Fluid to Solid Transition in Colloidal Suspensions of Thermo Responsive Core–Shell Soft Particles Interacting through Multi-Hertzian Pair-Potential: A Monte Carlo Study

Abstract

Maxime J. Bergman et al. have proposed a Multi-Hertzian (MH) pair-potential by modeling the core–shell structure of thermo-responsive poly (N-isopropyl acrylamide) (PNIPAM) soft microgel particles, which are known to be soft and can be over-packed beyond a volume fraction ϕ > 0.68. There have been no studies in the literature on the applicability of the MH pair potential to understand the phase behavior and dynamics of dense PNIPAM microgels. We report here the results of Monte Carlo (MC) simulations on PNIPAM microgel suspensions interacting with MH potential over a wide range of volume fractions (ϕ = 0.3–0.68), under over-packed conditions (ϕ = 0.68–1.0), and also in the temperature range of T = 15°C–30°C. MC simulations show a fluid (liquid-like ordered) to solid (crystalline) transition as a function of increasing volume fraction, ϕ, and a solid to fluid transition upon increasing temperature, T, which are in accordance with experimental observation. We also studied the dynamics of PNIPAM particles by computing the mean square displacement (MSD) as a function of Monte Carlo (MC) time for different volume fractions and at various temperatures. Although our simulations predict the phase behavior of PNIPAM suspensions similar to that observed in experiments, but failed to predict the reported experimental observations under over-packed conditions, viz., the report of sub-diffusive dynamics at small time scales by Joshi et al., which indicates the existence of entanglement of dangling polymer chains between shells of the neighboring PNIPAM microgel particles. Our simulations suggest the need for improvements in the MH pair-potential to account for the dangling polymer chains.