Understanding Creep in Vitrimers: Insights from Molecular Dynamics Simulations

Abstract

Vitrimers offer a promising sustainable alternative to conventional epoxies due to their recyclability. Vitrimers are covalent adaptive networks where some bonds can break and reform above the vitrimer transition temperature. While this can lead to desirable behavior such as malleability, this also leads to undesirable rheological behavior such as low-temperature creep. In this work, we investigate the molecular mechanisms of the creep of vitrimers using molecular dynamics simulations. The interplay between dynamic bonding with mechanical loading is modeled using a topology-based reaction scheme. The creep behavior is compared against cross-linked epoxies with dynamic reactions to understand the unique aspects related to dynamic bonding. It is found that the free volume that arises from tensile loads is reduced in vitrimers through dynamic bond rearrangement. An important feature that explains the difference in secondary creep behavior between conventional epoxies and vitrimers is the orientation of the dynamic bonds during loading. In vitrimers, the dynamic bonds preferentially align orthogonal to the loading axis, decreasing the axial stiffness during secondary creep, resulting in larger creep strain compared to epoxies. Over longer timescales, such increased strain leads to void growth, resulting in tertiary creep. Thus, chemistry changes or additives that can prevent the initial realignment of dynamic bonds, and therefore subsequent void growth, can be an effective strategy to mitigate creep in vitrimers.