Reprocessable, creep-resistant covalent adaptable networks synthesized using conventional free-radical polymerization conditions with piperidine-based and non-piperidine-based dynamic dialkylamino disulfide chemistry

Abstract

Conventional cross-linked polymers or thermosets cannot be melt-processed because of their permanent cross-links. Covalent adaptable networks (CANs) with covalent cross-links that are dynamic under a stimulus allow for reprocessing. Disulfide chemistry is widely used in CANs. In contrast to sulfur–sulfur bonds in dialkyl disulfides with a dynamic nature activated by internal or external catalysts, sulfur–sulfur bonds in dialkylamino disulfides have a lower bond dissociation energy, allowing for catalyst-free CANs. However, dialkylamino disulfides have only been applied in CANs synthesized under limited conditions. CANs made by free radical polymerization (FRP) of monomer with dialkylamino disulfide cross-linkers were previously synthesized at or near room temperature, highly unconventional FRP conditions, and the dialkylamino disulfides were based on substituted piperidine rings. Here, we show that these limitations are unnecessary to achieve robust CANs. With BiTEMPS methacrylate, a piperidine-based dialkylamino disulfide cross-linker, FRP of n-hexyl methacrylate with azo initiators leads to CANs with the same cross-link density and excellent reprocessability whether synthesized at room temperature or a conventional FRP temperature, 70 °C. We also synthesized a non-piperidine-based dialkylamino disulfide cross-linker, BiTEBES methacrylate. Using this cross-linker and FRP at 70 °C, we prepared catalyst-free poly(n-hexyl methacrylate) CANs that are reprocessable with full recovery of cross-link density. Their stress relaxation has a temperature dependence that is independent of cross-link density and an activation energy within uncertainty equal to the bond dissociation energy of the dialkylamino disulfide bond. The relaxation distribution breadth is a strong function of cross-link density. CANs made with 5 mol% BiTEBES methacrylate have very low cross-link density and exhibit single-exponential-decay stress relaxation. CANs made with 10 mol% BiTEBES methacrylate or 5 mol% BiTEMPS methacrylate and having factors of 5 to 9 higher cross-link density exhibit substantial relaxation distribution breadth. The BiTEBES-based CANs have excellent creep resistance at 70 °C, like that shown previously for BiTEMPS-based CANs, with creep strains of 0.75% or less after ∼14 h of 3.0 kPa stress. This work demonstrates the versatility of dialkylamino disulfide chemistry and its utility in developing catalyst-free CANs using conventional FRP conditions.