Effect of processing parameters of the continuous wet spinning system on the crystal phase of PVDF fibers

Poly(vinylidene difluoride) (PVDF) has been widely used in piezoelectric applications as films and nanofiber mats, but there are limited publications on piezoelectric wet-spun fibers. In this work, PVDF fibers were prepared using the wet spinning method, and the processing parameters, including the drawing ratio and heat setting temperature, were controlled in the continuous wet spinning system to increase the β-phase crystallinity of the fibers. In addition, the wet-spun PVDF fibers were compressed by a rolling press to eliminate voids in the fibers. Then, the compressed PVDF fibers were poled to align the molecular dipoles. The crystal structures of the PVDF fibers were investigated using X-ray diffraction and Fourier-transform infrared spectroscopy. Single filament tensile tests were performed to measure the tensile strength of the fibers. The morphologies of the PVDF fibers with respect to the processing parameters were observed by scanning electron microscope (SEM) and polarization optical microscopy. The piezoelectric constant of the prepared PVDF fibers was then measured using a d₃₃ meter. The wet-spun PVDF fibers showed the highest β-phase and piezoelectric constants when the drawing ratio and heat setting temperature were 6 and 150°C, respectively.