Responsive Gels: Volume Transitions I

280 Pages·1993·4.97 MB·English

by  Mitsuhiro Shibayama

109
Advances Science Polymer in
Responsive Gels:
Volume Transitions I
Editor: K. Dugek
contributions With by
M. Ilavsk#, .H Inomata, A. Khokhlov, M. Konno,
A. Onuki, .S Saito, M. Shibayama, R.A. Siegel,
.S Starodubtzev, .T Tanaka, ¥. .V Vasiliveskaya
With 241 Tables 6 and Figures
galreV-regnirpS
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Volume Editor:
Prof. K. Dugek
Inst. of Macromolecular Chemistry
Czech Academy of Sciences
162 06 Prague 6, Czech Republic
ISBN 3-540-56791-7 Springer-Verlag Berlin Heidelberg NewYork
ISBN 0-387-56791-7 Springer-Vedag NewYork Berlin Heidelberg
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Preface
Gels are cross-linked networks of polymers swollen with a liquid. Softness,
elasticity, and the capacity to store a fluid make gels unique materials. As our
society becomes richer and more sophisticated, and as we increasingly recognize
that natural resources are not unlimited, materials with better quality and
higher functional performance become more wanted and necessary. Soft and
gentle materials are beginning to replace some of the hard mechanical materials
in various industries. Recent progress in biology and polymer sciences
is unveiling the mystery of marvellous functions of biological molecules
and promises new development in gel technologies. All these factors bring
us to realize the importance and urgent need of establishing gel sciences and
technologies.
Due to the cross-linking, various properties of individual polymers become
visible on a macroscopic scale. The phase transition of gels is one of the most
fascinating and important phenomena that allows us to explore the principles
underlying the molecular interactions and recognition which exist in synthetic
and biological polymers. The polymer network changes its volume in response
to a change in environment; temperature, solvent composition, mechanical
strain, electric field, exposure to light, etc. The prediction and finding of the
phenomenon have opened the door to a wide variety of technological appli-
cations in chemical, medical, agricultural, electrical, and many other industrial
fields.
The volume phase transition in gels has its history. It was theoretically
predicted before it was discovered experimentally. However, the path from
theory to experiment was not so straighforward because the conclusion of the
theoretical analysis was that conditions for such a transition could hardly be
met experimentally.
Among the participants of the IUPAC International Symposium on
Macromolecular Chemistry in Prague in were 1965, the Editor of this volume
(K.D.) and Donald Patterson (D.P.) of the CRM in Strasbourg and later McGill
University in Montreal. D.P., well-known for his work in polymer solutions
thermodynamics, presented a paper in this area, and K.D. presented a theor-
etical paper on phase separation in Tgheilss,. however, concerned separation of
a liquid from a swollen gel as a result of deterioration of polymer-solvent
interaction or increasing crosslinking density during the crosslinking process
where dilutions during crosslinking played an important role [1].
At the time of the conference, D.P. and K.D. discussed the possible peculiar
shapes of the solvent chemical potential vs composition curves in swollen
vi ecaferP
polymer networks prepared at different dilutions during network formation and
values of the polymer solvent interaction parameter. Some of these curves
exhibited a minimum followed by a maximum, a condition necessary for
coexistence between two phases of different composition. Also at this sym-
posium, a paper was given by Oleg Ptitsyn [2] on globule-coil transition in
which he showed that a polyelectrolyte chain can undergo a collapse transition
if the polymer-solvent interaction or degree of ionization were changed. All this
inspired us in a deeper investigation of the phase equilibria in swollen polymer
networks.
The result of analysis showed that a thermodynamic transition between two
gels states differing in polymer concentration can be real and that the transition
can be brought about not only by a change in the interaction parameter
(temperffture) but also by deformation. To exhibit this phase transition, the gel
was to be prepared in the presence of a sufficient amount of diluent, its
crosslinking density had to be sufficiently high, and the solvent in which it was
swollen had to be rather poorer. The mechanistic explanation of the predicted
phase transition was as follows: the network chains, after removal of the diluent
after crosslinking, were rather supercoiled and had a tendency to assume more
relaxed (expanded) conformation; this tendency was resisted by a strong ten-
dency towards polymer segment association due to an unfavorable polymer -
solvent interaction (poor solvent). The balance between these two strong and
oppositely acting forces gave rise to the possibility of phase transition. However,
it had turned out that preparation of such non-ionic gels at a high content of
diluent and having high crosslinking density would be difficult due to a danger
of gel-liquid phase separation during preparation. It was clear that a strong
concentration dependence of the polymer solvent interaction parameter of the
swelling liquid would greatly facilitate the occurrence of phase transition.
