Phase transitions in dense thermo-responsive microgel suspensions: A Monte Carlo study

Abstract

Aqueous suspensions of sub-micron sized monodisperse particles exhibit structural ordering analogous that observed in atomic systems. Deionised suspensions of colloidal charged polystyrene particles exhibit long-range order for volume fraction I > 0.001, whereas suspensions of hard-spheres undergo crystallization at I > 0.48.In these dispersions the particle size is fixed and the temperature is not a controllable parameter to investigate the phase transitions like fluid to crystal (melting/freezing) and glass transition. Whereas aqueous suspensions of thermo-responsive poly(Nisopropylacrylamide) (PNIPAM) particles exhibit rich phase behaviour upon varying the temperature as well as osmotic pressure due to the variation in particle size as well as interparticle interaction U(r). Recently, Bergman et al [1]have proposed a Multi-Hertzian (MH) pair potential U(r), for PNIPAM microgels which are known to have an inhomogeneous dense polymer core and a thin shell of dangling polymer chains. MH pair-potential is expected to represent the interparticle interaction between PNIPAM microgel spheres up to their volume phase transition (VPT) temperature (VPT | 34qC. The applicability of this potential in predicting the phase behaviour, is verified only in the fluid region (i.e. volume fractions upto I = 0.49) of PNIPAM microgels. Its applicability for understanding the ordering, dynamics and the phase behaviour of microgels under dense conditions needs to be investigated. With this as the motivation, we have performed Monte Carlo (MC) simulations with MH potential over a wide range of volume fractions and results are reported here. Quite surprisingly, we found that that the fluid phase freeze in to a glassy state instead of a crystalline state upon increasingI.