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 ( p K a app ) 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 p K a app change depends on the local concentration of AA residues in the gel network. Nevertheless, it is found that the p K a app 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 p K a app values, and a good fit to the experimentally estimated p K a app values is obtained if a block copolymer is assumed.