Glassy correlations and microstructures in randomly crosslinked homopolymer blends

TL;DR

This paper develops a microscopic model for randomly crosslinked polymer blends, revealing how gelation and phase separation interplay, leading to glassy concentration fluctuations and microstructures like hexagonal or lamellar patterns.

Contribution

It introduces a Landau theory framework for gelation and phase separation in crosslinked polymer blends, analyzing glassy correlations and microstructure formation.

Findings

Mixed gels exhibit both thermal and persistent glassy concentration fluctuations.

Microphase separation occurs upon cooling, with structures influenced by composition and compressibility.

Microstructure length scales are determined by mesh size and can form various patterns.

Abstract

We consider a microscopic model of a polymer blend that is prone to phase separation. Permanent crosslinks are introduced between randomly chosen pairs of monomers, drawn from the Deam-Edwards distribution. Thereby, not only density but also concentration fluctuations of the melt are quenched-in in the gel state, which emerges upon sufficient crosslinking. We derive a Landau expansion in terms of the order parameters for gelation and phase separation, and analyze it on the mean-field level, including Gaussian fluctuations. The mixed gel is characterized by thermal as well as time-persistent (glassy) concentration fluctuations. Whereas the former are independent of the preparation state, the latter reflect the concentration fluctuations at the instant of crosslinking, provided the mesh size is smaller than the correlation length of phase separation. The mixed gel becomes unstable to microphase separation upon lowering the temperature in the gel phase. Whereas the length scale of microphase separation is given by the mesh size, at least close to the transition, the emergent microstructure depends on the composition and compressibility of the melt. Hexagonal structures, as well as lamellae or random structures with a unique wavelength, can be energetically favorable.