Effect of Crosslink Homogeneity on the Mechanochemical Activation of Elastic Polymer Networks

We use force-responsive mechanophores to investigate how network homogeneity impacts the molecular-scale force distributions in elastic polymer networks with different network topologies. Mechanochromic regularly-crosslinked networks are first synthesized by coupling tetrafunctional n-butyl acrylate star polymers with a difunctional naphthopyran crosslinker. Randomly crosslinked networks with comparable crosslink densities are then synthesized by free-radical polymerization of n-butyl acrylate with the same naphthopyran crosslinker, and all samples are characterized via simultaneous tensile tests and optical absorption measurements. We find that networks with randomly and regularly crosslinked topologies exhibit different onsets of mechanochemical activation, even when the bulk moduli of the materials are the same. These results suggest that network uniformity is critical for distributing stress evenly throughout polymer networks, and provides insight into how network structure can be manipulated to prepare materials with well-defined force-responsive properties.