pH-Dependent Equilibrium Swelling Properties of Hydrophobic Polyelectrolyte Copolymer Gels

The aqueous equilibrium swelling properties of a series of lightly cross-linked polyelectrolyte copolymers based on n-alkyl methacrylate esters (n-AMA) of various chain lengths and (dimethylamino)ethyl methacrylate (DMA) have been measured as a function of pH at 25 °C and constant ionic strength. Swelling is highly pH dependent, with water contents ranging from less than 10% (w/w) at high pH to greater than 90% at low pH. The extent of the transition from the collapsed hydrophobic state to the hydrophilic state and the pH interval over which it occurs depend on the comonomer composition. Increasing the proportion of n-AMA to DMA generally reduces the extent of the transition and shifts it to lower pH. Increasing the length of the n-AMA side chain (at constant comonomer ratio) causes a reduction in the extent of the transition, with a less systematic effect on the transition pH. The specific ionic composition of the solution, at a given pH and ionic strength, affects the extent of swelling. The ionic strength dependence of the equilibrium swelling has also been found to be affected by the ion valencies of the buffer system, as predicted by the Donnan equilibrium theory.