Long term evolution of morphology, melting, and crystal-crystal transitions facilitated by dynamic bond exchange in ethylene dynamic networks

Dynamic polymer networks with topology conserving exchange reactions (vitrimers) have emerged as a promising platform for sustainable and reprocessable materials. While the role of dynamic bonds on stress relaxation and viscosity is well documented, their role on crystallization kinetics is not well understood. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were studied to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers (100 – 1000 min) and increases with increasing crosslink density, ultimately taking weeks in the densest networks. An increase in melting temperatures (Tm) of 25-30 K is observed with isothermal crystallization over 43200 min. Wide angle x-ray scattering (WAXS) shows an unexpected crystal – crystal transition from the canonical orthorhombic to a proposed monoclinic phase, which is also tracked with optical microscopy (OM). Control experiments on a precise permanent network indicate that dynamic bonds are facilitating long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers.