7. Scanning electron microscopy ............................................................. 103
6.3. Results and discussion ............................................................................... 103
6.3.1. Dynamic interfacial tension ................................................................. 103
6.3.1.1. Influence of microgel concentration ............................................. 103
6.3.1.2. Influence of temperature ............................................................... 106
6.3.1.3. Influence of cross-link density ..................................................... 108
6.3.1.4. Influence of microgel size ............................................................ 109
6.3.2. Equilibrium state of the interfacial tension .......................................... 110
6.3.2.1. Influence of chemical composition of microgel ........................... 110
6.3.2.2. Influence of temperature ............................................................... 112
6.3.3. Discussion on the temperature dependence of interfacial tension ....... 114
6.3.4. Dynamic interfacial tension of microgel mixtures............................... 119
6.3.5. Interfacial dilatational rheology ........................................................... 121
6.3.6. Microgel-stabilized emulsions ............................................................. 123
6.3.6.1. Emulsions prepared at different oil/water ratios ........................... 123
6.3.6.2. Emulsions prepared at different microgel concentrations ............ 125
6.3.6.3. Summary of emulsions prepared with PVCL/NIPAm (1:1)
microgels… ..................................................................................................... 126
6.3.6.4. Emulsions prepared with microgels of different chemical
compositions ................................................................................................... 128
6.3.7. Materials based on microgel-stabilized emulsions .............................. 129
6.4. Conclusions ................................................................................................ 133
6.5. References .................................................................................................. 135
Chapter 7. Summary ............................................................................................... 141
Chapter 8. Outlook and Future Experiments ....................................................... 145
8.1. Interfacial behavior ................................................................................... 145
8.1.1. Microgels with ionizable groups or specific structures ....................... 145
8.1.2. Other techniques .................................................................................. 146
8.2. Potential applications and improvements ............................................... 146
8.2.1. Asymmetric colloidal particles ............................................................ 146
iv
8.2.2. New microgel-based capsules .............................................................. 147
8.2.3. Designed biodistribution of microgels for therapy .............................. 148
8.3. References .................................................................................................. 149
Acknowledgements .................................................................................................. 151
Curriculum Vitae ..................................................................................................... 153
Kurzfassung
I
Kurzfassung
Mikrogele sind ultraweiche intramolekulare vernetzte Polymernetzwerke. Eine
der wichtigsten Eigenschaften der Mikrogele ist die Grenzfläschenaktivität, die eine
große Rolle in ihre Anwendungen wie Herstellung von Filmen, Emulsionen oder
Kapseln spielt. Um den Einfluss der chemischen Zusammensetzung,
Vernetzungsdichte und Temperatur auf die Grenzflächenverhalten der Mikrogele zu
untersuchen, temperatur-sensitive Copolymermikrogele Poly(N-vinylcaprolactam-co-
N-isopropylacrylamide) (PVCL/NIPAm), Poly(N-vinylcaprolactam-co-N-
isopropylmethacrylamide) (PVCL/NIPMAm) und Homopolymeranaloga mit
unterschiedlicher Zusammensetzung und Vernetzungsgerad wurden synthetisiert und
charakterisiert.
Die Grenzfläschenaktivität der Copolymermikrogele wurden mit einem
Pendant-Drop-Tensiometer gemessen. Die Grenzfläschenspannung IFT der
Copolymermikrogele liegen zwischen den beiden IFT der Homopolymermikrogelen.
Dies bedeutet, dass die chemische Struktur die Grenzfläschenaktivität der
Copolymermikrogele beeinflusst. Die dynamische Grenzflächenspannung von
Mikrogelen mit niedrigerer Vernetzungsdichte verändert sich schneller als die
vonMikrogele mit höherer Vernetzungsdichte. Dies stimmt mit den
Simulationsergebnissen überein, dass die leicht vernetzten Mikrogele schneller an der
Luft/Flüssigkeit und Flüssigkeit/Flüssigkeit Grenzflächen als die dicht vernetzten
Mikrogele verbreiten, da die leicht vernetzten Mikrogele leichter verformbar sind
Weiterhin beweisen die Messungen mit einem Rasterkraftmikroskop, dass die
Verformbarkeit der Mikrogele auch an der Luft/Feststoff Grenzfläche mit dem
abnehmenden Vernetzungsgrad zunimmt. Außerdem ist die Abnahme der
dynamischen Grenzflächenspannung für PVCL/NIPAm und PVCL/NIPMAm
Kurzfassung
II
Mikrogelen bei T<VPTT (Volumenphasenübergangstemperatur) schneller als bei
T>VPTT, da die Verformbarkeit der Mikrogele sich mit steigender Temperatur
verschlechtert. Im weitern zeigen die Abmessungen der Dimensionen der Mikrogele
in zwei Zustände: auf einem festen Substrat und in einer Lösung , dass die Adsorption
der Mikrogele an eine Grenzfläche zu einer stärkeren Verformung als die Schwellung
in einer Lösing führt.
Abstract
III
Abstract
The interfacial activity is an important property of microgels which consist of
intramolecular cross-linked polymer networks, as it is involved in applications like the
fabrication of films, emulsions or capsules. To investigate the influence of the
chemical composition, cross-linking density and temperature on the interfacial
behavior of microgels, thermo-sensitive poly(N-vinylcaprolactam-co-N-
isopropylacrylamide) (PVCL/NIPAm), poly(N-vinylcaprolactam-co-N-isopropyl-
methacrylamide) (PVCL/NIPMAm) copolymer microgels and homopolymer
analogues were synthesized, and their properties were determined.
The interfacial activities of PVCL/NIPAm and PVCL/PNIPMAm copolymer
microgels were measured with a pendant drop tensiometer. The interfacial behaviors
of the copolymer microgels fall between that of homopolymer microgels. It means
that the chemical structure contributes to the variation in the interfacial tension. The
dynamic interfacial tension of microgels with lower cross-linking densities evolves
faster than that of microgels with higher cross-linking densities. This is consistent
with the simulation results that loosely cross-linked microgels spread faster at the
air/liquid and liquid/liquid interfaces than densely cross-linked ones. Meanwhile, the
dimensions of the adsorbed microgels on a solid substrate characterized with atomic
force microscopy indicate that the deformability of the microgels at the air/solid
interface also increases with the decreasing cross-linking degree. Besides, the
evolution of the dynamic interfacial tension for PVCL/NIPAm and PVCL/NIPMAm
microgels at T<VPTT (volume phase transition temperature) is faster than that at
T>VPTT because of the reducing microgel deformability with increasing temperature.
Furthermore, the comparison between the dimensions of microgels on the solid
Abstract
IV
substrate and in solution shows that adsorption of microgels induces higher
deformation than the swelling-induced expansion.
List of Abbreviations and Symbols
V
List of Abbreviations and Symbols
Chemicals
AAc acrylic acid
AAEM acetoacetoxyethyl methacrylate
ACMA 2, 2’-azobis(N-(2-carboxyethyl)-2-methyl-propionamidine)
AMPA 2, 2’-azobis(2-methylpropionamidine) dihydrochloride
BIS N, N’-methylenebisacrylamide
CTAB cetyltrimethylammonium bromide
NIPAm N-isopropylacrylamide
NIPMAm N-isopropylmethacrylamide
MAA methacrylic acid
PCL polycaprolactone
SDS sodium dodecyl sulfate
VCL N-vinylcaprolactam
VIm vinylimidazole
Instruments and methods
AFM Atomic Force Microscopy
DLS Dynamic Light Scattering
DSC Differential Scanning Calorimetry
FESEM Field Emission Scanning Electron Microscopy
JKR model Johnson-Kendall-Roberts model
NMR Nuclear Magnetic Resonance
PDT Pendent Drop Tensiometry
PSD Power Spectral Density
SANS Small-Angle Neutron Scattering
SLS Static Light Scattering
STEM Scanning Transmission Electron Microscopy
TEM Transmission Electron Microscopy
UV Ultraviolet
List of Abbreviations and Symbols
VI
General abbreviations and symbols
D Diffusion Coefficient
E' Storage Modulus
E" Loss Modulus
Volume Fraction of Polymer in Microgels
Effective Volume Fraction of Polymer in Microgels
G Shear Modulus
γ Interfacial Tension / Surface Tension
h Height
HIPE High Internal Phase Emulsion
LCST Lower Critical Solution Temperature
Mw Molecular Weight
MWCO Molecular Weight Cut-off
o/w emulsion Oil in Water Type Emulsion
Rcont Contact Radius
Rh Hydrodynamic Radius
Rp Radius of the Particle
Rs Microgel Radius by SLS
σsurf Gaussian Width of Microgel Surface
T Temperature
t Time
V:N Molar Ratio of VCL/NIPAm
V:NM Molar Ratio of VCL/NIPMAm
VPTT Volume Phase Transition Temperature
w/o emulsion Water in Oil Type Emulsion
Chapter 1. Introduction
1
Chapter 1. Introduction
1.1. Microgels
Microgels can be defined as a class of colloidal dispersions of gel particles
composed of cross-linked polymer network, with the particle size range of 10 – 1000
nm.[1]
They have been increasingly attractive in colloidal science and technology. As
an important class of soft matter, microgels find many applications in different fields
such as coating, medicine delivery and emulsion stabilization.[2-4]
Most of the aqueous
microgel systems are based on poly(N-isopropylacrylamide) (PNIPAm), poly(N-
vinylcaprolactam) (PVCL), poly(acrylic acid) (PAA) or poly(2-(diethylamino)ethyl
methacrylate) (PDEA).[1-4]
The special attention is focused on PNIPAm- or PVCL-
based microgels because of their temperature-responsive property which enables the
fabrication of stimuli-responsive materials.[2, 5]
1.1.1. Synthesis of microgels
The microgel synthesis is aimed to control the particle size and size
distribution, the colloidal stability and the selective incorporation of functional groups
in microgels.[1-3]
The possible synthetic routs for microgel preparation can be
summarized as following: (1) polymerization from monomers and cross-linkers in
homogeneous phase or in micro-droplets; (2) physical self-assembly or post cross-
linking of polymers in homogeneous phase or in micro-droplets; and (3) disruption
from macrogel by mechanical grinding or photolithographic techniques.[1, 3]
Figure 1.1 lists the vinyl monomers that are used in the preparation of
microgels investigated in this thesis. Among these monomers, N-vinylcaprolactam
(VCL), N-isopropylacrylamide (NIPAm) and N-isopropylmethacrylamide (NIPMAm)
Chapter 1. Introduction
2
are temperature-sensitive and nonionic. Acetoacetoxyethyl methacrylate (AAEM) is a
nonionic group but can react in a wide range of post modification reactions.[6]
Vinylimidazole (VIm) contains cationic group and acrylic acid (AAc) contains
anionic group. A bifunctional monomer N, N’-methylenebisacrylamide (BIS) with
two double bonds is the most widely used cross-linker for microgel synthesis (Figure
1.2).[1]
NO
N-Vinylcaprolactam (VCL)
O
O
O O
O
Acetoacetoxyethyl methacrylate (AAEM)
NH
O
N-isopropylacrylamide (NIPAm)
NH
O
N-isopropylmethacrylamide (NIPMAm)
N
N
Vinylimidazole (VIm)
OH
O
Acrylic acid (AAc)
Figure 1.1. Chemical structures of monomers used for microgel synthesis.
HN
HN
O O
N, N’-methylenebisacrylamide (BIS)
Figure 1.2. Chemical structure of cross-linker used for microgel synthesis.
Chapter 1. Introduction
3
Generally, the syntheses of microgels from monomers are initiated by ionic
radical initiators. Peroxide- and azo-based compounds are two kinds of initiators
commonly employed in microgel synthesis. For the synthesis of PNIPAm or
PNIPMAm microgels, potassiumperoxodisulfate (KPS) is frequently used.[1, 7-11]
However, the acidic environment caused by the decomposition of KPS can lead to the
hydrolysis of VCL.[12]
Thus, for the synthesis of microgels containing VCL unit, azo-
based initiators are frequently used, and two of them are shown in Figure 1.3. As
reported, the cationic initiator 2, 2’-azobis(2-methylpropionamidine) dihydrochloride
(AMPA) is an effective initiator for the synthesis of nonionic PVCL,[13]
PVCL/NIPAm[14]
and PVCL/AAEM,[6]
and cationic PVCL/AAEM/VIm microgels.[5]
For the incorporation of anionic functional group acrylic acid (AAc) into VCL-based
microgels by copolymerization, the cationic initiator AMPA is not suitable, due to its
using can cause coagulation during the particle formation. Therefore the cationic
initiator 2, 2’-azobis(N-(2-carboxyethyl)-2-methyl-propionamidine) (ACMA) instead
of AMPA is used in the synthesis of PVCL/AAEM/AAc microgels.[15]
N
N
H
H
H
N
N
H
H
H
Cl Cl
2, 2’-azobis(2-methylpropionamidine) dihydrochloride (AMPA)
N
N
NH
OH
ONH
HN
NH
HO
O
2, 2’-azobis(N-(2-carboxyethyl)-2-methyl-propionamidine) (ACMA)
Figure 1.3. Chemical structures of initiators used for microgel synthesis.
Chapter 1. Introduction
4
A powerful method for the synthesis of temperature-responsive microgels is
precipitation polymerization. During the synthesis, the monomer(s), cross-linker, and
initiator are dissolved in water. At the typical polymerization temperature (50 –
80 °C), free radicals are produced by the decomposition of initiator.[11]
Then the
polymerization is started with the attack of radicals on monomers, followed by chain
growth. As the polymerization temperature is much higher than the lower critical
solution temperature (LCST) of temperature-sensitive polymers (32 °C for both
PNIPAm and PVCL, and 43 °C for PNIPMAm), the growing polymer chains collapse
when a critical chain length is achieved and form precursor particles.[1-3, 14]
A
schematic mechanism of precipitation polymerization is presented in Figure 1.4.
Figure 1.4. The formation of microgels through precipitation polymerization.[3]
Reprinted with permission, copyright 2010 Springer.
It is proposed that the precursor particles grow by different ways: (1)
aggregation to form larger colloidally stable particles; (2) deposition onto formed
polymer particles; and/or (3) addition of monomers or macroradicals.[3]
During the
synthesis, the growing particles are shrunk and stabilized by electrostatic charges
originate from the initiator. Though in collapsed state, microgel particles still contain
a lot of water.[16]
Based on this, the precipitation polymerization is different from the
classical emulsion polymerization of water-insoluble monomers, which produces
latex particles with compact structures.[17]
After the polymerization is completed, the
Chapter 1. Introduction
5
reaction mixture is cooled down to room temperature (below LCST of polymer
chains). Then, microgel particles become swollen and develop a “hairy”
morphology,[3, 7]
which will be discussed in detail hereinafter. The stability of
microgel in swollen state relies on the steric mechanism through the formation of
hydrogen bonds between polymer segments and water molecules.[3, 16, 18]
The
precipitation polymerization offers numerous advantages for microgel preparation. It
is able to produce dispersions with low polydispersity and to control parameters such
as particle size,[19]
charge,[5, 15, 20]
and cross-link density.[21, 22]
It should be noted that
there are still some limitations for precipitation polymerization, such as high reaction
temperature, difficulty in synthesis of particles below 50 nm, and formation of sol
fration during the polymerization.[3]
The size of microgels can be controlled by the amount of surfactant added in
the synthesis.[23-25]
In the reaction mixture, the surfactant molecules adsorb on
precursor particles and make them more stable. With more surfactants added in the
synthesis, more precursor particles survive from coagulation.[23]
As a certain amount
of monomers, more growing particles means smaller sizes they are.[25]
The cationic
surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium
dodecyl sulfate (SDS) are commonly used in microgel synthesis. Figure 1.5 exhibits
the chemical structures of CTAB and SDS. As expected, the reported results show
that with the increase in the concentration of CTAB or SDS, the hydrodynamic size of
microgels decrease.[11, 14, 24]
Moreover, the presence of surfactant effectively prevents
the coagulation which leads to micro-scale particle and large polydispersity.[14, 26]
After the synthesis, the surfactant can be removed from the microgel solution by
centrifugation or dialysis.[25]
In addition, the size of microgels in surfactant-free
Chapter 1. Introduction
6
polymerization can be regulated by the temperature and concentration of monomers
or cross-linkers.[3]
N Br
Cetyltrimethylammonium bromide (CTAB)
O
S
O
OO
Na
Sodium dodecyl sulfate (SDS)
Figure 1.5. Two common surfactants used for the synthesis of micrgels.
1.1.2. Internal structure of microgels
Microgels consist of chemically intramolecular cross-linked polymer network.
Such kind of cross-linked internal structure makes microgels characteristically
different from other colloidal particles like polymer latex or colloidal supramolecular
aggregates.[17, 27]
The cross-linked polymer network provides microgels both swelling
capability in a good solvent and topological integrity.[28]
So far, the microgel
structures have been thoroughly investigated by scattering methods, differential
scanning calorimetry, rheology, nuclear magnetic resonance spectroscopy (NMR),
and atomic force microscopy (AFM).[13, 22, 28-32]
With higher reactivity, the cross-
linker (BIS) is normally consumed faster than monomers during synthesis. Therefore,
the distribution of cross-linker in microgels is inhomogeneous. Through small-angle
neutron scattering (SANS) as well as static light scattering (SLS) and dynamic light
scattering (DLS), the microgel structure has been proved to be a radial profile, where
the density of cross-linker decreases from the center towards the surface of
microgels.[7, 28, 33]
Chapter 1. Introduction
7
Due to the intrinsic cross-linked polymer network, microgels can undergo
volume phase transitions in response to environmental changes, such as temperature,
pH, solvent composition, ionic strength, and type of surfactant.[34-40]
Microgels
composed of temperature-responsive polymer like PNIPAm, PNIPMAm or PVCL,
experience a volume phase transition upon heating above a critical temperature close
to the LCST of corresponding linear polymers.[41, 42]
This typical temperature is called
volume phase transition temperature (VPTT) of microgels. Below VPTT, microgels
are swollen by solvent which is water in aqueous system. As temperature increases
above VPTT, a part of water is expelled from the inside of microgels due to the break
of hydrogen bond between water and polymer and increase in hydrophobic
interactions, which makes microgels shrink.[18, 41]
If containing charged groups,
microgels are also sensitive to the variation in pH of environment. For example, as pH
decrease from 6 to 4, more VIm groups in PVCL/AAEM/VIm copolymer microgel
are ionized. As a result, PVCL/AAEM/VIm microgels swell to a higher extent, due to
the increasing repulsiveness among more positive charged groups.[5]
It should be
noted that even in collapsed state, microgels still retain considerable amount of water
inside.[16, 41]
Figure 1.6 presents some important features of microgels, including the cross-
linked polymer network and radial profile with decreasing cross-linker density
towards the surface, as well as temperature- and pH-responsiveness. With such a
unique structure, microgels are highly porous, compliant and stimuli-responsive.
Upon these properties, microgels have great potential in applications like surface
coating, drug delivery, and emulsion stabilizer or capsule shells.[2, 4, 29, 43-46]
When
used in coatings and emulsions, the interfacial behavior of microgel is very important.
Chapter 1. Introduction
8
Figure 1.6. Some key features of microgels: internal structure and responsiveness to
environments like changes in temperature or pH.[3, 5-7, 47]
Reprinted with permission,
copyright 2004 American Institute of Physics, 2009 Royal Society of Chemistry, and
2010 Springer.
1.2. Nanoparticles at interfaces
The behavior of nanoparticles adsorbed at interfaces (air/liquid, liquid/liquid
and liquid/solid) has been extensively investigated, because of the wide application of
such particles in colloidal science and technology.[48]
One of the applications
concerning the interfacial behavior of nanoparticles is the famous “Pickering
emulsion”.[49, 50]
1.2.1. Conventional solid particles
For surfactant molecules in oil/water mixture systems, the hydrophilic-
lipophilic balance (HLB) determines the position of aggregated surfactants, whether
in water, oil or a third phase.[49]
In the case of rigid solid colloidal particles, the
Chapter 1. Introduction
9
position mainly depends on the contact angle which is made by the particle with
the interface. Figure 1.7 describes the positions of particles at the oil/water interface
with different . For hydrophilic particles, is normally less than 90o and a
larger part of the particle surface stays in water than in oil. In contrast, for
hydrophobic particles, is greater than 90o and a larger part of the particle surface
resides in oil than in water. The anchoring of particles to the interface is very strong,
which makes the particle-stabilized emulsions stable. The energy required to
remove of particle from the interface can be estimated by Equation 1.1.[49, 51, 52]
(Equation 1.1)
where is the radius of particle, is the oil/water interfacial tension. As the energy
for detachment of a particle from the interface is much higher than the thermal energy
, the particle is usually considered to be irreversibly attached.[52]
However, if
particles are very small (< 1 nm in radius), comparable to small surfactant molecules,
is comparable to thermal energy. Consequently, a constant particle exchange
occurs at the interface due to the displacement for extremely small particles.[49, 51]
In several recent reviews, some general rules have been summarized for
producing particle-stabilized emulsions: (1) particles should be partially wettable by
both oil and water; (2) the continuous phase of the emulsion is generally the one in
which a larger fraction of the particle surface resides; and (3) the interactions between
particles play an important role in emulsion-stabilizing.[52, 53]
The interfacial behavior
of particles can be controlled by size, shape, wettability, roughness, and the
interaction between particles. As a result, the properties of emulsions stabilized by
solid particles can be manipulated, to which the viscoelasticity of the interface is
highly relevant.[48-52, 54-59]
Chapter 1. Introduction
10
Figure 1.7. Solid particles at the oil/water interface. Upper part shows the position of
a spherical particle at a planner oil/water interface for contact angle (left),
(center) and (right). Lower part shows the corresponding
position of particles at a curved oil/water interface.[49]
Reprinted with permission,
copyright 2003 Elsevier.
1.2.2. Soft microgel particles
With an internal soft and porous structure, microgel particles have distinctly
different interfacial properties from classical rigid solid particles. Microgels, spherical
in solution, are strongly deformed and flattened at the oil/water interface.[60, 61]
It has
been revealed that the aqueous-born microgel particles reside mainly in the water
phase for both oil-in-water and water-in-oil emulsions.[52, 61]
Figure 1.8 presents a
schematic of microgel particles at the oil/water interface, which shows the differences
as compared to the interfacial behavior of rigid solid particles in Pickering emulsion
systems exhibited in Figure 1.7. Unlike rigid solid particles, the exact location of
oil/water interface is not clear for microgels. To some extent, the oil/water interface
could slightly penetrate inside the porous microgel.
A remarkable merit for microgel-stabilized emulsions or microgel-based
capsules is the sensitivity to environmental changes brought by stimuli-responsive
Chapter 1. Introduction
11
microgels, such as temperature-responsive PNIPAm or PVCL microgels.[43, 44, 62-66]
Stability of such emulsion systems is strongly reduced as temperature increases above
VPTT of microgels, which might lead to the break of emulsions.[44, 62, 63]
The collapse
of microgels incorporated in capsules causes contraction of the whole capsules, which
is very useful in the application of drug release.[43]
For microgels copolymerized with
ionic monomers like methacrylic acid (MAA), the interfacial property depends on pH,
ionic strength and oil polarity.[44, 64-67]
Figure 1.8. Left part shows the FreSCa cryo-SEM image of microgels at the
water/heptane interface viewed from the oil side, and the schematic representation of
microgel at the interface. Right part schematically describes the microgels at the
interface of oil in water (upper right) and water in oil (lower right) emulsions.[52, 61]
Reprinted with permission, copyright 2012 American Chemical Society.
The structure of latex particles at fluid/fluid interface can be directly observed
in situ through optical microscopy.[68, 69]
It has been also successfully employed for
microgel-stabilized emulsions.[70]
However, the resolving capability of optical
microscopy is limited. To understand the detail structure of microgels at the interface,
freeze-fraction cryo-scanning electron microscopy (cryo-SEM) is employed
usually.[60, 71, 72]
Through the observation on the surface of emulsion droplet stabilized
Chapter 1. Introduction
12
by PNIPAm/MAA microgels, several important features of the assembly structure are
summarized: microgels are highly deformed, interpenetrate, and are linked by
filaments.[52]
It can be easily related to the intrinsic “fluffy” structure of microgels as
shown in Figure 1.6. Recently, a special technique freeze-fracture shadow-casting
cryo-SEM (FreSCa) was successfully used on the study of assembly of
PNIPAm/MAA microgels adsorbed at a flat oil/water interface.[61]
The soft microgel
particles are also proved to be deformed with the surface chains stretching out at the
interface.
