Morphology alone does not predict compatibilizer performance of graft copolymers for polyethylene contaminated by polystyrene

Recycled plastics commonly suffer from inferior performance due to sorting error that lead to embrittlement, which limits their commercial adoption. Efforts to overcome performance degradation using macromolecular compatibilizers have been limited by the plethora of design choices and the relatively high cost of the compatibilizer compared to commodity plastics. In this work, we present a modular C-H activation and click chemistry approach to systematically probe the influence of architectural details of graft copolymers on their efficacy to improve the ductility of polyethylene (HDPE) with dispersed polystyrene (PS). Increasing the length of PS grafts at constant graft density from HDPE backbone acted to reduce the size of the dispersed PS phase. However, the impact of the graft copolymer on mechanical properties was strongly tied to the nature of the PS dispersed in the HDPE. Brittle-to-ductile transition of the PS controls the ability of the compatibilizer to improve ductility, irrespective of the improvement in the dispersion of the PS with graft copolymers. The polymer selection exhibits a greater effect than the architectural details of the graft copolymer compatibilizer on the overall mechanical response. For post-consumer recycled plastics, the molar mass of the polymers is unknown, which may lead to poor observed efficacy in the field for compatibilizers. This work suggests that improvements in efficacy with new potential compatibilizers for plastic recyclate should be approached cautiously due to sensitivity to details of the model polymers used to mimic waste plastics.