Syntheses and characterization of pH-sensitive hydrogel from poly(.gamma.-glutamic) acid

Novel pH-sensitive hydrogels were prepared through crosslinking of poly(γ-glutamic) acid (γ-PGA) by using 1,4-butanediol diglycidyl ether as a cross-linker, tetraethylammonium bromide as a catalyst, and reacting in DMSO. The dependence of the swelling ratio of the hydrogels on pH values, ionic strength and cross-linking degree was investigated. It was found that the pH-sensitive range was clearly extended through introducing multiple hydrogen bonds to the hydrogel network during the preparation. The swelling ratios of γ-PGA hydrogels increased with the increase in pH of the aqueous solution from 2 to 9. The swelling of the γ-PGA hydrogels was firstly controlled by the ionization of carboxyl groups in the hydrogels within the pH range from 2 to 5, and then controlled by breaking of the multiple hydrogen bonds in the hydrogels within the pH range from 5 to 9. The swelling ratios of γ-PGA hydrogels were also strongly dependent on the ionic strength of the medium and cross-linking degree of the hydrogels. Increasing the ionic strength and the crosslinking degree resulted in a decrease in the swelling ratio of the hydrogels.