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 °C and by alfa-relaxation above 35 °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 °C in comparison to 40 °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 °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 °C surprisingly promotes homo-crystal nucleation for the high-temperature crystallization below 120 °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 °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.