External flow-induced highly oriented and dense nanohybrid shish-kebabs: A strategy for achieving high performance in poly (lactic acid) composites

In situ nanofibrillation is regarded as an efficient and cost-effective technique to enhance polymeric materials' mechanical properties and crystallization behavior. Recently, a combination technique applying an intense external flow field and a polytetrafluoroethylene (PTFE) phase were shown to have the potential to enhance polymer properties. Using this approach, poly(lactic acid) (PLA)-based in situ nanofibrillar composites containing 1, 3, and 5 wt% PTFE were successfully prepared by a loop oscillatory push-pull molding (LOPPM) method in this study. The LOPPM approach applied a continuous intense dynamic oscillatory shear flow field during processing and in situ generation of well-aligned PTFE nanofibrils. Scanning electron microscopy (SEM) results showed that the in situ generation of nanofibrils served as the shish to induce the formation of highly oriented and dense nanohybrid shish-kebab superstructures in the composites. These superstructures were confirmed by twodimensional (2D)-small-angle X-ray scattering (2D-SAXS) and 2D-wide-angle X-ray scattering (2D-WAXS) techniques. Benefiting from oriented and dense nanohybrid shish-kebab superstructures, the in situ nanofibrillar PLA/ PTFE composites containing 3 wt% PTFE exhibited superior mechanical properties than pure PLA; the tensile strength, Young's modulus, and impact strength were 82.5 MPa, 4781.4 MPa, and 11.3 kJ/m2, respectively. Moreover, dynamic mechanical performance was significant improved as well. Overall, this work demonstrates a novel combination processing technique for the cost-effective, eco-friendly, and efficient design of PLA-based in situ nanofibrillar composites for engineering applications.