Recyclable polymer networks containing hydroxyurethane dynamic cross-links: Tuning morphology, cross-link density, and associated properties with chain extenders

Conventional polymer networks are incapable of being reprocessed or recycled in the melt state, leading to sustainability and economic losses associated with spent polymer networks. With the incorporation of hydroxyurethane dynamic cross-links resulting from reaction of tricyclocarbonate and telechelic amine-terminated polymer, polybutadiene (PB) networks and poly(tetramethylene oxide) (PTMO) networks are reprocessable with full property recovery after multiple molding steps. Two types of small-molecule chain extender are incorporated into these networks to tune their morphologies, cross-link densities and associated properties. Addition of m-xylylenediamine (XYL) chain extender to PTMO networks increases the hard-segment content and leads to nanophase separation, which is absent when XYL is not present, and the elimination of cold crystallization behavior that is observed in PTMO networks without XYL. Addition of XYL also increases the cross-link density and rubbery plateau modulus of both PB and PTMO networks, and the resulting materials remain fully reprocessable. In contrast, addition of divinylbenzene dicyclocarbonate (DVBDC) chain extender enhances the cold crystallization behavior of molded PTMO-based networks and their crystallinity. Addition of DVBDC reduces the cross-link density and rubbery plateau modulus of both PB and PTMO networks, and the resulting networks exhibit selective property losses after reprocessing. This study demonstrates the simple and effective use of chain extenders in achieving tunability of dynamic network properties to satisfy requirements in different potential applications. It also provides a better understanding of structure-property relationships in dynamic covalent networks from both chemical and physical standpoints.