Besides, microgel particles attached on a solid surface which can be
considered as the air/solid interface have been widely investigated by AFM.[73-75]
From the information provided by AFM measurement (the morphology and
dimensions like contact radius and height), the conformation of microgels upon
deformation can be obtained. From the theoretical point, the classic model deduced by
Johnson, Kendall and Roberts (JKR model) clearly describes the deformation of rigid
particles on a solid substrate (adhesion between elastic bodies).[76-78]
However, for
small soft particles microgels which highly deform upon particle adsorption, the JKR
model is not applicable. Recent theoretical effort provided a new scaling relationship
for nanoparticle deformation where the surface energy of the particle is considered
besides the other particle parameters which are all included in JKR model.[79]
It is
demonstrated that the JKR theory is suitable at the small deformation, whereas
significant deviation from the JKR theory occurs at the high deformation (comparable
with the particle size).
1.3. Challenges in the microgel research
The microgel research has been growing fast as microgels have specific
structures and wide applications. To increase the potential of microgels in both basic
Chapter 1. Introduction
13
research and practical applications, several key challenges need to be addressed.
Firstly, new controlled synthesis route of monodisperse microgels with specific
functional monomers and structures can expand the classes of microgels with broader
applications.[1, 3]
Secondly, a precise elucidation of the internal particle structure is
necessary for better understanding properties of microgels.[2]
Thirdly, the fabrication
of novel microgel-based hybrids, such as nanoparticle/microgel, drug/microgel and
polymer/microgel complexes, can be used in catalytic and release fields.[3, 80]
Finally,
new applications like microgel-stabilized emulsion, microgel-based capsules and
porous scaffolds, need to be paid more attention.[43, 52, 81, 82]
So far, a lot of work has been done in the microgel research. Apart from the
conventional emulsion- or precipitation-polymerization, microfluidics provides
significant potential in producing monodisperse microgel particles efficiently.[83]
Photo-cross-linking is another novel method to prepare microgels by irradiation of
photo-cross-linkable copolymers.[84]
Moreover, it is also very important to control the
spatial distribution of functional groups inside the microgel particles during
synthesis.[4]
Light and neutron scattering methods are very helpful in determining the
core-corona structure of microgels.[28]
Besides that, NMR characterization has also
been successfully used to detect the heterogeneous morphology of microgels.[13, 26]
Microgels can be applied to catalyst carrier and drug delivery. Hybrids of functional
microgels and metal nanoparticles or proteins like enzymes have been produced.[15, 80]
Furthermore, microgels can also be used to stabilize emulsions which are
environmental responsive.[52]
Other novel applications, such as microgel-based
capsules and three dimensional scaffolds, have been developed as well.[43, 81, 82]
These
new forms of materials greatly extend the applications of microgels.
Chapter 1. Introduction
14
In spite of the big achievement, further investigation and concerted effort are
still required to figure out the complicated relationship between microgel structures
and properties to develop new materials and microgel-based systems.
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[43] Berger S., Zhang H. P. and Pich A., Microgel-based stimuli-responsive
capsules, Advanced Functional Materials, 2009, 19, 554-559.
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X. S., Thermo- and pH-responsive microgels for controlled release of insulin,
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microcapsules in peptide and protein drug delivery, Advanced Drug Delivery Reviews,
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[48] Bresme F. and Oettel M., Nanoparticles at fluid interfaces, Journal of Physics-
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Chapter 2. Motivation and Aims
21
Chapter 2. Motivation and Aims
The interfacial activity is an important property of microgels for their
applications in fabrication of films, emulsions or capsules. Consisting of
intramolecular cross-linked polymer networks, microgels are highly deformable. Such
a property resembles the deformation of cells during, for example, the translocation
through a small vascular channel or the attachment and adhesion of cells on a solid
substrate.
The aim of the thesis is to investigate the influence of the chemical
composition, cross-linking density and temperature on the interfacial activity of
microgels. For this, thermo-sensitive PVCL/NIPAm and PVCL/NIPMAm copolymer
microgels as well as corresponding homopolymer microgels were synthesized.
Afterwards, their properties and internal structures were determined. Then, interfacial
behaviors of those microgels at different interfaces were investigated. Interfacial
activities of microgels at the air/liquid and liquid/liquid interfaces were measured with
a pendent drop tensiometer. The deformability of microgels upon the cross-linking
density was studied through dimensions of microgels on a solid substrate (detected by
AFM). The final goal is to describe the correlation between the interfacial behavior
and the microgel deformability depending upon the cross-linking density and
temperature.
Chapter 3. Synthesis of Microgels
23
Chapter 3. Synthesis of Microgels with Controlled Cross-
linking Density and Copolymer Composition
3.1. Introduction
The controlled synthesis of microgels is very useful from the point of view of
application as desired structures and properties can be achieved. By optimization of
the synthesis procedure, we can control the particle size and its distribution, the
monomer ratio, the cross-link density and the selective incorporation of functional
groups in microgels.[1-3]
So far, the mostly investigated microgel systems are based on
NIPAm or VCL because of the temperature-responsiveness.
Since 1980s, the polymerization of NIPAm-based microgels has been
extensively developed.[4-9]
Besides, NIPMAm-[10-13]
and VCL-based[14-20]
microgels
are also synthesized and their structures and properties are thoroughly investigated.
These temperature-responsive microgels were successfully copolymerized with
ionizable monomers like acrylic acid (AAc), vinylimidazole (VIm), or 4-
vinylpyridine.[17, 20-24]
In this way, the microgels are combined with pH-sensitivity,
increasing the potential for applications.[22, 25-28]
It is implied that VCL is more
biocompatible than acrylamides, due to the direct connection of amide group to
carbon-carbon backbone chain so that its hydrolysis will not form small amide
compounds.[29-31]
Thus, copolymerization of VCL and acrylamide could be an
interesting and practical method for biomedical application of thermo-sensitive
materials. Balaceanu et. al. have synthesized PVCL/NIPAm and PVCL/NIPMAm
copolymer microgels with a one-pot procedure, and found that the monomer units of
VCL and acrylamides are statistically distributed in the colloidal networks,
Chapter 3. Synthesis of Microgels
24
independent of the copolymer composition.[32, 33]
It is found that PVCL/NIPAm
microgels have VPTT around 35 °C independently from the copolymer composition;
while PVCL/NIPMAm microgels show a nonlinear increase of VPTT from 34 to 45
°C with the increase in NIPMAm fraction in copolymer structure.
Apart from the conventional microgels with statistically distributed monomer
units, novel core-shell microgels consisting two different chemical structures in the
core and shell parts are obtained by using a two-step synthesis procedure. For
example, the core-shell microgel systems of PNIPAm/NIPMAm are synthesized with
doubly temperature-responsiveness because the monomers have different phase
transition temperatures.[34-37]
The volume transition of such core-shell microgel can be
adjusted by the cross-link density and shell thickness. Recently, core-shell microgel
systems of VCL-core/NIPMAm-shell and NIPMAm-core/VCL-shell microgels have
been developed. They were investigated by DLS and a modified Flory-Rehner theory
on the correlated morphological changes during the volume transition temperature.[38]
In this work, we mainly focus on the copolymer microgels with a random
monomer distribution. It has been revealed that the structures and properties of
microgels strongly depend on the chemical composition and cross-linker density.[17, 33,
39] Therefore, we synthesize copolymer microgels based on VCL, NIPAm, and
NIPMAm with different monomer ratios, cross-linker densities and size distributions.
In this Chapter, besides the synthesis procedures, the characterization of microgels
like chemical composition by nuclear magnetic resonance (NMR) spectroscopy and
hydrodynamic size by DLS are presented as well. The NMR results show that the
monomer ratios in the copolymer microgels are consistent with the feeding ratios in
the synthesis. From the DLS results, it is found that the swelling property of microgels
represented by the normalized hydrodynamic radii ( ⁄ ) strongly depend
Chapter 3. Synthesis of Microgels
25
on the monomer ratio and the cross-linker concentration. Besides, the addition of
surfactant during the synthesis reduces the size of microgels as expected.
Furthermore, the viscosities of suspensions of microgels with different cross-linker
densities are checked with a viscosimeter. The critical mass concentration, as the
effective volume fraction is 1, increases with the cross-liker content of microgels.
Besides the neutral PVCL/acrylamide copolymer microgels, ionizable PVCL/AAEM-
based microgels are synthesized by the addition of VIm or AAc monomers. The
hydrodynamic size and zeta potential of PVCL/AAEM/VIm and PVCL/AAEM/AAc
microgels at different pH values are detected by DLS and Zetasizer, respectively. The
pH-responsiveness of these microgels indicates a successful incorporation VIm and
AAc groups into the copolymer microgels.
3.2. Experimental section
3.2.1. Materials
N-vinylcaprolactam (VCL), N-isopropylacrylamide (NIPAm) and N-
isopropylmethacrylamide (NIPMAm) (Sigma-Aldrich) were purified by high-vacuum
distillation at 80 °C before use. Acetoacetoxyethyl methacrylate (AAEM),
vinylimidazole (VIm) and acrylic acid (AAc) were obtained from Sigma-Aldrich and
used as received. The initiators 2, 2’-azobis(2-methylpropionamidine)
dihydrochloride (AMPA) (Sigma-Aldrich) and 2, 2’-azobis(N-(2-carboxyethyl)-2-
methylpropion-amidine) (ACMA) (Wako) were used as received. The cross-linker N,
N’-methylenebisacrylamide (BIS) and surfactant cetyltrimethylammonium bromide
(CTAB) and sodium dodecyl sulfate (SDS) were obtained from Sigma-Aldrich and
used as received. Deuteroxide (D2O) was obtained from KMF GmbH and used as
received.
Chapter 3. Synthesis of Microgels
26
3.2.2. Synthesis of microgels
PVCL/acrylamide copolymer microgels
PVCL/acrylamide copolymer microgels were synthesized according to
previously reported procedures.[32]
For copolymer microgels with different chemical
compositions, appropriate amounts of VCL, NIPAm and NIPMAm (see Tables 3.1
and 3.2) and 0.06 g of BIS (3 mol%) were added to 145 mL deionized water. To
synthesize microgels with constant chemical composition but variable cross-linking
degree, VCL (1.220 g), NIPAm (0.993 g) (VCL:NIPAm = 1:1 mol:mol) were used
and the amount of BIS was varied (0.05, 0.25, 1 and 3 mol% of monomers). To obtain
microgels with the same chemical composition and cross-linking degree but various
sizes, VCL (0.397 g), NIPMAm (1.820 g) (VCL:NIPMAm = 1:5 mol:mol) were used
and the concentration of surfactant CTAB was varied (0.37 mM and 1.10 mM). For
all synthesis, a double-wall glass reactor equipped with stirrer and reflux condenser
was purged with nitrogen. The solution of the monomers was placed into the reactor
and stirred for 1 h at 70 °C with purging with nitrogen. After that the 5 mL water
solution of initiator (5 mg/mL) was added under continuous stirring, the reaction was
carried out for 8 h.
After synthesis, all microgel suspensions were dialyzed by a Millipore
Labscale TFF system with Pellicon XL Filter (PXC030C50) for 4 days. During the
dialysis process the microgel solution was pumped continuously through the
membrane with a pore size much smaller than the size of microgels to remove small
organic molecules and oligomers. The waste-water drained away directly into the
sewer and fresh water was added to the dialyzed solution to keep the total volume
constant.
Chapter 3. Synthesis of Microgels
27
The polymerization technique used is the precipitation polymerization which
involves the formation of microgel particles by a homogeneous nucleation
mechanism. The process is carried out above the volume phase transition of the
microgels, while the particles are in collapsed state. Three sets of microgel samples
were synthesized: a) with variable molar ratios between VCL and NIPAm, and VCL
and NIPMAm respectively; b) with variable cross-linking degree and c) with variable
size.
PVCL/AAEM/VIm copolymer microgels
PVCL/AAEM/VIm copolymer microgels were synthesized with the same
procedure as PVCL/acrylamide microgels, which has been reported elsewhere.[17]
The
amounts of VCL, AAEM and VIm were 1.783, 0.321 and 0.071 g, respectively. The
mole ratio for VCL/AAEM/VIm is 9:1:0.5. Microgels were synthesized with different
amounts of BIS, which were 0.06, 0.02 and 0.007 g (3, 1 and 0.35 mol% of
monomers). The monomers were added in 145 mL water in a double-wall glass
reactor equipped with stirrer and reflux condenser. Then, the mixtures were stirred for
1 h at 70 °C with purging with nitrogen. After that the 5 mL water solution of initiator
(5 g/L) was added under continuous stirring, the reaction was carried out for 8 h. As a
reference, PVCL/AAEM microgels with monomer ratio of 9:1 were synthesized
under the same condition. After synthesis, PVCL/AAEM/VIm and PVCL/AAEM
microgels were dialyzed with a cellulose membrane (MWCO 12 000 – 14 000, Carl
Roth GmbH, Germany) against water for 4 days.
PVCL/AAEM/AAc copolymer microgels
PVCL/AAEM/AAc copolymer microgels were synthesized with a reported
procedure,[22]
which is different from that of VCL/AAEM/VIm. The mole ratio for
Chapter 3. Synthesis of Microgels
28
VCL/AAEM/AAc is 9:1:0.7. Microgels were synthesized with different amounts of
BIS, which were 0.06, 0.02 and 0.007 g (3, 1 and 0.35 mol% of monomers). The
cationic initiator AMPA induces the flocculation during synthesis because of the
existence of anionic AAc groups, and a widely anionic initiator potassium persulfate
(KPS) can lead to hydrolysis of VCL groups.[31]
Thus, another anionic initiator
ACMA was used and proved to be effective.[22]
VCL (1.877 g) and AAEM (0.338 g)
were added in 120 mL water in a double-wall glass reactor equipped with stirrer and
reflux condenser. To prevent the flocculation during synthesis, SDS (0.02 g) was
added to the reaction mixture. Then, the mixtures were stirred for 1 h at 70 °C with
purging with nitrogen. After that the 5 mL water solution of ACMA (0.07 g) was
added under continuous stirring. Precursor particle core rich in AAEM is formed after
the addition of initiator, due to the faster consumption of AAEM.[14]
After 5 minutes,
25 mL water solution of AAc (0.08 g) was added to the mixture. Then the reaction
was carried out for 8 h. After synthesis, VCL/AAEM/AAc microgels were dialyzed
with a cellulose membrane (MWCO 12 000 – 14 000, Carl Roth GmbH, Germany)
against water for 4 days.
3.2.3. Characterization methods
NMR spectroscopy
The composition of copolymer microgel samples were characterized with a
400 MHz Bruker L-S NMR spectrometer at room temperature (around 23 °C). The
calibration of the chemical shift in NMR spectra was made using TMS, based on
proton of D2O (4.8 ppm).[40]
Before NMR measurements, microgel samples were
lyophilized and then dissolved in D2O.
Chapter 3. Synthesis of Microgels
29
Dynamic light scattering (DLS)
The size of the microgel particles was measured with an ALV/LSE-5004 Light
Scattering (DLS) Multiple Tau Digital Correlator and Electronics with the scattering
angle set at 90°. The samples were measured at different temperatures (from 20 to 50
°C) and the temperature fluctuations were below 0.1 °C. Prior to the measurement
microgel samples were diluted with water for chromatography from Merck Millipore,
which is filtered through a 100 nm filter before use.
Zeta potential
Zeta potential measurements were carried out on a Malvern Zetasize NanoZS.
Before the measurements, the pH of microgel solutions were adjusted to different
values using 0.1 M HCl and NaOH aqueous solution. The measurements were
performed at 25 °C after an equilibration of at least 2 min.
Viscometry
The intrinsic viscosity of microgel suspension was determined by an automatic
Schott Micro Ostwald viscosimeter equipped with a water bath. The measurement
was conducted at 25 °C.
3.3. Results and discussion
3.3.1. PVCL/acrylamides-based neutral microgels
3.3.1.1.PVCL/NIPAm and PVCL/NIPMAm microgels
The copolymer microgels of PVCL/NIPAm and PVCL/NIPMAm with
different monomer ratios are obtained from Dr. Andreea Balaceanu. The synthesis
and size information can be found in the published paper and thesis.[32, 33]
These
results show that the monomer ratio in microgels can be controlled by tuning the
Chapter 3. Synthesis of Microgels
30
feeding ratio during synthesis. It was revealed that the monomer units in
PVCL/NIPAm and PVCL/NIPMAm microgels are statistically distributed in the
colloidal networks independent of the copolymer composition. As these microgels are
used in the investigation of interfacial behavior of microgels in other Chapters, here
we exhibit the basic information of the microgels extracted from Dr. Andreea
Balaceanu’s work. Table 3.1 shows the monomer ratios in PVCL/NIPAm and
PVCL/NIPMAm copolymer microgels, and the hydrodynamic radii of the microgels
below and above VPTT (20 and 50 oC). The normalized hydrodynamic radii
⁄ that represent the swelling properties of the microgels are shown as well.
Table 3.1. Monomer amounts and copolymer composition in PVCL/acrylamides
microgels; and the hydrodynamic radii of the microgels at 20 and 50 oC and the
swelling properties represented by ⁄ .[32, 33]
Smaplea VCL
(g)
NIPAm or
NIPMAm
(g)
VCL:acrylamide
(mol:mol),
measb
at 20
oC (nm)
at 50
oC (nm)
V:N (5:1) 1.905 1.905 4.6 366 160 2.29
V:N (1:1) 1.220 1.220 1.01 431 247 1.75
V:N (1:5) 0.437 0.437 0.3 399 190 2.10
V:NM (5:1) 1.872 0.342 5.04 360 183 1.96
V:NM (1:1) 1.16 1.06 1.08 371 209 1.77
V:NM (1:2) 0.78 1.43 0.48 419 204 2.06
V:NM (1:3) 0.592 1.623 0.33 472 241 1.96
V:NM (1:5) 0.397 1.820 0.2 444 216 2.06
a, V:N and V:NM represent PVCL/NIPAm and PVCL/NIPMAm, respectively; and
the ratios in the brackets are the expected monomer ratio of copolymer microgels. b,
measured molar ratio by NMR for PVCL/NIPAm and by Raman for
PVCL/NIPMAm.
Chapter 3. Synthesis of Microgels
31
3.3.1.2. PVCL/NIPAm microgel system with different cross-linking densities
Chemical composition by NMR
Microgels are composed of intramolecular cross-linked polymer networks.
The density of cross-linker plays an imperative role in the properties of either
microgel suspension or individual microgel particle.[39, 41]
We synthesized
PVCL/NIPAm (monomer ratio 1:1) copolymer microgels with different amounts of
cross-linker (BIS), and determined their monomer ratios with NMR. Figure 3.1 shows
the NMR spectra of PVCL/NIPAm (1:1) microgel with 3 mol% BIS content in D2O,
and the chemical structures of the VCL and NIPAm units. According to previous
work, the peak at around 1.2 ppm is assigned to CH3 protons (R1) of NIPAm unit; and
resonances at around 1.6, 2.5 and 3.3 ppm are assigned to CH2 protons unconnected
with N and C=O bond (R2), connected with N (R3) and near the C=O bond (R4) in the
ring of VCL unit, respectively.[18, 32]
Through multi peaks fitting in Origin software,
the slightly overlapped peaks of R1 and R2 are separated (Figure 3.1 b).
6 5 4 3 2 1 0
R4
NO
R4
R2R2
R2
R3
R3
NH
O
R1
R1
ppm
R2 (PVCL)
R1 (PNIPAm)
a
D2O
2.5 2.0 1.5 1.0 0.5 0.0
ppm
b
R2 (PVCL)
R1 (PNIPAm)
Figure 3.1. (a) 1H NMR spectra of PVCL/NIPAm (1:1) microgel with 3 mol% BIS;
(b) the separation of peaks R1 and R2 by multi peaks fitting, determining the monomer
ratios in the copolymer.
Chapter 3. Synthesis of Microgels
32
Based on the areas of R1 and R2 peaks, the monomer ratios of VCL and
NIPAm in the copolymer microgels are calculated, and the results are presented in
Table 3.2. It is found that the measured monomer ratios in copolymer microgels are
consistent with the initial feeding monomer ratio in the reaction mixture. Considering
the high conversion of the polymerization process and the narrow size distribution of
obtained microgels, we believe that the determined values are accurate and depict real
copolymer composition of microgels with a reasonably small error range.[32]
Table 3.2. Monomer ratios of PVCL/NIPAm microgels determined by NMR.
VCL
(g)
NIPAm
(g)
BIS
(g)
BIS
(mol%)
VCL:NIPAm
(mol:mol), theora
VCL:NIPAm
(mol:mol), measb
1.220 0.993 0.060 3 1 1.08
1.220 0.993 0.030 1.5 1 0.96
1.220 0.993 0.020 1 1 1.16
1.220 0.993 0.010 0.5 1 1.01
1.220 0.993 0.005 0.25 1 0.97
1.220 0.993 0.002 0.1 1 1.17
1.220 0.993 0.001 0.05 1 1.07
a, theoretic molar ratio; b, measured molar ratio.
It should be noted that the quantitative determination of the content of cross-
linker in copolymer microgel by NMR or other spectroscopies is very difficult. That is
because the amount of cross-linker in microgel is too small compared to monomers,
and signals of cross-linker are highly overlapped by those of monomers. However, the
qualitative changes in cross-linker density can be confirmed according to the variation
in properties of microgels.
In our synthesis protocols the total concentration of monomers was kept
constant to achieve a precise and reproducible synthesis. However, we changed the
amount of cross-linker BIS in the reaction mixture to obtain microgels with variable
Chapter 3. Synthesis of Microgels
33
cross-linking concentrations without considering the slight change of the total
monomer concentration, since the amount of BIS used here was much smaller
compared to the amounts of co-monomers, VCL and NIPAm. In the one-batch
precipitation polymerization, such a small difference in the monomer concentration
would not bring a big variation in the structure and the property of microgels.
Microgel size and temperature responsiveness
The hydrodynamic sizes of the obtained microgel samples are characterized
with DLS at different temperatures. The results summarized in Figure 3.2 show that
PVCL/NIPAm microgels exhibit temperature sensitivity and the volume phase
transition temperatures (VPTT) are around 35 °C, independent on the cross-linker
content.
20 30 40 50 60100
200
300
400
500a 3%
1.5%
1%
0.5%
0.25%
0.1%
0.05%
Rh (
nm
)
Temperature (oC)
20 30 40 50 60
1.0
1.5
2.0
2.5
3%
1.5%
1%
0.5%
0.25%
0.1%
0.05%
Rh /
Rh
, co
llap
sed
Temperature (oC)
b
Figure 3.2. (a) Hydrodynamic radii ( ) and (b) normalized hydrodynamic radii
( ⁄ ) of the PVCL/NIPAm microgels with different cross-linker contents
versus temperature. The legends show cross-linkers contents in mol%.
The measured hydrodynamic radii ( ) of microgels are normalized by
dividing the hydrodynamic radii in the collapsed state ( ), and plotted
versus temperature (Figure 3.2b). We can see that the swelling capability of microgel
Chapter 3. Synthesis of Microgels
34
decreases as the cross-linker density increase (the trend shown by the arrow). This
indicates that the microgel network becomes more compact and less elastic at higher
cross-linker density, leading to lower swelling capability. It is believed that in
aqueous solution at temperature above VPTT microgel particles in the collapsed state
still contain considerable amount of water[42]
and therefore should have larger size
compared to completely dried particles.
