(670i) Using Periodic Dynamic Polymers to Form Ordered Supramolecular Structures

In this talk, we highlight our recent work reporting the formation of ordered supramolecular structures using periodic dynamic polymers. The key message is that polymers with controlled bond periodicity along their backbone (i.e., periodic dynamic polymers), in contrast to polymers with randomly placed bonds, can display robust supramolecular assembly, analogous to that observed in small molecule or natural systems. Broadly, the repeated association and disassociation of reversible bonds in dynamic polymers results in temporarily cross-linked networks with a wide array of functional properties including high toughness, stimuli-responsiveness, self-healability, 3D printing, and reprocessability. Here, we 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 spontaneously. 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 expand upon the key aspects of polymer molecular design learned through this model system to design a novel shape memory polymer with record-high recovery stress (12.8 MPa) and energy density (18.9 MPa) 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. We explore alternative molecular architectures to better understand the nature of strain-induced supramolecular structure formation and generate more complex shape actuation. These discoveries show that periodic dynamic polymers represent an emerging class of materials in which well-defined molecular connectivity leads to enhanced material properties.