Polymorphic self-poisoning in poly(lactic acid): a new phenomenon in polymer crystallization

TL;DR

This paper reports a new self-poisoning phenomenon in poly(lactic acid) where a less stable crystal form interferes with the growth of the stable form, affecting crystallization behavior and mechanical properties.

Contribution

It introduces a novel type of self-poisoning in PLA involving alpha' form disrupting alpha form growth, supported by a growth rate model matching experimental data.

Findings

Alpha' form disturbs alpha form growth in PLA.

Lamellar thickness increases below growth rate minimum temperature.

Low fold surface free energy of alpha' form explains the phenomenon.

Abstract

Self-poisoning (SP) is ubiquitous in polymer crystallization, but has so far manifested itself visibly only as minima in growth rate vs. temperature in either monodisperse systems where e.g. unstable folded chains obstruct crystallization of stable extended chains, or in periodically segmented chains where unstable stems with n-1 segments disturb deposition of stable stems with n segments. Here we report a new type of self-poisoning found in poly(lactic acid) (PLA), where a less stable crystal form (alpha') disturbs growth of the stable form (alpha). While alpha requires strict up-down order of the polar chains, alpha' does not, hence is kinetically favoured. Unexpectedly, below the temperature of the growth rate minimum the lamellar thickness increases rather than drops, as in all other reported cases of polymer crystallization with decreasing temperature. A growth rate equation model is developed, giving good match with experiments, but revealing an unexpectedly low fold surface free energy of alpha' form. Due to SP of alpha, most practical fast-cooling processing gives the low-modulus alpha'-form grown close to Tg, explaining generally poor mechanical properties of the bio-friendly PLA.