Influence of cross-link density on rheological properties of temperature-sensitive microgel suspensions

The influence of the crosslink density on rheological properties of thermosensitive microgels was investigated. The temperature-sensitive hydrogel particles consisted of poly (N-isopropylacrylamide) (PNiPAM) chemically cross-linked with several different molar ratios of N,N'-methylenebisacrylamide. The variation of cross-link density leads to soft spheres that possess a different particle interaction potential and a different swelling ratio. With increasing temperature the microgel particles decrease in size and with it the effective volume fraction, which leads to strong changes in rheological properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted versus the effective volume fraction. Up to an effective volume fraction of 0.5 the microgels behaved like hard spheres and the maximum volume fraction, as determined from the divergence of the zero-shear viscosity, was mainly dominated by the polydispersity of the spheres and not by the cross-link density. The plateau modulus, on the other hand, revealed soft-sphere behavior and the interaction potential became softer with decreasing cross-linker content.