Effects of Crosslink Homogeneity on the High Strain Behavior of Elastic Polymer Networks

The topology of polymer networks affects critical material properties such as elastic modulus and toughness. While the effects of crosslink homogeneity and topological defects are well understood in low strain regimes, understanding their effects in the high strain regime has proven difficult because of premature network fracture. Here, we address this problem using a double network approach, in which the network of interest is swollen in a mixture of monomer and cross linkers that is then polymerized to generate a secondary network that helps dissipate stress, delay fracture, and "lock" the first network in a higher strain state. We examine the effect of crosslink homogeneity by comparing the low and high strain behaviors of randomly and regularly crosslinked butyl acrylate networks. Regularly-crosslinked networks are synthesized via coupling of n-butyl acrylate star polymers, while randomly crosslinked networks with comparable moduli are synthesized by free-radical polymerization. We find that networks with similar crosslinking densities exhibit similar low-strain behavior but that differences emerge in the high-strain regime, reflecting the growing importance of short network strands in transmitting stress in this limit.