Osmotic properties of poly(ethylene oxide) gels with localized charged units

Polyelectrolyte gels consisting of neutral poly(ethylene oxide) chains and charged polymethacrylate chains were studied at equilibrium in aqueous salt solutions. The swelling degree Q increases with the fraction of charged monomer units β however this growth is considerably weaker (Q∼β0.23 in 10−4 mol dm−3 NaCl) than the expected one. The shear modulus G varies with Q approximately according to the classical prediction G∼Q−1/3 and does not depend on β. This indicates that the neutral network chains determine the modulus and, hence, the elastic contribution to the swelling pressure. The osmotic contribution found from the data on free swelling and osmotic deswelling was separated into polymer and ionic parts. The polymer part scales as Q−9/4 like the osmotic pressure of semidilute aqueous solutions of poly(ethylene oxide). The ionic part is far less than the ideal-gas pressure of counterions, and this discrepancy increases with β. This is explained by two causes: localization of charged units within the network, resulting in the counterion condensation, and the electrostatic interactions in the gel.