Gradient Structure of Crystalline Morphology in Injection-Molded Polylactide Parts Tuned by Oscillation Shear Flow and Its Influence on Thermomechanical Performance

The inhomogeneity of the cooling rate and shear rate during polymer processing such as injection molding usually leads to the harvest of a layered structure, which is often difficult to tune. By introducing oscillation shear flow during the packing stage of an injection molding cycle, special injection-molded polylactide (PLA) parts with different thicknesses and crystallinity of skin layers were fabricated via controlling shear durations and oscillation frequencies, as revealed by microbeam wide-angle X-ray diffraction results. For the part of the 2000 mu m thick layer with 50% crystallinity, the heat distortion temperature and Vicat Softening temperature reach 96:6 and 159.3 degrees C, respectively. Moreover, the Young's modulus rises remarkably with the increase of thickness and crystallinity. Significantly, the maximal shear rate appeared 850 mu m from the surface, about 1068 s(-1) (up to 103 s(-1)), which plays a crucial role-for the formation of oriented crystalline morphology.