Polymorphic Self-Poisoning in Poly(Lactic Acid): A New Phenomenon in Polymer Crystallization

Self-poisoning 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), where a less stable crystal form (α^{'}) disturbs growth of the stable form (α). While α requires strict up-down order of the polar chains, α' does not, hence is kinetically favored. 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 α^{'} form. Delayed crystallization due to self-poisoning of α in practical fast-cooling processing not only gives the low-modulus α^{'} form, but also leads to an increased glassy amorphous fraction that results in embrittlement of the biofriendly poly(lactic acid) through physical aging.