New Insights into the Chain Dynamics and Microstructure of Highly Crosslinked Polymer Networks as a Function of Network Heterogeneity

Key physicochemical phenomena in polymer networks are critically impacted by the spatial distribution of crosslinks, i.e., network heterogeneity. Despite this fact, the chain dynamics and microstructure of heterogeneous networks, particularly bulk thermosets, have not been well characterized. To this end, we present a detailed investigation of novel photopolymerized thiol-ene networks. High glass transition temperatures and continuously tunable network heterogeneity were achieved by varying the stoichiometry between aromatic thiol and acrylate monomers. The chain dynamics and microstructure of these materials were characterized using a combination of dynamic mechanical analysis, solid-state nuclear magnetic resonance spectroscopy, and small angle x-ray scattering. We found that heterogeneous networks exhibited enhanced mobility deep in the glassy state, in contrast to their homogeneous counterparts. In addition, the heterogeneous and homogeneous networks were distinguished by their fractal structures. These new insights may help guide the future design of new crosslinked polymers with carefully controlled network heterogeneity.