Viscosity of PVCL/NIPAm microgel suspensions
It has been reported that the viscosity of suspensions of hard spheres depends
only on the volume fraction of particles.[43]
Such kind of concept has also been
successfully employed for colloidal suspensions, where the effective volume fraction
( ) needs to be considered.[39, 44]
An expression given by Batchelor for hard-sphere
system, which has been proved to be valid for microgel systems, is used for
describing the relationship between the relative viscosity and the effective volume
fraction.[39, 43, 44]
(Equation 3.1)
Here, the effective volume fraction can be substituted by , is the
mass concentration of the microgel dispersion and is the shift factor for converting
the mass concentration to the effective volume fraction. The relative viscosities of
diluted solutions PVCL/NIPAm (1:1) microgels with different cross-linker densities
are shown in Figure 3.3. The dash lines are fits according to Equation 3.1, and the
values of shift factor are summarized in Figure 3.4. We can see that the value of
increases with the decrease in cross-linker density. The variation tendency of with
cross-linker density here agrees with the reported results.[39]
Chapter 3. Synthesis of Microgels
35
A critical mass concentration can be calculated as ⁄ when
. If we intend to study the behavior of individual particle at the interface, the
microgel solutions should be diluted to certain concentration below . According
to Senff’s work, the effective volume fraction can be compared to the volume fraction
of the “dry” polymer which is calculated from the mass content and the polymer
density.[44]
From Figure 3.4, we can see that decreases with the reduction of
cross-linker content. It means that the microgels with lower cross-linker contents can
reach a certain effective volume fraction at lower concentrations compared to the
microgels with higher cross-linker contents. This might be due to the increasing
swelling capability of microgels with decreasing cross-linking densities, as described
in Figure 3.2b.
0.0 0.2 0.4 0.6 0.8 1.01.0
1.2
1.4
1.6
1.8
3%
1.5%
1%
0.5%
0.25%
0.1%
0.05%
re
l
c (wt%)
Figure 3.3. Relative viscosity of PVCL/NIPAm microgels with different cross-linker
contents at 25 °C versus the mass concentration of microgel dispersion. The cross-
linker content is given in mol% in the legends. The dash lines show fits based on
Equation 3.1.
Chapter 3. Synthesis of Microgels
36
0 1 2 30
10
20
30
40
50
0.00
0.05
0.10
0.15
0.20
0.25
cc
rit (
wt%
)
k
BIS content (mol%)
Figure 3.4. The values of shift factor k and critical mass concentration ( )
versus the cross-linker content for PVCL/NIPAm microgels.
3.3.1.3. Microgels synthesized with different amounts of surfactant
The addition of surfactant during the synthesis can reduce the size of
microgels.[1, 8]
The role of the surfactant in the microgel synthesis is to increase the
colloidal stability of the precursor particles and subsequently lower the size of the
primary particles as the number of the particles is increased. With the increase in the
concentration of surfactant, more precursor particles survive from the aggregation and
grow to a smaller size as the monomer amount is constant.[45]
Here, we add different amounts of CTAB to the reaction mixture during the
synthesis of PVCL/NIPMAm (1:5) copolymer microgel. Through DLS results (Figure
3.5), we can see that with the increase in the concentration of CTAB, the
hydrodynamic radii of microgels decrease. In the swollen state, the hydrodynamic
radius of PVCL/NIPMAm microgel is around 450 nm without CTAB during the
synthesis, while the radii of microgels are around 200 and 60 nm with 0.37 and 1.10
mM CTAB added during the synthesis, respectively. In the collapsed state, Rh of
PVCL/NIPMAm microgels decrease from 200 nm without CTAB to 20 nm with 1.10
Chapter 3. Synthesis of Microgels
37
mM CTAB. Although the microgels size varies with the amount of surfactant added
during synthesis, the swelling ability remains the same for microgels, regardless of
surfactant. It is because the surfactants do not change the chemical composition and
the cross-linker density of the microgels. From Figure 3.5b, it can be observed that
curves of ⁄ versus temperature of microgels synthesized with different
amounts of CTAB highly overlap with each other.
20 30 40 50 600
100
200
300
400
500
Rh (
nm
)
Temperature (oC)
NO CTAB
0.37mM CTAB
1.10mM CTAB
a
20 30 40 50 60
1.0
1.5
2.0
2.5
b
Rh / R
h,
co
lla
ps
ed
Temperature (oC)
NO CTAB
0.37mM CTAB
1.10mM CTAB
Figure 3.5. (a) Hydrodynamic radii ( ) and (b) normalized hydrodynamic radii
( ⁄ ) of the PVCL/NIPMAm (1:5) microgels synthesized with different
amounts of CTAB. The symbols of triangle, circle and square represent 0, 0.37 and
1.10 mM CTAB added during synthesis, respectively.
3.3.2. PVCL-based microgels with functional groups
By incorporation of monomers with ionizable groups, the temperature-
sensitive microgels can be endowed with pH-responsiveness. PVCL/AAEM
copolymer microgels with thermo-sensitivity have been developed by precipitation
polymerization and investigated extensively.[14-16, 46]
Pich and coworkers have
successfully incorporated VIm groups in PVCL/AAEM-based microgels.[17]
Recently,
PVCL/AAEM/AAc copolymer microgels are obtained through the copolymerization
of VCL, AAEM and AAc monomers.[22]
With the units of PVCL and ionizable groups
Chapter 3. Synthesis of Microgels
38
of VIm and AAc, the PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels exhibit
both temperature- and pH-sensitivity. Such temperature- and pH-sensitive microgels
may be used in drug release and catalysis.[22, 25]
3.3.2.1. PVCL/AAEM/VIm microgels
In the synthesis of PVCL/AAEM/VIm copolymer microgels with various
cross-linker contents (3, 1 and 0.35 mol%), the content of VIm in monomers is kept
constant at 5 mol%. After the synthesis and purification, PVCL/AAEM/VIm
microgels with different cross-linker densities are characterized by 1H NMR.
10 8 6 4 2 0
8 7 6
3
2
dc
b
ppm
a
1
N
N
1 2
3
Figure 3.6. 1H NMR spectra of PVCL/AAEM microgels with 1 mol% BIS (curve a),
and PVCL/AAEM/VIm microgels with BIS content of 3 mol% (curve b), 1 mol%
(curve c) and 0.35 mol%. The inset highlights the interesting part (6 – 8 ppm).
From Figure 3.6, we can see that in the 1H NMR spectra of
PVCL/AAEM/VIm microgels, peaks between 7 – 8 ppm belonging to protons in VIm
group appear, contrary to PVCL/AAEM microgel.[47]
The inset in Figure 3.6
highlights the interesting part (6 – 8 ppm), where peak 1, 2 and 3 represent protons of
CH between two nitrogen atoms (7.67 ppm), CH close to C–N (7.22 ppm) and CH
Chapter 3. Synthesis of Microgels
39
close to C=N (7.04 ppm), respectively. It shows that the incorporation of VIm group
in microgels is successful.
The hydrodynamic sizes of PVCL/AAEM/VIm microgels are detected by
DLS, and the results are presented in Figure 3.7. At 20 °C, the hydrodynamic radii of
microgels are around 350 nm for those synthesized with 3 and 1 mol% BIS, and 450
nm for those synthesized with 0.35 mol% BIS. However, microgels in collapsed state
are about 130 nm, regardless of cross-link density. That means the number of polymer
segments in a microgel particle does not change with cross-linker amount for
PVCL/AAEM/VIm microgels. Through the normalized hydrodynamic radii
( ⁄ ) of the PVCL/AAEM/VIm microgels, we can see that the swelling
capability of microgels with 1 mol% is close to those with 3 mol% BIS, but much
lower than those with 0.35 mol%.
20 30 40 50 60100
200
300
400
500
Rh (
nm
)
Temperature (oC)
0.35%
1%
3%
a
20 30 40 50 60
1
2
3
4
b
Rh / R
h,
co
lla
ps
ed
Temperature (oC)
0.35%
1%
3%
Figure 3.7. (a) Hydrodynamic radii ( ) and (b) normalized hydrodynamic radii
( ⁄ ) of the PVCL/AAEM/VIm microgels at pH 7. The symbols of
triangle, circle and square represent 3%, 1% and 0.35% BIS in mol added during the
synthesis, respectively.
As reported, the size of PVCL/AAEM/VIm microgels can be influenced by
pH of the solution.[17]
Figure 3.8 presents the pH-dependence of hydrodynamic radii
Chapter 3. Synthesis of Microgels
40
and zeta potential of PVCL/AAEM/VIm microgels synthesized with 1 mol% BIS.
The variation tendency of microgel size highly agrees the variation of zeta potential
with pH. As pH of the solution decreases from 8, of PVCL/AAEM/VIm microgels
increase gradually due to the ionization of VIm groups, which leads to the
enhancement of electrostatic repulsion in microgel particles (an increase in zeta
potential). When the solution pH is higher than 8, the zeta potential of microgels is
close to zero and the microgel size remains at a constant value as VIm units stay in
neutral state. A maximum of PVCL/AAEM/VIm microgel can be found at pH
around 3.5, which correlates with the maximum ionization of VIm groups. The
decrease of microgel size at pH < 3.5 is attributed to the screening effect of the
electrostatic repulsion between VIm groups by excess of protons. In the contrast, the
change in solution pH has no influence on the hydrodynamic radius of PVCL/AAEM
microgel, as the zeta potential does not change with pH. It has been confirmed that the
VIm content can also influence the microgel dimension.[17]
Here, we mainly focus on
the microgel synthesized with a constant concentration of VIm (5 mol%).
2 3 4 5 6 7 8 9 10 11100
200
300
400
500 VCL/AAEM/VIm
VCL/AAEM
Rh (
nm
)
pH
a
2 4 6 8 10-20
-10
0
10
20
Ze
ta P
ote
nti
al (m
V)
pH
VCL/AAEM/VIm
VCL/AAEMb
Figure 3.8. Influence of pH on (a) hydrodynamic radii and (b) zeta potential of
PVCL/AAEM/VIm and PVCL/AAEM microgels with 1 mol% BIS at 20 °C.
Chapter 3. Synthesis of Microgels
41
3.3.2.2. PVCL/AAEM/AAc microgels
It is difficult to determine the existence of AAc group in PVCL/AAEM/AAc
microgels by spectroscopy because AAEM unit also contain carboxyl groups.
Nevertheless, the influence of pH changes on microgel size suggests that the
incorporation of AAc in the copolymer microgel is successful. The hydrodynamic
radii of PVCL/AAEM/AAc microgels were detected by DLS, as shown in Figure 3.9.
20 30 40 50 60 70 8060
70
80
90
100
Rh (
nm
)
Temperature (oC)
0.35%
1%
3%
a
20 30 40 50 60 70 80
1.0
1.1
1.2
1.3
1.4
1.5
b
Rh / R
h,c
oll
ap
se
d
Temperature (oC)
0.35%
1%
3%
Figure 3.9. (a) Hydrodynamic radii ( ) and (b) normalized hydrodynamic radii
( ⁄ ) of the PVCL/AAEM/AAc microgels at pH 6. The symbols of
triangle, circle and square represent 3%, 1% and 0.35% BIS in mol added during
synthesis, respectively.
At 20 °C, the hydrodynamic radii of PVCL/AAEM/AAc microgels are around
95 nm for those synthesized with 3 and 1 mol% BIS, and 85 nm for those synthesized
with 0.35 mol% BIS. As temperature increases to 40 °C, the hydrodynamic sizes of
microgels with different cross-linker content decrease as microgel particles shrink.
When temperature is above 40 °C, microgels are in collapsed state, and the
hydrodynamic radii remain at a constant value, around 70 nm for microgel with 3
mol% BIS and 65 nm for microgels with 1 and 0.35 mol% BIS. Through the
normalized hydrodynamic radii ( ⁄ ) of the PVCL/AAEM/VIm
Chapter 3. Synthesis of Microgels
42
microgels, we can see that the swelling capability of microgels is independent on the
cross-linker content, unlike PVCL/AAEM/VIm microgels.
5 6 7 8 9 1060
90
120
150
180
-35
-30
-25
-20
-15
-10
Rh (
nm
)
pH
Zeta
Po
ten
tial (m
V)
Figure 3.10. Influence of pH on hydrodynamic radii (solid square) and zeta potential
(open square) of PVCL/AAEM/AAc microgels with 1 mol% BIS at 20 °C.
It should be noted that the PVCL/AAEM/AAc microgels are synthesized in
the presence of SDS. Without the addition of surfactant, coagulation occurs. The sizes
of them are smaller compared to those of PVCL/AAEM/VIm micorgels in either
swollen state or collapsed state.
3.4. Conclusions
We synthesized the copolymer microgels based on VCL, NIPAm, and
NIPMAm with different monomer ratios (the mole ratio of VCL to acrylamides varies
from 0.2 to 5). The chemical composition of copolymer microgels were determined
by NMR spectroscopy. It is found that the monomer ratio in copolymer microgel is
close to the initial feeding ratio during the synthesis.
Chapter 3. Synthesis of Microgels
43
PVCL/NIPAM (1:1) microgels with different cross-link densities (0.05 – 3
mol%) were obtained. The viscosity of microgel solution reflects the variation
tendency of cross-linker content. Microgels with lower cross-linker contents can reach
a certain effective volume fraction at lower concentrations compared to the microgels
with higher cross-linker contents. The hydrodynamic sizes of all microgels are
characterized by DLS at different temperatures. As expected, the swelling property of
microgels strongly depends on the synthesis condition such as monomer feeding ratio,
the concentrations of cross-linker, and the addition of surfactant.
Besides the neutral PVCL/acrylamide copolymer microgels, ionizable
PVCL/AAEM-based microgels are synthesized by the addition of VIm or AAc
monomers. The hydrodynamic size and zeta potential of PVCL/AAEM/VIm and
PVCL/AAEM/AAc microgels at different pH values are detected by DLS and
Zetasizer, respectively. The pH-responsiveness of these microgels indicates a
successful incorporation VIm and AAc groups into the copolymer microgels.
The successful synthesis of microgels with controlled cross-link density and
copolymer composition is achieved in this Chapter. The structures and interfacial
behaviors of these microgels will be in detail discuss in the following Chapters.
3.5. References
[1] Pelton R. and Hoare T., Microgels and their synthesis: An introduction In
Microgel Suspensions; Wiley-VCH Verlag GmbH & Co. KGaA: 2011, p 1-32.
[2] Pich A. and Richtering W., Microgels by precipitation polymerization:
Synthesis, characterization, and functionalization In Chemical Design of Responsive
Microgels; Pich A. and Richtering W., Eds. 2010; Vol. 234, p 1-37.
[3] Saunders B. R., Laajam N., Daly E., Teow S., Hu X. H. and Stepto R.,
Microgels: From responsive polymer colloids to biomaterials, Advances in Colloid
and Interface Science, 2009, 147-48, 251-262.
Chapter 3. Synthesis of Microgels
44
[4] Pelton R. H. and Chibante P., Preparation of aqueous lattices with N-
isopropylacrylamide, Colloids and Surfaces, 1986, 20, 247-256.
[5] Acciaro R., Gilanyi T. and Varga I., Preparation of monodisperse poly(N-
isopropylacrylamide) microgel particles with homogenous cross-link density
distribution, Langmuir, 2011, 27, 7917-7925.
[6] Meng Z. Y., Smith M. H. and Lyon L. A., Temperature-programmed synthesis
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Chapter 3. Synthesis of Microgels
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Chapter 3. Synthesis of Microgels
46
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Chapter 3. Synthesis of Microgels
47
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Chapter 4. Internal Structure of Microgels
49
Chapter 4. Internal Structure of Microgels by Light
Scattering and Microscopy
4.1. Introduction
Microgels are chemically cross-linked polymer networks, which are swollen
by good solvents. The incorporation of cross-linker into the microgel network is often
inhomogeneous, due to its faster consuming rate than monomers during the
copolymerization.[1]
This brings a complex and heterogeneous network structure,
affecting the properties of microgels.
The inhomogeneous structure of microgels has been investigated by proton
NMR transverse relaxation measurements.[2, 3]
As different kinds of proton mobility
are detected, the structure regions in microgel particles can be classified in two parts,
core and corona. In the core-corona model, a higher cross-linking degree in core part
is suggested. The scattering methods including small-angle neutron scattering (SANS)
as well as static light scattering (SLS) and dynamic light scattering (DLS) have also
been widely used to investigate the internal structure of microgels.[4-9]
The results
confirmed the core-corona structure of microgels. Such kind of structure has a radial
profile, where the density of cross-linker decreases from the center towards the
surface of microgels.[5]
Microscopy is a powerful method for direct observation of microgel particles.
The visualization of microgel particles has been successfully achieved by scanning
electron microscopy (SEM), transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM).[10-13]
From the microgel morphologies
observed by electron microscopes, the core-corona structure is recognized.[11, 14, 15]
Chapter 4. Internal Structure of Microgels
50
Cryo-SEM and Cryo-TEM are also useful approaches to capturing the microstructure
of microgel particles in solution state.[16-18]
Besides the electron microscope,
microgels tagged with fluorescein can also be visualized under confocal
microscopes.[16, 19]
However, with limited resolution, the confocal microscopes hardly
detect the detailed microstructure within the microgels. To determine structures of
soft samples at the nanoscale, atomic force microscopy (AFM) is widely used. The
AFM measurements can be carried on at trivial conditions, such as room ambient and
even under water, unlike SEM or TEM where high vacuum condition is needed. The
observation of PNIPAm-based microgels with AFM gives not only the morphology
but also the dimension information like contact radius and height.[20-22]
The low-
height corona part of microgels with heterogeneous structure can be readily captured
as AFM works well at the nanoscale. Moreover, the AFM measurements can be
implemented under water to detect microgels adsorbed at an interface.[23-26]
Microgel
particles, even in collapsed state, contain a lot of water in a solution.[27, 28]
The
particles are very soft and the fluffy polymer chains at the surface of microgels swing
in water. Therefore the in-situ characterization of microgels under water with AFM
can hardly observe the core-corona structure. However, the AFM observation on
microgel samples on solid substrates at dry state can show us an obvious core-corona
structure of microgel particles.[22]
Here, we investigate the internal structure of PVCL-based copolymer
microgels with SLS and microscopy methods (AFM and FESEM). The SLS results
show that PVCL/NIPAm and PVCL/NIPMAm microgels, rich in either VCL or
acrylamides, have core-corona structures with a radial profile. The observation of
PVCL/NIPAm copolymer and PVCL homopolymer microgels by AFM provides a
visualized evidence of core-corona structure that influenced by cross-linker content.
Chapter 4. Internal Structure of Microgels
51
We also deposit PVCL/NIPAm copolymer microgels on silicon wafer at the
temperature above the VPTT of microgels. The microgels are fixed on the solid
substrate in collapsed state due to the fast spin-coating process. The AFM observation
shows that the microgels do not have a core-corona structure as the fuzzy corona part
of the microgel contracts with the core part to form a single monolithic structure.
Additionally, PVCL/AAEM-based microgels with functional groups are characterized
with AFM and FESEM. It is interesting that the core-corona structure is found in
PVCL/AAEM /VIm microgels, but not in PVCL/AAEM/AAc microgels. Moreover,
the AFM observation on microgels of very low cross-linking density provides a
possible method to look into the domains in the cross-linked polymer networks.
4.2. Experimental section
4.2.1. Sample preparation
Microgel synthesis
PVCL/NIPAm microgels with different monomer ratios and cross-linker
contents are used in this Chapter. The synthesis and size characterization of microgels
are presented in detail in Chapter 3.
Deposition of microgels on a solid substrate
The deposition of microgels was carried out on a spin-coating machine
(Convac 1001S, Germany) with the speed of 3000 rpm for 1 min. The silicon wafer
used for deposition was cleaned by sonication in isopropanol for 10 min, dried in air
stream, and treated with UV/O2 for 12 min. The concentration of microgel solution
used for spin coating was 0.2 mg/mL. At this microgel concentration, sufficient
amount of particles could be well distributed on the silicon wafer without clustering
Chapter 4. Internal Structure of Microgels
52
effect. The quality of the samples was evaluated with a Zeiss Axioplan 2 optical
microscope. For each sample, a drop of 75 μL microgel solution was used. The
deposition process was conducted at room temperature (about 23 °C).
For samples prepared above VPTT, the microgel solution was incubated in a
water bath at 50 °C for around 10 min. The freshly cleaned silicon wafer was fixed on
the spin-coating machine and heated with an infrared lamp. The infrared lamp was
hung around 5 cm on the top of the spin-coating plate, and kept working during the
spin-coating process.
4.2.2. Characterization methods
Light scattering
The size of the microgel particles was measured with an ALV/LSE-5004 Light
Scattering (DLS) Multiple Tau Digital Correlator and Electronics with the scattering
angle set at 90°. The samples were measured at 20 °C and the temperature fluctuations
were below 0.1 °C. Prior to the measurement microgel samples were diluted with
water for chromatography from Merck Millipore, which is filtered through a 100 nm
filter before use. The SLS measurements were conducted on a Sofica with a
wavelength of 633 nm and a Fica with a wavelength of 495 nm at 20 °C. The
measured data were fitted with a model of polydisperse microgels.[5, 6]
Microscopy characterization
AFM images were recorded on Agilent 5500 and Veeco Nanoscope Atomic
Force Microscopes with a tapping mode. Cantilevers with resonance frequencies of
250 – 300 kHz and spring constants around 42 N/m were used. The data were
analyzed with Agilent Picoimage, Nanoscope Analysis and ImageJ software. FESEM
Chapter 4. Internal Structure of Microgels
53
images were obtained on a Hitachi S4800 high resolution scanning electron
microscope with field emission cathode at an accelerating voltage of 1 kV.
4.3. Results and discussion
4.3.1. Internal structure of microgels by light scattering
For a homogeneous sphere with the radius , the distribution of polymer mass
is uniform inside the particle, where the form factor is
{ [ ]
}
. However, for microgel particles, it has been
proved that the cross-link density is inhomogeneous in the particle, due to a higher
reactivity of cross-linker. It means the cross-link density inside the microgel is higher
than the outside part, which leads to a fuzzy surface of particles as shown in Figure
1.6. Stieger and coworkers[5]
proposed a model of the polymer mass profile of
microgel particle with radius and a Gaussian of width , implying that the
radius measured by scattering method is . In the inhomogeneous
model, the form factor is { [ ]
[
]}
. Such
model has been successfully employed in PNIPAm microgels.[5, 6]
Here we use SLS to
investigate copolymer microgels of PVCL/NIPAm and PVCL/NIPMAm. Each
system includes two copolymer compositions: one is VCL-rich and the other is
acrylamide-rich. The dependence of intensity on of PVCL/NIPAm and
PVCL/NIPMAm microgels is presented in Figure 4.1.
Chapter 4. Internal Structure of Microgels
54
1E-3
1E-5
1E-4
1E-3
0.01
0.1
I(q
) (a
.u.)
q (nm-1)
PVCL/NIPAm (5:1)
inhomogeneous microgel fit
homogeneous sphere fit
a
1E-31E-5
1E-4
1E-3
0.01
I(q
) (a
.u.)
q (nm-1)
PVCL/NIPAm (1:5)
inhomogeneous microgel fit
homogeneous sphere fit
b
1E-31E-5
1E-4
1E-3
0.01
0.1
I(q
) (a
.u.)
q (nm-1)
PVCL/NIPMAm (5:1)
inhomogeneous microgel fit
homogeneous sphere fit
c
1E-3
1E-5
1E-4
1E-3
0.01
I(q
) (a
.u.)
q (nm-1)
PVCL/NIPMAm (1:5)
inhomogeneous microgel fit
homogeneous sphere fit
d
Figure 4.1. Static light scattering profiles of (a) PVCL/NIPAm (5:1), (b)
PVCL/NIPAm (1:5), (c) PVCL/NIPMAm (5:1) and (d) PVCL/NIPMAm (1:5) at 20
°C. The dash-dotted lines represent homogeneous sphere fit, and the solid lines
represent inhomogeneous microgel fit.
Both inhomogeneous microgel fit and homogeneous sphere fit are performed
on the static light scattering profiles. We can see that the inhomogeneous microgel
model fits the experimental data very well, while the homogeneous sphere fit deviates
from the experimental result a lot. This means PVCL/NIPAm and PVCL/NIPMAm
copolymer microgels, rich in either VCL or acrylamide, are inhomogeneous in
polymer mass distribution.[5, 6]
Based on the inhomogeneous microgel fit, the values
of and are obtained and summarized in Table 4.1. For PVCL/NIPAm and
PVCL/NIPMAm copolymer microgels, the values of are around 50 nm,
Chapter 4. Internal Structure of Microgels
55
indicating the thickness of the fuzzy surface of the particle. The overall particle size
from SLS ( ) is smaller than the hydrodynamic radius from DLS ( ). It is attributed
it to that some dangling polymer chains attached to the microgel surface contribute to
the hydrodynamics.[5]
An interesting observation is that the difference between and
is much larger for VCL-rich copolymer microgels than NIPAm- and NIPMAm-
rich copolymer microgels. It could be originated from the different chemical
structures. However, the intrinsic reason is still unknown and needs further
investigation.
