Impacts of Network Heterogeneity on the Macroscopic and Molecular-Scale Force Responses of Polymer Network

We investigate the role of crosslink heterogeneity in determining the molecular-scale force responses of polymer networks using two sets of experiments. First, we investigate regularly and randomly-crosslinked networks of poly(n-butyl acrylate) prepared either via coupling of tetrafunctional star polymers (regular networks) or free-radical copolymerization of n-butyl acrylate with a difunctional crosslinker (random networks). We find that the random networks exhibit a significantly earlier onset of strain hardening relative to regular networks with the same modulus, which we attribute to the short strands in the random networks reaching their maximum extension at relatively low strains without being able to relax. We then explicitly probe the relative tension in the short and long strands of a triblock elastomer by incorporating force-responsive mechanophores in the middle of the rubbery midblocks of bidisperse samples. We find that the short strands activate far earlier than the long strands, supporting our hypothesis that the earlier onset of strain hardening is driven by short strands that are pinned to the glassy domains and are unable to relax. Together, these results demonstrate the importance of network topology, dispersity, and pinning of strands to chemical or physical crosslinks in determining both the macroscopic and molecular-scale force responses of polymer networks.