Phase transition and elasticity of protein-based hydrogels

Reported are specific materials characterizations of three protein-based polymers 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, which had been previously prepared and γ-irradiation cross-linked into elastic matrices. These polymers exhibit a hydrophobic folding and assembling transition on raising the temperature above a critical temperature, designated by T_t. Their equilibrium swelling ration, uniaxial tensile and dynamic shear behavior were studied. The effect of the transition on swelling and mechanical properties was demonstrated. Equilibrium swelling ratio below the transition temperature decreased with the increase of γ-irradiation dose. Above the transition temperature, hysteresis and frequency dependence were found in tensile loading-unloading tests and dynamic shear measurements, respectively. The rubber elasticity theory of random chain networks was applied to the data only below the transition temperature, where they may properly be considered random network hydrogels, to estimate molecular weight between cross-links and the Flory-Huggins interaction parameter.