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

Abstract

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 degrees C) compared with that of homocrystallites (</=140 degrees C). At 140 degrees C, the spherulite radius growth rate (G) for the stereocomplex crystallites (136.4 microm min(-1)) was an order of magnitude higher than those for the homocrystallites of PLLA (11.8 microm min(-1)) and PDLA (15.7 microm 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 (G(0)) and nucleation constant (K(g)) for the stereocomplex crystallites in the temperature range of 140-190 degrees C were estimated to be 3.56 x 10(12) microm min(-1) and 8.42 x 10(5) K(2), respectively. The G(0) value for stereocomplex crystallites was 1 and 2 orders of magnitude higher than those for the homocrystallites of PLLA (9.69 x 10(11) microm min(-1)) and PDLA (8.79 x 10(10) microm min(-1)), whereas the K(g) value for stereocomplex crystallites was twice those for the homocrystallites of PLLA (4.95 x 10(5) K(2)) and PDLA (4.20 x 10(5) K(2)).