(420f) Reprocessable Polymer Networks Based on Dynamic Covalent Bonds: Achievement of Full Crosslink Density Recovery after Recycling and Creep Suppression at High Temperature

Unlike thermoplastics, conventional thermosets and thermoset composites cannot be effectively recycled for high-value applications because the permanent nature of the covalent crosslinks prevents melt-state reprocessing. As a result, there are major sustainability and economic losses associated with conventional thermosets reaching their end of life. We have developed and/or applied several dynamic chemistries as replacements for permanent crosslinks. Two of these dynamic chemistries are aimed at developing replacements for non-reprocessable polyurethane networks. Urethane crosslinks have insufficient dynamic character to allow for reprocessing. Annual world-wide production of polyurethane is estimated to reach 58 billion pounds in 2021, with the majority being networks; thus, there are major sustainability and economic losses associated with the inability to reprocess polyurethane networks for high-value uses. We have shown that hydroxyurethane and thiourethane crosslinks are effective for reprocessing with full recovery of crosslinks after multiple reprocessing steps. These chemistries exhibit a dual nature of reversible and exchange reactions. With these dynamic chemistries involving functional groups, we have also determined that incorporation of nanofiller into the network can lead to deleterious effects on reprocessability if the nanofiller surfaces contain functional groups that can participate in the dynamic reactions associated with the crosslinks. Removing or replacing those surface functional groups prior to incorporation into the network can suppress or eliminate those deleterious effects. We have additionally shown that biobased monomers can be successfully employed in producing robust polyhydroxyurethane networks, adding yet another sustainability advantage to this approach. Finally, there is a problem with reprocessable polymer networks based on dynamic covalent bonds that has caused concern with regard to their viability for many applications: they exhibit creep at elevated temperatures. We will describe two approaches that we have used to address this issue, one leading to suppression of high-temperature creep and a second leading to virtual elimination of the high-temperature creep effect over a significant range of temperatures.