Stereocomplex Formation between Enantiomeric Poly(lactic acid)s. 12. Spherulite Growth of Low-Molecular-Weight Poly(lactic acid)s from the Melt

The spherulite growth of stereocomplex crystallites in the blend from low-molecular-weight poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] and poly(d-lactide) [i.e., poly(d-lactic acid) (PDLA)] from the melt, together with that of the homocrystallites in pure PLLA and PDLA films, was investigated using polarization optical miscroscopy. The spherulite growth of stereocomplex crystallites occurred at a wider temperature range (≤190 °C) compared with that of homocrystallites (≤140 °C). At 140 °C, the spherulite radius growth rate (G) for the stereocomplex crystallites (136.4 μm min-1) was an order of magnitude higher than those for the homocrystallites of PLLA (11.8 μm min-1) and PDLA (15.7 μm min-1), whereas the induction period was shorter for the spherulties of stereocomplex crystallites (0.0 min) than for the spherulties of homocrystallites of PLLA (2.6 min) and PDLA (0.7 min). In addition to these two factors, the higher spherulite density of stereocomplex crystallites compared with those of the homocrystallites of PLLA and PDLA resulted in rapid completion of overall crystallization of stereocomplex. The front factor (G0) and nucleation constant (Kg) for the stereocomplex crystallites in the temperature range of 140−190 °C were estimated to be 3.56 × 1012 μm min-1 and 8.42 × 105 K2, respectively. The G0 value for stereocomplex crystallites was 1 and 2 orders of magnitude higher than those for the homocrystallites of PLLA (9.69 × 1011 μm min-1) and PDLA (8.79 × 1010 μm min-1), whereas the Kg value for stereocomplex crystallites was twice those for the homocrystallites of PLLA (4.95 × 105 K2) and PDLA (4.20 × 105 K2).