J. Phys. Chem. 1994, 98, 11115-11118
11115
Volume Phase Transition of Thermosensitive Polymer Gels and Mechanism for Miscibility Seiji Katayama' and Yukio Akahori University of Shizuoka, School of Pharmaceutical Sciences 52-1, Yada Shizuoka, 422 Japan Received: March 9, 1994@
Temperature-dependent volume changes of the sodium methacrylate (SMA)/acrylamide (A)/N,N'-methylenebisacrylamide (Bis) copolymer gel (SMA gel), sodium stylenesulfonate (SSS)/A/Bis copolymer gel (SSS gel), and sodium 2-acrylamido-2-methylpropanesulfonate(SAMPS)/A/Bis copolymer gel (SAMPS gel) immersed in acetone-water mixtures were examined, as well as solvent-dependent volume changes of the gels. Thermoswelling volume change was observed for the SMA gel and thermoshrinking volume change for S S S and SAMPS gels. The results were found to be consistent with miscibility of the gels in acetonewater mixtures. That is, miscibility as the ratio of hydrophobic to hydrophilic characteristic is a factor determining the pattern of thermoswelling or thermoshrinking volume phase transition.
Introduction First referred to by covalently cross-linked polymer gels with an ionizable group undergo a reversible, discontinuous volume change with change in solvent composition, temperature, pH, and salt c~ncentration.~-llThis is a volume phase transition between a swollen and collapsed gel and has been generalized as a typical first-order phase transition. We have given much attention to temperature-dependent volume phase transition of polymer gels (thermoreversible polymer gels),'*-17 as well as solvent-dependent volume phase t r a n ~ i t i o n l ~from - ~ ~ the mechanochemical viewpoint. The present paper is concerned with patterns of volume phase transition of polymer gels induced by temperature change. Thermosensitive polymer gels undergo reversible volume change from a swollen to collapsed state via discontinuous volume change with change in temperature. The volume change of thermosensitive polymer gels has been studied for mechanochemical characteristics of polymer gels. For temperaturedependent volume change, three patterns have been observed: 12-14 shrunken-swollen behavior with discontinuous volume change, swollen-shrunken behavior with discontinuous volume change, and shrunken-swollen-shrunken behavior with two discontinuous volume changes. The first is a thennoswelling pattern, the second a thennoshrinking pattern, and the third a convex0 pattern. The theory underlying these patterns and their chemical conditions are still not understood. We have speculated that the relative amount of hydrophobic residue to hydrophilic residue incorporated in the gel polymer may play an important part in determining the patterns of volume phase transition. l 4 In the present experiment, temperature-dependent volume changes of several thennosensitive polymer gels are examined and the chemical conditions determining the patterns of volume phase transition are discussed. Thus, the following thermosensitive polymer gels were used: sodium stylenesulfonate/acrylamide (A)/N,N'-methylenebisacrylamide(Bis) copolymer gel, sodium 2-acrylamide-2-methylpropanesulfonate/ -is copolymer gel, and sodium methacrylate/A/Bis copolymer gel.
Experimental Section Acrylamide (A), N,N'-methylenebisacrylamide (Bis), and methacrylic acid (MA) were commercially obtained. Sodium @Abstractpublished in Advance ACS Abstracts, October 1, 1994.
stylenesulfonic acid (SSS) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) were from Toyosoutatsu Co. Ltd. and Nit0 Kagaku Co. Ltd., respectively. The monomer reagents were used without further purification (ca. 99%). Sodium methacrylate (SMA) and sodium 2-acrylamide-2-methylpropanesulfonate (SAMPS) were made by reacting MA and AMPS with sodium carbonate, respectively. SSS/A/Bis copolymer gel was prepared as follows: A (1 g), Bis (26.5 mg), and SSS (0, 17,70, 210,490, 1314, and 2628 mg) were dissolved in distilled water to a final volume of 20 mL in each case, and gel samples (a-g) with desired ion density were made by the copolymerization of solutions in micropipets at 50 "C for 1 h after addition of ammonium persulfate (initiator, 50 mg). In a similar manner, SAMPS/A/Bis copolymer gel samples (a-g) and SMA/A/Bis copolymer gel samples (a-f) were prepared by the copolymerization of SAMPS (0, 21, 63, 246, 501, and 1206 mg)/A (1 g)/Bis (26.7 mg) solutions and SMA (0, 18, 54, 171, 342, 689, and 1379 mg)/A (lg)/Bis (26.7 mg) solutions, respectively. The prepared gels all were washed in distilled water for a week and immersed in various acetone-water mixtures. After equilibrium had been reached, the diameter (d)of each cylindrical gel was measured and gel volume (V) was expressed by cubing the diameter.
