J. Phys. Chem. 1994,98, 9009-9012
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Volume Phase Transition of 2-(Acryloyloxy)ethyl Acid Phosphate/Acrylamide Copolymer Gel in Various Solvent Mixtures. Non-monotonic Variation of Phase Transition Threshold with Polymer Composition Seiji Katayama,. Yuji Takeshita, and Yukio Akahori School of Pharmaceutical Sciences, University of Shizuoka, 52-1. Yada Shizuoka- City, 422 Japan Received: March 23, 1994; In Final Form: July I , 1994’
Examination was made of changes in volume of 2-(acryloy1oxy)ethyl acid phosphate (AEAP)/acrylamide copolymer gel immersed in acetonitrile-water, methanol-water, tetrahydrofuran-water, dioxane-water, and dimethyl sulfoxide-water. The gel underwent reversible volume change from a swollen to a collapsed state via a discontinuous volume change with an increase in solvent composition, and the discontinuous volume change a t the transition point decreased with an increase in A E A P content. The transition point shifted first toward lower solvent content and then higher solvent content. This was a newly observed volume phase transition which was a combination of normal pattern and specific pattern recently found by us. This type of volume phase transition was characterized by the fact that phase transition threshold, Le., threshold solvent composition, varied with polymer composition in a non-monotonic way. This transition could be considered consistent with nonlinear change in miscibility of the gel polymer in solvent mixtures, that is, first increasing and then decreasing with an increase in AEAP.
Introduction Most covalently cross-linked gels with an ionizable group undergo reversible volume change from a swollen to a collapsed state via discontinuous volume change according to physical conditions such as solvent composition, pH, temperature, and salt concentration.’-11 This is a gel-gel volume phase transition for cross-linked gels with an ionizable group and is the first-order phase transition. Various patterns of volume phase transitions have been observed for a number of polymer gels.12-20 However, theoretical factors that govern the patterns of volume phase transition are still unknown. With respect to volume phase transitions induced by change in temperature, three types of volume phase transitions have been observed for sodium acrylate/ acrylamide/N,N’-methylenebis(acrylamide) (Bis) copolymer gel, N-isopropylacrylamide/Bis copolymer gel, and sodium vinylsulfonate/acrylamide/Bis copolymer ge1.”21 The first is a transition from a collapsed to a swollen gel via a discontinuous volume change with temperature (pattern I). The second is that from a swollen to a collapsed gel via a discontinuous volume change with temperature (pattern 2), and the third, that from a collapsed to a swollen gel and then from a swollen to a collapsed gel via two discontinuous volume changes with temperature (pattern 3). These were termed thermosensitive polymer gels, and the patterns of volume phase transition have been successfully explained by miscibility of the gel polymers in solvent mixtures. Then, relative amounts of hydrophobic group to hydrophylic group incorporated in the gel polymers have been presumed to be a primary factor to govern the patterns.2O The most usual volume phase transition induced by change in solvent composition was observed for acrylamide/sodium acrylate/N,N’-methylenebis(acry1amide)copolymer gel immersed in acetone-water mixtures.3.4 The equilibrium volume of the gel changed from a swollen to a collapsed state via a discontinuous volume change with an increase in acetone content. The discontinuous volume change at the transition point increased with ion density of the gel, and the transition point shifted toward higher acetone content. This type of volume phase transition has been observed for a number of polymer gels2 and may be named normal pattern. In contrast to this, another type of volume phase e Abstract
published in Advance ACS Absrracrs, August 1, 1994.
