Mixed polyelectrolyte/ionomer behavior of poly(methacrylic acid) gel upon titration

The titration of poly(methacrylic acid) gel with sodium methoxide was studied in water, methanol, dioxane, and their mixtures. There are three main regimes of the swelling behavior of the gels depending on the dielectric constant epsilon of the medium. In polar media with epsilon larger than a critical value epsilon 1, the gel swells monotonically with the increase of the degree of ionization, alpha, due to the osmotic pressure of counterions. In media with epsilon smaller than a critical value epsilon(2), the gel always shrinks with increasing a due to ion-pair formation with their aggregation to multiplets. In media with dielectric constants ranging from epsilon(1) to epsilon(2), the gel swells at alpha up to ca. 0.1 and then collapses. The collapse induced by ionization is in agreement with recent theoretical findings.