Polyelectrolyte gels were not considered at all, although they could have been
theoretically analyzed in view of the Ptitsyn's prediction of the globule-coil
transitions.
The first report on the gel-gel transition was presented in September 1967 at
the ts1 Prague Microsymposium on Marcomolecules [3]. A paper was sub-
mitted to the Journal of remyloP ecneicS and was published in 1968 [4]. One of
the referees wrote that it was questionable whether a paper should be published
on a phenomenon which could hardly be observed experimentally and re-
commended a reduction of the manuscript to about 50%. To meet, at least
partly, his wishes, we reduced the manuscript to about 70% by removing all
speculations about the possible concentration dependences of the interaction
parameter.
These circumstances may explain why it took ten years fort he phenomenon
to be experimentally observed after the prediction. In ,3791 prior to this finding,
Lon Hocker, George Benedek, and Tanaka realized that a gel scattered light,
and the light intensity fluctuated with time [5]. They established that the
scattering is due to the thermal density fluctuations of the polymneert work and
derived a theory that explained the fluctuation. These fluctuations are similar to
ecaferP vii
sound waves propagating in an elastic solid, which in this case is the polymer
network. Since the network moves in water, however, the sound wave does not
propagate, but decays exponentially with a relaxation time proportional to the
square of the wavelength of the sound wave.
Time = LengthZ/D
Here D is cooperative diffusion coefficient of the gel. Such a relationship applies
to the random or diffusive motions of molecules in a fluid; for example, ink
molecules in water. It is interesting that the same relation holds for a polymer
network even though all the polymers are connected into a single network.
In 1977, while studying the light scattering from an acrylamide gel, Shin-ichi
Ishiwata, Coe Ishimoto, and Tanaka found that the light intensity increased,
and the relaxation time became longer as the temperature was gradually
lowered [6]. They both diverged at a temperature of minus 17°C. Thus the
critical phenomena were found in gels.
The finding raised a question of ice formation, although such a possibility
was carefully checked and eliminated by the measurement of the refractive index
of the gel. Such a question could be answered once and for all, tihfe temperature
at which the scattering diverged was raised to much above the freezing
temperature. So, many pieces of the gel were placed in acetone-water mixtures
with concentrations ranging from 0% to 100%, hoping to find a proper solvent
in which the gel would become opaque at room temperature. The next day, all
the gel pieces were found to be transparent. But surprisingly, the gels in the
lower acetone concentrations were swollen, and the gels in the higher acetone
concentrations were collapsed. This meant that the gel volume changed dis-
continuously as a function of acetone concentration. The volume transition was
found in gels 10 year after the first theoretical prediction [7].
The experiments were repeated but were not reproducible: Acrylamide gels
were made anew with various recipes and their swelling curves were determined
as a function of acetone concentration, but they were all continuous. It took a
couple of months to recognize that the gels that showed the discontinuous
transition were old ones, that is, gels prepared a month earlier and left within the
tubes in which they were polymerized. Subsequent experiments were all carried
out on "new" gels, and, therefore, underwent a continuous transition. At that
time all the "old" gels were used up, and none were left in the laboratory.
Later the difference between the new and old gels wasi dentifieads ionization
which induced an excess osmotic pressure within the gels leading to the
discontinuous transition [8]. Hydrolysis was graduallyt aking place in the gel in
a mildly high pH solution used at gelation. This explanation was experimentally
proven by artificially hydrolyzing the gel and observing the increase in the
discontinuity of the volume transition.
The theoretical formulation indicates that the gel transition should be
universally observed in any gel. Many gels of synthetic and natural origin have
been studied and the universality of the phase transition in gels seems to have
been well established [9-11].
viii ecaferP
This volume contains the first part of short reviews with emphasis on the
authors' work to show the present activity and state of knowledge in the field of
volume transitions in gels. Part II will appear in Volume 110. Unfortunately, a
few of the leading groups were not able to prepare a review in time due to their
overcommitments.
