One-Step Radical-Based Compatibilization of PE/PP Blends via Dynamic Covalent Crosslinking: Creating Interfacial Reversible Crosslinks 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 challenging, and the poorly bonded interfaces in PE/PP mixtures severely limit their 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 crosslinking in PE CANs and can be directly applied to PE/PP immiscible blends, enhancing tensile strain-at-break and toughness by forming dynamic covalent crosslinks, 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 crosslinkers that have inherent resonance stabilization or are stabilized by 2-vinylnaphthalene (VN) as an additive. All tested crosslinkers 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 crosslinker and VN as an additive. BTMA-based PE/PP CANs showed more than a 3-fold increase in the strain-at-break and a roughly 6-fold increase in toughness compared to corresponding PE/PP blends; these improvements were retained after reprocessing. In the nonreactive compatibilization approach, we added 10 wt % BTMA-based PE/PP CANs to PE/PP blends, resulting in an up to 6-fold increase in the strain-at-break and a 9-fold increase in toughness. These results demonstrate that the formation of interfacial dynamic covalent crosslinks enables compatibilization of immiscible PE/PP blends by two methods: (1) radical-based reactive processing of PE/PP blends with suitable dynamic covalent crosslinks and vinyl additives, producing compatibilized CANs, and (2) addition of low levels of PE/PP CANs as compatibilizers to PE/PP blends.