(639a) Recyclable Dynamic Covalent Polymer Networks: Roles of Viscoelasticity and Rheology in Reprocessing and Robust Sustainable Response

Global production of conventional crosslinked polymer networks and their composites, i.e., thermosets and thermoset composites, was estimated to have exceeded 100 billion pounds of polymer in 2020. Unfortunately, thermosets cannot be melt-reprocessed into high-value products because permanent crosslinks prevent melt flow. Examples include rubber tires and polyurethane (PU) foam, with major economic and sustainability losses as a result. Here, we report on research employing simple one-step or two-step reactions to produce networks with dynamic covalent crosslinks that are robust at use conditions but allow for melt-state reprocessing multiple times at high temperature. We have developed two approaches that allow for melt-state reprocessing of polymer networks and network composites. The first involves networks and their composites synthesized directly by chain-growth reactions from monomer and/or polymer containing carbon-carbon double bonds and strictly dissociative dynamic chemistry, i.e., bond dissociation leading to radical formation at higher temperature and bond reformation at lower temperature. The second involves networks and their composites synthesized by step-growth reactions involving functional groups and involving both dissociative and associative dynamic chemistry; in particular, we have made reprocessable PU networks as well and PU-like networks from polyhydroxyurethane (PHU) and polythiourethane (PTU). With appropriate processing conditions, it is possible to reprocess the networks, i.e., recycle spent networks, with full recovery of crosslink density and associated properties. This presentation will focus on the critical roles of viscoelastic and rheological responses both in proving the recyclability with full crosslink density recovery (rubbery plateau modulus is proportional to crosslink density according to Flory's ideal rubber elasticity theory) and in addressing the differences in temperature-dependent responses exhibited by dynamic covalent networks that are strictly dissociative from those with associative character. An “Achilles’ heel” is associated with dynamic covalent networks, i.e., they are subject to creep at elevated temperature. Such elevated-temperature creep response can disqualify the use of dynamic covalent networks in place of permanently crosslinked networks in many applications. We have addressed this limitation in two ways. First, we have replaced a fraction of dynamic covalent crosslinks with permanent crosslinks. leading to still reprocessable networks but with greatly suppressed creep response. Second, we have used dynamic chemistry with a high activation energy, allowing for reprocessability at high temperature but with the dynamic chemistry essentially fully arrested well above room temperature, e.g., 70-80 °C. In other cases, the presence of dynamic covalent chemistry near room temperature may allow for designed of materials with optimized properties, especially where self-healing behavior may be key.