Liquid-Crystalline Polymer Gels VI. Preparation and Swelling Behavior of Cross-Linked Hydrogels of Poly(L-glutamic acid) Possessing Cholesteric Order

Abstract

Cross-linked gels of poly(L-glutamic acid) (PGA) possessing cholesteric liquid-crystalline order were prepared from the corresponding liquid-crystalline gels of poly(γ-benzyl L-glutamate). The PGA hydrogels were found to hold original liquid-crystalline order in the neutral pH range, but became isotropic at higher pH values. The phase change was reversible. The swelling with the increase of pH occurred through two stages. During the first stage that begins around pH=6, a finger-print texture was observed under crossed polarizers. After the second stage of the swelling that begins around pH=9, the finger-print texture was mostly disappeared. From a detailed study on the dissociation of PGA inside the gel, the first stage is attributed to a process of expansion of partially dissociated helical PGA chains and the second stage is the full expansion of fully dissociated PGA chains.