(272j) Structure-Property Relationships of Dynamic Covalent Polymer Networks Made By Reactive Processing of Ethylene-Containing Polymers with Comparisons to Thermosets

Plastics such as polyethylene (PE) and other ethylene-containing polymers enable modern life through their advantageous properties and broad applicability. However, current methods for recycling spent plastics such as re-extrusion with additives result in property degradation over time. Some plastics may be permanently cross-linked into thermosets to enhance their properties at a further expense of their recyclability. Here, we developed a melt-state reactive process to upcycle ethylene-containing polymers such as PE (both virgin and waste materials) and ethylene/1-octene copolymers (EOCs) into reprocessable polymer networks containing dynamic covalent cross-links based on bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide (also known as BiTEMPS methacrylate). We utilized radical initiator species to graft BiTEMPS methacrylate cross-links between molten polymer chains, resulting in cross-linked polymer networks with robust properties at use conditions that also boast reprocessability at elevated-temperature processing conditions. We were able to investigate pertinent structure-property relationships between precursor polymers and the dynamic networks made from them by varying properties of the precursor PE and EOCs used to make dynamic networks such as melt flow index, 1-octene content, and crystallinity (proxy for hard-block content in EOCs). We also studied these relationships in permanently cross-linked thermosets made from PE and EOCs via our reactive process without BiTEMPS methacrylate. Our dynamic networks exhibited full cross-link density and thermomechanical property recovery after reprocessing, whereas their thermoset counterparts could not be reprocessed. Importantly, we were able to extrude an EOC dynamic network as a proof-of-principle experiment. Our very recent work has demonstrated that our reactive processing approach can be extended to polymers containing ethylene units on side chains, e.g., poly(n-hexyl methacrylate).