Supramolecular self-assembly of periodic dynamic polymers

Polymer networks formed through dynamic noncovalent or covalent bonds exhibit robust and tunable mechanical properties (e.g., tough, self-healable, and stimuli-responsive). In nature, these networks are often hierarchically-ordered to perform precise functions and assemble via cooperative interactions of many weak bonds as opposed to independent association of a few strong bonds. Here, we use these principles to design linear, flexible polymer chains with periodically-placed and directional dynamic bonds that collectively assemble into supramolecular nanofibers. We show that when the overall molecular weight (M_n) is below the polymer’s critical entanglement molecular weight (M_c), robust self-assembly of supramolecular nanofibers occurs. The formation of nanofibers increases the bulk film modulus by over an order of magnitude and delays the onset of terminal flow by more than 100°C. We also apply these molecular design principles to design a novel shape memory polymer with record-high recovery stress and energy density based on the formation of strain-induced supramolecular nanostructures. While initially polymer chains adopt an amorphous structure (M_n > M_c), during strain the polymer chains align and form strong directional dynamic bonds, which trap the stretched polymer chains in a highly elongated state. These examples highlight the potential to use ordered supramolecular networks formed from polymers with defined backbone periodicity to enhance and control material properties.