Results and Discussion Volume Change of Thermosensitive Polymer Gels in Acetone-Water Mixtures. Figure 1 shows plots of the swelling ratio (VIVO)of SSS/A/Bis copolymer gels immersed in acetone-water mixtures versus acetone concentration. The value (VIVO)is the ratio of the final network volume to initial network volume. As the acetone concentration increased, the nonionic gel (sample a) underwent gradual volume change from a swollen to collapsed state (curve a). When ion density (SSS content) increased, a curve with an inflection point was first observed for sample b, and curves with discontinuous volume change were observed for samples c-g. The volume change at the transition point increased with increase in ion density, and the transition point shifted toward a higher acetone content. The volume change was the same as that observed for conventional cross-linked gels with an ionizable Figure 2 shows volume change of SAMPS gel samples immersed in acetone-water mixtures. As ion density (SAMPS content) increased, gradual volume change was observed for sample a and then discontinuous volume change for samples
0022-3654/94/2098-11115$04.50/00 1994 American Chemical Society
11116 J. Phys. Chem., Vol. 98, No. 43, 1994
Katayama and Akahori
a b c defg
e-
@ O 0 DMO
moo.ao o
f
I
60
I61o/p I
0
2e
0 DAO
0
40
f 20
0
8
56% I
I
10
0
-10
a
4
12
Volume (VNo)
Figure 1. Volume change of sodium stylenesulfonate/acrylamide/Nflmethylenebisacrylamide copolymer gel (samples a-g) immersed in
acetone-water mixtures.
0 -
C
.0 +
e
c
a
b cdef
0
@ A D -
8 8 m
+
a
0
-
20
60-
80-
A m 0
00
9''
70
-
50
-
30
-
10
-
DOA
a A
A @
& d
-
P
W O I
A
DDoA 00 A
& a
A
A:
POJI e r
100
.OA
A
-
40
Figure 4. Temperature-dependent volume change of the sodium methacrylate/acrylamide/N,W-methylenebisacrylamidecopolymer gel
(sample f) immersed in acetone-water mixtures.
I
5
16
Diameter (mm)
-10
8
1; I ' 4
a
12
16
20
Diameter (mm)
Figure 5. Temperature-dependent volume change of the sodium stylenesulfonate/acrlamide/N,W-methylenebisacrylamidecopolymer
ab c d e f g h
10
-
. O
5
'38
.O
30 -
e%'
u
5 8 8
O&A
A
0 &A
A A
a
* I
?t
50
0
2
3
0 -
A
7090
-
gs A I
I
4
b-f. The volume change at the transition point and the transition point (solvent composition) were the same as in the case of the SSS gel. Similar volume change was observed for SMA gels, as shown in Figure 3. Consequently, these volume changes all were the same as those for conventional cross-linked gels with an ionizable group2 and could thus be explained theoretically.4 Temperature-DependentVolume Change of Thermosensitive Polymer Gels. Figure 4 shows plots of an equilibrium
gel (sample f) immersed in acetone-water mixtures. volume of the SMA gel sample (f) immersed in acetone-water mixtures versus temperature. The ordinate indicates diameter of the cylindrical gel and the abscissa, temperature. The gel samples immersed in acetone-water mixtures of 61% or more acetone remained in the collapsed state over the experimental range of temperature. Gel samples in 56% or less remained in the swollen state. Gel samples immersed in 60, 59, 58, and 57% acetone-water mixtures showed the swollen state in a high-temperature region and the collapsed state in a lowtemperature region. At intermediate temperatures, discontinuous volume changes appeared. Transition temperatures were 49, 36, 20, and 0 "C for gels in 60, 59, 58, and 57% acetone. The discontinuous volume change at the transition temperature thus occurs at a lower level of temperature with decrease in acetone content. The volume change is the same as the previous result for sodium acrylate/A/Bis copolymer gel and thus shows the thermoswelling pattern. l4 Temperature-dependentvolume change of the SSS gel sample (f) is shown in Figure 5. The gel samples immersed in acetonewater mixtures of 65% acetone or less remained swollen throughout the experimental range of temperature. Gel samples in acetone-water mixtures of 71% or more acetone remained collapsed. Gel samples in mixtures of 66, 67,68, 69, and 70% acetone showed the collapsed state in a high-temperature region and the swollen state in a low-temperature region. Discontinuous volume change occurred at intermediate temperatures of
J. Phys. Chem., Vol. 98, No. 43, 1994 11117
Thermosensitive Polymer Gels 65%
64 %
.u h
2i?
