Swelling of strong polyelectrolyte hydrogels in polymer solutions: effect of ion pair formation on the polymer collapse

A series of strong polyelectrolyte hydrogels based on acrylamide and 2-acrylamido-2-methylpropane sulfonic acid sodium salt or [(Methacrylamido)propyl]trimethyl ammonium chloride were prepared by free-radical crosslinking copolymerization. Their swelling behavior was investigated in a good solvent (water), in polymer melt and in aqueous polymer solutions. Poly(ethylene glycol) of various molecular weights was used as the linear polymer in the swelling experiments. The equilibrium volume swelling ratio q(v) of the hydrogels in water scales with the network charge density f by the relation q(v) proportional to f(0.66). The volume of the hydrogels in the polymer melt decreases as the number of segments, y, on the polymer increases. It was shown, for the non-ionic hydrogels, that this deswelling occurs due to the mixing entropy of the linear polymer in the external solution, whereas, for the ionic gels, due to trapping of counterions caused by the decreased polarity of the medium. The network-polymer interaction parameter chi(23) was found to vary with y by the relation chi(23) = constantly. The hydrogels also deswell in aqueous polymer solutions as the polymer concentration increases. This deswelling occurs smoothly or jumpwise, as a first-order phase transition, depending on the ionic group content of the hydrogels. The Flory-Huggins theory correctly predicts the swelling and collapsing behavior of the hydrogels in polymer solutions if the variation of the effective charge density depending on the medium polarity is taken into account. (C) 2000 Elsevier Science ltd. All rights reserved.