Polymer networks formed through dynamic noncovalent or covalent bonds exhibit robust and tunable mechanical properties (e.g., tough, elastic, self-healable, stimuli-responsive, and reconfigurable). Here, we present our work on linear, flexible polymer chains with periodically-placed and directional dynamic bonds that collectively assemble into secondary supramolecular networks of nanorods and nanofibers under various conditions. We show that when the overall molecular weight (Mn) is below the polymer’s critical entanglement molecular weight (Mc), robust self-assembly of a secondary network of supramolecular nanofibers within the linear polymer melt occurs. However, for Mn &gt; Mc, fiber formation is restricted, despite identical chemical composition of the chains (i.e., dynamic bond concentration). 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 polymer chains initially adopt an amorphous structure (Mn &gt; Mc), the polymer chains align during strain and form strong directional dynamic bonds, which trap the stretched polymer chains in a highly elongated state. The formation of ordered supramolecular structures from building blocks constrained onto the backbone of linear polymers offers an exciting mechanism to enhance and control material properties.

Constrained supramolecular polymerization within linear polymer melts at equilibrium and under strain

ACS Fall 2021

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