Role of Dynamic Bonds on Crystallization in Polyethylene Vitrimers

Crystallization of linear polyethylene (PE) has been extensively investigated for decades. Here, we study the effects of dynamic bonds on the crystallization in PE vitrimers by preparing networks with exactly 8, 10, and 12 carbons (C₈-C₁₂) between dynamic boronic ester crosslink points. The C₁₂ networks crystallize immediately upon cooling, while C8 networks only show evidence of crystallization after waiting multiple weeks at 20 °C. The C₁₀ networks show intermediate initial crystallization. Samples were aged at room temperature for 30 days, and in all cases, monotonic increase in melting temperature (T_m) is observed which is attributed to dynamic bonds allowing local rearrangements of network strands in the amorphous regions or at the crystal-amorphous interface. A control sample of linear PE shows instant crystallization and no substantial long term T_m evolution. Initial melting temperatures are analyzed in the Hoffman-Weeks framework, and reasonably predict the long-time T_m of orthorhombic crystal. Upon aging, a transition in the morphology is observed via wide-angle x-ray scattering and the time required for the transition tracks with dynamic bond density. Finally, we demonstrate that step annealing can result in PE networks with T_m approaching 100 °C.