Electrostatic expansion of polyelectrolyte microgels: Effect of solvent quality and added salt

We report dynamic light scattering data for a PNIPAM based microgel copolymerised with 2% ionic groups. The hydrodynamic radius is measured as a function of temperature (which varies the solvent quality) and added salt concentration (c_S) from salt-free conditions up to c_S = 0.1 M. Incorporation of the ionic co-monomer leads to an increase of the volume phase transition temperature with respect to a non-ionic microgel from T≃306 K to T≃317 K in salt-free water. The hydrodynamic radius varies as R_H∝c_S^-0.05 in good solvent (T≃278-298 K) and R_H∝c_S^-0.15 in poor solvent (T≃313-319 K). Deep into the poor solvent regime the microgel is collapsed for all salt concentrations and R_H∝c_S⁰. Data are fitted to the Flory-Rehner model modified to account for the osmotic pressure of the counterions through the Donnan model. We find that the Flory-Rehner-Donnan (FRD) model does not fully account for the experimental observations, particularly at low ionic strengths, possibly due to the influence of electrostatic excluded volume. The disagreements between theory and experiments are greatly reduced if a cross-linking density three times lower than that expected from synthesis is assumed. The scaling theory exponents for the variation of gel size with temperature and added salt are not in agreement with the experimental results presented.