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

Crosslink heterogeneity and topological defects have been shown to affect the moduli of polymer networks in the low strain regime. Probing their role in the high strain regime, however, has been difficult because of premature network fracture. Here, we address this problem by using a double network approach to investigate the high-strain behavior of both randomly and regularly crosslinked networks with the same backbone chemistry. Randomly crosslinked poly(n-butyl acrylate) networks with target molecular weights between crosslinks of 5-30 kg/mol were synthesized via free-radical polymerization, while regularly-crosslinked poly(n-butyl acrylate) networks with molecular weights between crosslinks of 7-38 kg/mol were synthesized via crosslinking of tetrafunctional star polymers. Both types of networks were then swollen in a monomer/crosslinker mixture, polymerized to form double networks, and characterized via uniaxial tensile testing. Interestingly, the onset of strain stiffening occurred later in regular networks than in random networks with the same modulus, but was well-predicted by the target molecular weight between crosslinks of each sample. These results indicate that the low- and high-strain behavior of polymer networks result from different molecular-scale features of the material, and suggest that controlling network architecture offers new opportunities to both further fundamental understanding of architecture-property relationships and design materials with independently controlled moduli and strain-stiffening responses.