Tuning High-Density Polyethylene Microstructure and Properties from Known Distributions of Dynamic Bonds.

Over 50% of plastic waste comes from a single class of polymers called polyolefins. Most recycling strategies fail to preserve the broad molecular mass distribution of these polyolefins, from which they derive their unique combination of processability and mechanical strength. Here, we show that incorporation of urethane-based dynamic bonds into high-density polyethylene (HDPE) can circumvent this issue, by strengthening the amorphous phase through interchain supramolecular interactions, as opposed to traditional entanglements from high molecular mass chains. We show that many key properties of these dynamic HDPE polymers, including percent crystallinity, melting temperature, lamellae and amorphous thicknesses, and bond association, are determined by the distribution of bond-to-bond spacings along the chain backbone and are predicted using polymer physics theories. Moreover, we find that a mixed backbone HDPE dynamic polymer exhibits mechanical properties that exceed HDPE and approach the strain-hardening behavior seen in ultrahigh molecular weight polyethylene along with a unique display of long-range supramolecular order that persists in the melt state. This work illuminates key principles governing how the placement of dynamic bonds influences bulk material properties and provides a framework for toughening semicrystalline polymers and designing chemical recycling processes based on controlling bond-spacing distributions.