The mechanisms for the transition from the isotropic melt to the three-dimensional ordered lamellae of crystalline polymers remains highly debated. On this area of interest, polyethylenes have served as models due to the simplicity of the molecule and availability of narrow and precise chain lengths in the widest molecular weight range. Polyethylene-like molecules with equidistant placement of a co-unit along the backbone are unique models to study the effect of intermolecular staggering of co-units and methylene runs on the crystallization kinetics. Recent studies of polymorphism, melting behavior and crystallization kinetics of polyethylenes with halogens (as pendant), acetals (in-chain), and other moieties under rapid and isothermal crystallization from the melt have shown discrete minima in the temperature coefficient of the crystallization kinetics. The crystallization rate minima correlate with transitions between two polymorphic structures that differ in packing symmetry. The minima or retardation of crystallization kinetics in systems with halogen moieties is attributed to self-poisoning at the growth front, while for polyacetals differences in nucleation and growth between the two polymorphs at the transition temperature also contribute to the observed deep minima. These precision systems provide excellent models for testing events that take place at the crystal growth front. Because these polymers can assemble in two crystalline forms with drastic differences in chain conformation and nucleation barriers, approaching the transition between both forms, large effects of the kinetically favored form retarding the growth of the more stable one can be easily detected. Besides the impact on present and future polymer crystallization theories, control over different layered crystalline structures demonstrates the ability of these novel systems to generate new polyethylene-based materials based on nanostructures at the lamellar and sub-lamellar level not feasible in classical branched polyethylenes.

Precision polyethylenes as models to probe events taking place during polymer crystallization and at the lamellae growth front

ACS Spring Meeting 2021

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