Clarifying the Relaxation Mechanisms of Dissociative-Type Dynamic Covalent Polymer Networks Containing Some Fractions of Percolated or Non-Percolated, Static Cross-Links

We characterize and consider the mechanisms governing the stress relaxation responses of dissociative-type poly(n-hexyl methacrylate) (PHMA) and poly(n-lauryl methacrylate) (PLMA) dynamic covalent polymer networks (also known as covalent adaptable networks, or CANs), some containing small fractions of percolated or non-percolated, static cross-links. CANs were prepared via free-radical copolymerizations of HMA or LMA with low levels (≤ 5 mol%) of BiTEMPS methacrylate) (BTMA), a dissociative dynamic covalent cross-linker containing exchangeable dialkylamino sulfur-sulfur bonds, and/or bis(2-methacryloyl)oxyethyl disulfide (DSDMA), a cross-linker containing a dialkyl disulfide bond which is static in the absence of catalyst. First, we demonstrate the (re)processability and property recovery of the CANs. We then show that CANs without percolated, static cross-links have activation energies of stress relaxation that are dominated by exchanges of BTMA cross-links rather than by cooperative segmental relaxations of polymer backbone segments, i.e., the alpha relaxation. We also show that, in CANs with percolated, static cross-links, the (sub-)segmental relaxations of side chains, i.e., the beta relaxation, facilitate bulk stress relaxation and govern the associated activation energy. Our judicious choices of PHMA and PLMA as the CAN matrices enabled our conclusions, as PHMA and PLMA exhibit very different activation energies of their respective cooperative segmental mobilities.