Reprocessable covalent adaptable networks with excellent elevated-temperature creep resistance: facilitation by dynamic, dissociative bis(hindered amino) disulfide bonds

Abstract

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.