Phase transition and elasticity of protein-based hydrogels

Reported are specific materials characterizations of three protein-based po lymers comprised of repeating pentapeptide sequences, namely (GVGVP)(251), (GVGIP)(260) and (GVGVP GVGVP GEGVP GVGVP GVGVP GVGVP)(n)](GVGVP) where G = glycine, V = valine, P = proline, I = isoleucine, and E = glutamic acid, w hich had been previously prepared and gamma -irradiation cross-linked into elastic matrices. These polymers exhibit a hydrophobic folding and assembli ng transition on raising the temperature above a critical temperature, desi gnated by T-t. Their equilibrium swelling ratio. uniaxial tensile and dynam ic shear behavior were studied. The effect of the transition on swelling an d mechanical properties was demonstrated. Equilibrium swelling ratio below the transition temperature decreased with the increase of gamma -irradiatio n dose. Above the transition temperature, hysteresis and frequency dependen ce were found in tensile loading-unloading tests and dynamic shear measurem ents, respectively. The rubber elasticity theory of random chain networks w as applied to the data only below the transition temperature, where they ma y properly be considered random network hydrogels, to estimate molecular we ight between cross-links and the Flory-Huggins interaction parameter.