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

Area of Science:

• Polymer Science
• Materials Science
• Crystallization Kinetics

Background:

• Self-poisoning is a known phenomenon in polymer crystallization, typically observed as growth rate minima.
• Previous instances involved unstable folded or segmented chains interfering with stable crystal growth.

Purpose of the Study:

• To identify and characterize a novel self-poisoning mechanism in poly(lactic acid) (PLA).
• To investigate the impact of this self-poisoning on PLA crystallization kinetics and material properties.

Main Methods:

• Experimental observation of poly(lactic acid) crystallization kinetics.
• Development and application of a growth rate equation model.
• Analysis of crystal form kinetics and lamellar thickness variations.

Main Results:

• A new self-poisoning mechanism was identified in PLA, where the less stable α′ crystal form inhibits the growth of the stable α form.
• An unexpected increase in lamellar thickness was observed below the growth rate minimum.
• The model revealed a low fold surface free energy for the α′ form.

Conclusions:

• Self-poisoning in PLA involves the kinetic hindrance of stable α crystal growth by the less stable α′ form.
• This phenomenon, particularly in fast-cooling processes, leads to the formation of the low-modulus α′ phase and increased amorphous content.
• The resulting embrittlement of PLA is attributed to physical aging of the amorphous fraction.