Rheo-Optical Study on the Viscoelastic Relaxation Modes of a Microgel Particle Suspension around the Liquid-Solid Transition Regime

Dynamic viscoelasticity and dynamic birefringence of a microgel system were investigated around the liquid-solid transition concentration to clarify the molecular origin of the viscoelastic response of the microgel system. The complex modulus showed viscoelastic liquid-like behavior at concentrations, c, below a threshold c(jamming) for random close packing of the microgels, whereas viscoelastic solid-like behavior at c > c(jamming). The imaginary part of the complex strain-optical coefficient changed its sign with increasing angular frequency omega in a liquid-like regime, suggesting that the stress and birefringence involved three relaxation mechanisms. Utilizing the stress-optical rule, SOR, for each relaxation process, the complex shear modulus at c < c(jamming) was separated into the orientational stress of network strands in temporary contacted microgels, deformation of particle, and the redistribution process, from high frequencies in this order. On the other hand, in a solid-like regime at c > c(jamming), the ordinary SOR held well with a single stress-optical coefficient, C, implying that only one stress origin is dominant, which can be attributed to the orientational stress of densely packed microgels in permanent contact. The strain-optical coefficients evaluated using the Onuki-Doi theory reproduced the measurements qualitatively and supported these assignments.

Automated summary

The dynamic viscoelasticity and dynamic birefringence of a microgel system were investigated to clarify the molecular origin of the viscoelastic response. Different behaviors were observed at concentrations below and above a critical value for random close packing, with multiple relaxation mechanisms involved in the liquid-like regime and a single dominant stress origin in the solid-like regime. The measurements were qualitatively supported by the Onuki-Doi theory.