Functionalized microgel swelling: comparing theory and experiment

A comprehensive gel swelling model accounting for the effects of added salt, counterion/polyelectrolyte charge condensation, inter-cross-link chain length distribution, polyelectrolyte chain stiffness, and direct charge-charge repulsion between fixed polymer network charges has been applied to predict water fraction profiles in -COOH-functionalized microgels based on poly(N-isopropylacrylamide). The model can successfully order the microgels according to their rheologically measured water fractions and explains key differences in observed microgel swelling according to the different functional group and cross-linker distributions in the microgels. The cross-linking efficiency is used as an adjustable variable to match the magnitude of the different model predictions with the experimental water contents from rheological measurements. The resulting cross-linking efficiency predictions are correlated with the ability of the different comonomers to facilitate chain transfer and/or radical termination in the polymerization environment. The model can capture the differing responses of the microgels in the presence of different salt concentrations and can account for the impact of many key physical parameters and heterogeneities in microgel swelling which the Flory-Huggins model cannot directly address.