Microphase Separation in Dynamically Crosslinked Polymers

Permanently crosslinked polymers are ubiquitous due to their favorable mechanical properties. However, these materials are unsustainable as they cannot be reprocessed. In contrast, polymer networks that can rearrange their topology without depolymerization, e.g., via the formation of reversible crosslinks ("vitrimers"), are reprocessable and are promising materials for a circular economy. The bond exchange between atom pairs in vitrimers is facilitated by reversible chemical reactions, such as transesterification or disulfide bond exchange. This enables favorable mechanical, rheological, self-healing, adhesive, and shape memory properties in a vitrimer, whose underlying mechanisms remain unresolved. Here, we report a hybrid molecule-dynamics – Monte Carlo (MD-MC) simulation study that shows the microphase separation of crosslinkers in a reversibly crosslinked polymer network, consistent with recent experiments. The time and length scales of the microstructures in a polymer network are found to be strongly correlated to the crosslink bond lifetime, concentration, and points of crosslinks in polymer chains. As a consequence, we observe a hierarchical meso to nanostructure that is incompatible in the networks. The work has fundamental implications for understanding and rational design of vitrimers and other dynamically crosslinked polymer networks.