Extrudable and Highly\nCreep-Resistant Covalent Adaptable\nNetworks Made from Polyethylene and Ethylene/1-Octene Copolymers by\nReactive Processing with Aromatic Disulfide Cross-Links

Polyolefins like polyethylene (PE)\nand ethylene-based\ncopolymers\nare widely used in consumer and industrial applications due to their\nversatility, the diversity and tunability of their properties, and\ntheir theoretical recyclability at elevated temperatures. However,\ntheir recycling rates are markedly low, and, though the cross-linking\nof PE enhances its properties through the creation of a networked\narchitecture, the resulting thermoset known as PEX is rendered completely\nunrecyclable. Incorporating associative or dissociative dynamic covalent\nbonds as cross-links into plastics like PE is a promising route both\nto make use of spent plastics (via “upcycling” them)\nand to generate recyclable alternatives to unrecyclable thermosets\nlike PEX. Such materials are known as covalent adaptable networks\nor CANs (also called vitrimers if the cross-links are exclusively\nassociative). Here, we present a method for imbuing ethylene-based\npolymers with aromatic disulfide dynamic covalent cross-links, resulting\nin robust, reprocessable CANs. Radical-based reactive processing of\nPE and ethylene/1-octene-based copolymers with 1 wt % dicumyl peroxide\nand 5 wt % bis(4-methacryloyloxyphenyl) disulfide (BiPheS methacrylate\nor BPMA) successfully resulted in CANs which fully recovered their\ncross-link densities and associated thermomechanical properties after\nmultiple reprocessing cycles. These CANs demonstrate remarkable elevated-temperature\ncreep resistance due to the high-temperature thermal stability and\nhigh temperatures required for exchanges of the BiPheS-based cross-links.\nBiPheS-based cross-links in PE and ethylene-based copolymer CANs also\nenable their (re)processability via extrusion at elevated temperatures,\nwith property recovery demonstrated with extrusion temperatures as\nhigh as 260 °C, thereby indicating the feasibility of extending\nour approach to industrial scales and processes as well as other rigorous\napplications.