Extrudable and Highly Creep-Resistant Covalent Adaptable Networks Made from Polyethylene and Ethylene/1-Octene Copolymers by Reactive Processing with Aromatic Disulfide Cross-Links

Abstract

Polyolefins like polyethylene (PE) and ethylene-based copolymers are widely used in consumer and industrial applications due to their versatility, the diversity and tunability of their properties, and their theoretical recyclability at elevated temperatures. However, their recycling rates are markedly low, and, though the cross-linking of PE enhances its properties through the creation of a networked architecture, the resulting thermoset known as PEX is rendered completely unrecyclable. Incorporating associative or dissociative dynamic covalent bonds as cross-links into plastics like PE is a promising route both to make use of spent plastics (via “upcycling” them) and to generate recyclable alternatives to unrecyclable thermosets like PEX. Such materials are known as covalent adaptable networks or CANs (also called vitrimers if the cross-links are exclusively associative). Here, we present a method for imbuing ethylene-based polymers with aromatic disulfide dynamic covalent cross-links, resulting in robust, reprocessable CANs. Radical-based reactive processing of PE and ethylene/1-octene-based copolymers with 1 wt % dicumyl peroxide and 5 wt % bis(4-methacryloyloxyphenyl) disulfide (BiPheS methacrylate or BPMA) successfully resulted in CANs which fully recovered their cross-link densities and associated thermomechanical properties after multiple reprocessing cycles. These CANs demonstrate remarkable elevated-temperature creep resistance due to the high-temperature thermal stability and high temperatures required for exchanges of the BiPheS-based cross-links. BiPheS-based cross-links in PE and ethylene-based copolymer CANs also enable their (re)processability via extrusion at elevated temperatures, with property recovery demonstrated with extrusion temperatures as high as 260 °C, thereby indicating the feasibility of extending our approach to industrial scales and processes as well as other rigorous applications.