Poisoning by Purity: What Stops Stereocomplex Crystallization in Polylactide Racemate?

Area of Science:

• Polymer Science
• Materials Science
• Crystallization Science

Background:

• Stereocomplex (SC) crystals enhance poly(lactic acid) (PLA) heat resistance and mechanical properties.
• SC crystallization of poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) often fails at practical cooling rates, leading to homochiral crystal (HC) formation.
• Understanding SC crystallization limitations is crucial for optimizing PLA material performance.

Purpose of the Study:

• To investigate the reasons behind the cessation of SC crystallization in PLLA/PDLA racemates.
• To differentiate between nucleation and growth limitations in SC crystallization.
• To explore factors influencing SC crystallization, such as molecular weight and initial melt conditions.

Main Methods:

• Differential scanning calorimetry (DSC) to analyze thermal transitions and crystallization behavior.
• Optical microscopy to visualize crystal morphology and growth.
• X-ray scattering to study crystalline structures and phase formation.
• Experiments with varying molecular weights and pre-melt conditions.

Main Results:

• SC crystallization growth is primarily halted by the accumulation of rejected, nearly pure enantiomers (PLLA or PDLA) ahead of the SC growth front.
• The presence of residual SC seeds or starting from a partially molten state improves SC crystallization.
• Homochiral crystal (HC) growth can be inhibited by the surrounding polymer blend, explaining the resumption of SC growth at lower temperatures.
• SC-promoting nucleating agents do not resolve the issue of SC crystallization cessation, indicating a growth-related problem.

Conclusions:

• The primary impediment to SC crystallization in PLLA/PDLA is not nucleation but rather the growth kinetics limited by enantiomeric rejection.
• Local compositional fluctuations in the melt lead to the rejection of excess enantiomers, hindering SC crystal growth.
• Optimizing PLA properties through SC crystallization requires addressing growth limitations rather than solely focusing on nucleation strategies.