Effect 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, quantifying 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 crosslinkers that is then polymerized to generate a secondary network that helps dissipate stress. 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 the randomly crosslinked networks exhibit a much earlier onset of non-Gaussian strain stiffening behavior than the comparable regularly crosslinked networks, indicating the importance of strand length distribution, and particularly the shortest strands, in determining the high strain behaviors of elastic polymer networks.