Schematic illustration of the polylactic acid spherulite growth rate–crystallization temperature curve and polymorphic self-poisoning mechanism.

Self-poisoning effects influence the nucleation and growth kinetics of synthetic polymers and protein crystals. For synthetic polymers, self-poisoning affects the industrial production of thermoplastics; in the case of protein crystals, it may be linked to neurodegenerative diseases.

Unlike traditional impurity-induced poisoning, where foreign substances inhibit crystal growth, the toxin in self-poisoning is the long-chain molecule itself. These chains adopt metastable folded conformations that attach to the crystal growth surface. Their sufficiently long lifetimes block the more thermodynamically favorable, less-folded growth sites, thereby suppressing crystal growth.

The functional soft materials innovation team at Xi'an Jiaotong University (XJTU) has unveiled a novel self-poisoning mechanism based on the biodegradable polymer polylactic acid (PLA). This mechanism does not arise from misfolded chain conformations, but rather from the polymer's polymorphic crystalline structure.

During PLA crystallization, a less-ordered and thermodynamically less-stable crystal phase forms more rapidly, which in turn hinders the growth of the more ordered, thermodynamically stable phase.

Combined with experimental results and theoretical calculations, this phenomenon — caused by polymorphic competition — has been named ‌polymorphic self-poisoning‌. This mechanism is likely also present in other polyesters and in commodity plastics such as isotactic polypropylene and nylon, particularly under industrial processing conditions with high supercooling.

The study was published online in Physical Review Letters under the title Polymorphic self-poisoning in poly(lactic acid): a new phenomenon in polymer crystallization.