  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.
