Peculiar crystallization and viscoelastic properties of polylactide/polytetrafluoroethylene composites induced by in-situ formed 3D nanofiber network

This study comprehensively investigates the development of in-situ polylactide (PLA)/polytetrafluoroethylene (PTFE) nanofibrillated composites. PLA based composites containing 1, 3 and 5 wt% PTFE were prepared by melt compounding in a twin-screw extruder. SEM images demonstrated that interconnected PTFE nanofiber networks were generated with uniform distribution throughout the PLA matrix. Melt viscoelastic properties of nanofibrillated composites were significantly improved due to the PTFE nanofiber network. This increase was even more pronounced when the PTFE content was raised from 1 to 3 wt%, which could be considered as the percolation threshold network concentration range of PTFE fibers. However, the overall viscosity of composites was lower than that of PLA when the amount of PTFE was increased to 5 wt% due to the lubricating effect of PTFE nanofibers, which could ease PLA processing. With the increase in PTFE content, the transcrystallization of PLA around PTFE was improved and the crystallization half-time was remarkably decreased. Accordingly, the PLA crystallization kinetics was enhanced. The PLA/PTFE nanocomposite extrudates were also injection molded. SEM images of the skin layer of the injection molded nanocomposites exhibited an oriented structure in which PLA chains crystallized around the PTFE nanofibrils, whereas an amorphous structure was observed for the core layer. The crystalline structure and orientation, as well as thermal properties of the injection molded PLA/PTFE nanocomposites, were assessed by wide and small-angle X-ray scattering and differential scanning calorimetry, respectively.