ABSTRACT
One method to improve the properties of covalent adaptable networks (CANs) is to reinforce them with a fraction of permanent cross-links without sacrificing their (re)processability. Here, a simple method to synthesize poly(n-hexyl methacrylate) (PHMA) and poly(n-lauryl methacrylate) (PLMA) networks containing static dialkyl disulfide cross-links (utilizing bis(2-methacryloyl)oxyethyl disulfide, or DSDMA, as a permanent cross-linker) and dynamic dialkylamino sulfur-sulfur cross-links (utilizing BiTEMPS methacrylate as a dissociative dynamic covalent cross-linker) is presented. The robustness and (re)processability of the CANs are demonstrated, including the full recovery of cross-link density after recycling. The authors also investigate the effect of static cross-link content on the stress relaxation responses of the CANs with and without percolated, static cross-links. As PHMA and PLMA have very different activation energies of their respective cooperative segmental mobilities, it is shown that the dissociative CANs without percolated, static cross-links have activation energies of stress relaxation that are dominated by the dissociation of BiTEMPS methacrylate cross-links rather than by the cooperative relaxations of backbone segments, i.e., the alpha relaxation. In CANs with percolated, static cross-links, the segmental relaxation of side chains, i.e., the beta relaxation, is critical in allowing for large-scale stress relaxation and governs their activation energies of stress relaxation.
ABSTRACT
Conventional cross-linked polymers cannot be reprocessed because of the presence of permanent covalent cross-links, preventing reuse and recycling. Covalent adaptable networks (CANs) employ dynamic covalent bonds that undergo dynamic reactions under external stimulus, allowing recyclability of these network materials. Hindered urea chemistry is one of the recently discovered dissociative dynamic chemistries. While hindered urea bonds have traditionally been exploited in the synthesis of step-growth type CANs, the use of hindered urea bonds in the synthesis of chain-growth-type dynamic networks has only been narrowly explored. Here, we present a simple, catalyst-free, fast method to synthesize a hindered-urea-based dynamic cross-linker that can undergo a free radical polymerization with vinyl-type monomers or polymers to form reprocessable CANs. Using this cross-linker, we developed dynamic polymethacrylate networks that can be (re)processed at 80 °C. These dynamic covalent networks exhibit full recovery of cross-link density after multiple recycling steps; they are only the second chain-growth network synthesized directly and exclusively from carbon-carbon double bond monomers to demonstrate such recovery. Unlike other dissociative dynamic polymer networks, polymethacrylate networks that contain dissociative dynamic hindered urea bonds do not flow and maintain their network structure even at high temperature (300 °C). Despite its relatively fast reprocessability, the network showed delayed and extremely slow stress relaxation at the processing temperature. This work offers a simple approach to obtain reprocessable addition-type networks based on hindered urea bonds while revealing the limitations of stress relaxation experiments in relationship to the processability of some dynamic polymer networks.
