Polyampholyte gels of a cross-linked polyanion or polycation network into which an oppositely charged polyion was immobilized Their pH-induced swelling/shrinking characteristics

摘要

Three polyampholyte gels (Gl to G3) composed of acrylic acid (AA), 1-vinylimidazole (VI) and N-isopro-pylacrylamide (NIPA) were prepared: Gl with a pre-gel solution (pH=12) containing NIPA (700 mM), AA (150 mM), VI (150 mM) and N, N-methylenebis(acrylam-ide) (10 mM); G2 with the same pre-gel, except for containing poly(acrylic acid) (PAA) (150 unit mM) instead of AA; and G3 with the same pre-gel, except for containing poly(l-vinylimidazole) (PVI) (150 unit mM) instead of VI. The immobilization of PAA (M_w = 205,000) and PVI (M_w = 35,900) resulted in transparent G2 and G3, respectively, while G3 with another PVI (M_w = 193,000) was opaque. Potentiometric titrations and swelling measurements of an anionic and a cationic copolymer gel (as a control sample) suggested that each polyampholyte gel exhibits an isoelectric point (pI) at pH=5.6, at which the electrostatic attraction between the carboxylate and imidazole ions leads to gel collapse. Indeed, all the ampholyte gels underwent a collapse transition at pH around the pI upon cyclic pH changes (first increased and then decreased). The pH region where gel collapse was observed, however, was broader for G2 and G3 (with immobilized polyions) than for Gl (random ter-polymer gel). The swelling/deswelling characteristics of transparent and opaque G3 gels were different from each other, and also from those of Gl and G2. These results were discussed in terms of the role of hydrogen bonding of the amide with the carboxyl or imidazole groups in the collapse transition of the ampholyte gels.