Polymer crystal-melt interfaces and nucleation in polyethylene

TL;DR

This paper develops a new theoretical approach to understand the crystal-melt interfaces in polyethylene, revealing that nucleation via a metastable rotator phase is favored, which could extend to other polymers.

Contribution

It introduces a self-consistent pressure field model for polymer interfaces and combines it with free energy estimates to analyze nucleation pathways.

Findings

Rotator-phase nucleation is favored over crystal nucleation in polyethylene.

The new model can predict nucleation barriers and rates accurately.

Methods are extendable to other polymer systems.

Abstract

Kinetic barriers cause polymers to crystallize incompletely, into nanoscale lamellae interleaved with amorphous regions. As a result, crystalline polymers are full of crystal-melt interfaces, which dominate their physical properties. The longstanding theoretical challenge to understand these interfaces has new relevance, because of accumulating evidence that polymer crystals often nucleate via a metastable, partially ordered "rotator" phase. To test this idea requires a theory of the bulk and interfacial free energies of the critical nucleus. We present a new approach to the crystal-melt interface, which represents the amorphous region as a grafted brush of loops in a self-consistent pressure field. We combine this theory with estimates of bulk free energy differences, to calculate nucleation barriers and rates via rotator versus crystal nuclei for polyethylene. We find rotator-phase nucleation is indeed favored throughout the temperature range where nucleation is observed. Our methods can be extended to other polymers.