Dynamic covalent polymer networks and network composites: Excellent cross-link density recovery and elevated-temperature creep resistance after multiple recycling steps

Conventional crosslinked polymers and their composites cannot be melt-reprocessed into high-value products because permanent crosslinks prevent melt flow. Examples include spent rubber tires and polyurethane (PU) foam, with major economic and sustainability losses resulting from the inability to upcycle these spent materials or have them become a part of the circular economy. Although dynamic covalent networks (DCPNs) and their composites allow for melt-state reprocessability, an "Achilles heel" has been identified with DCPNs that undergo exchange reactions, also called vitrimers: "For applications in typical vitrimer processing of rigid networks, the creep is highly undesirable in most application where elastomers are typically used." (Denissen et al., Chem. Sci. 2016) For example, significant creep in tires made with dynamic crosslinks would eliminate any possible commercial application. We show that elevated-temperature (T) creep can be strongly suppressed or arrested in both addition-type and step-growth-type DCPNs and their composites, including biobased and synthetic, by use of certain chemistries where the dynamic nature is entirely reversible (dissociative) or both reversible and exchange (associative). Alkoxyamine amine and dialkylamino disulfide reversible dynamic chemistries yield DCPNs and DCPN composites that exhibit excellent recovery of crosslink density after reprocessing at 130-160 degrees C and arrest of creep at 80 degrees C. PU networks their variants, e.g., polyhydroxyurethane and polythiourethane, have combined associative and dissociative dynamic chemistry and can exhibit excellent post-reprocessing crosslink density recovery, elevated-T creep resistance, and potential for monomer recovery. We will explain the underlying causes for the elevated-T creep resistance. These achievements of crosslink density recovery after reprocessing and elevated-T creep resistance are important in moving DCPNs from the research lab to commercial application.