Propylene–Ethylene Copolymer Covalent Adaptable Networks Synthesized by Resonance-Stabilized, Radical-Based Reactive Processing with Excellent Elevated-Temperature Creep Resistance

Abstract

Low-crystallinity propylene–ethylene copolymer (PEC) thermoplastics exhibit creep in the melt and semicrystalline states. To enhance creep resistance while maintaining reprocessability, dynamic covalent cross-links are introduced through one-step, radical-based reactive processing to create covalent adaptable networks (CANs). During reactive processing, it is essential to suppress β-scission of propylene repeat units. To promote the formation of resonance-stabilized macroradical intermediates, a methacrylate-based cross-linker bis(4-methacryloyloxyphenyl) disulfide (BPMA) is replaced with a phenylacrylate-based cross-linker bis(4-phenacryloyloxyphenyl) disulfide (BPST) and styrene and divinylbenzene, vinyl aromatic additives, are incorporated. The use of BPST but not BPMA leads to percolated PEC CAN formation. Adding vinyl aromatic additives reduces the disparity in cross-linking capability between BPMA and BPST. The resulting PEC CANs show markedly improved elevated-temperature creep resistance compared to neat PEC. Relative to thermoplastic PEC, the best-performing PEC CAN suppresses >99% of viscous creep at 160 °C (melt state) over 600 s and >98% at 100 °C (semicrystalline state) over 10,000 s. This top-performing PEC CAN is reprocessable through compression molding and twin-screw extrusion, achieving full recovery of cross-link density and tensile properties. These results showcase a promising one-step strategy for producing recyclable PEC CANs with enhanced creep resistance in melt and semicrystalline states, addressing critical limitations of low-crystallinity polyolefins.