50-
c
3010
Thermoswelling volume phase transition
Miscibilitycurve with an UCST
Y
. Thenoshrinking volume transition transnion
5
10
15
Miscibility Curve with a LCST
20
Diameter (mm)
Figure 6. Temperature-dependent volume change of the sodium 2-acrylamide-2-methylpropanesulfonate/acryl~de/~~-methylenebisacrylamide copolymer gel (sample e) immersed in acetone-water
mixtures. 67, 53, 40, 35, and 28 "C. The discontinuous volume change thus occurs at a lower level of temperature with increase in acetone content. The volume change is the same as the previous result for N-isopropylacrylamide/Biscopolymer gel and thus shows the thermoshrinking pattem.15 Volume change of the thermoshrinking pattem of the volume phase transitin was observed for the SAMPS gel sample (e), as shown in Figure 6. The gel sample immersed in acetone-water mixtures of 65% acetone or less remained swollen over the experimental range of temperature, and the gel sample in 72% or more acetone remained collapsed. However, the gel samples in 64,66,68,70, and 7 1% acetone collapsed at high temperature and swelled at low temperature. Discontinuous volume change was observed for the gel samples in 64,66,68, and 71% acetone at transition temperatures of 62, 40, 22, 0, and -12 "C. The discontinuous volume change thus occurs at a lower level of temperature with increase in acetone content. Various temperature-dependentvolume phase transitions may be explained as follows. Thermoreversible characteristics of polymers are generally either those that are soluble in water at high temperature and insoluble at low temperature, Le., thermosoluble polymers with water as a good or poor solvent at these temperatures, respectively, or secondly polymers that are soluble in water at low temperature and insoluble at high temperature, i.e., thermoinsoluble polymers with water as a good or poor solvent at these temperatures, respectively. This concept may easily be extended so as to include the case of polymer gels. Assuming that polymers soluble in water are essentially equivalent to swollen gels and polymers insoluble in water to collapsed gels, thermosoluble gels may swell at high temperature and collapse at low temperature, and thermoinsoluble gels may collapse at high temperature and swell at low temperature. The former are thermoswelling gels and the latter, thermoshrinking gels.14 Thus, thermoreversible characeristics of gels can be essentially characterized by miscibility of polymers in water in the range of experimental temperature. Miscibility then changes according to chemical property of gel polymers, because gel polymers usually contain hydrophilic and hydrophobic residues in the side chains. When the overall hydrophilic character of a gel dominates over the hydrophobic one, thermosoluble characteristics become dominant, since the gel polymer is soluble in water at high temperature and insoluble in water at low temperature. When the overall hydrophobic character in a gel dominates over the hydrophilic one, thermoinsoluble characteristics may appear, since the gel polymer is insoluble in water at high temperature and soluble in water at low temperature. When both are comparable, thermosoluble and thermoinsoluble
Convex type of volume phase transition
Miscibilitycurve of mirror image pattern
Miscibilitycurve Of sandglass pattem
Figure 7. Relationships between various thermoreversible volume phase transitions of polymer gels and miscibility of polymer solutions. Thermoswelling (upper) and thermoshrinking (middle) volume phase transitions appear on miscibility curves with UCST or LCST, respec-
tively. The convex (bottom) volume phase transition is seen on the miscibility curve of a mirror image or sandglass. characteristics mentioned above may appear successively. These thermoreversible characteristics of gel polymers are markedly different from those in solvents other than water, because miscibility is dependent on not only the chemical property of the gel polymer but also that of the solvents. Organic solvents are generally poor for hydrophilic gel polymers, while water is a good solvent for them. Water-organic solvent mixtures of different compositions thus show a wide range of miscibility for hydrophilic gel polymers, from good to poor. Accordingly, thermoreversible behavior of gel polymers in solvent mixtures may appear in a poorer level than that of gel polymers in water. This is a reasonable explanation for thermoswelling and thermoshrinking behaviors observed for thermoreversible gels in water-acetone