Table 4.1. Summary of parameters obtained by inhomogeneous microgel fit of SLS
data and hydrodynamic radius from DLS.
Samples (nm) (nm) (nm)a (nm)
b
V:N (5:1)c 206 51 308 366
V:N (1:5) 295 47 389 399
V:NM (5:1) 198 45 288 351
V:NM (1:5) 336 47 430 444
a, ; b, is the hydrodynamic radius from DLS; c, V:N and V:NM
represent the monomer ratio of VCL:NIPAm and VCL:NIPMAm in copolymer
microgels.
4.3.2. Microscopy observation of microgels with different cross-linking degrees
Using proton transverse relaxation NMR and Flory temperature-induced
volume transition theory, Balaceanu and coworkers have revealed that PVCL
homopolymer microgels have a core-corona structure, and the polymer segment
distribution is influenced by cross-linker concentration.[3]
However, the direct
microscopy observation on the structure of PVCL homopolymer microgels is seldom
reported. Herein, we used AFM to characterize PVCL homopolymer microgels. From
Chapter 4. Internal Structure of Microgels
56
the images in Figure 4.2, we can see that PVCL homopolymer microgels also show
core-corona structures.
Figure 4.2. AFM topography images of PVCL homopolymer microgels with different
BIS contents: 6 mol% (a and d), 4 mol% (b and e), and 1.6 mol% (c and f). Graphs
(d), (e) and (f) are the magnified scanning of square area in (a), (b) and (c),
respectively. The lines in d), e) and f) highlight the height of corona part of microgels.
Through the cross section profile (Figure 4.3), we can obtain the height and
contact angle of microgel particles on silicon wafer, as summarized in Table 4.2. We
can see that as the amount of cross-linker decreases, the height and contact angle of
microgels decreases. For PVCL microgels with 1.6 mol% BIS, the contact angle is
even lower than 10°, and the cross section profile is very flat, indicating a strong
deformation of microgels on the solid surface.
Chapter 4. Internal Structure of Microgels
57
0 100 200 300 400 500 600 700
0
20
40
60
80
1.6 mol% BIS
4 mol% BIS
nm
nm
6 mol% BIS
Figure 4.3. The height cross-section profiles of PVCL homopolymer microgels with
different BIS contents at the position indicated by white lines in Figure 4.3 d, e and f.
Table 4.2. Dimensions of PVCL microgels with different cross-linker (BIS) contents
deposited on the silicon wafer detected by AFM.
BIS content
(mol%)
Contact radius
( , nm)
Height
( , nm)
Contact angle
( , °)
6 255 72 70
4 268 44 46
1.6 291 12 8
Besides the PVCL microgels, the PNIPAm-based microgels also have core-
corona structures, detected by cryo-FESEM or AFM.[16, 17, 22, 29]
Figure 4.4 shows the
AFM images of some PVCL/NIPAm (1:1) microgels varied in cross-linker contents.
We can see that copolymer microgels exhibit the representative core-corona structure
in AFM images, especially in phase images. Due to the low density of polymer
network, the corona part of microgel is very thin in thickness, which can be found in
the height cross-section profile (around 2 nm). As cross-linker content in microgels
decreases, the size of corona part becomes larger. The same as PVCL microgels, the
height of PVCL/NIPAm copolymer microgels decreases with the cross-linker content.
Chapter 4. Internal Structure of Microgels
58
0 1 2 3 4 5
0
50
100
1.6 1.8 2.00
2
4g
nmm
0 1 2 3 4 5
0
20
40
60
1.4 1.6 1.8-2
0
2
nmm
h
0 1 2 3 4 5
0
10
20
30
0.0 0.5 1.0 1.5
0
1
2
i
nmm
Figure 4.4. AFM results of PVCL/NIPAm microgels with different BIS contents: 3
mol% (a, d and g), 1 mol% (b, e and h), and 0.5 mol% (c, f and i). Graphs (a), (b) and
(c) are topography images; (d), (e) and (f) are phase images; (g), (h) and (i) are the
height cross-section profiles of microgels at the position indicated by white lines in
(a), (b) and (c). The insets in (g), (h) and (i) highlight the height of corona part of
microgels (~ 2 nm).
The dimensions of PVCL/NIPAm copolymer microgels upon cross-linking
density are summarized in Table 4.3. We can see that the height and the contact angle
of microgels on the solid substrate decrease with the reduction of cross-linker content.
For microgels with 3 mol% BIS, the height is around 100 nm and the contact angle is
30 degree, whereas for microgels with 0.05 mol% BIS, the height is less than 10 nm
Chapter 4. Internal Structure of Microgels
59
and the contact angel is even less than 1 degree. The contact radius ( ) of
microgels increases with the cross-linker content: from around 500 nm for microgels
with 3 mol% BIS to more than 2200 nm for microgels with 0.05 mol% BIS. The
results show that the microgels, especially with low cross-linker contents, are highly
spread on the solid substrate. The deformation of microgels upon cross-linking
density will be discussed in detail in Chapter 5.
Table 4.3. Dimensions of PVCL/NIPAm (1:1) microgels with different cross-linker
contents deposited on the silicon wafer detected by AFM.
BIS content
(mol%)
Contact radius
( , nm)
Height
( , nm)
Contact angle
( , °)
3 510 97 30.0
1.5 862 53 12.0
1 915 39 9.8
0.5 1204 27 6.5
0.25 1295 20 2.7
0.1 1354 11 1.4
0.05 2241 8 0.8
PVCL/NIPAm (1:1) microgels with 3 mol% BIS are deposited on the silicon
wafer at 50 °C which is above the VPTT of microgels. The sample is characterized
with AFM, and the results are shown in Figure 4.5. Compared with the sample
prepared at room temperature (Figures 4.2a, d and g), the size of microgel deposited
above VPTT is much smaller. Furthermore, the microgels do not show a core-corona
structure, in both topography and phase images. From Figure 4.5c, we can see that the
ends of the height cross-section profile do not have flat part with low height as seen
for samples prepared at room temperature.
Chapter 4. Internal Structure of Microgels
60
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
0.35 0.40 0.45
0
5
10
mnm
c
Figure 4.5. (a) Topography and (b) phase images of PVCL/NIPAm (1:1) microgels
with 3 mol% BIS deposited on silicon wafer at 50 °C; (c) the cross-section profiles of
microgels at the position indicated by white lines in (a).
As temperature increases from room temperature to higher ones above VPTT,
the polymer chains collapse and the microgels shrink to a smaller size. It is indicated
that the volume of the corona part is reduced more than the core part during the
volume phase transition.[30]
The fuzzy corona part of the microgel contracts with the
core part to form a single monolithic structure.[5, 6]
Some work has been reported on
the in situ observation of microgels under water by AFM.[23-26]
The volume phase
transition of microgels on a substrate was probed through increasing temperature.
However, the microgel particles are restricted due to a strong adhesion between the
polymer chains and the substrate, especially for microgels with low cross-linking
density.[23, 24, 26]
Therefore, the microgel size changes significantly in the vertical
direction (height) but slightly in the horizontal direction (contact radius). For
microgels with high cross-linker contents, the restriction of the adsorption, though
Chapter 4. Internal Structure of Microgels
61
still exists, is not very strong and the particles can shrink at both height and contact
radius.[26, 31]
Here, most water is expelled out from the microgel upon the fast spin-
coating process and the whole process for the sample preparation is kept at a
temperature above VPTT. Thus, the microgels are fixed on the solid substrate in
collapsed state. The result should better reflect the morphology and size of microgels.
4.3.3. Polymer network domains in microgels observed by AFM
Polymer gels, including microgel, consist of the three-dimensional network of
flexible polymer chains with chemical or physical cross-linking.[32]
It is assumed that
the cross-linked network structure can be divided into small domains centered with
cross-linking junctions.[32, 33]
Keerl et al. infered internal nanophase separated ‘dirty
snowball’ structure in PNIPAm/NIPMAm copolymer microgels at the phase
transition temperature from SANS results, and the size of collapsed segment is around
10 nm.[34]
A direct observation of PNIPAm and poly (acrylamide) gel surfaces has
been successfully implemented on AFM by Suzuki and coworkers.[35]
Sponge-like
domains of 100 nm size and of 10 nm size were found in PNIPAm and PAAm (poly
acrylamide) gel, respectively. Normally, the observation of microgels by AFM is
executed on the samples with high or medium cross-linking degree. The overlaps of
layers of polymer networks make the surface of microgel smooth under AFM. Here,
we use AFM to study the PVCL/NIPAm microgels with a very low BIS content (0.05
mol%) specifically.
As shown in Figure 4.6, the sponge-like domains is clearly found. In the
topography images, the dark-colored meshes are surrounded by the bright-colored
domains, like holes distributed on a polymer film. The average size of sponge-like
domains, analogous to the center-to-center distance of neighboring holes, can be
Chapter 4. Internal Structure of Microgels
62
estimated by power spectral density (PSD) profiles converted from AFM images.[36]
From Figure 4.6c, the peak of PSD profile is located at 23 μm-1
, which means the
average domain size in PVCL/NIPAm (0.05 mol% BIS) microgels is around 43 nm. It
should be noted that even with very low cross-link degree, the overlaps of polymer
network layers could not be avoided, that obstructs the accuracy of estimating the
domain size. Therefore, the domain size obtained here is larger than that gained from
scattering methods.[34]
The average diameter of holes in Figure 4.6a, which can be
considered as polymer network mesh, is around 17 nm. It is bigger than the mesh size
(around 2.5 nm) estimated for PNIPAm microgels with higher cross-linker content (~
1 mol%) by Saunders.[7]
0 20 40 60 80 100 120 140
-4
-3
-2
-1
0
1
Lo
g (
PS
D/n
m2)
d-1 (m
-1)
c
Figure 4.6. (a) Topography and (b) phase images of sponge-like domains detected by
AFM in PVCL/NIPAm microgels with a low BIS content (0.05 mol%); (c) a 2D
power spectral density plot of image (a), d in the horizontal axis is the center-to-center
distance of dark dots.
4.3.4. Microscopy observation of PVCL-based microgels with functional groups
According to the reported procedures,[11, 37]
PVCL/AAEM/VIm and
PVCL/AAEM/AAc microgels with different cross-linking degrees were synthesized.
As shown in Chapter 3, the incorporation of functional groups brings the copolymer
microgels temperature and pH-sensitive properties. The morphologies of
Chapter 4. Internal Structure of Microgels
63
PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels were obtained with AFM and
FESEM, shown in Figures 4.7 and 4.8.
The microscopy results show that PVCL/AAEM/VIm microgels exhibit
distinct core-corona structures, regardless of cross-linking degree. The corona-like
shell in dark shadows surrounding the PVCL/AAEM/VIm microgel particles are
found in FESEM images, which has also been reported by Pich et al. before.[11]
As the
AFM is much more sensible at the small scale, the corona part of microgels is more
distinguishable in AFM images than FESEM images. In the height cross-section
profile of PVCL/AAEM/VIm microgels, it is obviously found the corona part with a
very low height outside the core part (Figure 4.9a). From the center toward the edge
of microgel, the height decreases slightly at the beginning, then drops sharply to a low
position, and goes down slowly to the end of microgel particle.
Figure 4.7. AFM topography (a, b and c) and FESEM (d, e and f) images of
PVCL/AAEM/VIm microgels with different BIS contents: 3 mol% (a and d), 1 mol%
(b and e), and 0.35 mol% (c and f).
Chapter 4. Internal Structure of Microgels
64
Compared to PVCL/AAEM/VIm microgels, the PVCL/AAEM/AAc microgels
hardly show core-corona structure in both AFM and FESEM morphologies
represented in Figure 4.8. The height cross-section profile decreases sharply from the
flat top to the substrate surface (Figure 4.9b). It should be noted that the synthesis
procedure of PVCL/AAEM/AAc microgels differs from that of PVCL/AAEM/VIm
microgels. A different initiator and an extra surfactant SDS used in
PVCL/AAEM/AAc microgel synthesis might be the reason leading to a structure
without fluffy corona part. However, an additional surfactant CTAB is used in the
synthesis of PVCL microgels, but it does not make the core-corona structure vanish
(Figures 4.3 and 4.4).
Figure 4.8. AFM topography (a, b and c) and FESEM (d, e and f) images of
PVCL/AAEM/AAc microgels with different BIS contents: 3 mol% (a and d), 1 mol%
(b and e), and 0.35 mol% (c and f).
Chapter 4. Internal Structure of Microgels
65
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
40
80
120
160
m
nm
a
0.0 0.2 0.4 0.6 0.8 1.0
0
40
80
120 b
m
nm
Figure 4.9. Cross-section profiles of (a) PVCL/AAEM/VIm and (b) PVCL/AAEM/
AAc microgels with 1 mol% BIS at the position indicated by white lines in Figures
4.7 and 4.8. The arrows show the positions of two ends of the microgel particles.
The dimension of PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels on
the silicon wafer detected by AFM is summarized in Table 4.4. As expected, for both
kinds of microgels, the height and contact angle decrease, whereas the contact radius
increases with the decreasing cross-linking density. Compared with PVCL/NIPAm
(1:1) microgels (Table 4.3), changes in microgel size upon deposition on the solid
substrate are much less for PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels.
Contact angles of particles on the solid substrate are around 50 – 60° for
PVCL/AAEM/VIm microgels and 65 – 70° for PVCL/AAEM/AAc microgels. Both
values are higher than those of PVCL/NIPAm microgels with similar cross-linking
density.
The result indicates that PVCL/AAEM-based microgels are stiffer and spread
less on the silicon wafer compared with PVCL/NIPAm and PVCL microgels with
similar cross-linking densities. This is probably due to the incorporation of AAEM
monomers into the microgel. It has been found that AAEM segments are mostly
accumulated in the core of mcirogels due to their higher reactivity than VCL
monomers.[11, 38]
The accumulation of AAEM segments makes the core more rigid
which contributes to high values of heights and contact angles.
Chapter 4. Internal Structure of Microgels
66
Table 4.4. Dimensions of PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels with
different cross-linker (BIS) contents deposited on the silicon wafer detected by AFM.
Samplesa Contact radius
( , nm)
Height
( , nm)
Contact angle
( , °)
PVAV_3%BIS 313 148 62
PVAV_1%BIS 329 144 61
PVAV_0.35%BIS 387 126 53
PVAA_3%BIS 90 113 70
PVAA_1%BIS 94 109 65
PVAA_0.35%BIS 99 105 65
a, PVAV and PVAA represent PVCL/AAEM/VIm and PVCL/AAEM/AAc
microgels, respectively.
As references, the PVCL/AAEM and PVCL/AAEM/VIm (0.9 % VIm)
microgels with 1 mol% BIS are synthesized as well. The obtained microgels are
detected with AFM, as shown in Figure 4.10. It can be found that PVCL/AAEM
microgels do not exhibit apparent corona-like shell in AFM height morphology,
consistent with the reported results.[11]
However, the cross-section profile indicates the
existence of the corona part with fuzzy polymer chains surrounding the core part.
From the cross-section profile, the corona part is very small, around 50 nm from the
end to core part. Even containing very few amount of VIm (0.9 mol%),
PVCL/AAEM/VIm microgels have obvious core-corona structure in AFM
topographic morphology. Compared to PVCL/AAEM/VIm microgels with the same
amount of BIS but a higher content of VIm (5 mol%), as shown in Figure 4.8b, the
corona part of PVCL/AAEM/VIm (0.9 mol% VIm) microgels is smaller. The height
cross-section profiles indicate distances from the core to the surface are about 150 nm
for low VIm content microgels (0.9 mol%, Figure 4.10d), and around 250 nm for high
VIm conten microgels (5 mol%, Figure 4.9a).
Chapter 4. Internal Structure of Microgels
67
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
nmm
c
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150 d
mnm
Figure 4.10. AFM topography images of (a) PVCL/AAEM, and (b) PVCL/
AAEM/VIm with 0.9 mol% VIm. (c) and (d) show the cross-section profiles of
microgels at the position indicated by white lines in (a) and (b). The arrows show the
positions of two ends of the microgel particles. The BIS content is 1 mol% for all
microgels.
4.4. Conclusions
In this Chapter, we investigate the internal structure of PVCL-based
copolymer microgels with SLS and microscopy methods (AFM and FESEM). The
SLS results show that PVCL/NIPAm and PVCL/NIPMAm microgels, both with
different monomer ratios in copolymer, have core-corona structures with a radial
profile as reported for PNIPAm microgels previously. The characterization of
PVCL/NIPAm copolymer and PVCL homopolymer microgels by AFM gives
morphologies and dimensions (contact radius and height) of microgels on a solid
substrate. It provides visualized evidence of core-corona structure that influenced by
cross-linker content. The AFM characterization of PVCL/NIPAm copolymer
microgels deposited on a solid substrate at the temperature above VPTT reveals the
structure of microgels in the collapsed state, which is highly consistent with the
reported results by SANS and SLS. The fuzzy corona part of the microgel contracts
with the core part to form a single monolithic structure. Besides, PVCL-based
Chapter 4. Internal Structure of Microgels
68
microgels with functional groups are characterized with AFM and FESEM. It is
interesting that the core-corona structure is found in PVCL/AAEM /VIm microgels,
but not in PVCL/AAEM/AAc microgels, which might be originated from their
different synthesis procedures. Additionally, the AFM observation on microgels of
very low cross-linking density provides a possible method to look into the domains in
the cross-linked polymer networks.
4.5. References
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[24] Wiedemair J., Serpe M. J., Kim J., Masson J. F., Lyon L. A., Mizaikoff B. and
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thermoresponsive hydrogel particles, Langmuir, 2007, 23, 130-137.
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nanoscale mechanics of single poly(N-isopropylacrylamide) microgels across the
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linker density of P(NIPAM-co-AAc) microgels at solid surfaces on the
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deformation of responsive microgels at oil/water interfaces: A step forward in
understanding soft emulsion stabilizers, Langmuir, 2012, 28, 15770-15776.
[30] Balaceanu A., Synthesis, morphology and dynamics of microgels with different
architectures, PhD Thesis, RWTH Aachen University, Aachen, Germany, 2013.
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Adhesion and mechanical properties of PNIPAM microgel films and their potential
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[32] de Gennes P.-G., Scaling Concepts in Polymer Physics; Cornell University
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Chapter 4. Internal Structure of Microgels
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[34] Keerl M., Pedersen J. S. and Richtering W., Temperature sensitive copolymer
microgels with nanophase separated structure, Journal of the American Chemical
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[35] Suzuki A., Yamazaki M. and Kobiki Y., Direct observation of polymer gel
surfaces by atomic force microscopy, Journal of Chemical Physics, 1996, 104, 1751-
1757.
[36] Kannaiyan D., Cha M.-A., Jang Y. H., Sohn B.-H., Huh J., Park C. and Kim
D. H., Efficient photocatalytic hybrid Ag/TiO2 nanodot arrays integrated into
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2431-2436.
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Chapter 5. Deformation and Spreading of Microgels
73
Chapter 5. Deformation and Spreading of Copolymer
Microgels at the Air/Water and Air/Solid Interfaces
5.1. Introduction
Deformability plays an important role in not only synthetic materials, but also
natural ones. A typical example in nature is that the extraordinary flexibility of
mammalian cells enables them to deform in order to adapt the blood circulation
environment.[1, 2]
In a simulation of renal filtration of soft particles, microgels can
translocate through pores that are only one tenth of the particle diameter under
physiologically relevant pressures.[3]
Such a translocation depends on the
deformability of microgels which is controlled by the cross-linking density.[1, 3]
It
gives a hint for the design of soft micro/nano particles with desired in vivo
biodistributions.[1]
Besides the translocation of cells or soft particles, the in vitro cell
culture on a solid substrate also concerns the deformation and adhesion of cells.[4, 5]
A
similar deformation has also been observed for microgels deposited on a solid
substrate.[6]
Microgels are soft particles consisting of an intramolecular cross-linked
network of polymer chains swollen with a solvent. They are currently investigated
due to their role in applications including coating,[7, 8]
drug delivery and release,[9, 10]
sensors,[11, 12]
cell culturing substrates[6, 13]
and emulsion stabilizer.[14, 15]
Although a
completely unanimous definition on microgel size range is absence, it is generally
accepted that the swollen particles are in the range of 10 – 1000 nm.[13, 16, 17]
Macromolecular elasticity confers deformability with enhanced colloidal stability of
suspensions of microgel. These unique features invoke for improved understanding of
Chapter 5. Deformation and Spreading of Microgels
74
interfacial behavior of soft colloids and their dynamics at a fluid/fluid or fluid solid
interfaces. In particular microgels exhibit a radial gradient in the cross-linking density
resulting in a core-shell structure.[18]
Adsorption of such tree-dimensional branched
macromolecules enhances steric repulsion of the chains; therefore surface-induced
scission of covalent bonds will need to be considered depending on the cross-linking
density and the strength of adsorption.[19, 20]
The dynamics of interfacial attachment in a fluid medium is of interest and
applicability to several physical and biological systems.[21]
From the theoretical point,
the classic model deduced by Johnson, Kendall and Roberts (JKR model) clearly
describes the adhesion between elastic bodies.[21-23]
According to this model, a contact
radius of an elastic sphere with radius on a solid substrate can be expressed
as ⁄ ⁄ , where is the particle shear modulus and W is the work
of adhesion. This scaling relation indicates that to achieve a large contact area and
consequently enhanced adhesion requires large particles (
) with very low
shear modulus (soft particles, ).[24]
On the other hand, this formula works
very well for strongly cross-linked large particles which are subjected by a small
shape deformation during adhesion.
However, in the case of weakly cross-linked soft small nanoparticles, for
example microgels, which highly deform upon particle adhesion, the JKR model is
not applicable. Recent theoretical effort provided a new scaling relationship for
nanoparticle deformation as ⁄ , where the surface energy of the
particle is considered besides the other particle parameters which are all included in
JKR model.[24]
They demonstrated that the JKR theory is applicable at small values of
the deformation ( ), whereas significant deviation from the JKR theory is
observed at higher deformations (comparable with the particle size).
Chapter 5. Deformation and Spreading of Microgels
75
In this Chapter we try to address the issue of the interfacial attachment of
microgels depending on the cross-linking density. For this purpose, we investigated
monodisperse colloidally stable PVCL/NIPAm microgels synthesized with different
amounts of cross-linker. We assessed the interfacial tension of the microgel at the
water/air interface. Particular emphasis is on the adsorption dynamics depending on
the cross-linking density. Adsorbed microgels on the solid surface were investigated
by mapping the profile and measuring the contact radius with scanning force
microscopy.
We demonstrate that the softness of microgel colloids strongly influence the
dynamics of adsorption rather than the equilibrium air/water interfacial tension. It is
remarkable that the time needed for soft particles to adsorb at the air/water interface
can be three orders of magnitude faster than that for stiffer ones. Insight on the
adsorption dynamics could be gained by the computer simulation. The data shows that
two effects concur and lead to enhanced adsorption dynamics: low friction coefficient
for loosely cross-linked microgels combined with the chain flexibility which sustains
deformation and spreading at the interface. These results are complemented by the
detailed analysis of the adsorption of microgels on the solid surface. We note in this
regard that the density of the polymer network limits the spreading of microgels at an
interface, while in solution opposes the expansion by the osmotic pressure from the
solvent. It is also found that a loosely cross-linked microgel can spread and cover a
large surface area with a radius of two- to four-fold greater than the hydrodynamic
radius of the microgel in the swollen state.
Chapter 5. Deformation and Spreading of Microgels
76
5.2. Experimental section
5.2.1. Sample preparation
Microgel synthesis
PVCL/NIPAm microgels with different monomer ratios and cross-linker
contents are used in this Chapter. The synthesis and size characterization of microgels
are presented in detail in Chapter 3.
