Analysis of particle–solvent interactions in pH-responsive cross-linked microgel systems

Viscometry, static light scattering and turbidimetry studies were performed to investigate the interaction between pH-responsive microgels consisting of methacrylic acid (MAA) and ethyl acrylate (EA) copolymers cross-linked with di-allyl phthalate (DAP) and the solvent environment. The characteristics of microgel particles and their corresponding interaction potential were manipulated by varying the following: (a) degree of neutralization of MAA groups; α (b) salt concentrations and (c) microgel cross-linked density. Three independent interaction parameters, i.e. Huggins constant, Kh, second virial coefficient, A2 and apparent second virial coefficient, Bapp, which describe the degree of particle–solvent interactions, were obtained from these techniques. A scaling relationship between the three parameters given as: B app / K h ∝ α M ¯ w A 2 (where M ¯ w represents the weight-average molecular weight), was proposed. Good agreement was observed, where all the data for the three different salt concentrations of each microgel with different cross-linked densities collapsed onto a single master curve. This suggested that all three techniques can accurately predict the degree of interaction between microgel particle and solvent environment. Turbidimetry, being the simplest among the three techniques is the most convenient method.