J. Phys. Chem. 1992, 96, 5209-5210
5209
Chemical Condition Responsible for Thermoswelling or Thermoshrinking Type of Volume Phase Transition in Gels. Effect of Relative Amounts of Hydrophobic to Hydrophilic Groups in the Side Chain Seiji Katayama School of Pharmaceutical Sciences, University of Shizuoka, 52- 1 Yada, Shizuoka- City, 422 Japan (Received: February 18, 1992; In Final Form: April 27, 1992)
Thermoreversiblevolume phase transitions were observed for three polyacrylamide derivative gels, acrylamide (Am)sodium (Bis) copolymer gel (SACgel), Amsodium vinylacetate (SVA)-Bis copolymer acrylate (SAC)-N,N'-methylenebis(acry1amide) gel (SVA gel), and Amsodium allylacetate(SAA)-Bis copolymer gel (SAA gel). SACand SVA gels underwent thermoswelling volume changes, and SAA gel, thermoshrinking volume change. The chemical conditions that cause a gel to thermoswell or thermoshrink can be clearly determined from the relative molar ratio of a hydrophobic group to the hydrophilic group in the gel. The transition from thermoswelling to thermoshrinking occurs with a change in the relative molar ratio from 1 to 2.
introduction Polymer gels with an ionizable group undergo a reversible, discontinuous volume change with that in solvent composition, temperature, pH, and salt concentrati~n.l-~ This involves a volume phase transition between swollen and shrunken gels and is now considered as a first-order phase transition. Volume phase transitions have come of interest in recent years because of scientific and technological importance. Our interest in this study was directed toward specific volume changes induced by changing the gel temperature.6-20 Such change is thermoreversible and is of the following three types that occur with increasing temperature: (1) shrunken/swollen, (2) swollen/shrunken, and (3) shrunken/s~ollen/shrunken.~~* However, the chemical conditions for these have yet to be determined. Therefore, it has been assumed that the hydrophilic or hydrophobic character of polymer gels may be essential to these changes. This study was thus conducted to determine these conditions based on thennoreversible data for three polymer gels: acrylamide (Am)sodium acrylate (SAC)-N,N'methylenebis(acry1amide) (Bis) copolymer gel (SAC gel), Amsodium vinylacetate (SVA)-Bis copolymer gel (SVA gel), and Am-sodium allylacetate (SAA)-Bis copolymer gel (SAA gel).
SVA GEL
SAC GEL
1
-CH.jCHI
SAA GEL
Experimental Section Acrylamide (Am), NJV-methylenebis(acry1amide) (Bis), acrylic acid (Ac), vinylacetic acid (VA), allylacetic acid (AA), and acetone were commercially obtained and all of adequate purity unless otherwise noted. Sodium acrylate (SAC), sodium vinylacrylate (SVA), and sodium allylacrylate (SAA) were prepared by reacting sodium carbonate with Ac, VA, and AA. SAC,SVA, and SAA gels were prepared as follows. For SAC gel, Am (1 .O g), SAC (400 mg), and Bis (26.7 mg) were dissolved in distilled water to a final volume of 20 mL. The solution in micropipets (ad. = 1.6 mm) was polymerized at 50 OC for 1 h after adding ammonium persulfate (40 mg). SVA gels (A-F) were prepared by copolymerization of the aqueous solutions (20 mL) containing Am (1.0 g), SVA (574.5 mg), and Bis (26.7 mg). SAA gel
samples (A-G) were prepared by copolymerization of aqueous solutions (20 mL) containing Am (1.0 g), SAA (649.2 mg), and Bis (26.7 mg) under the same conditions. The prepared gels were washed in distilled water for a week after being discharged from the micropipets. The temperatures of the gels immersed in acetone-water mixtures were held at the desired levels. After thermal equilibrium was attained (ca. 1 week), the gel diameter (6) was measured and the volume (V) estimated by cubing the diameter. Results and Discussion Data for the temperature-dependent volume of SAC gel are shown in Figure 1 . A gel immersed in a mxiture of 60% acetone remained collapsed at low temperatures. As the temperature of the gel increased, the collapsed gel underwent a discontinuous volume change at 55 OC and swelled. The swollen gel remained unchanged during subsequent temperature increases. The same was observed for gels immersed in 59 and 58% mixtures, the former showing a discontinuous volume change at 40 OC and the latter at 5 OC. The transition temperature would thus appear to shift to a higher level with an increase in acetone content. A volume phase transition induced by an increase in temperature can thus be characterized by shrunken-swollen behavior with discontinuous volume change. However, discontinuous volume changes were not observed for the gels immersed in mixtures above 61%and below 57%,the former remaining collapsed and the latter being swollen within the temperature region. This behavior may thus be considered a thermoswelling characteristic of polymer gels. Figure 2 shows the temperature-dependent volume behavior of SVS gel immersed in acetone-water mixtures. The gels in mixtures of 35, 40, 43, and 45% acetone collapsed at low temperature and swelled a t high temperature. In an intermediate temperature region, a discontinuous volume change was noted at 0, 12, 35, and 50 OC. However, gels in mixtures below 34% remained swollen in the temperature region, and gels above 46% remained collapsed. SVA gel would thus appear to exhibit the same volume behavior as SACgel and thus display thermoswelling. Figure 3 shows the temperature-dependent volume behavior of SAA gel immersed in acetone-water mixtures. SAA gel collapsed at high temperature and swelled at low temperatures. A discontinuous volume change at +23, +12, and -2 OC was observed for gels in 49, 50, and 51% mixtures. However, below 48%, they remained swollen in the temperature region studied. Above 52%, they remained collapsed in the same temperature region studied. SAA gel thus underwent a discontinuous volume change from a collapsed state at high temperatures to a swollen state at low temperatures, and the transition temperature shifted to a lower level with an increase in acetone content. This is in
0022-3654/92/2096-5209%03.00/0 0 1992 American Chemical Society
5210 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
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Figure 1. Temperature-dependent volume behavior of acrylamide-sodium acrylate-Bis copolymer gel immersed in 55, 58, 59, and 60% acetonewater mixtures. The gel sample was prepared by radical copolymerization of a solution containing acrylamide (1 .O g/20 mL), sodium acrylate (400 mg/20 mL), and Bis (26.7 mg/20 mL).
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Figure 2. Temperature-dependent volume behavior of acrylamidesodium vinylacetate-Bis copolymer gel immersed in 22.5, 35.0, 40.0, 43.0, and 45.0% acetone-water mixtures. The gel sample was prepared by radical copolymerization of a solution containing acrylamide (1.0 g/20 mL), sodium vinylacetate (574.5 mg/20 mL), and Bis (26.7 mg/20 mL).
VIV.
Figure 3. Temperature-dependent volume behavior of acrylamide-sodium allylacetateBis copolymer gel immersed in 47,49, 50, 51, and 55% acetone-water solvent mixtures. The gel sample was prepared by radical copolymerization of a solution containing acrylamide (1.0 g/20 mL), sodium allylacetate (649.2 mg/20 mL), and Bis (26.7 mg/20 mL).
marked contrast to the results for SACand SVA gels and should thus be considered a thermoshrinking characteristic of gels. Isopropylacrylamide and the copolymer gels all show this feature. M16-19
Letters On the basis of the present results, SACand SVA gels exhibit thermoswelling and SAA gel exhibits thermoshrinking. The conditions involved for the thermoswelling and thermoshrinking can thus be determined from the chemical composition of a gel. SACgel has a side chain (-COONa) that functions as a hydrophilic moiety. SVA gel has a side chain (