Deposition of microgels on a solid substrate
The deposition of microgels was carried out on a spin-coating machine
(Convac 1001S, Germany) with the speed of 3000 rpm for 1 min. The silicon wafer
used for deposition was cleaned by sonication in isopropanol for 10 min, dried in air
stream, and treated with UV/O2 for 12 min. The concentration of microgel solution
used for spin coating was 0.2 mg/mL. At this microgel concentration, sufficient
amount of particles could be well distributed on the silicon wafer without clustering
effect. The quality of the samples was evaluated with a Zeiss Axioplan 2 optical
microscope. For each sample, a drop of 75 μL microgel solution was used.
5.2.2. Characterization methods
Dynamic light scattering (DLS)
The size of the microgel particles was measured with an ALV/LSE-5004 Light
Scattering Multiple Tau Digital Correlator and Electronics with the scattering angle
set at 90°. The samples were measured at 20 and 50
°C, below and above VPTT of the
microgels, respectively. The temperature fluctuations were below 0.1 °C. Prior to the
measurement microgel samples were diluted with doubly distilled water and filtered
through 2.5 µm filter.
Chapter 5. Deformation and Spreading of Microgels
77
Dynamic surface tension
The dynamic surface tension of microgel solutions was measured through the
pendant drop method on a DSA 100 tensiometer (Kruss, Germany). Aqueous
dispersions of microgels were prepared with deionized water (Milli-Q Academic A-10
system, Millipore). The microgel solutions used for surface tension measurement are
of 0.2 mg/mL, the same as used for spin-coating. To reduce the experimental errors
from the evaporation of the droplet, the needle was put in a cuvette with some water
at the bottom sealed with a para film. The setup is shown in Figure 5.1.
Figure 5.1. Schematic illustration of the pendent drop sealed in a cuvette with water
at its bottom in the surface tension measurement.
Atomic force microscopy
Atomic force microscopy (AFM) images were recorded on an Agilent 5500
Atomic Force Microscope in tapping mode. Cantilevers with resonance frequencies of
250 – 300 kHz and spring constants around 42 N/m were used. Picoimage software
provided by Agilent was used to analyze the recorded AFM results.
5.2.3. Computer simulation of microgels at the interface
We consider two microgel particles of nearly equal molecular weight differing
in the cross-linking density, presented in Figure 5.2. The microgels have an ideal
network structure similar to diamond lattice: each cross-link has the functionality four
Chapter 5. Deformation and Spreading of Microgels
78
(originates four subchains). To provide a “spherical” shape of the particle, the
network is placed into a sphere with further deleting the outer beads. Weakly cross-
linked microgel consists of 64 subchains, each of 19 beads; 571 beads form dangling
chains so that the total number of the beads in the particle is equal to 1787. The
densely cross-linked particle consists of 1785 beads including 212 subchains, each of
8 beads, and 89 beads in the dangling chains, as shown in Figure 5.2.
Figure 5.2. Initial (before annealing) diamond lattice structures of weakly (a) and
highly (b) cross-linked microgels. The total number of beads in the microgel, the
number of the beads per subchain, and the number of the subchains are 1787, 19, 64
(a) and 1785, 8, 212 (b), respectively.
Explanation of the observed evolution of the surface tension coefficient can be
done based on the mobility of the microgels and their volume in the solution (water).
All the issues are studied with the dissipative particle dynamics (DPD) simulation
technique.[25, 26]
Detailed description of the simulations can be found in literatures.[27,
28]
Each microgel particle, depicted in Figure 5.3, was placed in a cubic box of a
constant volume with periodic boundary conditions in
the x, y and z directions. We set the total number density in the system as
, so that the total number of the microgel and solvent particles (denoted as M and S,
Chapter 5. Deformation and Spreading of Microgels
79
respectively) in the box was . Repulsion between the particles of the
system is quantified by interaction parameters and which can be related
to the Flory-Huggins parameters according to the formula:
at .[27, 29]
Annealing of the microgels (transformation of the initial
structure, Figure 5.2, into equilibrium one) was performed in good (
) and poor ( and ) solvents.
Figure 5.3. The absolute value of the total displacement of the microgel as a
function of time steps t for different solvents and cross-linking densities: (a) dense
microgel in poor (red circles) and good (black squares) solvents; (b) dense (red
circles) and loose (black squares) microgels in the poor solvent; (c) dense (red circles)
and loose (black squares) microgels in the good solvent.
After attaining the equilibrium swelling degree, an average velocity of the
center of mass of the microgel was examined. The velocity is defined as a
derivative of the coordinate of the center of mass with respect to time ,
⁄ ⁄ , which can be approximated as a displacement of the center
Chapter 5. Deformation and Spreading of Microgels
80
of mass during short time interval : the shorter the interval, the better the
approximation. In the computer simulations, we measured the absolute value of
every simulation steps and plotted the total displacement | |
| | as a function of time . Mobility of the microgel is
different in good and poor solvents, as seen in Figure 5.3. The linear fit of is
quite accurate and average velocity of the microgel is determined by a slope of the
straight. We can see that the dense microgel has higher mobility in the poor solvent.
The ratio of the average velocities (slopes) of the dense microgel in different solvents
is
⁄ .
The same behavior is observed for the loose microgel (not shown). In both
cases, the microgel has a compact form in the poor solvent. The characteristic size of
the microgels, which is defined as a square root of the mean square radius of gyration,
was measured in units of the box cell: ,
,
and
. The swelling coefficient was defined as ⁄ :
and . Therefore, the difference in the mobility can be
explained by a model of impenetrable spheres: the friction coefficient obeys to the
Stokes’ law and proportional to the radius of the microgel. The arguments in favor of
this statement can be extracted from Figures 5.3 b and c: the velocities of both
microgels in the poor solvent are practicaly identical like their sizes,
⁄
, and mobility of the dense microgel in the good solvent (water) is 20% higher:
⁄ . Therefore, if we deal with equal weights of the dense and loose
microgels in the good solvent, the probability to reach the surface for the loose
particles is higher because they have approximately threefold excess in volume,
(
⁄ ) despite of 20% reduction in the mobility.
Chapter 5. Deformation and Spreading of Microgels
81
In order to simulate a water/air interface, we considered two incompatible
liquids (denoted by W and A) in the simulation box which were strongly segregated,
( and ). One of the liquids (“air”, A) was considered to be
poor for the microgel (M), , whereas both dense and loose microgels were
in the swollen state in the second liquid, . Such choice of the parameters
provided localization of the microgels at the interface with their further spreading.
Indeed, the high interfacial energy of bar water\air interface, , is
thermodynamically unfavorable if one of the subphases contains the microgels. Some
of them will be localized at the interface reducing the area of water/air contacts and,
as a result, the interfacial energy.
Computer simulations were performed in a box of a constant volume
with periodic boundary conditions in the x and y directions. The
upper part of the box was filled by a poor solvent (A) which occupied 30% of the box.
The bottom part of the box was filled by the good solvent (W). The dense and the
loose microgels were first pre-annealed in one-component good solvent (
and ). Then the corresponding swollen microgel was placed into
the bottom subphase in such a way to make a contact with the interface through one or
two polymer beads. Such contact was enough to keep the particles at the interface. In
the simulation, the lateral size of the microgel is described as (a square root of the
mean square of components of the radius of gyration), the contact diameter of the
microgel at the interface. The vertical size is , the height of the microgel at the
interface.
Based on the computer simulation demonstrated above, we obtain the
spreading dynamics and equilibrium shape of the particles at the interface.
Chapter 5. Deformation and Spreading of Microgels
82
5.3. Results and discussion
5.3.1. Dependence of microgel modulus on the cross-linking density
The shear modulus is a representative parameter to describe the elasticity of
gels. A theory about predicting of the shear modulus of microgels from the
information of size and cross-linking density has been deduced and proved to be valid
on PNIPAm-based microgels,[30]
(
) (
) ⁄
(Equation 5.1)
where is the shear modulus of the microgel particle, is the volume fraction of
polymer within the particle, is the polymer volume fraction in the collapsed
microgel particle, is the average number of monomers between cross-links, and
is the monomer size.
Here the volume fraction of polymer in the particle can be considered as
. Assuming that the polymer mass does change with the
collapse of the particle, we can calculate that (
) ⁄
(
) ⁄
. The value of
is the average volume of the segment between cross-links,
thus
, where is the total number of cross-links in the
particle and is the volume of the microgel in the collapsed state. From the Flory
temperature-induced volume transition theory,[31, 32]
the value of can be obtained.
According to Equation 5.1, the shear modulus of microgels particles is
assessed. As seen from Figure 5.4, the shear modulus of microgels, regardless of the
cross-linking density, increases with temperature. We can also find a transition
temperature for microgel modulus at around 35 °C, equal to the volume phase
transition temperature. Besides, the shear moduli of microgels at a certain temperature
Chapter 5. Deformation and Spreading of Microgels
83
increase with the cross-linker content, as seen in Figure 5.5. This explains the
phenomenon that the higher cross-linker content leads to a lower swelling capability.
20 30 40 5010
100
1000
G (
kP
a)
Temperature (oC)
3%
1.5%
1%
0.5%
0.25%
0.1%
0.05%
Figure 5.4. Shear modulus revised according to the Flory shear modulus concerning
the polymer networks in solvent.
0.1 1
100
1000
50 oC
20 oC
G (
kP
a)
BIS content (mol%)
Figure 5.5. Dependence of shear modulus of microgels on cross-linker content.
It has been reported that the modulus of the individual microgel particle can be
obtained from the AFM indentation force measurement,[6, 30, 33, 34]
or calculated from
the plateau shear modulus of concentrated microgel suspensions.[35]
However, both
methods involve theoretical assumptions and inevitable experimental errors. In this
Chapter 5. Deformation and Spreading of Microgels
84
work, we mainly estimate the shear modulus of the individual microgel to obtain the
qualitative relation of the mechanical property and cross-linking density of the
microgel. For this reason, we adapt the predicting calculation. As the cross-linker
content increases from 0.05 to 3 mol%, the shear modulus becomes ten times higher
(Figure 5.5). The results firmly depict that the less cross-linking in the particle, the
softer is it.
5.3.2. Interfacial activity of microgels at the air/water interface
To study the effect if the cross-linking concentration on the microgel
deformation (spreading) at the air/solid interface, we first discuss the adsorption of
microgels at the water/air interface. In this regard, drops of aqueous microgel
suspensions are formed in air and the interfacial tension is measured as a function of
time. In a pendent drop tensiometry (PDT) the drop profile is automatically detected
and fitted with the Young-Laplace equation, extracting the interfacial tension value
.[36, 37]
Adsorption of microgels at the air/liquid interface lowers the system free
energy which translates into an effective interfacial tension reduction, therefore by
monitoring as a function of time we can obtain information about the adsorption
kinetics. Figure 5.6 shows that regardless of the cross-liking degree of the microgel
the surface tension decreases and eventually reaches a steady state with an
equilibrium value equal to about 46 mN/m. Since the microgel contains 50 mol%
VCL, we expect the surface tension to be different from previously reported
equilibrium surface tension for PNIPAm microgels (~ 42 mN/m).[38]
Remarkably the time required to achieve a steady state of the surface tension
increases with the particle cross-linking density. To make a more explicit comparison
we plotted in Figure 5.6b the half time , defined by the inflexion point
Chapter 5. Deformation and Spreading of Microgels
85
corresponding to the time it takes for the surface tension .[39]
is the surface tension at time , is the surface tension the instant that the drop is
formed and is assumed equal to the surface tension of water (72 mN/m at 23 °C),[40]
and is the surface tension at the steady state. The data suggest that weakly cross-
linked microgel particles exhibit marked adsorption rates compared to highly cross-
linked ones. In principle, adsorption depends on the concentration and requires the
diffusion and spreading of the microgels to the air/water interface. The surface tension
data do not differentiate between the two processes. Nevertheless, it has been
demonstrated that the interfacial activity is related to the microgel deformation
(spreading) at interfaces.[38, 41-44]
In the next section, we propose computer simulations
which explain the data on the surface tension.
1 10 100 100040
45
50
55
60
65
70 a
3%
1.5%
1%
0.5%
0.25%
0.1%
0.05%
(m
N/m
)
Drop age (s)
0.1 1
1
10
100
t* (
s)
BIS content (mol%)
b
Figure 5.6. (a) Dynamic surface tension for microgels with different cross-linker
contents. (b) half time correspond to the inflection point of the dynamic surface
tension curves (a) (see text for detail). The microgel concentration was kept constant
and corresponds to 0.2 mg/mL.
Chapter 5. Deformation and Spreading of Microgels
86
5.3.3. Conformation of microgels at the air/water interface
The spreading dynamics of dense and loose microgels is presented in Figure
5.7 as a function of the lateral size of the microgel (a square root of the mean
square of components of the radius of gyration) versus time steps. A slope of the
tangent to each point of the fitting curve characterizes a rate of the spreading of the
polymer microgel in x and y directions. One can see that both microgels increase the
lateral size with time from the size in the bulk solution ( ) up to equilibrium
value at the interface ( ). The loose microgel spreads faster in the beginning of
the process because of the lower elastic moduli. Slowing of the microgel spreading at
the late stage is due to approaching the equilibrium state (plateau value of ).
Therefore, we can conclude that faster spreading of the loose microgel is another
contribution to the faster decay of the surface tension coefficient for the case of
weakly cross-linked microgels.
Figure 5.7. The average lateral diameter of the adsorbed dense and loose microgel
particle as a function of time steps.
Chapter 5. Deformation and Spreading of Microgels
87
Comparison of the equilibrium shapes of the loose and dense microgels at the
interfaces is presented in Figure 5.8. One can see that the loose microgel has more flat
shape with characteristic diameter , and height
in
comparison with the denser microgel ,
. Therefore, one
needs a smaller number of the loose microgels in order to cover the water/air interface
and its saturation occurs earlier with the loose particles.
Figure 5.8. Snapshots of equilibrium structures of the dense (upper) and loose
(bottom) microgels at interfaces.
5.3.4. Dimensions of microgels varied in the cross-linking degree
The morphologies of microgel particles with different cross-linker
concentrations captured with AFM and corresponding dimensions (contact radius and
height) have been shown in Chapter 4 (Figure 4.2 and Table 4.3). Different form solid
rigid spheres, the cross-linking density of microgels increases towards the particle
center.[18]
There are some polymer chains dangling on the particle surface, which
results in fuzzy particle surface.[6, 45]
Such an inhomogeneous internal structure,
namely the core-corona structure, can be figured out from the AFM images and
corresponding height profiles. We believe that the exterior part of the microgel on the
Chapter 5. Deformation and Spreading of Microgels
88
solid substrate is the corona formed from fluffy polymer chains. The results in
Chapter 4 show that the microgels, especially with low cross-linker contents, are
highly spread on the solid substrate.
Figure 5.9 shows characteristic dimensions of microgels in solution and
adsorbed on the solid surface. When the microgel adsorbs from the bulk solution to a
solid substrate, several areas can be distinguished: i) a thick dense core of the
microgel determines the height h; ii) a thin extended outer layer of the microgel
defines the contact radius ; iii) the most expanded microgel conformation in
solution defined by the hydrodynamic radius ; and iv) the collapsed state of the
microgel in solution above VPTT is represented by .
Figure 5.9. (a) Height image of dehydrated microgel on solid surface and (b) a
scheme of the height profile and (c) is density profile of the swollen and collapsed
microgel according to reference[18]
to highlights the relation between the dense core
and height, as well as hydrodynamic radius and contact area.
Chapter 5. Deformation and Spreading of Microgels
89
While the AFM measurements give the size of microgels adsorbed on the soild
substrate (contact radius and height), the DLS results provide the size of microgels in
either swollen or collapsed state in bulk solution. The hydrodynamic radii of
microgels in both swollen and collapsed states measured with DLS at 20 and 50 oC,
respectively are shown in Figure 5.10. The hydrodynamic radii of microgels in
solution state increase if the cross-linker concentration is reduced. This effect is due to
the formation of the loose colloidal networks with larger amount of the dangling
chains on the surface what leads to the increase of hydrodynamic radius.[31, 45, 46]
However, this trend can be observed when the cross-linker content is above 0.25
mol%. As the cross-linker content is lower than 0.1 mol% during synthesis, the
hydrodynamic radii of microgels are only around 400 nm. That might be due to the
cross-linking is not enough to hold polymer chains in the microgel network when
small amount of cross-linker is used in synthesis.
0 1 2 3
0
800
1600
2400
Rh, collapsed
Rh, swollen
Mic
rog
el d
imen
sio
ns (
nm
)
Cross-linker amount (mol%)
acont
height
Figure 5.10. Microgel dimensions in contact with the solid surface and in solution
depending on the cross-linker content. The shape of the microgel on SiO2 is
described by its height and contact radius denoted by . For comparison, the
hydrodynamic radius in swollen ( ) and collapsed state ( ).
Chapter 5. Deformation and Spreading of Microgels
90
5.3.5. Deformation of microgels
The contact area is systematically larger that the radius of the particle in the
swollen state, unless the microgel is highly cross-linked. We note in this regard that at
an interface the density of the network limits the spreading of the microgel, while in
solution opposes the expansion by the solvent (osmotic pressure). Curiously, the
loosely cross-linked particle spreads and covers a large surface area with a radius of
two- to four-fold greater than the hydrodynamic radius of the microgel in the swollen
state (Figure 5.11). This indicates that the spreading involves a significant
deformation (stretching) of the microgel relative to that induced by the swelling. In
the swollen state, the particle contains 80 – 90 % water,[31]
which partially evaporates
upon dehydration in ambient conditions and the microgel should in principle shrink at
the surface (air is the bad solvent).
In principle, when a colloidal particle with a radius in contact with a solid
substrate, the ratio of the reduced height in the vertical direction describes
the extent of deformation of the particle (Figure 5.9).[47]
For instance, a hard sphere
does not deform ( ), whereas a highly compliant soft particle flattens and the
deformation may approach its diameter ( ). Microgels exhibit a radial gradient
in the cross-linker concentration resulting in a core-shell structure. As a consequence,
the shell and core respond differently to the environment cue, because of a higher
cross-linking degree at the interior of the particle. Expansion of the shell provides
colloidal stability whereas the densest core delimits their size in the collapsed state. A
balance between the expansion by the solvent and the elasticity of the network defines
the equilibrium size of the microgel particle. However, it is not known that to which
extent the presence of an interface modifies this balance and therefore affects the
shape and size of the microgel particle.
Chapter 5. Deformation and Spreading of Microgels
91
Figure 5.11 indicates that the highly cross-linked microgel placed on a surface
assumes a hemispherical shape with a contact radius close to the hydrodynamic radius
of the particle in solution and the height is limited by the compressibility of the dense
core. As the cross-linker content is reduced from 3 to 1 mol%, the height decreases
with the intramolecular cross-linking, whereas the contact radius changes slightly.
The deformation increases and becomes saturated at around 90% relative to the
diameter of the microgel in the collapsed state. Above this limit, deformation of the
loosely cross-linked microgel undergoes a small change, despite an increase an
increase of more than 500% of the contact radius relative to the radius of the
expanded state.
Figure 5.11. The contact radius, normalized to the hydrodynamic radius in the
swollen state, as a function of the cross linker concentration (solid symbol). Reduced
height of the microgel, normalized to the hydrodynamic radius in the collapsed state,
as a function of the cross linker concentration (open symbol).
Chapter 5. Deformation and Spreading of Microgels
92
The contact radius and height results from a balance between spreading which
tends to maximize the contact area and the restoring network elasticity which tends to
minimize deformation of the microgel on the solid surface. In contrast, the
hydrodynamic radius of the microgel in solution results from a balance of osmotic
pressure and network elasticity of the microgel. The data suggest that adsorption of
microgels induces deformation exceeding the swelling-induced expansion. In
particular, a loosely cross-linked microgel sustains large deformation due to swelling,
and adsorption further amplifies the strain and eventually may induce uncontrolled
CC-bonds scission leading to fragmentation of the microgel.[19, 20]
5.4. Conclusions
This Chapter studies adsorption and deformation of microgels at the air/water
and air/solid interfaces. Through systematic variation of microgel cross-linking degree
based on PVCL/NIPAm. The dynamic light scattering and tensiometer were used to
determine size and surface activity of the microgels respectively. The conformation of
microgels at the air/water interface was simulated. The results show that faster
spreading happens for the loose microgel which takes more flat shape compared to the
dense microgel. To characterize deformation of the microgel on the solid substrate,
the microgel samples were deposited on the silicon wafer and dried. We used AFM to
investigate the morphology as well as determine the contact radius and height of the
adsorbed microgels. The data indicate that the deformability of the microgels
increases with decrease of the cross-linking degree. The results suggest that
adsorption of microgels induces deformation exceeding the swelling-induced
expansion. In particular, a loosely cross-linked microgel sustains large deformation
Chapter 5. Deformation and Spreading of Microgels
93
due to swelling, and adsorption further amplifies the strain and eventually may cause
CC-bonds scission inducing destruction of the microgel.
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Chapter 6. Microgels at the Oil/Water Interface
97
Chapter 6. Behavior of Temperature-Responsive Copolymer
Microgels at the Oil/Water Interface
(The results and descriptions of this Chapter have been published in the paper: Yaodong Wu,
Susanne Wiese, Andreea Balaceanu, Walter Richtering, Andrij Pich. Behaviour of
Temperature-responsive Copolymer Microgels at the Oil/Water Interface, Langmuir, 2014,
30, 7660–7669.)
6.1. Introduction
Microgels have become increasingly important in polymer and colloidal
science due to their attractive properties and potential applications. So far, a lot of
work on synthesis of microgel particles has been done and mostly such colloids are
prepared in an aqueous medium and consist of water-soluble polymers, such as
poly(N-isopropylacrylamide) (PNIPAm), poly(N-isopropylmethacrylamide)
(PNIPMAm), poly(N-vinylcaprolactam) (PVCL), poly(acrylic acid) (PAA) or poly(2-
(diethylamino)ethyl methacrylate) (PDEA).[1]
Among these microgels, special
attention has been focused on PNIPAm-, PNIPMAm- and PVCL-based microgels due
to their temperature-sensitive behavior. These polymers feature the lower critical
solution temperature (LCST) and their corresponding microgels have the volume
phase transition temperature (VPTT), close to the LCST of linear polymers.[2-6]
The
temperature sensitivity of these polymers facilitates the fabrication of “switchable” or
“stimuli-responsive” materials.[7, 8]
Indeed, the responsive microgels are effectively
used as the drug carrier or the emulsion stabilizer.[7-11]
As the drug carrier, microgel
particles can be used in either a dispersed state or an assembly state, e.g. colloidosome
capsules.[8]
The application of microgels in the synthesis of colloidosomes or emulsions is
based on the adsorption of microgel particles at the oil/water interfaces. The oil/water
Chapter 6. Microgels at the Oil/Water Interface
98
interfacial tension is reduced when microgel particles are adsorbed onto the interface
and this effect leads to the stabilization of emulsions.[7, 10-13]
The microgel-stabilized
emulsion is a new kind of particle-stabilized emulsion, referred by Richtering as
“Mickering emulsion” compared to the traditional “Pickering emulsion”.[13]
Compared to surface-modified latexes or other inorganic/polymer hybrid particles
used in the conventional Pickering emulsions, microgels are easy to synthesize, highly
deformable at the interface and importantly environment-responsive.[7, 10-12]
The
emulsion stability is deeply influenced by the environment because of the
temperature- and pH-sensitivity of microgels.[7, 10-16]
Through the studies of the
interfacial behavior of modified inorganic nanoparticles at the liquid-liquid interfaces,
it was revealed that the assemblies of nanoparticles can be controlled by tuning the
size of the nanoparticle, the chemical characteristics of the ligands, and environment,
such as temperature and pH.[17-20]
Until now, the interfacial tension in the Pickering
emulsion system has been widely investigated, and some theoretical models have
been developed; nevertheless, these theories are normally based on the hard latexes or
inorganic particles.[17-21]
Microgels are colloidal particles whose behavior falls
between that of hard spheres and ultra-soft systems (e.g., polymer coils in solution).[22,
23] The assembly laws deduced from the interfacial behavior of hard particles at the
oil/water interface seem not to be completely suitable for microgels, since soft
microgel particles are highly deformed at the oil/water interface.[16, 24-26]
With the temperature-sensitive unit, NIPAm, and the pH-sensitive unit, MAA,
the interfacial behavior of PNIPAm-co-MAA microgels, strongly depends on the
temperature and the pH, and so does the emulsion stability.[7, 9-11, 14, 24]
The interfacial
properties of PNIPAm-based microgels as a function of temperature were studied by
Monteux and coworkers. A minimum interfacial tension was found around VPTT of
Chapter 6. Microgels at the Oil/Water Interface
99
the microgel. They assumed that the decrease in the interfacial tension with
temperature below VPTT came from the formation of denser layers of microgels at
the interface; and that the increase in the interfacial tension with temperature above
VPTT arose from loosely packed aggregations of microgels at the interface.[12]
Compared to the microgel with ionizable groups, neutral microgels should be more
suitable for investigating the interfacial behavior because the lack of electrostatic
interaction would make the system much simpler.
