Critical effect of exchangeable units distribution on properties of vitrimers: Quantitatively controlling dynamics thus suppressing creep

Vitrimers usually exhibit reprocessability and recyclability due to the reversible crosslinking bonds, thus these materials are expected to be applied as sustainable materials. However, the vitrimer elastomers are susceptible to creep because of their inherent exchange nature, which limits their practical application. The key to improve the creep resistance at service temperature relies on controlling and regulating the intrinsic dynamic properties. Therefore, an in-depth study of the effect of molecular variables on dynamic properties was useful for the design and application of vitrimers. exchangeable units distribution is one of the major molecular variables to tune the dynamics, but it has hardly been studied. Herein, vitrimer elastomers with different uneven degrees of exchangeable units distribution (n) was prepared to study the quantitative relationship between exchangeable units distribution and dynamic behaviors. Due to the uneven exchangeable units distribution, the crosslink-rich and crosslink-poor regions existed simultaneously in networks. The relaxation time of resulting networks increased and then decreased with n due to the combined effects, and samples with moderate uneven distribution showed the longest relaxation time. At the same time, the relaxation times for all samples were short enough to reprocess at elevated temperatures. What is more, the activation energy (Ea) and topology freezing transition temperature (Tv), two important intrinsic parameters for controlling the dynamics, increased linearly with n values. In the meantime, the creep strains and creep rates decreased with n, revealing improved creep resistance. This work provides a strategy to control the dynamics by adjusting the exchangeable units distribution thus enhancing creep resistance. And the proposed strategy and resulting quantitative models might be applied to other vitrimers, which could be utilized to prepare desired vitrimers with exact network designs for various applications.