transition was recently observed for 2-(acryloy1oxy)ethyl acid phosphate (AEAP)/acrylamide (Am) copolymer gel and sodium 2,2’-bis(acrylamido)acetate/acrylamide copolymer gel in acetone water mixtures.12 In this case, the equilibrium volume of the gels changed from a swollen to a collapsed state via a discontinuous volume change with an increase in acetone content. The discontinuous volume change at the transition point decreased with an increase in ion density, and the transition point shifted toward higher acetone content. In this manner, the volume behavior was definitely different from that of normal pattern. This type of volume phase transition may be named reverse pattern, because the pattern appears to turn upside and down, right and left, compared with that of normal pattern. While investigating these volume phase transitions, a volume phase transition with a combination of both patterns was newly found and designated a volume phase transition of combined pattern. As is presented here, this is the volume phase transition observed for 2-(acry1oyloxy)ethyl acid phosphate (AEAP)/Am copolymer gels in several solvent mixtures not including water-acetone mixtures. The same type of volume phase transition has been recently observed for N-arylacrylamide/acrylamide/sodium acrylate copolymer gel in acetone-water or dioxane-water mixtures. In this regard, it will be reported in the forthcoming paper.22 Although the AEAP/Am copolymer gel immersed in a c e t o n e water mixtures showed a volume phase transition of reverse pattern,12 a volume phase transition of combined pattern was currently observed for the same gel when immersed in acetonitrile water, methanol-water, tetrahydrofuran-water, dioxane-water, and dimethyl sulfoxide-water mixtures. In the present paper, a detailed examination was made of this type of volume phase transition, together with a discussion of the relation to gel miscibility in the following. Experimental Section 2-(Acryloy1oxy)ethyl acid phosphate (AEAP) was obtained from Daihachi Co. Ltd. and used without further purification (>98%). Acrylamide and ammonium persulfate (initiator) were obtained from Wako and Tokyo Kasei Co. Ltd. Acetonitrile, methanol, tetrahydrofuran,dioxane, and dimethyl sulfoxide were commercially obtained (>99%). The solvents were used for preparations of various solvent mixtures following several distillations. The solvent mixtures were prepared by mixing each
0022-3654/94/2098-9009$04.50/0 0 1994 American Chemical Society
Physical Chemistry, Vol. 98,No. 36, I994
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Figure 1. Volume phase transition (combinedpattern) of 2-(acryloyloxy)ethyl acid phosphate (AEAP)/acrylamide copolymer gel immersed in a C e t 0 nitrile-water mixtures. Curves A, B, C, D, and E indicate gel samples containing AEAP at 0.142, 0.2, 0.56, 1.4, and 2.8 g, respectively, with acrylamide, 1 g. solvent in distilled water at various molar ratios. AEAP (0.142, 0.2, 0.56, 1.40, and 2.8 g) and acrylamide (1 g) were dissolved in distilled water to a final volume of 20 mL in each case. Gel samples (A-E) were thus prepared by copolymerization of the solutions in micropipetsat 50 "C, 1 h, after additionof ammonium persulfate (50 mg). All gels were washed in flowing water for 2 days following removal from the micropipets. The gels were immersed in solvent mixtures after cutting them into cylindrical pieces ca. 5 mm long. After equilibrium had been reached, the cylindrical gel diameter was measured. Gel volume was expressed by cubing the diameter. These sample preparation procedures were mostly the same as previously reported.'2~~3-*~
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Results and Discussion Figure 1 shows thevolumechange of 2-(acryloy1oxy)ethyl acid phosphate (AEAP)/acrylamide (Am) copolymer gel (samples A-E) when immersed in acetonitrilewater mixtures. Equilibrium volume of the gel in distilled water or solvent mixtures of low acetonitrile content decreased gradually with an increase in AEAP (ion density). This is essentially different from volume changes observed for usual cross-linked gels with an ionizable group, because the gel volume generally increases with an increase in ion d e n ~ i t y .The ~ difference may be due to a specific property of constituent AEAP in the gel. AEAP has two functional groups which cross-link and ionize the gel polymers, with consequent gel collapse and swelling, respectively. The former effect cancels out the latter, since the equilibrium volume actually decreased with an increase in AEAP. As acetonitrile content increased, equilibrium volume of the gel samples (A-E) first decreased gradually and then reached a collapsed state via a discontinuous volume change at intermediate acetonitrile content. The discontinuous volume change (volume difference between a swollen and a collapsed gel) at the transition point decreased gradually with an increase in AEAP (samples E-A). The transition point shifted first toward lower acetonitrile content with an increase in AEAP (samples E-C). Subsequent increase in AEAP, however, caused the transition point to shift toward higher acetonitrile content (samples C-A). The results from samples E-C indicate a volume phase transition of normal pattern, while for gel samples C-A,a volume phase transition of reverse pattern. Gel sample C then gives a curve profile a t a crossing point from normal to reverse pattern. As a consequence, the overall volume change of the AEAP gel in acetonitrile-water mixtures indicates a volume phase transition of combined pattern, Le., a combination of normal and reverse patterns. Essentially the same was observed for AEAP/Am gel samples immersed in methanol-water, tetrahydrofuran-water, dioxane-water, and dimethyl sulfoxide-
