(154ao) Upcycling Virgin and Waste Polyethylene to Reprocessable Dynamic Covalent Networks Via Free-Radical Grafting of Dialkylamino Disulfide Bonds

Plastics enable modern life through their advantageous properties and broad applicability. Regardless of their type or use, plastics are challenging to recycle efficiently. Current methods for recycling spent thermoplastics such as re-extrusion with additives result in property degradation over time and the relegation of these downcycled polymers to low-value applications. Plastics may be permanently cross-linked into thermosets, yet permanent cross-links prevent these plastics from being processed and molded into new shapes at high temperature. An emerging avenue to mitigate these sustainability problems involves enriching waste plastics with dynamic covalent bonds as chemical cross-links. By introducing dynamic covalent cross-links, previously thermoplastic materials exhibit robust mechanical properties characteristic of conventional thermosets yet maintain their reprocessability at high temperatures. Using reactive batch processing, we upcycled virgin and waste low-density and high-density polyethylene into covalent adaptable networks (CANs) via melt-state, free-radical grafting of a cross-linker capable of dynamic dialkylamino disulfide (BiTEMPS) chemistry onto polymer chains. Unlike PE thermosets, our PE CANs are reprocessable and recover their thermomechanical properties after reprocessing. We have further shown that, in the absence of crystallinity, high-temperature creep behavior of the CANs is dominated by the exclusively dissociative reversible dynamic chemistry of the cross-linker. This observation also demonstrates the utility of this dissociative dynamic chemistry of high activation energy at suppressing creep in networks exhibiting different viscoelastic behavior. We will also describe cases of other thermoplastics that are able to be upcycled into CANs using our simple method.