(31a) Controlling Stress Relaxation, Deformation, and Crystallinity in Semi-Crystalline Networks Using Dynamic Covalent Bonds

Covalent adaptable networks (CANS) are a unique paradigm in polymeric materials whereby exchangeable bonds are leveraged to enable thermoplastic-like flow and reprocessability into thermoset networks. The choice of dynamic bond chemistry and polymer matrix can be combined to imbue CANs systems with a nearly-limitless combination of mechanical and rheological properties to enable myriad applications ranging from dynamic biomaterial platforms to shape memory devices. Despite the promise of such systems, there remains a lack of fundamental understanding of how the coupling of molecular-scale bond exchange kinetics, segmental relaxations, and network dynamics govern the material properties of any given system. One system of particular interest are semi-crystalline CANs, which could enable the recyclability of high-performance materials. In this work, we introduce a method to incorporate exchangeable thioester bonds into semi-crystalline thiol-ene networks to form tough and strong thermoplastics that can be dynamically reconfigured in the presence of catalytic nucleophiles via thiol-thioester exchange. Because thiol-thioester exchange enables robust bond exchange at ambient temperatures with kinetics that can be tuned by the choice of nucleophilic catalyst, these attributes combined with the modularity of thiol-ene systems make this chemistry a model system to explore fundamental questions about the interplay of bond exchange and crystallinity on emergent network properties. Using a combination of oscillatory rheology, differential scanning calorimetry, and x-ray scattering, we fully characterize the rheological properties and morphology of these materials as a function of network chemistry and catalyst strength. We show that the kinetics of thiol-thioester exchange dictate both the rheological response of these materials as well as the crystallization of these networks. Furthermore, we use these fundamental insights to construct shape-memory materials by leveraging directed crystallization and melting to drive controlled deformation. Finally, we show that the incorporation of photolatent catalysts affords spatiotemporal control over exchange and crystallization dynamics and enables hands-free manipulation and actuation.