Rates of transesterication in epoxy-thiol vitrimers

TL;DR

This study investigates how chemical structure and physical properties influence transesterification rates and vitrification temperatures in epoxy-thiol vitrimers, aiding in optimizing their processability.

Contribution

It introduces a simple method using click chemistry for creating epoxy-thiol vitrimers and analyzes how chemical and physical factors affect their bond exchange and transition temperatures.

Findings

Transesterification activation energy varies with chemical structure and catalyst concentration.

Softer networks exhibit lower vitrification temperatures despite higher activation energies.

Physical stiffness significantly influences the elastic-plastic transition temperature.

Abstract

Vitrimers, an important subset of dynamically crosslinked polymer networks, have many technological applications for their excellent properties, and the ability to be re-processed through plastic flow above the so-called vitrification temperature. We report a simple and efficient method of generating such adaptive crosslinked networks relying on transesterification for their bond exchange by utilising the `click' chemistry of epoxy and thiols, which also has the advantage of a low glass transition temperature. We vary the chemical structure of thiol spacers to probe the effects of concentration and the local environment of ester groups on the macroscopic elastic-plastic transition. The thermal activation energy of transesterification bond exchange is determined for each chemical structure, and for a varying concentration of catalyst, establishing the conditions for the optimal, and for the suppressed bond exchange. However, we also discover that the temperature of elastic-plastic transition is strongly affected by the stiffness (dynamic rubber modulus) of the network, with softer networks having a much lower vitrification temperature even when their bond-exchange activation energy is higher. This combination of chemical and physical control factors should help optimise the processability of vitrimer plastics.