Melt Spinning of Poly(vinylidene fluoride) Fibers and the Influence of Spinning Parameters on beta-Phase Crystallinity

摘要

The effects of melt-spinning and cold-drawing parameters on the formation of beta-phase crystallinity in poly(vinylidene fluoride) (PVDF) fibers and ways of increasing such crystallinity were studied. Fibers were melt-spun with four different melt draw ratios (MDRs) and were subsequently cold-drawn at different draw ratios (lambda). The maximum lambda value in cold drawing was dependent on the MDR used in melt spinning. The crystalline structure of the fibers was studied mainly with differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results showed that the degree of crystallinity in the fibers was determined by the MDR and that before cold drawing the crystalline structure of the fibers was predominantly in the alpha form. By cold drawing, alpha-phase crystallites could be transformed into the beta phase. It was established that, under certain conditions of melt spinning and cold drawing, PVDF fibers of up to 80 crystallinity, mainly in the beta form, could be prepared. It was further proposed that fibers spun at a sufficiently high MDR consist to a large extent of extended-chain crystals, and this greatly affects the melting point of PVDF. Thus, DSC melting-point data were shown to be insufficient for determining the crystalline phase of PVDF.