It is known that the variation of temperature induces a change in the
hydrophilic/hydrophobic balance of microgels, consequently leading to a size
variation. However, both the size and the hydrophilic/hydrophobic balance have a
great influence on the interfacial behavior of colloidal particles.[18, 19]
Which one of
them is more imperative in the change in the interfacial tension of microgel systems
with temperature is still unclear. A recent paper from Li et. al. reports the adsorption
kinetics of PNIPAm microgels at the oil/water interface. It is demonstrated that the
spreading of microgels at the oil/water interface, governed by microgel deformability,
plays an important role in the decline rate of the interfacial tension.[27]
In this Chapter, we investigate the interfacial behaviors of these microgels. In
our previous work,[28]
we demonstrated that in PVCL/NIPAm and PVCL/NIPMAm
microgels there is a statistical distribution of the monomer units in the colloidal
networks independent of the copolymer composition. This gives a straightforward
possibility to study systematically the influence of the chemical structure on the
interfacial behavior of microgels. So far the influence of the copolymer structure and
the cross-linking degree of microgels on the oil/water interfacial tension has not been
investigated thoroughly. The equilibrium interfacial tension and the time for reaching
the equilibrium state can be considered as the thermodynamic aspect and the kinetic
Chapter 6. Microgels at the Oil/Water Interface
100
aspect, respectively. Dependences of these two aspects on temperature, chemical
composition and cross-linker degree of microgels are analyzed. The results indicate
that both the chemical composition of copolymer microgels and the cross-linking
degree have a governing influence on the deformation-controlled interfacial behavior
of microgels at the toluene/water interface. The former strongly affects the volume
phase transition temperature which determines the temperature-dependence of the
softness of microgels, and the latter directly influences the softness of mcirogels.
Furthermore, we did a primary study on the application of microgels based on
the interfacial activity. The emulsions stabilized by PVCL/NIPAm and
PVCL/NIPMAm microgels are prepared. It is found that the type of the emulsion
relies on the chemical composition of microgels. PVCL/NIPAm microgels and
PVCL/NIPMAm microgels with low NIPMAm contents tend to stabilize w/o
emulsions, while PVCL/NIPMAm microgels with high NIPMAm contents are likely
to make o/w emulsions. For PVCL/NIPAm microgels, a HIPE is obtained at the
oil/water volume ratio of 3/7. Unlike the interfacial activity, the type of emulsion is
not influenced by the cross-linking density.
6.2. Experimental section
6.2.1. Microgel synthesis
PVCL/NIPAm and PVCL/NIPMAm microgels with different monomer ratios
and cross-linker contents are used in this Chapter. The synthesis and characterization
of microgels are presented in detail in Chapter 3.
Chapter 6. Microgels at the Oil/Water Interface
101
6.2.2. Interfacial tension
The dynamic interfacial tension of microgels at the toluene/water interface was
measured through the pendant drop method on a DSA 100 tensiometer (Kruss,
Germany) equipped with a thermostat. Aqueous dispersions of microgels were
prepared with deionized water (Milli-Q Academic A-10 system, Millipore) previously
saturated with toluene. A drop of toluene (previously saturated with deionized water)
was formed in microgel dispersions, and then dynamic interfacial tension was
determined through rapid acquisition of the drop image, edge detection, and fitting of
the Laplace-Young equation conducted by computer automation. As toluene is lighter
than water, we formed a toluene drop from bottom to up using a curved needle, as
shown in Figure 6.1. For the temperature dependence experiment, fresh interfaces
were produced at each temperature.
Figure 6.1. Schematic illustration of the pendent drop from bottom in the interfacial
tension measurement.
6.2.3. Interfacial dilatational rheology
The interfacial dilatational rheology was recorded by the DSA 100 tesiometer
(Kruss, Germany) on which the interfacial tension measurements were implemented.
For the dilatational rheology measurement, an oscillating drop module (ODM, Kruss,
Germany) was installed on the tesiometer. The drop area of was oscillated at the
Chapter 6. Microgels at the Oil/Water Interface
102
frequency of 0.2 Hz at different temperatures for homopolymer microgels of PVCL,
PNIPAm and PNIPMAm with 3 mol% BIS. The measurements started 5 min after the
formation of the drop. The Fourier analysis of the raw data was performed with the
software packages supplied by the manufacturer.
6.2.4. Preparation of microgel-stabilized emulsions
Microgel-stabilized emulsions were prepared by homogenizing the microgel
dispersion with proper amounts of chloroform using an Ultra Turrax T25
homogenizer (17 mm head) operating at the speed of 8000 rpm for 3 min. The total
emulsion volume was 10 mL. The o/w ratio (v/v) ranged from 1/9 to 9/1. The
concentration of microgels in the aqueous phase varied from 0.05 – 0.5 wt%.
6.2.5. Preparation of emulsion-templated materials
The emulsion-template materials were prepared with the same procedure for
emulsion preparation. Appropriate amount of polycaprolactone (PCL, Mw ~ 80 k),
purchased from Sigma-Aldrich and used as received, was dissolved in chloroform.
For the porous scaffold-like materials, the microgel solution (0.5 wt%) of
PVCL/NIPAm (1:1) with 3 mol% BIS was used, and the PCL/chloroform solution (4
wt%) was used as the oil phase. The mixture with oil/water ratio of 3/7 was
homogenized with Ultra Turrax T25 homogenizer (17 mm head) operating at 8000
rpm for 3 min. To obtain porous materials, the mixture was freeze-dried.
For the emulsion-template colloidosomes, the microgel solution (0.015 wt%)
of PVCL/NIPMAm (1:5) microgels with 3 mol% BIS, and the PCL/chloroform
solution (2 wt%) was used as the oil phase. The mixture with oil/water ratio of 1/9
was homogenized with Ultra Turrax T25 homogenizer (17 mm head) operating at
8000 rpm for 3 min. Then the mixture was stirred with a magnetic stirrer under
ambient condition to remove chloroform by evaporation overnight.
Chapter 6. Microgels at the Oil/Water Interface
103
6.2.6. Fluorescence microscopy
Fluorescence microscopy was performed on a Zeiss Axioplan 2 microscope
equipped with XBO 75 illuminating system (Xenon lamp) and using a filter
565 nm and 620 nm. To facilitate the fluorescence microscopy characterization,
the oil phase contains Nile Red with the concentration of 0.1 μg/mL.
6.2.7. Scanning electron microscopy
Field emission scanning electron microscopy (FESEM) was carried out on
Hitachi S-4800 field-emission SEM. The freeze-dried samples were broken with
tweezers and stick to the sample holder before the measurement. The samples of
colloidosomes were prepared by placing a drop of emulsion on silicon wafer. Before
being placed into the SEM, the samples were dried under ambient conditions.
6.3. Results and discussion
6.3.1. Dynamic interfacial tension
6.3.1.1. Influence of microgel concentration
Due to the fact that no monomers carrying acidic or basic groups or ionic
surfactants were used for microgel synthesis, we can consider that only small amount
of ionizable groups in the microgel structure originates from the initiator residues
incorporated into polymer chains. In this situation we consider the investigated
microgels as purely temperature-sensitive colloids where no pH-sensitive effects are
present. The characterizations of microgels, such as chemical composition by NMR,
size at different temperatures by DLS and morphology by AFM, are presented in
Chapters 3 and 4. Because of the different reactivity ratios of the monomers and the
cross-linker, microgels synthesized have a radial density profile in the internal
structure, where the segment density gradually decays at the surface.[4, 29, 30]
Chapter 6. Microgels at the Oil/Water Interface
104
Figure 6.2 shows the interfacial tension of toluene against aqueous solutions of
PVCL/NIPMAm microgels at different concentrations. It is obvious that the
interfacial tension decreases with time, and eventually approaches an equilibrium
value. The time for reaching the equilibrium interfacial tension decreases with the
increase in the concentration of microgels. The plots, especially at low
microgel concentrations, interpret the classical kinetics of interfacial tension with
three distinct regimes (Figure 6.2b), which is similar to the dynamic interfacial
behaviors of polymer-grafted inorganic nanoparticles and proteins.[17, 31]
The first
stage is a kind of induction regime, where the interfacial tension decreases slightly. In
the second regime, a sharp decline in the interfacial tension is observed. In the last
regime, the interfacial tension reaches a quasi-equilibrium value.
1 10 100 1000 10000
10
20
30
40
0.0014 mg/mL
0.0046 mg/mL
0.0145 mg/mL
0.046 mg/mL
0.145 mg/mL
0.28 mg/mL
(m
N/m
)
Drop age (s)
a
Figure 6.2. (a) Time dependence of the toluene/water interfacial tension with
PVCL/NIPMAm (5:1) microgels of various concentrations at 24 °C. The red lines
show the fitting results based on Equation 6.1. (b) Schemetic showing of three
regimes of the plots marked on a typical curve (0.046 mg/mL in Graph
a).
Chapter 6. Microgels at the Oil/Water Interface
105
Generally, the decrease of interfacial tension in the second regime can be
explained by the adsorption of microgel particles to the toluene/water interface. In the
early stage, microgels within the vicinity of toluene drop adsorb onto the interface,
which induces the gentle reduction of interfacial tension. The diffusion of microgel
particles from the bulk solution to the interface increases the interface coverage. After
a certain period of time, the interface coverage reaches a critical value, and then a
steep decline in the interfacial tension is observed and maintains the status for a short
time from several to tens of seconds. When the interface coverage approaches to the
maximum value, the interfacial tension reaches an equilibrium value. For microgels of
lower concentrations, a longer time is needed for the adsorption of sufficient amount
microgel particles from bulk solution to the toluene/water interface. Therefore, the
induction time increases with the decrease of microgel concentration. When the
concentration of microgel is very low (here lower than 0.0014 mg/mL), the induction
time is so long that the toluene droplet becomes unstable before the second regime
starts. The induction period is only found for dilute microgel solutions. In the
measurable time scale, the induction time is difficult to be detected for concentrated
microgel solutions, because the dynamic interfacial tension decreases very fast to a
stable level.[17, 31]
One possible explanation for the acceleration of the interfacial
tension decay is that the capillary force between the particles at the interface that
increases with the particle concentration makes the soft microgels easier deformed.[27]
It should be noted that in the conventional Pickering emulsions, solid particles seldom
lead to appreciable changes in oil/water interfacial tension.[16, 32, 33]
However,
microgels are organic colloidal particles, and their deformation leads to a higher
number of polymer segments adsorbed at the interface.[27]
This leads to a significant
decrease in the interfacial tension.
Chapter 6. Microgels at the Oil/Water Interface
106
To obtain the equilibrium interfacial tension, an empirical equation (Hua-
Rosen equation) is employed (Equation 6.1).
(Equation 6.1)
where , and are the initial interfacial tension, the equilibrium interfacial
tension and the interfacial tension detected at time , respectively. is a
semiempirical parameter having the unit of time and corresponds to the time when
, and is a dimensionless constant. The Hua-Rosen equation has
been successful for small molecular surfactant systems, macromolecular systems and
globular protein systems.[34-36]
We carried out the fitting of experimental data using
Equation 6.1 for samples with medium microgel concentrations, which had typical
decay. In Figure 6.2, we can see a good agreement between the fitting
results and the experimental data.
6.3.1.2. Influence of temperature
It has been reported that temperature has a considerable influence on the
interfacial/surface tension of small molecular surfactants, polymers, and colloidal
particles.[34, 37-41]
Normally, as temperature increases, the diffusion rate increases.[34]
That means faster adsorptions of solutes and/or dispersed substances occur at higher
temperatures, reflected by the faster interfacial/surface tension decay rate. However,
through the experimental results of interfacial tension of temperature-sensitive
polymers, such as PNIPAm, a different temperature dependence of surface tension
were observed.[39, 40]
Below LCST, the rate of the surface tension reduction increases
with the increase of temperature, which is similar to the behavior of non-ionic
surfactants.[34, 42]
On the contrary, the surface tension decay rate decreases as
temperature increases above LCST. Kawaguchi et al. attributed the slowdown of
surface tension decay to aggregation of PNIPAm chains which retarded the diffusion
Chapter 6. Microgels at the Oil/Water Interface
107
of polymers from the bulk solution to the interface.[39]
Zhang and Pelton supposed that
the unfolding of PNIPAm at the air/water interface above LCST prolonged the time
for reaching a steady-state surface tension.[40]
They adopted the same explanation to
the similar phenomena observed in the case of PNIPAm microgels.[41]
0 100 200 300 400 500
5
10
15
20
25
30
35
24.0 oC
27.5 oC
31.0 oC
35.5 oC
39.5 oC
43.0 oC
(m
N/m
)
Drop age (s)
Figure 6.3. Time dependence of the toluene/water interfacial tension with
PVCL/NIPMAm (5:1) microgels of 0.15 mg/mL at different temperatures. The red
lines show the fitting results based on Equation 6.1.
The dynamic interfacial tension of toluene against aqueous microgel solutions
was measured at various temperatures. At each temperature, the interfacial tension of
freshly formed interface was measured. As shown in Figure 6.3, the dynamic
interfacial tension represents a similar trend versus time at different temperatures,
which decays with time and finally approaches to an equilibrium value as discussed
before. A good agreement between the fitting through Equation 6.1 and the
experimental data was also found in plots at different temperatures.
However, the equilibrium interfacial tension and the time for reaching the equilibrium
status behave differently at different temperatures. The resulting Hua-Rosen
parameters, describing the dynamic interfacial tension, from the fittings in Figure 6.3
Chapter 6. Microgels at the Oil/Water Interface
108
are presented in Table 6.1. The lowest value of was found at around 31 °C. A
similar change of with temperature was found in the dynamic surface tension of
linear PNIPAm solution.[39]
It was mentioned that the value of might reflect the
hydrophobic character of the surfactant, but there is no direct evidence to prove this.
However, others’ work show that temperature has no special influence on the value of
.[40, 41]
Table 6.1. Dynamic interfacial tension parameters of Hua-Rosen equation for
PVCL/NIPMAm (5:1) microgels of 0.15 mg/mL at different temperatures (fitting in
Figure 6.3).
(°C) (mN/m) (s)
24.0 8.5 1.2 1.94
27.5 7.1 1.0 2.06
31.0 5.6 0.5 0.81
35.5 5.2 5.4 1.09
39.5 5.6 51.3 2.26
43.0 6.6 124.6 3.32
6.3.1.3. Influence of cross-link density
Cross-link density plays an important role in the microgel structure and
consequently influences the property of either the individual microgel particle or the
entire microgel suspension.[4, 43-45]
Figure 6.4 presents the dynamic interfacial tension
of toluene against the solutions of microgels with different cross-linking densities. We
can obviously see that microgels with lower cross-linker concentrations, which are
softer, have the faster kinetics of reduction of interfacial tension with time. A similar
phenomenon was observed by Pelton’s group on the dynamic surface tensions of
PNIPAm microgels at the air/water interface.[41]
It was proposed that the rate of
surface tension lowering was partially influenced by the rate of particle spreading
Chapter 6. Microgels at the Oil/Water Interface
109
after the adsorption onto the interface. In Chapter 5, both the experimental data and
the computer simulation results of microgels at the air/water interface have shown that
a faster spreading happens for weekly cross-linked microgels compared to denser
ones.
10 100 1000
10
20
30
40
3%
1%
0.25%
0.05%
(m
N/m
)
Drop age (s)
Figure 6.4. Dynamic interfacial tension of toluene against the solutions of microgels
with different cross-linking densities at 24 °C. The values in the legend represent the
molar ratio of cross-linker, BIS, to monomers added during synthesis. The
concentration of microgel solution is 0.15 mg/mL.
6.3.1.4. Influence of microgel size
It was reported that the size of nanoparticles strongly affects their interfacial
behaviors.[17, 18]
To figure out whether the size of microgel particles can influence the
equilibrium interfacial tension, we synthesized a series of microgels with the same
chemical composition, but of different sizes. By adding different amounts of
surfactant CTAB during the synthesis, the size of microgel can be tuned.[46]
From
Figure 6.5, we can see that the more CTAB is used in synthesis, the smaller are
microgels obtained. However, the changes in the microgel size have no impact on the
interfacial tension of microgel solutions against toluene, both below and above VPTT.
Chapter 6. Microgels at the Oil/Water Interface
110
Thus, within the range of microgel size investigated here, the effect of size on the
interfacial tension can be ignored. It ensures the comparison of the interfacial
behaviors of microgels with different compositions, which are also varied in size,[28,
47] is reasonable.
0 100 200 300 400 500 60010
15
20
25
30
35
100 1000
NO CTAB
0.37 mM CTAB
1.10 mM CTAB
(m
N/m
)
Drop age (s)
a
Rh (nm)
0 100 200 300 400 5005
15
25
35
10 100
b
NO CTAB
0.37 mM CTAB
1.10 mM CTAB
(m
N/m
)Drop age (s)
Rh (nm)
Figure 6.5. Dynamic interfacial tension of PVCL/NIPMAm (1:5) microgels solution
(0.15 mg/mL), synthesized with different amounts of surfactant CTAB, against
toluene. (a) Below VPTT (24 °C) and (b) above VPTT (49 °C). The inset shows the
size distributions of microgels, which decrease with the increase in CTAB
concentration. The temperature trends of hydrodynamic radii of microgels are shown
in Figure 3.5.
6.3.2. Equilibrium state of the interfacial tension
6.3.2.1. Influence of chemical composition of microgel
The variation of the equilibrium interfacial tension ( ) and the size of
copolymer microgels with the molar ratio of acrylamide at room temperature are
summarized in Figure 6.6. It is found that the size of PVCL/NIPAm microgels has no
significant change with the change in the fraction NIPAm (around 400 nm in ),
while the size of PVCL/NIPMAm microgels increases slightly with the fraction of
NIPMAm (from 350 to 450 nm in ).[28]
As synthesized in the presence of CTAB,
Chapter 6. Microgels at the Oil/Water Interface
111
the corresponding homopolymer microgels are much smaller than the copolymer
microgels. Consequently, an irregular trend of size variation with the chemical
composition is found. However, the values of for PVCL/NIPAm and
PVCL/NIPMAm systems increase with the fraction of NIPAm and NIPMAm in the
copolymer. It does not follow the changing trend of size at all. For both
PVCL/NIPAm and PVCL/NIPMAm copolymer microgel systems, the change in
is very slight as the fractions of NIPAm and NIPMAm increase from 0 to 0.5. As the
fractions of NIPAm and NIPMAm increase from 0.5 to 1, the values of increase
significantly.
0.0 0.2 0.4 0.6 0.8 1.0
8
9
10
11
12
100
200
300
400
500
eq
(mN
/m)
Fraction of NIPAm in PVCL/NIPAm
a
Rh (
nm
)
0.0 0.2 0.4 0.6 0.8 1.0
8
10
12
14
16
100
200
300
400
500
eq
(mN
/m)
Fraction of NIPMAm in PVCL/NIPMAm
b
Rh (
nm
)
Figure 6.6. Variation of and with the fraction of (a) NIPAm and (b) NIPMAm
in copolymer microgels at 24 °C. The concentration of microgel solution is 0.15
mg/mL.
Most of the microgels investigated here were synthesized without CTAB,
except the homopolymer PVCL, PNIPAm and PNIPMAm microgels and
PVCL/NIPMAm copolymer microgels varied in size. For these microgels, a small
amount of CTAB was used. We believe that no CTAB residues are left in the solution
after the 4-day dialysis with a continuous fresh water exchange. If there were CTAB
Chapter 6. Microgels at the Oil/Water Interface
112
residues left in the solution, they would lead to the same interfacial behavior of PVCL,
PNIPAm and PNIPMAm microgels. However, they are different from each other
(Figure 6.6). The PVCL/NIPMAm microgels of different sizes (synthesized in the
presence of CTAB) and a control sample prepared without CTAB have the same
interfacial behavior (Figure 6.5). From this we conclude that CTAB has been
successfully removed from microgel samples during the dialysis step.
6.3.2.2. Influence of temperature
The equilibrium interfacial tensions at different temperatures for
PVCL/NIPAm and PVCL/NIPMAm microgels are presented in Figure 6.7. At the
same temperature, of PVCL homopolymer microgels is always lower than those
of PNIPAm and PNIPMAm homopolymer microgels. The value of for PNIPAm
microgels decreases with temperature firstly, and reaches a minimum around VPTT,
then increases with temperature slightly when temperature is higher than VPTT. This
kind of transition temperature for interfacial tension is consistent with the reported
results about the interfacial tension of NIPAm-based microgels.[12]
For PVCL and
PNIPMAm microgels, the values of also decrease with temperature below VPTT,
however, stay at a plateau value above VPTT. The interfacial behaviors of
PVCL/NIPAm or PVCL/NIPMAm copolymer microgels fall between those of PVCL
and PNIPAm or PNIPMAm homopolymer microgels, and depend on the chemical
compositions of microgels.
Figures 6.7c and d show the kinetics of interfacial tension reduction as
reflected by for microgels at different temperatures. We can see that the values of
remain at a constant level below VPTT and increase with temperature above VPTT.
The kinetics of interfacial tension ( ) also depends on the chemical compositions of
copolymer microgels. The increase in for PVCL- and PNIPAm-rich microgels
Chapter 6. Microgels at the Oil/Water Interface
113
above VPTT is more obvious compared to that for PNIPMAm-rich systems. It means
that at higher temperature (above VPTT), the reduction of interfacial tension for
PVCL- and PNIPAm-rich microgel systems is slowed down to a larger extent
compared to PNIPMAm-rich microgel systems. Though the reason for that is still
unknown, such a dependence of the interfacial behavior on the chemical composition
could be made use of in the application of microgels.
20 25 30 35 40 454
6
8
10
12
14 PVCL
V:N (5:1)
V:N (1:1)
V:N (1:5)
PNIPAm
eq (
mN
/m)
Temperature (oC)
a
20 25 30 35 40 45 504
8
12
16 PVCL
V:NM (5:1)
V:NM (1:1)
V:NM (1:2)
V:NM (1:3)
V:NM (1:5)
PNIPMAm
eq (
mN
/m)
Temperature (oC)
b
20 25 30 35 40 45
0
50
100
150
200
250
300
PVCL
V:N (5:1)
V:N (1:1)
V:N (1:5)
PNIPAm
t* (
s)
Temperature (oC)
c
20 25 30 35 40 45 50
0
50
100
150
PVCL
V:NM (5:1)
V:NM (1:1)
V:NM (1:2)
V:NM (1:3)
V:NM (1:5)
PNIPMAm
t* (
s)
Temperature (oC)
d
Figure 6.7. Dependence of the interfacial tension and time to equilibrium on
temperature for microgels with different ratios of VCL:NIPAm (a and c) and
VCL:NIPMAm (b and d). V:N and V:NM represent PVCL/NIPAm and
PVCL/NIPMAm, respectively; and the ratios in the brackets are the monomer ratio of
copolymer microgels. The concentration of microgel solution is 0.15 mg/mL.
Chapter 6. Microgels at the Oil/Water Interface
114
The intrinsic reason for the difference between the interfacial behavior of
PVCL, PNIPAm and PNIPMAm microgels observed here is still not understood. It is
revealed in this paper that these copolymer microgels have the similar internal
structure, regardless of the chemical composition (SLS results in Figure 4.1), and that
the size of microgels has no impact on the interfacial behavior (Figure 6.5). The
probable reason for such a difference we can think out is the variation in the chemical
structure. However, it is still an open question how the chemical structure has such an
influence on the interfacial behavior observed here. Though the reason for that is still
unknown, such a dependence of the interfacial behavior on the chemical composition
could be made use of in the application of microgels.
