Relationship between Reaction Kinetics and Chain Dynamics of Vitrimers Based on Dioxaborolane Metathesis

TLDR

Vitrimer processability depends strongly on the temperature dependence of viscosity. The study analyzes temperature‑dependent viscoelasticity of dioxaborolane‑based vitrimers. The gel‑to‑sol transition arises from a reverse reaction at two‑site interchain cross‑linking points with excess cross‑linker, creating two non‑cross‑linking sites. The viscoelastic measurements reveal sol‑to‑gel and reverse gel‑to‑sol transitions; above the gel point, decross‑linking accelerates more slowly than Rouse relaxation, broadening the plateau, which indicates that the dynamic covalent bond lifetime exceeds the Rouse time and that strand relaxation is governed by the low activation energy of dioxaborolane metathesis.

Abstract

Processability of vitrimers strongly relies on the temperature dependence of viscosity. In this study, we analyzed temperature-dependent viscoelasticity of vitrimers based on the dioxaborolane metathesis reaction. A sol-to-gel transition process and a reverse gel-to-sol process are observed in the linear viscoelasticity with increasing content of the cross-linker. The latter gel-to-sol process is owing to a reverse reaction between a two-site interchain cross-linking point with an excess cross-linker, forming two noncross-linking sites. For samples above the gel point, the increasing temperature leads to a weaker acceleration of the decross-linking process than the Rouse-type relaxation, and accordingly, broadening of the plateau region. This trend is easily visualized in samples slightly above the gel point for which the stress relaxation arising from the Rouse-type relaxation and the decross-linking process are not well separated over time. This temperature-dependent behavior reflects a case that the lifetime of the dynamic covalent bond is significantly larger than the Rouse time of the network strands. As a result, the stress borne by a strand relaxes immediately upon decrosslinking, and thus, the low activation energy of the dioxaborolane metathesis reaction governs the strand relaxation.