Unexpectedly large entropic barrier controls bond rearrangements in vitrimers

TL;DR

This study uncovers that a large entropic barrier, rather than enthalpic factors, predominantly controls bond exchange rates in vitrimers, influencing their viscoelastic properties and sol-gel transition behavior.

Contribution

It reveals that the entropic barrier significantly impacts bond rearrangements in vitrimers, a novel insight into their fundamental viscoelastic mechanisms.

Findings

Crosslink density does not affect chain dynamics significantly.

Vitrimers undergo a sol-gel transition consistent with classical gelation theory.

Large negative activation entropy slows bond exchange despite low activation enthalpy.

Abstract

Vitrimers are a relatively new class of polymeric materials containing associative covalent dynamic bonds that make them recyclable by design. However, the fundamental mechanisms controlling their viscoelastic properties remain poorly understood. Our detailed studies of relaxation dynamics and viscoelastic behavior of model vitrimers revealed that the density of dynamic covalent crosslinks has no influence on chain dynamics (beyond a weak change in the glass transition temperature), yet it strongly affects the linear viscoelasticity of vitrimers. Increasing the crosslink density induces a sol-gel transition consistent with predictions of classical gelation theory, demonstrating its applicability to vitrimers. Remarkably, the temperature-dependent analysis of the bond rearrangement time reveals an unexpectedly large negative activation entropy in the transition state that strongly slows down the bond exchange process despite its relatively low activation enthalpy. This insight explains the unusual long timescale for bond rearrangement in vitrimers and highlights the significance of entropy in controlling the viscoelasticity of dynamic covalent networks.