6.3.3. Discussion on the temperature dependence of interfacial tension
We can easily find turning points on the curves (Figure 6.7). Before
such kind of point, the value of decreases monotonously with the increase in
temperature. Here, the turning point can be considered as the transition temperature of
the interfacial behavior, and its exact position was defined as the intersecting point of
two trend lines (approximate slopes). According to our previous work, the phase
transition temperature of microgels can be determined by DSC.[28]
The transition
temperatures determined by DSC and the curves are summarized in Table
6.2. We can see that the interfacial behavior of microgels is highly consistent with the
phase behavior of microgels. For PVCL/NIPAm microgels, both kinds of transition
temperatures are not influenced by the monomer ratio and remain in a certain range,
although the phase transition temperatures (34 – 36 °C) are slightly higher than the
transition temperature derived from the interfacial behavior (31 – 33 °C). For
PVCL/NIPMAm microgels, both transition temperatures depend on the monomer
Chapter 6. Microgels at the Oil/Water Interface
115
ratio in copolymers. When the VCL:NIPMAm ratio is higher than 1, the transition
temperatures are around 34 – 36 °C. However, if the VCL:NIPMAm ratio decreases
from 1 to 0.2, the transition temperatures increase rapidly to 45 °C. The results
indicate that the behavior of copolymer microgels with VCL:NIPMAm ratios above
and below 1 is dominated by VCL and NIPMAm, respectively. The transition
temperatures of follow exactly the same trends of the VPTT of microgels which
stands for exclusively the chemical structure of microgels. Considering the
independence of on copolymer microgel size (Figure 6.5), it can be inferred that
the chemical constitution of microgels has a dominant influence on the interfacial
tension.
Table 6.2. Comparison of transition temperature of the interfacial behavior and that of
phase behavior of microgels with different monomer ratios.
Samplea Phase transition temperature by
DSC (°C)[28]
Transition temperature of γ
(°C)
V:N (5:1) 34.9 32.7
V:N (1:1) 35.5 31.8
V:N (1:5) 35.0 31.7
V:NM (5:1) 33.8 32.6
V:NM (1:1) 35.9 32.4
V:NM (1:2) 41.3 40.8
V:NM (1:3) 42.3 42.0
V:NM (1:5) 44.6 45.0
a, V:N and V:NM represent PVCL/NIPAm and PVCL/NIPMAm, respectively; and
the ratios in the brackets are the monomer ratio of copolymer microgels.
It is believed that the reduction of interfacial tension is caused by the
adsorption of microgel particles at the oil/water interface. At the early stage, the
adsorption of nanoparticles is expected to be a diffusion-controlled process. For
Chapter 6. Microgels at the Oil/Water Interface
116
nonionic nanoparticles, the process can be quantitatively described as the following
equations through the short time ( ) and the long time ( )
approximations,[17, 34, 42]
, or (
⁄)
;
, or (
⁄)
(Equations 6.2)
where is the initial interfacial tension and is the equilibrium interfacial tension,
is the bulk concentration of nanoparticles, is the interfacial density of
nanoparticles, and is the diffusion coefficient. For the adsorption of nonionic
surfactant, a mechanism of mixed diffusion-activation is suggested, where a linear
relationship exists between ⁄ and .[17, 34, 42]
Here, is the diffusion
coefficient of the particles at different temperatures and is the particle diffusion
coefficient at 24 °C. From the interfacial tension, to obtain the values of , the
interfacial density of particles Γ is needed, as seen in (
⁄)
and
(
⁄)
. However, as the microgels are very soft and deformable,
the size of the particles adsorbed at the interface is hardly known. As a consequence
of that, the interfacial density of microgel particle is not possible to obtain. Therefore,
the relative value of ⁄ is calculated.
The plots of ⁄ versus were obtained from DLS and calculated
through the approximations in Equations 6.2 (Figure 6.8). We found that the
⁄ plot according to light scattering data has a good linear dependence
in the small range of investigated here. Compared to that, the calculation based
on Equations 6.2 shows a different result. When T < VPTT, the values of ⁄
depend on linearly, and are also located near the fitted line of light scattering
Chapter 6. Microgels at the Oil/Water Interface
117
results for both approximations. That means the change in diffusion coefficients
derived from interfacial tension (through Equations 6.2) with temperature is
consistent with the one in the bulk solution (light scattering results). But when T >
VPTT, the values of ⁄ calculated with Equations 6.2 deviate far away
from the fitting line of light scattering results. It indicates that the adsorption of
microgels onto the oil/water interface is not simply controlled by diffusion
mechanism, especially above VPTT.
0.0031 0.0032 0.0033 0.0034-3
-2
-1
0
1
2
T > VPTT T < VPTT
DLS
IFT_short time
IFT_long time
linear fit for DLS
ln(
DT/D
0)
T-1 (K-1)
Figure 6.8. Dependence of apparent diffusion coefficients for PVCL/NIPMAm (5:1)
microgels on temperature. The values were obtained from DLS results (square) and
calculated through short time (circle) and long time (triangle) approximation by
Equations 6.2.
At the equilibrium state a balance exists between the adsorption and
desorption at the oil/water interface for the small molecular surfactant or the solid
inorganic particles in the Pickering emulsion.[17, 34]
The increase in temperature can
bring stronger thermal fluctuations that result in a weak particle attachment at the
fluid interfaces.[17, 48]
In our work, the oil drop was freshly formed in each experiment.
At the temperature above VPTT, the interfacial tension of the freshly formed droplet
Chapter 6. Microgels at the Oil/Water Interface
118
decays, as usual. That means the adsorption of microgels onto the interface is steady.
According to the literatures about the microgel-stabilized emulsion, the increase of
temperature leads to the breaking of emulsion, where macroscopic floccules are
observed.[7, 12]
However, even in the broken emulsions, microgels are found to be
adsorbed at the oil/water interface.[49]
So far, none has found significant desorption for
microgel particles from the oil/water interface even upon increasing temperature.
Monteux and co-workers reported the same interesting phenomena for
PNIPAm-based microgels, where the variation of the interfacial tension versus
temperature presents a minimum value around VPTT. They correlated the decrease in
the interfacial tension below VPTT to the increasingly compact packing of microgel
particles at the interface, and the increase in interfacial tension above the VPTT to the
aggragates of microgels at the interface at high temperatures.[12]
It has also been
revealed that soft microgels deform at the interface.[13, 14, 24, 26]
At T < VPTT, the
swollen microgels are able to deform easily. They can spread their dangling chains at
the particle surface to cover the interface as much as possible. That is the reason some
chain bridges were found between adjacent microgels in some reports.[13, 14, 24]
The
results of dynamic interfacial tension of microgels with different cross-link densities,
shown in Figure 6.4, strongly support this view. With lower cross-linker
concentrations, microgels are softer, thus have faster kinetics of the decay of
interfacial tension with time. However, at T > VPTT, the microgel particles deform
less. Without good spreading ability, a higher number of particles and a closer
packing density are needed to reach the maximum coverage of the interface. Besides,
the mobility at the high packing density might be much slower due to crowding or
jamming of colloidal particles at the interface.[50-52]
Therefore, a strong increase of
Chapter 6. Microgels at the Oil/Water Interface
119
is observed at T > VPTT. The adsorption of microgels below and above VPTT is
schematically shown in Figure 6.9.
Figure 6.9. The adsorption of microgels below and above VPTT.
Such a deformation-controlled mechanism could also explain the
independence of the interfacial tension on the microgel size shown in Figure 6.5.
Assuming that the use of surfactant in the synthesis does not change the internal
structure and the cross-linking concentration of microgels, the deformability of the
copolymer microgels should remain the same. Therefore, the interfacial tension
decays for PVCL/NIPMAm microgels synthesized with different amounts of CTAB
highly overlap to each other.
6.3.4. Dynamic interfacial tension of microgel mixtures
The temperature dependence of the interfacial behaviors of PVCL and
PNIPAm microgels are similar to each other in both and , but are different to
that of PNIPMAm microgels, especially in the kinetics represented by (Figure 6.7).
With the increase in temperature, microgel particles shrink because of the change in
the hydrophilic and the hydrophobic interactions.[2-6]
The decrease in the size results
in a denser microgel coverage at the interface, which consequently lower the
Chapter 6. Microgels at the Oil/Water Interface
120
interfacial tension.[12]
When the temperature is higher than VPTT, the increasing
thermal fluctuation and the loose aggregation lead to a slight increase in the value of
, and a sharp decrease in the kinetics of the interfacial tension decay (increase in
). However, the kinetics of the interfacial tension decay of NIPMAm-rich microgels
seems not to be affected by temperature. It indicates that microgels with chemical
structures rich in NIPMAm are probably easy to adsorb at the oil-water interface.
The interfacial tension of the mixing solution of PVCL and PNIPMAm
homopolymer microgels were investigated at different temperatures, i.e. 24, 37, and
44 °C. They are below, between, and above the VPTT of PVCL and that of
PNIPMAm homopolymer microgels, respectively (Figure 6.10). At 24 °C, of the
PNIPMAm system is much higher than that of the PVCL system, while of them are
very close to each other (Figure 6.7). Therefore, two reduction slopes with a small
time interval in the plot are observed. A similar result was obtained at 37 °C,
but the time interval between two interfacial tension reducing slopes is longer.
Because the temperature is above VPTT of PVCL microgels, the rate of the interfacial
tension reduction for PVCL microgels becomes slower as temperature increases.
However, the temperature has no effect on the kinetics of the dynamic interfacial
tension of the PNIPMAm microgel system. In the mixed solution of PVCL and
PNIPMAm microgels, only one step is observed in the dynamic interfacial tension
curve. With a faster kinetics, PNIPMAm microgels adsorb onto the interface earlier,
which decrease to the first plateau. Due to a slower kinetics, the influence of PVCL
microgels on begins to appear later than the first plateau, inducing a second plateau
in the plot. When temperature is above the VPTT of PNIPMAm microgels,
the difference between of PVCL and PNIPMAm is very slight, although the gap in
Chapter 6. Microgels at the Oil/Water Interface
121
of them is even larger (Figure 6.7). Therefore, only one plateau without any step is
observed in the plot at 44 °C.
0 100 200 300 400 5005
15
25
35
0 100 200 300 400 500 600 7005
15
25
0 100 200 300 400 500 600 7005
15
25
35
24 oC
(m
N/m
)
Drop age (s)
37 oC
(m
N/m
)
44 oC
(m
N/m
)
Figure 6.10. Dynamic interfacial tension of mixture solution of PVCL and
PNIPMAm at 24, 37 and 44 °C. Arrows show the steps between two kinds of quasi-
equilibrium induced by different microgels. The weight ratio of PVCL and
PNIPMAm in the mixture is 1:1, and the total concentration of microgels is 0.15
mg/mL.
6.3.5. Interfacial dilatational rheology
The interfacial tension results show that microgels are good surface active
materials, with high application potential in the emulsion stabilization field. To
further understand the structure of microgels at the toluene/water interface, the
dilatational rheology of the interface was investigated. Figure 6.11 shows the
temperature trend of the dilatational modulus of the toluene/water interface with
PVCL, PNIPAm and PNIPMAm homopolymer microgels with 3 mol% BIS. We can
Chapter 6. Microgels at the Oil/Water Interface
122
see that the storage moduli (elastic moduli, ) of the interface are higher than the loss
moduli (viscous moduli, ) for all microgel solutions at the temperatures between 24
to 45 °C. The values of moduli are of the same order as the reported results of
PNIPAm-co-MAA microgels at the heptane/water interface and PNIPAm microgels
at the air/water interface, but two orders lower than the results for hard latex
particles.[15, 53, 54]
20 25 30 35 40 45 501E-4
1E-3
0.01a
E'
E''
Dil
ati
on
al m
od
ulu
s (
N/m
)
Temperature (oC)
20 25 30 35 40 45 501E-4
1E-3
0.01
b
E'
E''
Dil
ati
on
al m
od
ulu
s (
N/m
)
Temperature (oC)
20 25 30 35 40 45 501E-4
1E-3
0.01
c
E'
E''
Dilata
tio
nal m
od
ulu
s (
N/m
)
Temperature (oC)
Figure 6.11. The dilatational modulus of the toluene/water interface with (a) PVCL,
(b) PNIPAm and (c) PNIPMAm microgels with 3 mol% BIS. The concentration of
microgels is 0.15 mg/mL.
The work of Cohin et al. shows that the compression dilatational surface
moduli for PNIPAm microgels at the air/water interface decrease with the increase in
temperature till VPTT and then increase with temperature.[53]
However, the
Chapter 6. Microgels at the Oil/Water Interface
123
temperature trend of the dilatational surface modulus for linear PNIPAm polymer
solution remains a plateau below LCST, and has a clear increase above LCST.[54]
For
all microgels studied here, the values of of the oil/water interface increase with
temperature. This means the elasticity of the oil/water interface adsorbed with
microgels increase as temperature increase. However, compared to linear polymers,[54]
the increasing rates of the interfacial elasticity for microgel systems are much lower.
The loss modulus of the interface increases slightly with temperature for
PVCL microgels, but remains at a certain level for PNIPAm microgels. For
PNIPMAm microgels, the values of of the interface decrease with temperature.
The results indicate that the viscous property of the oil/water interface with the
adsorption of microgels depends on the chemical structure of microgels. As
temperature increases, a decline is only found in the viscous property of PNIPMAm
microgels-adsorbed interface. That means the interface with the adsorption of
PNIPMAm microgels is more solid-like than those covered by PVCL and PNIPAm
microgels above VPTT. Typical winkles were found on the drop surface during the
volume contraction above VPTT for PNIPMAm microgels, leading to inaccuracy of
the results based on picture analysis of drop shape. Thus, the interfacial dilatational
rheology measurements for the PNIPMAm system were only implemented below
VPTT.
6.3.6. Microgel-stabilized emulsions
6.3.6.1. Emulsions prepared at different oil/water ratios
Figure 6.12 shows the photographs of the emulsions prepared with
PVCL/NIPAm (1:1) microgels at different oil/water volume ratios. Macroscopic
phase separation is found for these mixtures except the one with the oil/water ratio of
Chapter 6. Microgels at the Oil/Water Interface
124
3/7. For the samples prepared with the oil/water ratios of 1/9 and 2/8, the emulsions
are at the lower part of the mixtures. For the sample prepared with the oil volume
fraction of 30%, the entire mixture is at the emulsion state without macroscopic phase
separation. For samples with the oil volume content more than 40%, the emulsions are
at the upper parts of the mixtures.
Figure 6.12. Photographs of emulsions prepared with PVCL/NIPAm (1:1) microgels
of 3 mol% BIS at the oil/water volume ratios from 1/9 to 9/1. The concentration of
microgels in water phase is 0.5 wt%. The pink color of the samples originates from
Nile Red.
Figure 6.13 shows the fluorescence microscopy results of the emulsions. It can
be clearly found that the continuous phase is the oil stained by Nile Red, which shows
red color under the fluorescence microscope. That means the emulsions are of water-
in-oil (w/o) type, different from the reported results of the oil-in-water (o/w)
emulsions based on PNIPAm/MAA microgels.[7, 10, 11, 55]
Several characteristic
emulsion statuses are found in the fluorescence optical microscopy results. The
emulsions prepared with the oil/water ratios less than 3/7 are fluid, and the aqueous
drops are dispersed very well in the oil phase without collision (Figure 6.13a). By
contrast, the droplets in the emulsion with the oil/water ratio of 3/7 (Figure 6.13b)
have a higher density, similar to the high internal phase emulsions (HIPE) prepared
with PNIPAm/MAA microgels.[9, 56]
For the emulsion prepared with the oil/water
ratio of 4/6, the droplets collide with the neighbor ones and exhibit irregular
Chapter 6. Microgels at the Oil/Water Interface
125
polygonal shapes rather than spheres (Figure 6.13c). As the oil/water volume ratios
range from 5/5 to 9/1, the emulsions are unstable under the pressure of coverslip. The
coalescence happens quickly at the beginning of optical microscopy measurement.
Small aqueous droplets with collision are fused into bigger continuous phases with
small amount of oil between them (Figure 6.13d). After several minutes, the
coalescence becomes very slow and the system tends to be dynamically stable.
Figure 6.13. Fluorescence microscopy of the emulsions prepared with the oil/water
volume ratio of (a) 1/9, (b) 3/7, (c) 4/6 and (d) 6/4, showing different typical emulsion
statuses. The oil phase stained by Nile Red exhibits red color under the fluorescence
microscope. The concentration of microgels in water phase is 0.5 wt%.
6.3.6.2. Emulsions prepared at different microgel concentrations
The emulsions with the oil/water ratio of 3/7 prepared with different microgel
concentrations are shown in Figure 6.14. For systems with higher concentrations of
microgels (0.5 and 0.25 wt%), HIPE-like emulsions are obtained. When the
concentration of microgels is low (0.1 and 0.05 wt%), macroscopic phase separation
Chapter 6. Microgels at the Oil/Water Interface
126
occurs and a thin emulsion layer is located at the interface of water and oil phases. As
microgel particles play the role of stabilizer in the system, a sufficient amount of
particles is needed for the formation and subsequent stability of emulsions. We also
prepared emulsions using the microgels with different cross-linker contents, but found
no significant influence on the formation of emulsions.
Figure 6.14. Photographs of emulsions prepared with PVCL/NIPAm (1:1) microgels
of 3 mol% BIS at the oil/water volume ratio of 3/7. The concentration of microgels in
water phase varies from 0.05 to 0.5 wt%.
6.3.6.3. Summary of emulsions prepared with PVCL/NIPAm (1:1) microgels
The statuses of emulsions prepared with PVCL/NIPAm (1:1) microgel
solutions and chloroform are summarized in Table 6.3. It is found that the emulsions
are of w/o type, regardless of the microgel concentration and the oil/water ratio. The
emulsions were prepared with an Ultra Turrax, having a very strong shearing force.
Before the operation of the Ultra Turrax, the aqueous phase is at the upper part and
the oil phase is at the lower part because chloroform has a higher density than water.
For the samples with the oil/water ratio lower than 3/7, some water is brought into the
oil phase during the strong shearing, and a w/o emulsion forms at the lower part. For
the sample with the oil/water ratio of 3/7, the water phase and the oil phase are mixed
perfectly. As the volume fraction of the water phase is close to that of the internal
phase of HIPE (74%), the w/o HIPE in the entire mixture is formed. For samples with
the oil/water ratio above 3/7, an appropriate amount of oil is brought into the upper
Chapter 6. Microgels at the Oil/Water Interface
127
water phase to form w/o HIPE. It is interesting that the emulsion with the oil/water
ratio of 2.5/7.5 which has a closer ratio to 74% compared to that of 3/7, is not of
HIPE.
Table 6.3. Summary of the emulsions prepared with PVCL/NIPAm (1:1) microgel
solutions and chloroform.
Cross-linker
content
Oil/water
ratio (v/v)
Microgel
concentration
Emulsion status
3 mol% 1/9 0.5 wt% w/o at the lower part
2/8 0.5 wt% w/o at the lower part
2.5/7.5 0.5 wt% w/o at the lower part
3/7 0.5 wt% w/o HIPE in the entire mixture
0.25 wt% w/o HIPE in the entire mixture
0.1 wt% unstable
0.05 wt% unstable
4/6 0.5 wt% w/o HIPE at the upper part
5/5 0.5 wt% w/o HIPE at the upper part
6/4 0.5 wt% w/o HIPE at the upper part
7/3 0.5 wt% w/o HIPE at the upper part
8/2 0.5 wt% w/o HIPE at the upper part
9/1 0.5 wt% w/o HIPE at the upper part
1.5 mol% 1/9 0.5 wt% w/o at the lower part
2/8 0.5 wt% w/o at the lower part
3/7 5 wt% w/o HIPE in the entire mixture
0.05 mol% 1/9 0.5 wt% w/o at the lower part
2/8 0.5 wt% w/o at the lower part
3/7 0.5 wt% w/o HIPE in the entire mixture
Chapter 6. Microgels at the Oil/Water Interface
128
6.3.6.4. Emulsions prepared with microgels of different chemical compositions
The effect of the chemical composition of microgels on the emulsion is studied
as well, and the results are summarized in Table 6.4. At the oil/water ratio of 1/9, w/o
emulsions are obtained for PVCL/NIPAm microgel systems, regardless of the
monomer ratios of VCL/NIPAm. For PVCL/NIPMAm microgels with the
VCL/NIPMAm ratios of 5:1 and 1:1, the emulsions are of w/o type; and for that with
the VCL/NIPMAm ratio of 1:5, o/w emulsions are obtained. It is interesting that
stable o/w emulsions can be gained with PVCL/NIPMAm (1:5) microgels even at a
low microgel concentration (Figure 6.15), unlike PVCL/NIPAm systems (Figure
6.14). In Figure 6.15, we can see that the oil droplets, showing red color under
fluorescence observation, are dispersed in the continuous water phase.
Table 6.4. Summary of the emulsions prepared with PVCL/NIPAm and
PVCL/NIPMAm microgels of different chemical ratios.
Microgelsa Microgel concentration Emulsion status (oil/water = 1/9)
V:N (3:1) 0.5 wt% w/o at the lower part
V:N (1:1) 0.5 wt% w/o at the lower part
V:N (1:3) 0.5 wt% w/o at the lower part
V:NM (5:1) 0.5 wt% w/o at the lower part
V:NM (1:1) 0.5 wt% w/o at the lower part
V:NM (1:5) 0.5 wt% o/w at the lower part
V:NM (1:5) 0.015 wt% o/w at the lower part
a, V:N and V:NM represent PVCL/NIPAm and PVCL/NIPMAm, respectively; and
the ratios in the brackets are the monomer ratio of copolymer microgels.
Chapter 6. Microgels at the Oil/Water Interface
129
Figure 6.15. Optical microscopy of the emulsions (oil/water = 1/9) prepared with
PVCL/NIPMAm (1:5) microgels of 3 mol% BIS at the concentration of 0.015 wt%.
(a) is the bright field observation and (b) is the fluorescence observation. The insets in
(a) is the photograph of the emulsion.
6.3.7. Materials based on microgel-stabilized emulsions
The emulsion-template materials have been developed in different forms, such
as 3D porous scaffold,[57]
2D nanoporous film,[58]
and colloidosomes.[59]
The 3D
porous scaffold is an ideal matrix for the cell cultivation applied in tissue
engineering.[60, 61]
Functional nanoparticles are embedded into the pore walls of
scaffolds to make the materials multi-functional, more suitable for the biomedical
application.[62, 63]
As described above, the w/o emulsion prepared with PVCL/NIPAm (1:1)
microgel at the oil/water ratio of 3/7 resembles HIPE which is usually employed for
the fabrication of scaffold.[57]
The HIPE obtained mentioned above was freeze-dried
and observed with FESEM. As shown in Figure 6.16, the fragments of the emulsion
droplets can be found in the cross section of the freeze-dried sample. With a high
resolution in Figure 6.16b, we can see both close-packed hexagonal arrays and loosely
packed clusters, which have been reported for microgels adsorbed at the oil/water
interface of the emulsions observed by cryo-SEM.[13, 14, 24]
The packed microgels form
Chapter 6. Microgels at the Oil/Water Interface
130
thin films at the interface. We believe that such a packing layer structure of microgels
at the oil/water interface is essential in the stability of the emulsions.
Figure 6.17. FESEM images of the cross section of freeze-dried sample of the
emulsion prepared with PVCL/NIPAm (1:1) microgel at the oil/water ratio of 3/7.
Figure (b) is the magnified view of the place marked by the rectangle in Figure (a).
The freeze-dried emulsion is macroscopically floccule-like. To make a
stronger supporting frame, the chloroform solution of PCL and aqueous solution of
microgels were homogenized with Ultra Turrax at the same condition for the
preparation of the HIPE described above. The sample was freeze-dried and observed
with FESEM. As shown Figure 6.17, the freeze-dried sample is like a porous material.
