Entropic phase separation in polymer--vitrimer melts

TL;DR

This study models and simulates vitrimer melts, revealing they can undergo phase separation without energetic interactions, with phase behavior influenced by the number of functional sites.

Contribution

It adapts a free energy model for dissociative networks to associative vitrimers and validates predictions with molecular dynamics simulations, highlighting phase separation mechanisms.

Findings

Vitrimer melts can phase separate without energetic interactions.

The phase diagram resembles that of dissociative systems.

Critical conversion degree inversely relates to functional sites per chain.

Abstract

Traditional plastics demand a choice between durability (thermosets) and reprocessability (thermoplastics). Vitrimers are a recent class of polymer network combining both these qualities. Their increased cost of production can be offset by mixing them with a traditional thermoplastic; however, phase separation in such blends can lead to inhomogenous materials. In this paper, we adapt an existing model for the free energy of dissociative polymer networks to their associative, vitrimeric counterpart. We test the accuracy of the model's predictions by comparing them with the results of novel molecular-dynamics simulations. We demonstrate that such melts can undergo phase separation even in the absence of energetic interactions between the components. We find furthermore that the phase diagram of the melts is qualitatively similar to that of dissociative systems, and that the critical degree of conversion for the onset of phase separation depends reciprocally on the number of function sites per vitrimer chain.