Covalent Adaptable Networks Made by Reactive Processing of Highly Entangled Polymer: Synthesis-Structure-Thermomechanical Property-Reprocessing Relationship in Covalent Adaptable Networks

Reactive processing provides a simple approach for grafting dynamic covalent cross-linkers onto linear or branched polymers, resulting in covalent adaptable networks (CANs). We synthesized poly(n-hexyl methacrylate) (PHMA) CANs from neat, entangled PHMA using radical-based reactive processing to graft the dynamic covalent cross-linker called BiTEMPS methacrylate (BTMA) between PHMA side chains. By tuning the BTMA loading, we achieved a range of cross-link densities and characterized how stress relaxation, elevated-temperature creep, and reprocessability are affected by cross-link density. The grafting of dynamic covalent cross-links to the PHMA side chains allowed for a novel comparison of our CANs to the reversible, entangled polymers described by sticky reptation theory, in which long-time relaxation occurs by the unraveling of backbone entanglements, a process enabled by the dissociation and subsequent exchange of side-chain "stickers." We observed two stress relaxation regimes in our CANs that required their fitting to a linear combination of two stretched exponential decay functions to extract pertinent stress relaxation parameters. The apparent activation energies of stress relaxation and creep viscosity are the same within experimental uncertainty for these PHMA CANs, verifying the shared mechanisms governing the temperature dependence of their viscoelastic responses, independent of CAN cross-link density. Notably, the activation energies for these PHMA CANs made by linking BTMA between side chains are ∼50–60% of those reported previously for PHMA CANs made with BTMA cross-links between the chain backbones. These outcomes demonstrate the importance of synthesis-structure-property-reprocessing relationships in CANs that may be made by various methods or by varying the position or incorporation of dynamic cross-linkers in the CAN structure.