References
.1 K Du~ek )7691( J Polym icS C 16:1289
.2 Ptitsyn ,BO Kron ,BA Eisner EY )5691( IUPAC International Symposium on Macromolecular
Prague, Chemistry Preprint P747
.3 Patterson D K, Du~ek )7691( transition A in induced networks polymer swollen by intramolecu-
lar Gels Polymer Microsymposium condensation, and Macromol. Inst. Solutions, Concentrated
.mehC Abstract Prague, F2
.4 K, Patterson Du~ek D )8691( J Polym icS A-26:1209
.5 Tanaka ,T Hocker ,OL Benedek ,BG )3791( J Chem Phys 1515:95
.6 Tanaka ,T Ishiwata ,S Ishimoto ,C )7791( Phys veR Lett 93 : 474
.7 Tanaka T )8791( Phys veR Lett 40:820
.8 Tanaka T, Fillmore ,JD Nishio Sun S-T, .I wolsiwS G, Shah A )0891( Phys veR Lett 45:1636
.9 Hrouz ,J '~skvatI ,M Ulbrich K, ke6epoK J (t981) Eur J Polym 17:361
.01 M, Hrouz Ilavsk~ J, Ulbrich K )2891( Polym Bull 7:107
.11 Amiya .T Tanaka T )7891( 20:1162 Macromoleeules
Karel ke~uD
Institute of Macromolecular Chemistry, Czechoslovak Academy of ,secneicS 16206 Prague ,6
aikavolsohcezC
Toyoichi Tanaka
Cambridge, Institute Technology, Massachusetts of ,AM ASU
Table of Contents
Phase Transition and Related Phenomena of Polymer Gels
M. Shibayama, T. Tanaka .................................
Theory of Phase Transition in Polymer Gels
A. Onuki .............................................. 63
Conformational Transitions in Polymer Gels:
Theory and Experiment
A. Khokhlov, S. Starodubtzev, V.V. Vasilevskaya .............. 123
Effect of Phase Transition on Swelling and Mechanical
Behavior of Synthetic Hydrogels
M. Ilavsk~ ............................................. 173
Volume Phase Transition oNf- Alkylacrylamide Gels
S. Saito, M. Konno, H. Inomata ............................ 207
Hydrophobic Weak Polyelectrolyte Gels:
Studies of Swelling Equilibria and Kinetics
R.A. Siegel ............................................ 233
Author Index Volumes 100 - 109 .......................... 269
Subject Index .......................................... 273
Volume Phase Transition and Related Phenomena
of Polymer Gels
Mitsuhiro Shibayama* and Toyoichi Tanaka
Department of Physics, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA
This review covers the recent advances in studies of the volume phase transition and critical
phenomena of polymer gels mostly carried out in our group from 1973 to the present. We aimed here
to discuss intensively (i) the basic understanding of the transition from the viewpoints of structure,
dynamics, kinetics, and equilibrium thermodynamics, (ii) technological applications of the volume
transition, and (iii) the relation between the phase transition and biological interactions.
List of Symbols and Abbreviations ................................. 3
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Thermodynamics of Volume Phase Transition of Gels ................... 10
2.1 van der Waals Fluids .................................... 10
2.2 Classic Theory for Gels ................................... 11
2.3 Prediction of Discontinuous Volume Phase Transition
with Respect to Temperature ................................ 14
2.4 Improved Equation of State ................................ 16
3 Dynamics of Gels .......................................... 18
3.1 Collective Diffusion of Gel Networks ........................... 18
3.2 Dynamic Light Scattering (DLS) .............................. 19
3.3 Spatial Inhomogeneity and Non-Ergodicity of Gels .................. 22
4 Microscopic Structure of Gels: SANS ............................. 23
4A Polymer Solutions ...................................... 24
4.2 Neutral Gels ......................................... 24
4.3 Weakly Ionized Gels ..................................... 27
5 Critical Phenomena of Gels ................................... 32
5.1 Dynamic Light Scattering .................................. 32
5.2 Gel-Solvent Friction ..................................... 33
5.3 Calorimetry and Universality Class of the Volume Phase
Transition of Gels ...................................... 34
5.4 Small.Angle Neutron Scattering .............................. 37
6 Kinetics of Swelling ........................................ 37
6.1 Spherically Symmetric Gels ................................. 37
6.2 Asymmetric Gels ....................................... 39
6.3 Critical Kinetics ....................................... 44
* Permanent address: Department of Polymer Science and Engineering, Kyoto Institute of
Technology, Matsugasaki, Kyoto 606, Japan
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2 M. Shibayama and T. Tanaka
7 Fundamental Interactions for Volume Phase Transition cf Gels ............. 45
7.1 ~an der Waals Interaction ................................. 46
7.2 Hydrophobic Interaction .................................. 47
7.3 Hydrogen Bonding ...................................... 49
7.4 Electrostatic Interaction ................................... 49
8 Environment Sensitive Gels ................................... 51
8.1 Thermo-Sensitive Gels .................................... 51
8.2 Solvent Sensilixe Gels .................................... 52
8.3 Ion and pH Sensilixe Gels 52
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4 Light Sensitive Gels ..................................... 53
8.5 Eleclric Field Sensitive Gels ................................ 53
8.6 Biochemically Sensitive Gels ................................ 53
8.7 StJess Sensitive Gels ..................................... 54
9 Multiple Pha, es .......................................... 55
10 Concluding Remarks ....................................... 58
11 References ............................................. 60