Revisiting the Elasticity of Tetra-Poly(ethylene glycol) Hydrogels

The elasticity of polymer gels was conventionally analyzed based on the classical rubbery theories considering predominantly the entropic stress. Nevertheless, the recent studies of Yoshikawa et al. revealed that the extrapolation of the plots of elastic modulus against T to 0 K gave a negative characteristic modulus intercept, G_int < 0. They suggested that the elastic modulus stemmed not only from an increase of free energy owing to the reduced conformational entropy but also from a reduction of free energy owing to the facilitation of a favorable polymer/solvent interaction upon deformation. In this study, we choose a tetra-poly(ethylene glycol) gel (Tetra-PEG gel) dissolved in water/ethylene glycol mixtures as a model system and investigate how the elasticity of the gel samples changes with the quality of the solvent. We find that the negative G_int should stem from an improvement of the solvent quality with decreasing T. Owing to this improvement, the network strands exhibit a more expanded conformation at lower T, leading to an extra softening effect.