Glassy Alfa-Relaxation Promotes Surprising Homo-Crystal Nucleation in the Low-Molar-Mass Enantiomeric Poly(lactic acid) Blend

Low-molar-mass enantiomeric poly(lactic acid) (PLA) blends commonly harvest stereo-complex crystals that hold alternatingly packed enantiomeric polymers favoring more intermolecular nucleation than intramolecular nucleation. Structural relaxation in polymer glassy states often promotes early-stage crystal nucleation for the high-temperature crystallization. By means of fast-scanning differential scanning calorimetry measurements, we first identified enthalpy relaxation as dominated by beta-relaxation below 35 degrees C and by alfa-relaxation above 35 degrees C in the glassy states of the symmetric enantiomeric PLA blends (20 kDa, L/D = 7:3). We then employed the two-stage Tammann analysis on crystal nucleation during isothermal annealing for 300 s at 30 degrees C in comparison to 40 degrees C, followed with isothermal crystallization at various high temperatures. The results of high-temperature crystallization showed that in comparison to the parallel cases without prior annealing, the annealing with beta-relaxation at 30 degrees C promotes stereo-complex crystal nucleation, probably because only very small stereo-complex crystal nuclei could survive due to their relatively high melting points. In contrast, the annealing with alfa-relaxation at 40 degrees C surprisingly promotes homo-crystal nucleation for the high-temperature crystallization below 120 degrees C, probably because the relatively large-scale but limited molecular cooperation under these circumstances favors intramolecular chain-folding nucleation of homo-crystals more than intermolecular fringed-micelle nucleation of stereo-complex crystals. The homo-crystal nuclei could not survive at a temperature of 120 degrees C and above and hence leave the rest stereo-complex crystal nuclei as dominant again for the high-temperature crystallization. Our observations revealed a strong competition between intermolecular crystal nucleation and intramolecular crystal nucleation at the early stage of polymer crystal nucleation.

Automated summary

The relaxation of structure in poly(lactic acid) blends promotes crystal nucleation, and the annealing at different temperatures has different effects on the nucleation process.