Study of the compositional heterogeneity in poly(N-isopropylacrylamide-acrylic acid) microgels by potentiometric titration experiments

The compositional distribution of acrylic acid (AA) residues in poly(N-isopropylacrylamide-acrylic acid) [poly(NIPAM-AA)] microgels has been investigated using alkaline titration experiments. It is found that the apparent acid dissociation constant (pKaapp) of the AA residues increases with the degree of dissociation. This is due to the electrostatic effect that arises as AA residues dissociate, and the pKaapp change depends on the local concentration of AA residues in the gel network. Nevertheless, it is found that the pKaapp change is independent of the overall AA content. This indicates that the AA residues in poly(NIPAM-AA) microgels are clustered and not uniformly distributed in the gel network. A two-phase model based on the Donnan concept is developed to predict pKaapp values, and a good fit to the experimentally estimated pKaapp values is obtained if a block copolymer is assumed.