pH induced swelling of PVP microgel particles--a first order phase transition?

The pH-induced swelling of poly(2-vinylpyridine) microgel particles was studied using dynamic light scattering. The increase in particle diameter with decreasing pH was modeled using a well-established thermodynamic model for microgel swelling. The Maxwell construction was applied to extend the model and yield a prediction for a pH-responsive microgel across the entire pH range. The model predicts a first order phase transition for polymer-solvent combinations with a Flory interaction parameter, χ, greater than a critical value χ(c). The modified theory compared favorably to the dynamic light scattering data for the hydrodynamic diameter of microgel particles based on 2-vinylpyridine at different pH values. In particular, the swelling transition is both predicted theoretically and observed experimentally to occur at a pH lower than the pK(a) of the polymer.