Fair Investigation of Cross-Link Density Effects on the Bond-Exchange Properties for Trans-Esterification-Based Vitrimers with Identical Concentrations of Reactive Groups

摘要

In this research, we demonstrate effects of cross-link density on the bond-exchange property of trans-esterification-based vitrimers. In most of the past literature, the change of cross-link density is inevitably accompanied by the change of concentration of reactive groups, which disturbs extraction of pure effects of cross-link density. For pursuing fair comparison, the concentrations of free OH groups (i.e., the reactive group for trans-esterification) and catalysts are here kept the same among samples with different cross-link densities. This is achieved by using an amorphous low-T-g polyester possesing multiple COOH side groups (PE-COOH), di-epoxy compound (1,4-butanediol diglycidyl ether), and mono-epoxy compound (butyl glycidyl ether). The di-epoxy acts as a cross-linker agent that reacts with COOH groups in PE-COOH, while the mono-epoxy reacts but does not work as the cross-linker, and thus the cross-link density can be tuned by the fraction of the di-epoxy compound (f(di-epoxy)) in the total epoxy compounds. Importantly, both epoxy compounds generate a free OH group in reacting with COOH, and thus the change of f(di-epoxy) is not accompanied by the change of the free OH concentration in the system. Rheological measurements and tensile tests first reveal the systematic tuning of cross-link density depending on f(di-epoxy) We then investigate effects of the cross-link density on the stress-relaxation rate at high temperatures and activation energy (E-a) for the bond exchange, revealing an interesting phenomenon, that is, the fastest relaxation rate and the lowest Ea for the largest cross-link density sample. The results are discussed in terms of entropically driven acceleration of bond exchange for the perturbed network segments.