The fragments of the emulsion droplets can be also found in the cross section of the
material. Compared with the freeze-dried emulsion samples without PCL, the pores
are smaller in the PCL-containing “porous material”. The high-resolution images
(Figures 6.17b and c) of an open “droplet” show that the microgels are embedded in
the wall of the “porous materials” constructed by polymers (PCL) in the oil phase.
Such kind of materials containing microgels could be used in tissue engineering or
cell cultivation with the advantage of the environmental stimuli controlled release.
Chapter 6. Microgels at the Oil/Water Interface
131
Figure 6.17. FESEM images of the cross section of the porous materials from the w/o
emulsions (oil/water = 3/7). Figures (b) and (c) are the magnified views of the places
marked by rectangles in (a) and (b), respectively. The aqueous phase contains
PVCL/NIPAm (1:1) microgels at the concentration of 0.015 wt% and the oil phase
contains PCL (Mw ~ 80 k) at the concentration of 4 wt%.
The emulsion-template colloidosomes are a kind of novel capsules with
permeability and have been developed rapidly in the last decade.[8, 59, 64-67]
As
mentioned above, the o/w emulsions can be obtained by mixing PVCL/NIPMAm (1:5)
microgel solutions and chloroform at the oil/water ratio of 1/9 (Table 6.4 and Figure
6.15). It has been reported that microgel-based capsules through an emulsion template
can be successfully obtained by mixing the microgel solution and polymer/chloroform
solution.[8, 66]
The preparation process of such capsules is schematically shown in
Figure 6.18.
At the first stage, the PCL/chloroform solution is prepared and added to the
microgel solution. Then the mixture is homogenized with Ultra Turrax to obtain a
stable o/w emulsion (Figure 6.18a). During the emulsification process, the microgel
particles adsorb to the oil/water interface and stabilize chloroform droplets. At the
second step, the chloroform is removed, and the water-insoluble PCL chains
precipitate on the inner surface and bind the microgel particles together (Figure 6.18b).
Chapter 6. Microgels at the Oil/Water Interface
132
As a result, capsules are formed with a PCL wall containing integrated microgel
particles.
Figure 6.18. Schematic representation of the capsule preparation: (a) the
emulsification process of the mixture of PCL/chloroform solution and aqueous
microgel solution, where microgels adsorb to the oil/water interface and stabilize
chloroform droplets; (b) after the removal of chloroform, capsules are formed with a
surface that has integrated PCL chains and microgels.
The morphologies of the capsules are characterized with FESEM. Figure 6.19
shows the typical examples of the colloidosome capsules. Due to the inhomogeneous
size of emulsion droplets, the capsule sizes are not uniform. From the FESEM images,
we can see that the capsule diameter ranges from around 5 to 20 μm. On the surface
of the capsules, the microgel particles are hexagonally close-packed, similar to the
reported results of both soft microgels and stiff solid latex at the surface of
colloidosome capsules.[8, 59, 64, 66]
From the high-resolution images, we can see that the
microgel particles are entrapped within the PCL layer, and small holes exist on the
wall. These holes would make the capsules permeable. As PCL is a biodegradable
polymer, the capsule wall can be broken with the degradation of PCL. Both effects
increase the potential of the formed microgel-based capsules in the biomedical
application like drug release. It should be noted that due to the coalescence of
Chapter 6. Microgels at the Oil/Water Interface
133
neighboring droplets in the emulsion, the coagulation among adjacent capsules is
found (Figure 6.19c).
Figure 6.19. FESEM images of the colloidosomes from the o/w emulsions (oil/water
= 1/9). The aqueous phase contains PVCL/NIPMAm (1:5) microgels at the
concentration of 0.015 wt% and the oil phase contains PCL (Mw ~ 80 k) at the
concentration of 2 wt%. The insets show the magnified views of the places marked by
the rectangles, and the arrows indicate the holes on the wall of capsules.
6.4. Conclusions
In this Chapter, the interfacial behaviors of the temperature-responsive
copolymer microgels based on VCL and two acrylamides, NIPAm and NIPMAm,
with various monomer ratios, are investigated. A classic kinetics of the interfacial
tension decay with three distinct regimes is found in the dynamic interfacial tension
plots of the copolymer microgels. This is similar to the dynamic interfacial behavior
of inorganic nanoparticles and proteins. The equilibrium interfacial tensions are
obtained through the data fitting using the empirical equation (Hua-Rosen equation).
We looked through the influences of monomer ratio and temperature on the
interfacial behavior of microgels. As the copolymer microgels have the similar
internal physical structure, regardless of the chemical composition, and the size of
microgels has no impact on the interfacial behavior, the variation of the interfacial
tension for different microgels should be derived from the chemical structure. The
Chapter 6. Microgels at the Oil/Water Interface
134
increase in monomer ratio of NIPAm or NIPMAm in microgels leads to an increase in
the interfacial tension. Furthermore, a transition of the interfacial behaviors of
microgels around VPTT is observed, which is highly consistent with the phase
behavior of microgels. Below VPTT, the equilibrium interfacial tensions of all
microgel systems decrease as temperature increases. Above VPTT, the equilibrium
interfacial tensions remain at a certain level for PVCL- and PNIPMAm-rich microgel
systems, and increase slightly for PNIPAm-rich microgel systems. When the
temperature is below VPTT, the interfacial behavior of microgels complies with a
diffusion-controlled process. However, when the temperature is above VPTT, the
diffusion-controlled mechanism seems not to be suitable for the interfacial assembly
of microgels studied here. It is proposed that the deformability (softness) of microgels
plays an imperative role in the interfacial behavior. Because of the reducing
deformability of microgel with increasing temperature, the evolution of the dynamic
interfacial tension for microgel at T < VPTT is faster than that at T > VPTT. The
dynamic interfacial tension results of the microgel systems with different cross-
linking densities strongly support this point.
The emulsions stabilized by PVCL/NIPAm and PVCL/NIPMAm microgels
are prepared. The type of the emulsion depends on the chemical composition of
microgels. It is found that PVCL/NIPAm microgels and PVCL/NIPMAm microgels
with low NIPMAm contents are inclined to make w/o emulsions, while
PVCL/NIPMAm microgels with high NIPMAm contents are prone to o/w emulsions.
For PVCL/NIPAm microgels, a HIPE is obtained at the oil/water volume ratio of 3/7.
The cross-linking density has no significant to the type of emulsions. Based on the
two kinds of emulsions prepared, two different emulsion-template materials can be
Chapter 6. Microgels at the Oil/Water Interface
135
made. Some preliminary results of the porous scaffold materials based on the w/o
HIPE and the colloidosome capsules based on the fluid o/w emulsion are presented.
The study of interfacial behavior of microgels at the oil/water interface in this
work might provide a new insight for better understanding the properties of microgels
at the fluid interfaces and future applications, related to the stabilization of emulsions
or the fabrication of microgel-based capsules for drug deliveries.
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Chapter 7. Summary
141
Chapter 7. Summary
We synthesized PVCL-based thermo-sensitive copolymer microgels and
investigated the influence of the chemical composition, cross-linking density and
temperature on their interfacial behavior.
The polymerization technique used is the precipitation polymerization which
involves the formation of microgel particles by a homogeneous nucleation mechanism.
Three sets of microgel samples were synthesized: a) with variable molar ratios
between VCL and NIPAm, and VCL and NIPMAm respectively; b) with variable
cross-linking degree and c) with variable size. The chemical composition of
copolymer microgels were determined by NMR spectroscopy. The results show that
the monomer ratio in copolymer microgel is close to the initial feeding ratio during
the synthesis. The hydrodynamic dimensions of all microgels are characterized by
DLS at different temperatures. As expected, the swelling property of microgels
strongly depends on the chemical structure, the cross-linking density and the presence
of the surfactant. Besides the neutral PVCL/NIPAm and PVCL/NIPMAm copolymer
microgels, ionizable PVCL/AAEM-based microgels are synthesized by the addition
of VIm or AAc monomers. The hydrodynamic size and zeta potential of
PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels at different pH values are
detected by DLS and Zetasizer, respectively. The pH-responsiveness of these
microgels indicates a successful incorporation VIm and AAc groups into the
copolymer microgels.
The internal structure of PVCL-based copolymer microgels were investigated
with SLS and microscopy methods (AFM and FESEM). The SLS results show that
Chapter 7. Summary
142
PVCL/NIPAm and PVCL/NIPMAm microgels, both with different monomer ratios in
copolymer, have the core-corona structure with a radial profile as reported for
PNIPAm microgels previously. The characterization of PVCL/NIPAm copolymer and
PVCL homopolymer microgels by AFM gives morphologies and dimensions (contact
radius and height) of microgels on a solid substrate. It provides visualized evidence of
a core-corona structure depending on the cross-linking density. The AFM
characterization of PVCL/NIPAm copolymer microgels deposited on a solid substrate
above VPTT reveals the structure of microgels in the collapsed state, where the
corona part is hardly to observe. Besides, PVCL-based microgels with functional
groups are characterized with AFM and FESEM. It is interesting that the core-corona
structure is found in PVCL/AAEM /VIm microgels, but not in PVCL/AAEM/AAc
microgels, which might be originated from their different synthesis procedures. In
addition, the AFM observation on microgels of very low cross-linking density
provides a possible method to look into the domains in the cross-linked polymer
networks.
We further studied adsorption and deformation of microgels at the air/water
and air/solid interfaces. The surface activity of the PVCL/NIPAm microgels varied in
cross-linking degree was detected with a tensiometer and the conformation of
microgels at the air/water interface was simulated. The results show that faster
spreading happens for the loose microgel which takes more flat shape compared to the
dense microgel. The contact radius and height of the adsorbed microgels, extracted
from AFM images, indicate that the deformability of the microgels increases with
decrease of the cross-linking degree. The results suggest that adsorption of microgels
induces deformation exceeding the swelling-induced expansion. In particular, a
loosely cross-linked microgel sustains large deformation due to swelling.
Chapter 7. Summary
143
Furthermore, the interfacial behaviors of PVCL/NIPAm and PVCL/NIPMAm
microgels varied in the monomer ratio, cross-linking degree and size are investigated.
A classic kinetics of interfacial tension with three distinct regimes, similar to the
dynamic interfacial behavior of inorganic nanoparticles and proteins, are found in the
dynamic interfacial tension plots of copolymer microgels. The equilibrium interfacial
tensions are obtained through the data fitting using the empirical equation (Hua-Rosen
equation).
As the copolymer microgels have the similar internal physical structure,
regardless of the chemical composition, and the size of microgels has no impact on
the interfacial behavior, the variation of the interfacial tension for different microgels
should be derived from the chemical structure. A transition of the interfacial
behaviors of microgels around VPTT is observed, which is highly consistent with the
phase behavior of microgels. Below VPTT, the equilibrium interfacial tensions of all
microgel systems decrease as temperature increases. Above VPTT, the equilibrium
interfacial tensions remain at a certain level for PVCL- and PNIPMAm-rich microgel
systems, and increase slightly for PNIPAm-rich microgel systems. When the
temperature is below VPTT, the interfacial behavior of microgels complies with a
diffusion-controlled process. However, when the temperature is above VPTT, the
diffusion-controlled mechanism seems not to be suitable for the interfacial assembly
of microgels. It is proposed that the deformability (softness) of microgels plays an
imperative role in the interfacial behavior. Because of the reducing deformability of
microgel with increasing temperature, the evolution of dynamic interfacial tension for
microgel at T < VPTT is faster than that at T > VPTT. The dynamic interfacial
tension results of microgel systems with different cross-linker densities strongly
support this point.
Chapter 7. Summary
144
Last but not the least, we used PVCL/NIPAm and PVCL/NIPMAm microgels
varied in the monomer ratio and cross-linking degree to produce microgel-stabilzed
emulsions. The type of the emulsion depends on the chemical composition of
microgels. It is found that PVCL/NIPAm microgels and PVCL/NIPMAm microgels
with low NIPMAm contents are inclined to make w/o emulsions, while
PVCL/NIPMAm microgels with high NIPMAm contents are prone to o/w emulsions.
For PVCL/NIPAm microgels, a HIPE is obtained at the oil/water volume ratio of 3/7.
The cross-linker density has no significant to the type of emulsions. Based on the two
kinds of emulsions prepared, two different emulsion-template materials can be made.
They are porous scaffold materials based on the w/o HIPE and colloidosome capsules
from the fluid o/w emulsion.
Chapter 8. Outlook and Future Experiments
145
Chapter 8. Outlook and Future Experiments
The interfacial behaviors of PVCL-based microgels on the solid surface or at
the oil/water interface are investigated in this thesis. The results can provide a new
insight for better understanding the properties of microgels at interfaces and future
applications, like the stabilization of emulsions or the fabrication of microgel-based
capsules for drug deliveries. However, the topic of soft microgel particles at the
interface is very complex, and needs further investigation. Here we propose some
concepts for the future experiments.
8.1. Interfacial behavior
8.1.1. Microgels with ionizable groups or specific structures
The ionizable groups in functional microgels play an important role in the
structure and consequently the property of microgels. A lot of work has been done to
investigate the interfacial behavior of PNIPAm-based ionizable microgels. The charge
of the microgel influences the assembly structure of microgel layers at the interfaces.
However, the intrinsic role of charges still needs further investigation. Recently, the
core-shell microgels with different components in core and shell parts are obtained.[1]
This enables the study on the influence of spatial distribution of functional groups on
the interfacial behavior of microgels. Further experiments could be carried out with
the following sets:
1) Polyelectrolytic or ampholytic microgels with acidic or basic groups
located in the core or shell of the particle;
Chapter 8. Outlook and Future Experiments
146
2) Core-shell microgels with different VPTTs in core and shell parts, like
PVCL-core/NIPMAm-shell and PNIPMAm-core/PVCL-shell;
3) Core-shell microgels with stiff insensitive core, polystyrene for example,
and soft sensitive shell, like NIPAm or VCL; or reverse ones.
It should be noted that there are many sorts of microgels with unique
structures, and specific properties. They are worth well investigating.
8.1.2. Other techniques
Many other techniques, such as interfacial rheology and cryo-FESEM and
liquid-cell AFM, have also been used to characterize interfacial layers. Interfacial
rheology is very useful in investigating the mechanical property of microgel layers at
the fluid interface and their response to external stimuli. Cryo-FESEM is another
practical tool to detect the assembly microgel suprastructures at the liquid/liquid
interfaces. However, either rheometer or cryo-FESEM cannot take into account both
the bulk state and visualization of microgel structures at fluid/fluid interfaces. Liquid-
cell AFM offers a potential in observing the microgel structure at the fluid interface in
situ. To achieve this aim, we need choose the oil phase carefully, due to a strong
fluctuation of the fluid interfaces. Silicone oil is a possible option as its viscosity is
much higher than water. The interface of water and silicone oil could be relatively
stable for the liquid-cell AFM characterization of adsorbed microgels.
8.2. Potential applications and improvements
8.2.1. Asymmetric colloidal particles
We have shown that the adhesion of microgels on a solid substrate is very
strong. With such a feature, microgels can be applied to microgel-supported adhesives
Chapter 8. Outlook and Future Experiments
147
which are useful in papermaking industry. Besides the indeformable silicon wafer
used in this work, microgels can also be deposited on a soft stretchable substrate,
PDMS for example. After the deposition, we can stretch the substrate. Then the
mcirogel particles on the substrate surface would be elongated in the stretching
direction.[2]
With some cross-linking treatments, like UV irradiation of light sensitive
groups, the stretched microgel particles would be fixed in an asymmetric shape. After
removed from the substrate, anisotropic particles can be obtained.
Figure 8.1. Fabrication of asymmetric colloidal particles upon stretching.
Another potential application of microgels on a solid substrate is to form
asymmetric hybrid colloids. Metal nanoparticle precursors can be incorporated inside
the microgels with functional groups, for instance, HAuCl4 in PVCL/AAEM/AAc
microgels.[3]
As demonstrated in Chapters 4 and 5, the polymer density decrease from
the center to the surrounding of the particle profile. If such a structure is exposed in
UV light for an appropriate time, Au nanoparticles would only form in a superficial
layer where the UV light reaches. After eluted from the substrate, asymmetric
colloidal hybrids can be produced.
8.2.2. New microgel-based capsules
The preliminary fabrication of microgel-based emulsions, as well as capsules
and scaffolds is successful in this work. The properties of microgel-stabilized
emulsions, like stability and stimuli responsiveness, need to be studied in detail. For
Chapter 8. Outlook and Future Experiments
148
the future application of the capsules and porous scaffolds, several issues should be
addressed. Firstly, the parameters for producing such kinds of materials from
emulsion need to be optimized to obtain monodisperse capsules and interconnected
porous scaffolds. Secondly, more experiments, like drug load and release or
mechanical test, need to be implemented to ensure that microgel-based capsules and
scaffolds are effective materials for biomedical application. Thirdly, polymers with
the glass transition temperature or melting temperature close to the VPTT of
microgels can be used to increase the temperature sensitivity of the formed capsules.
8.2.3. Designed biodistribution of microgels for therapy
In nature cells can deform to translocate through small vascula and recover to
original shape without loss of physiological functions. As microgels are soft porous
particles, they can deform like cells when translocating through a channel much
smaller than the particle size. In a simulation of renal filtration of soft particles,
microgels passed through pores which were ten times smaller than the particle
diameter under physiologically relevant pressures.[4]
Such a property could be applied
to anti-cancer therapy for which the translocation of drug carriers through tumor
vascular is demanded (Figure 8.2).[5, 6]
Additionaly, the deformability (softness) also
governs the biodistribution and circulation times of soft gel particles in blood.[7]
Because of the diverse retention sizes of human organs, the circulation time and
distribution of gel particles in the body differ upon the organ. Therefore we can
design microgels with an appropriate softness for target therapeutic applications due
to their preferential accumulation in a tissue.
Chapter 8. Outlook and Future Experiments
149
Figure 8.2. Microgel-based nanocarriers for tumour targeting of long-circulating
therapeutics by the translocation of deformable particles loaded with drugs through
tumor vascula.
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catalytically active gold-polymer microgel hybrids via a controlled in situ reductive
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Acknowledgements
151
Acknowledgements
At the end of the thesis, I would like to express my gratitude to many people who
helped me during my four-year doctorate study in Aachen.
Firstly, I would like to thank Prof. Dr. Andrij Pich for giving me a good opportunity of
pursuing my PhD study in the Functional and Interactive Polymers group at DWI. I sincerely
appreciate him for his kind support in the development of my research work and useful
discussions with him.
I am thankful to Dr. Ahmed Mourran for his constructive suggestions and kind helps
on AFM measurements and Prof. Dr. Igor Potemkin for his help on the computer simulation
support. I would also like to thank Prof. Dr. Walter Richtering from the Institute of Physical
Chemistry (IPC) at RWTH Aachen University for his valuable comments and interesting
discussion on my PhD work. Thanks to Dr. Xiaomin Zhu for his kind help in my arriving in
Aachen. In addition, the financial support from the China Scholarship Council (CSC) for my
first three years of PhD study in Germany is highly appreciated.
I am grateful for all my other colleagues at DWI and ITMC who helped me during my
memorable years. Many thanks to Dr. Andreea Balaceanu, Ms. Karla Dörmbach, Ms. Garima
Agrawal, Ms. Ricarda Schröder, Mr. Dominic Kehren and Mr. Christian Willems for the work
side by side since I was in Aachen. Thanks to Ms. Angela Huschens and Mr. Rainer Hass for
their kind help as well. The kind help from Ms. Susanne Weise at IPC/RWTH for light
scattering measurements and Mr. Rustam A. Gumerov at Physics Department in Lomonosov
Moscow State University for computer simulation is also appreciated. To my other colleagues,
I appreciate your kindness and will never forget the happy time with you guys of working
together, funny outing-trips and delicious BBQs. Although I do not list all your names here, I
cherish everything in my heart.
Acknowledgements
152
Thanks to my dear Chinese friends I met in Aachen: Dr. Hailin Wang, Dr. Yingchun
He, Dr. Zhirong Fan and Mrs. Fan – Yunfei Jia, Dr. Qizheng Dou, Mrs. Dou – Dr. Cheng
Cheng and their cute daughter – Tangdou, Dr. Jingbo Wang and Mrs. Wang – Yuting Zhao,
Dr. Lei Li, Dr. Heng Zhang, Ms. Huihui Wang, Mr. Qingxin Zhao, Ms. Yelin Bao, Ms. Helin
Li, Ms. Chi Zhang, Mr. Yongliang Zhao, Ms. Yanqing Li, Ms. Qianjie Zhang, Mr. Baolei Zhu,
Mr. Huan Peng, Mr. Chao Liang, Ms. Yanlan Zheng, Mr. Hang Zhang, Mr. Zhihua Hong, Mr.
Baochun Wang and Mrs. Wang – Congling Wang, Ms. Yumei Wang, Mr. Zhi Chen, Mr.
Kang Han, Ms. Xiaolin Dai and Mr. Lei Wu. I will cherish the happy memory of having lunch
together, hanging out from time to time and sport games every week. Besides, I would also
like to thank my friends who chatted with me by phones or emails. Wherever you guys are, in
China, UK or USA, you always encouraged me to get free from the trapped thoughts. Thank
you!
Lastly but most sincerely, I would like to express my greatest appreciation to my
family for their warm support and selfless love to me. Confucius said: “While your parents
are alive, do not journey afar. If a journey has to be made, your direction must be told.” My
dear mother, father, grandma, uncles and aunts: now I am completing my far journey. I did
not make such a journey alone (and I cannot make it alone), but with all of you backing me
and as well along with so many lovely friends. I love you very much!
Yaodong Wu
08 July, 2014
153
Curriculum Vitae
Personal Details
Family Name: Wu
First Name: Yaodong
Birth Date: 31. December 1986
Birth Place: Dongzhi, Anhui, China
Nationality: Chinese
Education
09/2003 – 07/2007
Bachelor of Engineering, Material Science and Engineering, Tianjin University,
Tianjin, China
09/2007 – 07/2010
Master of Science, Polymer Chemistry and Physics, Fudan University, Shanghai,
China
09/2010 –
Doctorate study in Macromolecular Chemistry, RWTH Aachen University, Aachen,
Germany
154
List of publications
Papers
1. Yaodong Wu, Susanne Wiese, Andreea Balaceanu, Walter Richtering, Andrij
Pich. Behaviour of Temperature-responsive Copolymer Microgels at the
Oil/Water Interface, Langmuir, 2014, 30, 7660–7669.
2. Yaodong Wu, Cheng Cheng, Jinrong Yao, Xin Chen, and Zhengzhong Shao.
Crystallization of Calcium Carbonate on Chitosan Substrates in the Presence of
Regenerated Silk Fibroin, Langmuir, 2011, 27, 2804–2810.
3. Florian Schneider, Andreea Balaceanu, Artem Feoktystov, Vitaliy Pipich,
Yaodong Wu, Jürgen Allgaier, Wim Pyckhout-Hintzen, Andrij Pich, Gerald
Schneider. Monitoring the Internal Structure of Poly(N-vinylcaprolactam)
Microgels with Variable Crosslink Concentration, submitted.
4. Yaodong Wu, Rustam A. Gumerov, Andrey A. Rudov, Igor I. Potemkin, Andrij
Pich, Ahmed Mourran. Statics and Dynamics of Interfacial Attachment of
Microgel Colloids, in progress.
Conference contributions
1. Yaodong Wu, Ahmed Mourran, Andrij Pich. Deformation and Adhesion of
Microgels on a Solid Substrate, Poster Presentation at Bayreuth Polymer
Symposium 2013, Bayreuth, Germany, September 15th
– 17th
, 2013.
2. Yaodong Wu, Susanne Wiese, Andreea Balaceanu, Walter Richtering, Andrij
Pich. Behaviour of Thermo-sensitive Copolymer Microgels at the Oil/Water
Interface, Poster Presentation at 27th Conference of the European Colloid and
Interface Society, Sofia, Bulgaria, September 1th
– 6th
, 2013.
3. Yaodong Wu, Susanne Wiese, Andreea Balaceanu, Walter Richtering, Andrij
Pich. Behaviour of Thermo-sensitive Copolymer Microgels at the Oil/Water
Interface, Poster Presentation at IUPAC 10th International Conference on
Advanced Polymers via Macromolecular Engineering, Durham, UK, August 18th
– 22nd
, 2013.