Bridging polymer network scales: crosslinks as fundamental structural units, and emergent multiphysics phenomena

There are deep, yet elusive, connections between the structure of a polymer network and its bulk properties. The structure is both hierarchical and statistical in nature, with local order among monomers depending on entropy, pairwise interactions, and external stimuli (e.g. electromagnetic fields); and the connectivity and polydispersity of polymer chains within the network depending on the stochastic process of polymerization. Here we use statistical mechanics to bridge between the monomer and polymer chain scales, then explore different ways of bridging between the chain and network scales, i.e. we explore different polymer network models. Specifically, polymer network models are used to connect network structure to different emergent multiscale phenomena such as flexoelectricity (i.e. coupling between electrical polarization and strain gradients), conventional electromechanical actuation modes, and asymmetric actuation of elastomers. Remarkably, different network models yield notably distinct predictions, both quantitatively and qualitatively. This underscores the need for a new approach to relate macroscopic deformations to chain-level deformations. A new approach is introduced which unifies previously disparate polymer network models and provides more intuitive insights into emergent phenomena in specific scenarios. We contend that it prompts a shift from regarding a single polymer chain as the fundamental unit to considering crosslinks as potentially more suitable fundamental units within the broader network. To conclude, we discuss how this new approach provides a potential path forward for efficient and rational modeling of networks with statistical variations in chain molecular weights, crosslink functionalities, and crosslink geometries.