Precursors in stereo-complex crystals of poly(L-lactic acid)/poly(D-lactic acid) blends under shear flow

To improve the mechanical and the thermal performance of poly(lactic acid) materials, this work focuses on the formation of stereo-complex crystals by blending poly(l-lactic acid) (PLLA) with poly(d-lactic acid) (PDLA). The resulting structure was analyzed using time-resolved in situ X-ray scattering, optical microscopy, differential scanning calorimetry and viscoelastic measurements. The objective of this study is to investigate the effect of shear flow imposed prior to crystallization on higher-order structure formation and acceleration of stereo-complex crystal growth of PLLA and PDLA blends using a wide spatial scale analysis and viscoelastic measurements. Density fluctuations of 100 nm scale were observed prior to nucleation by in situ simultaneous wide- and small-angle X-ray scattering measurements. These density fluctuations grew with time and the intensity increased with increasing shear rate. Furthermore, the results revealed that the PLLA and PDLA chains were only partially interpenetrated; consequently, stereo-complex crystals could grow only in the mixed PLLA/PDLA phase. The correlation length of density fluctuation prior to nucleation was strongly dependent on the mixed phases.