One-Step Radical-Based Compatibilization of PE/PP Blends via Dynamic Covalent Cross-Linking: Creating Interfacial Reversible Cross-Links and Utilizing Their Block- or Graft-Like Structures as Compatibilizers

Polyethylene (PE) and isotactic polypropylene (PP) constitute the majority of global polymer production and are major components in the plastic waste stream. Their similar physical properties make effective separation for recycling challenging, and the poorly bonded interfaces in PE/PP mixtures severely limit mechanical performance. A one-step radical-based reactive process was recently developed to create covalent adaptable networks (CANs) from thermoplastic PE; we also modified it to produce PP CANs by suppressing β-scission via resonance stabilization. Here, we show that this modified, solventless process produces dynamic cross-linking in PE CANs and can be directly applied to PE/PP immiscible blends, enhancing tensile strain-at-break and toughness by forming dynamic covalent cross-links, including at blend interfaces. We also show that compatibilized PE/PP CANs, with interfacial PE/PP dynamic linkages, can act as PE/PP blend compatibilizers. We tested several dynamic disulfide cross-linkers that have inherent resonance stabilization or are stabilized by 2-vinylnaphthalene (VN) as an additive. All tested cross-linkers with resonance stabilization form PE/PP CANs; the best results were achieved with bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide (BTMA) as a dynamic covalent cross-linker and VN as an additive. BTMA-based PE/PP CANs showed more than a threefold increase in strain-at-break and roughly a sixfold increase in toughness compared to corresponding PE/PP blends; these improvements were retained after reprocessing. In the non-reactive compatibilization approach, we added 10 wt% BTMA-based PE/PP CANs to PE/PP blends, resulting in up to a sixfold increase in strain-at-break and a ninefold increase in toughness. These results demonstrate that formation of interfacial dynamic covalent cross-links enables compatibilization of immiscible PE/PP blends by two methods: (1) radical-based reactive processing of PE/PP blends with suitable dynamic covalent cross-links and vinyl additives, producing compatibilized CANs, and (2) addition of low levels of PE/PP CANs as compatibilizers to PE/PP blends.