Effect of melting survived nanocrystal on flow-induced crystallization of poly(L-lactic acid)

Usually, poly(l-lactic acid) (PLLA) needs to be kept for a sufficiently long time (e.g., several hours) at crystallization temperature to achieve high crystallinity without adding nucleating agents. It is still challenging to develop the economically viable approach for enhancing the crystallinity of PLLA both in academia and industry. In this study, the isothermal crystallization time to reach the crystallinity of about 35% can be shortened to a few mins, by simply extruding the near melting point (NMP) melt of PLLA through a capillary die at 175 degrees C with a shear stress of 45 kPa. In addition, the crystalline morphology of extrudates was developed from the cylindrites to the spherulites when the annealing temperature (T-a) of NMP melt increased from 175 to 184 degrees C. By comparison, only a few of spherulites were exhibited on the extrudates of supercooled (SC) melt which extruding at 175 degrees C. Further, the later stage of the melting process, especially after the disappearance of lamellar stacks, was examined by in situ small/wide-angle X-ray scattering (SAXS/WAXS). The SAXS curves of melting survived nanocrystals within the NMP melt could be fitted by using a form factor for polydisperse cylinders. The fitting results showed that the height of cylindrical nanocrystals continued to increase while the diameter rapidly decreased during the melting process. Our findings suggested that melting survived nanocrystals can drastically enhance the PLLA crystallinity, mainly through an acceleration of nucleation process, which can be further facilitated by shearing.