Microstructure and rheological properties of thermo-responsive poly(N-isopropylacrylamide) microgels

The microstructure and rheological properties of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgels cross-linked with methylenebis-acrylamide (BA) were examined by dynamic light scattering and rheological techniques. As the temperature was increased from 10 to 50 °C, the particles diameter decreased by approximately two times near the volume phase transition temperature, T_v of between 30 and 35 °C. The addition of salt to the microgel dispersion provides competition for the water molecules hydrating the PNIPAM chains thus weakening the PNIPAM-H₂O hydrogen bonds and the microgel progressively deswelled. The validity and limitation of the semi-empirical approach to model charged soft microgel particles developed previously were tested on this thermo-responsive system. A variable specific volume, k was introduced to convert the mass concentration to effective volume fraction. With increasing concentration, inter-particle repulsive force was enhanced, which overcame the osmotic force inside the soft particle, resulting in the expulsion of solvent from the swollen particles, and the particle shrank. The viscosity data for PNIPAM microgels at varying solution temperatures and ionic strength showed excellent agreement with the modified Krieger-Dougherty (K-D) model.