Linear and Nonlinear Viscoelasticity of Concentrated Thermoresponsive Microgel Suspensions

TL;DR

This study combines experiments and theory to analyze the complex viscoelastic behavior of concentrated pNIPAM microgel suspensions across temperature and concentration ranges, revealing non-monotonic changes driven by size and interaction shifts.

Contribution

It introduces a minimalistic model linking microgel size and interparticle interactions with viscoelastic properties, validated by quantitative experimental comparisons.

Findings

Non-monotonic viscoelastic changes with temperature

Good agreement between theory and experiments

Predictions for nonlinear rheological properties

Abstract

This is an integrated experimental and theoretical study of the dynamics and rheology of self-crosslinked, slightly charged, temperature responsive soft Poly(N-isopropylacrylamide) (pNIPAM) microgels over a wide range of concentration and temperature spanning the sharp change in particle size and intermolecular interactions across the lower critical solution temperature (LCST). Dramatic, non-monotonic changes in viscoelasticity are observed with temperature, with distinctive concentration dependences in the dense fluid, glassy, and soft-jammed states. Motivated by our experimental observations, we formulate a minimalistic model for the size dependence of a single microgel particle and the change of interparticle interaction from purely repulsive to attractive upon heating. Using microscopic equilibrium and time-dependent statistical mechanical theories, theoretical predictions are quantitatively compared with experimental measurements of the shear modulus. Good agreement is found for the nonmonotonic temperature behavior that originates as a consequence of the competition between reduced microgel packing fraction and increasing interpar-ticle attractions. Testable predictions are made for nonlinear rheological properties such as the yield stress and strain. To the best of our knowledge, this is the first attempt to quantitatively understand in a unified manner the viscoelasticity of dense, temperature-responsive microgel suspensions spanning a wide range of temperatures and concentrations.