Properties of polyamide gels with varying chain stiffness and length synthesized in the swollen state

Abstract

Cross-linked networks of rigid, semirigid, and flexible polyamides were polymerized in solution. When transferred to DMAc the volume of the rigid and semirigid networks did not change significantly, but the flexible networks swelled several hundred percent. When gels previously equilibrated in DMAc were immersed in mixtures containing increasing amounts of nonsolvent, they shrank by an amount that depended on chain flexibility: The flexible and semirigid gels shrank to 30% to 40% of their as-synthesized volume, while the rigid gels shrank to only about 60% of their as-synthesized volume. Furthermore, the volume of the rigid gel decreased smoothly with the change in solvent composition. Unlike that of the rigid network, the volumes of the semirigid and flexible gels sharply decreased over a rather narrow range of solvent composition. Semirigid gels prepared at lower polymer concentration shrank over a narrower solvent composition than their counterparts prepared at higher polymer concentration. All gels changed from transparent through translucent to opaque as they shrank. The slope of the deswelling curves and the developing turbidity indicate that the collapse of the networks is not a first-order transition. Uniaxial compressive stress-strain curves were obtained from several of the solvent-equilibrated gels. When Treloar's equation relating modulus to molecular weight between cross-links, M_c, in swollen gels of flexible chains was employed, a reasonably good agreement was found between M_c values obtained from the modulus and those calculated from structural formulae of semirigid as well as flexible gels equilibrated in DMAc.