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Thermosets and thermoset composites constitute an extraordinary challenge for recycling and participation in a circular economy because their permanent covalent cross-links prevent spent thermosets from being melt processed into new products. With annual world-wide production in the tens of billions of kilograms, the inability to recycle thermosets into high-value products represents major economic and sustainability losses. While recent research into polymer networks with dynamic covalent cross-links has indicated promise for reprocessability at common melt-state processing temperatures, a crucial shortcoming has been identified: such reprocessable networks and network composites commonly exhibit creep at use conditions due to their dynamic nature, which may prevent their use in applications that require long-term dimensional stability. Here, we use a strategy based on nitroxide-mediated polymerization to synthesize reprocessable networks and network composites containing alkoxyamine dynamic bonds. The resulting networks, including those synthesized from lab-grade polybutadiene and industrial-grade natural rubber/carbon black composites, exhibit full cross-link density recovery and essentially no creep at 80 °C, where alkoxyamine cross-links are nearly static, after multiple molding cycles at 140/160 °C, where alkoxyamine cross-links are dynamic. This capability to “turn on” and “arrest” dynamic chemistry over a relatively narrow temperature window is attributed to the high activation energy (∼120 kJ/mol) and thus strong temperature dependence of the alkoxyamine dissociation reaction. With this key element of high activation energy for the dissociation reaction in systems undergoing dynamic reversion or the dynamic exchange reaction in vitrimers, it is possible to design covalent network materials with acute temperature response allowing for reprocessability with outstanding elevated-temperature creep resistance.

Conventionally cross-linked polymer networks known as thermosets contain permanent cross-links which prevent their recyclability, leading to major sustainability and environmental challenges. To overcome this problem, covalent adaptable networks (CANs) containing dynamic covalent bonds have been studied over the past two decades. Because of their dynamic nature, CANs are capable of undergoing reversible or exchange reactions rendering them reprocessable, offering a sustainable alternative to thermosets. However, unlike thermosets with static cross-links, CANs are considered to be highly susceptible to creep especially at elevated temperature, which limits their utility in many high-value applications. Here, we use the dynamic cross-linker bis(2,2,6,6-tetramethylpiperidin-1-yl) disulfide methacrylate (BiTEMPS methacrylate) in the free radical polymerization of reprocessable poly(hexyl methacrylate) networks with different degrees of cross-linking. Full recovery of cross-link density was achieved after multiple recycling steps. We show that BiTEMPS chemistry is capable of arresting creep at elevated temperature up to 90 °C. Poly(hexyl methacrylate) networks containing 5 mol% BiTEMPS exhibited almost no creep with strain values of 0.07% and 0.38% at 70 °C and 90 °C, respectively, after 13.9 h of continuous, 3 kPa shear stress. This excellent creep resistance is comparable to the creep response of static networks. The temperature-dependent viscosity of a BiTEMPS-cross-linked dissociative network calculated from creep data followed an Arrhenius relationship. The viscous flow activation energy from creep and the stress relaxation activation energy were very similar to the bond dissociation energy of disulfide bonds in BiTEMPS, indicating that the creep and stress relaxation mechanisms are both dominated by the dynamic chemistry in the network. This work indicates that BiTEMPS chemistry offers a simple method to synthesize CANs with excellent elevated-temperature creep resistance while achieving full recovery of cross-link density after recycling.

Reprocessable polymer networks with dynamic covalent bonds exhibit thermoplastic-like properties at elevated processing temperatures while maintaining thermoset responses under service conditions, ...