mixtures. A previous study on temperature-dependent volume change of sodium acrylate/A/Bis copolymer gel, sodium vinylacetate/ A/Bis copolymer gel, and sodium allylacetate/A/Bis copolyemr gel indicated that these gels showed a thermoswelling or thermoshrinking volume change according to the ratio of a hydrophobic residue to a hydrophilic residue on the side chains of the gels, which was defined as a balance factor.14 On the basis of the generalized rule, a gel with a balance factor 1 should show thermoswelling volume change and those with balance factors 2-4, thermoshrinking volume change. The thermoswelling volume change of the SMA gel and thermoshrinking volume changes of SAMPS and SSS gels would thus appear to comply with these factors, being in most cases 1 for the SMA gel, 4 for the SAMPS gel, and 6 for the SSS gel. The balance factor for the SSS gel may have been overestimated owing to an aromatic residue differing from an aliphatic residue. This
11118 J. Phys. Chem., Vol. 98, No. 43, 1994 is basically consistent with the result by Ito et ~ 1 that. cloud ~ ~ points for various acrylamide-derived polymer solutions appear according to the number of hydrophobic and hydrophilic residues in the side chains. Volume change of thermosensitive gels can be correlated to the miscibility of a binary system (polymer and solvent mixtures) as follows. Volume phase transition for thermoswelling gels, characterized by a shrink-swell cycle with increase in temperature, is consistent with the phase transition on a miscibility curve with UCST (upper level in Figure 7). Volume phase transition for thermoshrinking gels characterized by a swellshrink cycle corresponds to that on a miscibility curve with LCST (middle level in Figure 7). A convex volume phase transition is correlated to a phase transition on a miscibility curve with a sandglass or mirror image pattern (bottom in Figure 7). When a gel changes from a shrunken to a swollen state with increase in temperature and then from a swollen to a shrunken state, phase transition from a two-phase to one-phase and from a one-phase to two-phase state with temperature may appear on the miscibility curve with a sandglass (or a mirror image pattern), as shown in Figure 7. The present data thus provide strong support for the possibility of thermoswelling and thermoshrinking volume phase transitions induced by change in temperature. The factors that determine the patterns of the temperature-dependent voluem phase transition are consistent with miscibility of gel polymers in acetonewater mixtures, and the primary factor is the relative ratio of hydrophobic and hydrophilic residues on the side chains of the gels.
References and Notes (1) Tanaka, T. Sci. Am. 1981, 244, 110
Katayama and Akahori (2) Tanaka, T.; Fillmore, D. J.; Sun, S.-T.; Nishio, I.; Swislow, D.; Shah, A. Phys. Rev. Lett. 1980, 45, 1636. (3) Katayama, S. In Mechanochemistry; Sasabe, H., Ed.; Maruzen Press: Tokyo, 1989; pp 56-118. (4)Hirokawa, Y.; Tanaka, T.; Katayama, S. In Microbial Adhesion and Aggregation; Marshall, K. C., Ed.; Springer Verlag: Berlin, 1984; pp 177-188. (5) Ilavsky, M.; Hrouz, J.; Ulbrich, K. Polym. Bull. 1982, 7, 107. (6) Ilavsky, M.; Hrouz, J.; Havlicek, I. Polymer 1985, 26, 1514. (7) Ricka, J.; Tanaka, T. Macromolecules 1984, 17, 2916. (8) Ohmine, I.; Tanaka, T. J. Chem. Phys. 1982, 77, 5725. (9) Katayama, S.; Myouga, A,; Akahori, Y. Polym. Bull. 1992, 28, 227. (10) Katayama, S.; Myouga, A,; Akahori, Y. J. Phys. Chem. 1992, 96, 4698. (1 1) Katayama, S.; Kazama, S.; Yoshioka, H. J. Phys. Chem. 1992,96, 2023. (12) Katayama, S.; Ohata, A. Macromolecules 1985, 18, 2781. (13) Katayama, S. Polymer 1991, 32, 558. (14) Katayama, S. J. Phys. Chem. 1992, 96, 5209. (15) Hirokawa, Y.; Tanaka, T. J . Chem. Phys. 1984, 81, 6379. (16) Katayama, S.; Takeshita, Y.; Akahori, Y. Polymer 1993,34,2677. (17) Amiya, T.; Hirokawa, Y.; Hirose, Y .; Li, Y .; Tanaka, T. J. Chem. Phys. 1987, 86, 2375. (18) Katayama, S.; Yamazaki, F.; Akahori, Y. J. Phys. Chem. 1992, 96, 9585. (19) Katayama, S.; Yamazaki, F.; Akahori, Y. J. Phys. Chem. 1993, 97, 290. (20) Katayama, S.; Takagi, C.; Akahori, Y. Polym. Bull. 1993,30,333. (21) Katayama, S.; Hirokawa, Y.; Tanaka, T. Macromolecules 1984, 17, 2641. (22) Amiya, T.; Hirokawa, Y.; Li, Y.; Tanaka, T. J. Chem. Phys. 1987, 86, 2375. (23) Katayama, S.; Takeshita, Y.; Akahori, Y. To be prepared. (24) Ito, S.; Mizoguchi, K.; Suda, M. In Research Report; Research Institute for Polymers and Textiles, Ed.; Tsukuba, Japan, 1984; pp 7, 144.