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AEAP/Acrylamide Copolymer Gel
The Journal of Physical Chemistry, Vol. 98, No. 36, I994 9011
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volume ( V / V1. Figure5. Volume phase transition (combinedpattern)of 2-(acryloy1oxy)ethyl acid phosphate (AEAP)/acrylamide copolymer gel immersed in dimethyl sulfoxide-water mixtures. Curves A, B, C, D, and E indicate gel samples containing AEAP at 0.142, 0.2, 0.56, 1.4, and 2.8 g, respectively, with acrylamide, 1 g. 70 60
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threshold) as a function of AEAP content: 0 ,acetonitrile; W, methanol; A, tetrahydrofuran; 0 , dioxane; 0, dimethyl sulfoxide. samples C-A shift toward higher solvent content. It may, therefore, be concluded that the AEAP/Am copolymer gel shows a volume phase transition of combined pattern when immersed in these solvent mixtures. Figure 6 shows the transition solvent composition as a function of AEAP content. Transition solvent composition in each solvent mixture first increased and then decreased with an increase in AEAP content, and the turning point from increasing to decreasing appeared a t an AEAP content of ca. 100 mM. This suggests that the transition point, Le., phase transition threshold, varies with polymer composition in a nonmonotonic way. This may thus be the most remarkable characteristic of the volume phase transition of combined pattern. The volume phase transition of combined pattern should occur in the same manner as those of normal and reverse patterns. The major factors in determining whether a gel exhibits the volume phase transition of normal or reverse pattern are as follows: ion and cross-link density in the gel and miscibility of the gel polymer in solvent mixtures. In the former case, the ion group causes the gel to swell and the cross-link group to collapse. Increase in ion density causes a discontinuous volume change at the transition point to increase, while increase in cross-link density causes it to decrease.4 These factors thus determine the final equilibrium volume of cross-linked gels with an ionizable group and the extent of discontinuous volume change, and eventually whether the gel exhibits a phase transition of normal or reverse pattern. On the other hand, miscibility of a gel in solvent mixtures is a primary factor in determining the transition solvent composition a t a discontinuousvolume change, as follows. Miscibility is essentially
due to intermolecular interactions between the gel polymer and solvent mixtures and therefore is considerably affected by their chemical properties. In our previous w ~ r k , I ~it*has * ~been found that the chemical property of hydrophilicity or hydrophobicity ofgel polymers ischaracterized by therelative ratioof hydrophobic group to hydrophilic group incorporated in the gel polymers, and therefore it is responsible for miscibility of the gel polymers in solvent mixtures. Thus, introducing a hydrophilic group into a gel causes miscibility of gel polymers in solvent mixtures and introducing a hydrophobic group eliminates miscibility. On the other hand, miscibility can range from good to poor, depending on whether a solvent mixture is soluble or insoluble to gel polymers. Most organic solvents are generally poor even for hydrophilic gel polymers, while water is a good solvent for the gel polymers. Water-organic solvent mixtures of different compositions therefore show a wide range of miscibility for the hydrophilic gel polymers, from good to poor. As a consequence, introducing a hydrophilic group causes the gel to be soluble even in mixtures of higher solvent content (poor solvent), and introducing a hydrophobic group causes the gel to be soluble even in mixtures of lower solvent content (good solvent). This explains the shift of the transition point toward higher solvent content or lower solvent content. For these reasons, miscibility is also an important factor in determining whether polymer gels exhibit a volume phase transition of normal or reverse pattern. The same consideration is thus the case for the AEAP/Am copolymer gel, as below. Increase in AEAP in AEAP/Am copolymer gel should result in synchronous increasing in ion and cross-link groups, since AEAP is a bifunctional monomer (cross-linker) with an ionizable group. The effect of an ion group on gel swelling was less than that of a cross-link group