Bullvalene "Ball Joint" Fluxionality Modulates Mechanics and Dynamic Fragility in Polymer Glass.

Dynamic fragility of polymer glasses describes how steeply a material's viscosity changes as it passes through its glass transition; stronger glasses have a less steep transition than fragile glasses. Fragility is an important parameter in determining practical service temperature windows, where mechanical properties remain predictable. At the molecular level, polymer chain flexibility is usually a key driver of fragility, ultimately dictating a balance of local molecular relaxation versus longer range segmental motion. Herein, we present molecular fluxionality as a new motif to control fragility by exploiting bullvalene Hardy-Cope rearrangements within glassy poly-(methyl methacrylate) networks. Thermosets cross-linked with bullvalene consistently show lower fragility (i.e., stronger glass) relative to static adamantane-derived control networks. Such strengthening through sigmatropic rearrangements within a hydrocarbon cage presents new insight into the impact of local molecular motion on glass formation and the applications of such materials.