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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-Tg 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 (fdi-epoxy) in the total epoxy compounds. Importantly, both epoxy compounds generate a free OH group in reacting with COOH, and thus the change of fdi-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 fdi-epoxy. We then investigate effects of the cross-link density on the stress-relaxation rate at high temperatures and activation energy (Ea) 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.
Hayashi et al. (Thu,) studied this question.