Stabilizing the Hexagonal Close Packed Structure of Hard Spheres with Polymers : Phase diagram, Structure, and Dynamics

TL;DR

This study uses Monte Carlo simulations to explore how adding polymers to colloidal hard spheres can stabilize the hexagonal close packed structure, revealing phase behavior, structure, and dynamics of the mixture.

Contribution

It provides the first detailed free-energy based phase diagram for colloid-polymer mixtures, confirming conditions for HCP stabilization and analyzing structural and dynamic properties.

Findings

HCP phase stability occurs at high polymer concentrations

Broad fluid-solid coexistence region identified

Metastable gas-liquid coexistence observed

Abstract

We study the phase behaviour of a binary mixture of colloidal hard spheres and freely-jointed chains of beads using Monte Carlo simulations. Recently Panagiotopoulos and coworkers predicted [Nat. Commun. 5, 4472 (2014)] that the hexagonal close packed (HCP) structure of hard spheres can be stabilized in such a mixture due to the interplay between polymer and the void structure in the crystal phase. Their predictions were based on estimates of the free-energy penalty for adding a single hard polymer chain in the HCP and the competing face centered cubic (FCC) phase. Here we calculate the phase diagram using free-energy calculations of the full binary mixture and find a broad fluid-solid coexistence region and a metastable gas-liquid coexistence region. For the colloid-monomer size ratio considered in this work, we find that the HCP phase is only stable in a small window at relatively high polymer reservoir packing fractions, where the coexisting HCP phase is nearly close packed. Additionally we investigate the structure and dynamic behaviour of these mixtures.