Swelling, structure, and phase stability of compressible microgels.

Matthew Urich,Alan R Denton

doi:10.1039/c6sm02056a

Abstract

Microgels are soft colloidal particles that, when dispersed in a solvent, swell and deswell in response to changes in environmental conditions, such as temperature, concentration, and pH. Using Monte Carlo simulation, we model bulk suspensions of microgels that interact via Hertzian elastic interparticle forces and can expand or contract via trial moves that allow particles to change size in accordance with the Flory-Rehner free energy of cross-linked polymer gels. We monitor the influence of particle compressibility, size fluctuations, and concentration on bulk structural and thermal properties by computing particle swelling ratios, radial distribution functions, static structure factors, osmotic pressures, and freezing densities. For microgels in the nanoscale size range, particle compressibility and associated size fluctuations suppress crystallization, shifting the freezing transition to a higher density than for the hard-sphere fluid. As densities increase beyond close packing, microgels progressively deswell, while their intrinsic size distribution grows increasingly polydisperse.

Highlights

Microgels are soft, compressible colloidal particles, typically composed of cross-linked polymer networks that, when dispersed in a solvent, can swell significantly in size and can respond sensitively to environmental changes
From simulations of N = 500 particles initialized on an fcc lattice, we analyzed the equilibrium particle swelling ratio, structural properties, and osmotic pressure over a wide range of densities
The results presented below represent statistical averages of particle coordinates and radii over 1000 independent configurations, separated by intervals of 100 Monte Carlo (MC) steps, following an initial equilibration phase of 5 Â 104 MC steps, where a single MC step is defined as one combined trial displacement and size change of every particle

Summary

Introduction

Compressible colloidal particles, typically composed of cross-linked polymer networks that, when dispersed in a solvent, can swell significantly in size and can respond sensitively to environmental changes. Tunable particle size results in unusual materials properties, with practical applications to filtration, rheology, and drug delivery.[5,6,7,8]. Over the past two decades, numerous experimental and modeling studies have characterized the elastic properties of single microgel particles[9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24] and the equilibrium and dynamical behavior of bulk suspensions.[25,26,27,28,29,30,31,32,33,34,35,36] Connections between singleparticle properties, such as swelling ratio, and bulk properties, such as osmotic pressure, thermodynamic phase behavior, pair structure, and viscosity, have been probed experimentally by static and dynamic light scattering, small-angle neutron scattering, confocal microscopy, and osmometry.[37,38,39,40,41,42,43,44,45] While the swelling/ deswelling behavior of microgels has been extensively studied, the full implications of elasticity and compressibility of these soft colloids for bulk suspension properties remain only partially understood

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