Robust Interfacial Cylindrites of Polylactic Acid Modulated by an Intense Shear Flow Field

Abstract

Evolution of crystalline morphology/structure of PLA was investigated by combination of homemade fiber-pulling device and polarized optical microscopy, where the intense shear field within the range of the actual processing conditions was modulated by the pulling speed of glass fiber (GF). Sporadic dispersion of PLA spherulites in the quiescent melt indicated no nucleation ability of GF. High density of row-nuclei was induced by the strong shear flow (120 s–1) around the pulled GF surface. Also, these nuclei hindered the lateral extension of lamellae and forced growth direction perpendicular to the GF axis, leading to the formation of the semicircle cylindrite. As the shear rate rose to 280 and 420 s–1, nuclei density on the surface of pulled GF was further increased, and the cylindrites became more compact, generating the fan-shaped and brush-like morphology, respectively. As detected by two-dimension wide-angle X-ray diffraction, the cylindrites showed increased crystallinity and degree of orientation with the shear rate. Thereupon, the remarkable enhancement in the interfacial strength between the PLA matrix and GF was obtained, showing the upmost increase of 143% for the sheared sample (420 s–1) compared to the quiescent counterpart. These interesting results are of importance to bridge the gap between the crystalline morphology/structure of PLA and the shear flow quantitatively, offering helpful guidance to design the interfacial crystalline structure of PLA composites during the pratical processing.