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

Abstract

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 cⱼₐₘₘᵢₙg for random close packing of the microgels, whereas viscoelastic solid-like behavior at c > cⱼₐₘₘᵢₙg. The imaginary part of the complex strain-optical coefficient changed its sign with increasing angular frequency ω 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 cⱼₐₘₘᵢₙg, 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.