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

Abstract

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.

BiTEMPS methacrylate dynamic covalent cross-linker providing rapid reprocessability and extrudability of covalent adaptable networks: high-yield synthesis with strong selectivity for disulfide linkages

“…With the structure and composition of the BTMA-S2 having been verified, we sought to compare the dynamic characteristics of BTMA-HMA CANs made with 5 mol% of BTMA-S2 as compared with 5 mol% of BTMA-S n . BTMA-S n has been the subject of extensive studies by Torkelson and co-workers, 33–39 showing robust dynamic covalent characteristics for reprocessing, stress relaxation, and elevated-temperature creep resistance of CANs containing BTMA. To study the effects of pure disulfide BTMA and oligosulfide-containing BTMA, HMA was cross-linked with either BTMA-S n or BTMA-S2 by free-radical polymerization using AIBN at 70 °C to produce BTMA-S n -HMA CANs and BTMA-S2-HMA CANs (see Fig.…”

“…Dialkylamino disulfide compounds have a dynamic covalent bond that has been of recent scientific interest because of the catalyst-free activation of the disulfide dynamic chemistry, commercially available building blocks, and amenability for free-radical processing of existing commodity polymers. 29–38 Upon thermal triggering, a dialkylamino disulfide compound dissociates into two sulfur radical moieties, thereby initiating the dynamic nature. This chemistry was first reported by Otsuka and coworkers in the context of CAN materials.…”

“…This chemistry was first reported by Otsuka and coworkers in the context of CAN materials. 29 Over the last several years, this chemistry has been utilized for the preparation of CANs based on polyurethanes 30,31 and polymethacrylates 32–35 as well as traditional commodity polymers such as low-density and high-density polyethylene 36 and ethylene/1-octene copolymers. 37,38 Recently, Torkelson and coworkers have shown that this chemistry can be used to produce melt-extrudable CANs made from ethylene/1-octene copolymers cross-linked with bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide, also known as BiTEMPS methacrylate (BTMA), which makes it a potentially attractive choice for industrial applications.…”

“…37 However, the current methods to synthesize dialkylamino-disulfide compounds present significant drawbacks. For instance, dialkylamino-disulfide compounds have been synthesized either by a three-step protocol via protection–deprotection group chemistry in the presence of a strong base, followed by laborious chromatographic purification techniques, as reported by Otsuka and coworkers, 29 or by a simpler one-step approach reported by Torkelson and coworkers 33–39 that is, however, lower yielding and impure. The CANs previously synthesized or prepared with BTMA also typically exhibited a yellow tint, possibly due to the presence of some polysulfide impurities.…”

“…Here, we show an in-depth analysis of BTMA synthesized using our previously reported method 33–39 and identify the presence of oligosulfide linkages (BTMA-S n ). Through purification and optimization, we describe an alternative single-step protocol for the synthesis of BTMA.…”