John Torkelson1
Northwestern University1
Covalent adaptable networks (CANs), sometimes called vitrimers, offer promise for overcoming long-standing recycling and polymer circularity issues associated with conventional, permanently crosslinked thermosets that cannot be recycled for high-value use. It is also vital to demonstrate that properties of reprocessable networks can be optimized to meet the demands for high-performance materials. For example, it is important that the dynamic covalent bonds in CANS are robust at use conditions, including elevated temperatures where creep needs to be minimal, while ensuring reprocessabiltiy at yet higher temperatures that are well below conditions where degradation occurs. We have designed several classes of CANs that show that such performance demands can be met while contributing to polymer circularity and sustainability. Additionally, in many cases, such CANs can be derived from biobased or waste starting materials. We wiill provide examples ranging from non-isocyanate polyurethane networks, including both polyhydroxyurethane networks and non-isocyanate polythiourethane networks, that exhibit non-catalyzed dynamic chemistry of a dual associative and dissociative nature and can be made from a variety of biobased sources. We will also provide examples in which some polyolefins, including thermoplastic polyethylene, can be upcycled into CANs via simple one-step reactive processing with a radical initiator and an appropriate covalent crosslinker with dynamic chemistry that is strictly dissociative in nature, e.g, a dialkylamino disulfide or other disulfide-based crosslinker. For example, waste thermoplastic polyethylene can be can be transformed into crosslinked polyethylene exhibiting robust properties and reprocessabiltiy. Additionally, because of the dissociative nature of the dynamic chemistry, these CANs offer the possibility of being processed by conventional, continous melt-processing methods such as melt extrusion.