on gel collapsing, because the equilibrium volume of the gel immersed in various solvent mixtures actually decreased with an increase in AEAP. On the other hand, the effect of an ion group in the gel on miscibility may be in competition with that of the cross-link group on miscibility, because of a synchronous increase in ion and cross-link groups. When the former dominates the latter, miscibility of the gel may increase with an increase in AEAP, and therefore the transition point may shift toward higher solvent content (poor solvent), whereas the discontinuous volume change at the transition point may decrease gradually with an increase in AEAP. This is a volume phase transition of reverse pattern observed for the AEAP/Bis copolymer gel in acetone-water mixtures.12 In contrast, when the latter dominates the former, miscibility of the gel may decrease with an increase in AEAP, and therefore the transition point may shift toward lower solvent content (good solvent), whereas the discontinuous volume change at the transition point may decrease gradually with AEAP. This is a volume phase transition of normal pattern, as mentioned above. For these reasons, miscibility of the AEAP/Am gel in solvent mixtures may first decrease and then increase with an increase in AEAP, because of the synchronous increase in ion and cross-link groups. This may permit the transition point to shift first toward lower solvent content and then toward higher solvent content, whereas discontinuous volume change may decrease monotonously with an increase in AEAP. This explains the volume phase transition of combined pattern observed for the AEAP/Am copolymer gel in the present solvent mixtures. A swollen gel generally corresponds to a solubilized state (one phase) in a polymer/solvent binary system and a collapsed gel, an insolubilized state (two phases) in the binary system.19 The transition point from a swollen to a collapsed gel should coincide with that from a one- to a two-phase state on the miscibility curve. The volume phase transition of combined pattern is thus closely correlated to a miscibility curve with UCST (upper critical solution temperature), as shown in Figure 7. The transitions along thecontour line A, which is like a parabola (normal pattern),
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The Journal of Physical Chemistry, Vol. 98, No. 36, 1994
increasing and then decreasing, and the change would be eventually ascribed to the specific property of AEAP characterized by bifunctional character with an ion group.
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References and Notes
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Volume phase transition Miscibility curve d combined pattern with an UCST Figurel. Schematicrepresentation of therelationshipbetwen thevolume phase transition with the combined pattern and the miscibility curve of the polymer/solvent binary system. Vand I$ on the abscissa indicate gel volume and molar fraction, respectively. The vertical axis indicates composition of solvent mixtures or temperature. The contour lines A and B indicate boundary lines for volume phase transitions of normal and reverse patterns.
possibly correspond to those on the upward slope of the miscibility curve, while those along the contour line B, which is like an inverse parabola (reverse pattern), to those on the downward slope of the miscibility curve. The crossing point from the volume phase transition of normal pattern to that of reverse pattern may thus be in accord with a critical point (UCST) on the miscibility curve. The AEAP/Am copolymer gel in several solvent mixtures showed the volume phase transition of combined pattern. This could be accounted for by a nonlinear change in miscibility of the gel polymer for solvent mixtures with an increase in AEAP, first
(1) Tanaka. T. Sci. Am. 1981. 244. 110. (2) Katayama, S. In Mechankhemisrry; Sasabe, H., Ed.; Maruzen Press: Tokyo, 1989;pp 56-118. (3) Tanaka, T.; Fillmore, D.J.; Sun, S.-T.; Nishio. I.: Swislow. D.;Shah, A. Phys. Rev.Left. 1980,45, 1636. (4) Hirokawa, Y.; Tanaka, T.; Katayama, S . In Microbial Adhesion and Aggregation; Marshall, K. C., Ed.; Springer Verlag: Berlin, 1984;pp 177188. (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,II,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.;Yamazaki, F.; Akahori, Y. J . Phys. Chem. 1992,96, 9585. (13) Katayama, S.;Yamazaki, F.; Akahori, Y. J . Phys. Chem. 1993,97, 290. (14) Katayama, S.;Takagi, C.; Akahori, Y. Polym. Bull. 1993,30,333. (15) Katayama, S.;Hirokawa, Y.; Tanaka, T. Macromolecules 1984,17, 2641. (16) Amiya, T.; Hirokawa, Y.; Li, Y.; Tanaka, T. J . Chem. Phys. 1987, 86. 2375. (17) Amiya, T.; Hirokawa, Y.;Hirose, Y.;Li, Y.;Tanaka, T. J . Chem. Phys. 1987,86,2375. (18) Katayama, S.;Ohata, A. Macromolecules 1985,18,2781. (19) Katayama, S.Polymer 3991, 32, 558. (20) Katayama, S.J. Phys. Chem. 1992,96,5209. (21) Hirokawa, Y.;Tanaka, T. J . Chem. Phys. 1989,81,6378. (22) Katayama, S.;Shimizu, M.;Akahori, Y. To be prepared. (23) Katayama, S.;Takeshita, Y.; Akahori, Y . Polymer 1993.34, 2677.
