Structure and Dynamics of High-Density Polyethylene Cross-linked by an Azidotriazine via Reactive Extrusion

Abstract

Azidotriazine incorporated into high-density polyethylene (HDPE) by reactive extrusion gives rise to covalent adaptable networks (CANs) while simultaneously enhancing the PE chain mobility at the crystal–amorphous interface, altering the semicrystalline morphology and the molecular dynamics. Increasing azidotriazine T-N3 loading lowers the equilibrium melting temperature and reduces the degree of crystallinity, indicating that the dynamic cross-links interfere with polyethylene chain packing during crystal growth. Isothermally crystallized HDPE revealed the emergence of two distinct lamellar populations, reflecting modified crystallization kinetics. Small- and wide-angle X-ray scattering revealed that the triazine-based grafts, dynamic cross-links, and ungrafted byproducts are preferentially located in the restricted amorphous phase adjacent to the crystalline lamellae. There, they act as plasticizers by reducing the crystalline lamellar thickness and the effective fold surface free energy while promoting lamellar thickening at lower temperatures as compared to bulk HDPE. Dielectric spectroscopy further revealed two segmental processes corresponding to two liquid-to-glass temperatures with enhanced segmental mobility with increasing T-N3 loading. These results demonstrate that covalent adaptable networks of semicrystalline polymers prepared via reactive extrusion can exhibit unanticipated structural and dynamic features that reflect the chemical composition and the topology of the final products. Furthermore, results from present azidotriazine-based CANs are compared to earlier dioxaborolane-based vitrimers as well as to permanently cross-linked PE with a homogeneous distribution of cross-links.