Restricting Chain Mobility in Semicrystalline Polyethylene and Polypropylene via Dynamic Covalent Cross-links: Major Effects on Elevated-Temperature Creep, Heat Distortion Temperature, and Wear Resistance

We demonstrate that dynamic covalent cross-links restrict chain mobility and suppress molecular relaxation in semicrystalline polyethylene (PE) and isostatic polypropylene (PP) covalent adaptable networks (CANs) produced by one-step, radical-based reactive processing. This is evidenced by increased resistance of the amorphous domains to solvent swelling and diminished temperature-dependent interfacial relaxation. These molecular-level constraints have major effects on elevated-temperature creep resistance, heat distortion temperature (HDT), and wear performance. Creep behavior was evaluated under relatively high loads across a range of temperatures. At lower temperatures, CAN formation reduces creep resistance due to decreased crystallinity. In contrast, substantial improvements are observed at temperatures approaching but below the melt transition. At 110 °C for PE and 140 °C for PP, creep strain in CANs is reduced by 88% and 61%, respectively, relative to the precursor polymers. Furthermore, dynamic covalent cross-links increase the service temperature of CANs, with HDT improvements of 16 °C in PE CANs and 11 °C in PP CANs. In wear testing, PE CANs exhibit markedly improved resistance, with wear volume reduced by over one-third compared to neat PE. In contrast, PP CANs show decreased wear resistance, with increased wear volume attributed to 2 low-molecular-weight polymer generated via β-scission during radical-based reactive processing. Overall, we show that dynamic covalent cross-linking can sustainably enhance key engineering properties of semicrystalline polyolefins under targeted service conditions.