Elastic energy in microscopically phase-separated swollen polymer networks

The paper analyzes a microscopic regime of strain, different from the one c onventionally considered, that presumably takes place in swollen polymers s howing strong microscopic phase separation, such as ion-exchange resins in water. Such systems show linear dependence of the elastic pressure on swell ing in contrast to the Flory-Rehner theory and its modifications. The prese nt work proposes a simple model that predicts this kind of behavior. Swelli ng is considered as a non-affine 'inflation' of the hydrophobic matrix by s mall aggregates of water molecules ('droplets') adsorbed by highly hydrophi lic groups, whereas the macroscopic dimensions of the sample change as a re sult of the compression of the 'films' separating the droplets. This compre ssion is then analyzed along the classical lines. In the case of the Dowex resins a partial test of the model based on the reported shear moduli showe d reasonable agreement with experiment. (C) 2001 Elsevier Science Ltd. All rights reserved.