Creep-resistant, extrudable, and recyclable polyolefin covalent adaptable networks incorporating azine cross-links via reactive processing

Abstract

Polyolefins, encompassing polymers such as polyethylene (PE) and ethylene-based copolymers, are the world's most widely produced class of plastics due to their versatility and facile property tunability. Although these materials are recyclable, in practice their recycling rates are low due to a range of issues, including thermomechanical degradation. Furthermore, converting polyolefins into thermosets, such as PEX (made by cross-linking PE to form a permanent network), may yield property improvements but it comes at the cost of reprocessability and, thus, recyclability. The development of covalent adaptable networks (CANs), which utilize dynamic rather than static cross-links, has provided a promising avenue for improving the sustainability of polyolefin-based materials. By incorporating dynamic covalent bonds (associative or dissociative in nature) into polyolefin precursor materials, the resulting polyolefin CANs may exhibit improved properties and full recovery of these properties upon recycling. Here, we used radical-based reactive processing to introduce associative azine dynamic cross-links into a variety of ethylene-based polymers, yielding robust, reprocessable CANs. By reacting various PEs and ethylene/1-octene copolymers with 0.6 wt% dicumyl peroxide and 5 wt% bis(4-methacryloyloxybenzylidene)hydrazine (BiBeN methacrylate, BBMA), we obtained CANs that exhibited full recovery of cross-link density and thermomechanical properties after multiple cycles of remolding, as well as substantial, elevated-temperature creep resistance and amenability to industrially relevant processing methods (e.g., injection molding and extrusion). Finally, compared with our previous studies using aromatic-disulfide-based and dialkylamine-disulfide-based cross-linkers, our BBMA-based polyolefin CANs exhibited improved cross-link density and elevated-temperature creep suppression, required less radical initiator, and were reprocessable at lower temperature.