Electrospun polyvinylidene fluoride (PVDF) nanofiber has shown its advantages of producing piezoelectric devices. Although the effect of electrospinning parameters on the formation of β crystalline phase has been investigated, their mixed effect has not been fully understood. In this study, two dependent design of experiments were utilized to systematically control the transformation of crystalline phases by three typical electrospinning parameters, i.e. applied voltage, needle tip diameter and feeding flow rate, and the mixed interaction of these parameters in improving the fraction of β crystalline phase was investigated. The structures and the contents of crystalline phases within PVDF nanofibers were analyzed by Fourier transform infrared spectra and x-ray diffraction patterns. The results of single factorial experiments showed that all of three electrospinning parameters have a significant effect on the formation and fraction of β-phase crystallinity of electrospun PVDF nanofibers. Furthermore, both the fraction and the crystallinity of β phase initially increase and then decrease with an increase of applied voltage, and consistently decrease with the needle-tip diameter. However, the β-phase fraction parabolically increases with the increasing feeding flow rates, whereas its crystallinity linearly deceases. The orthogonal experimental results demonstrated that the mixed effect of three electrospinning parameters is different from that of single parameter on the fraction of β-phase crystalline, and that the feeding flow rates among three parameters have the greatest effect. Taken together, these results suggested a new strategy to improve the β-phase fraction of electrospun PVDF fibers via sequence of priority of electrospinning parameters.

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The electrospun PVDF (Polyvinylidene Fluoride) nanofiber web is commonly agreed on a kind of new sensitive materials for the sensor testing the dynamic pressure and energy harvesting, and has the characteristics of fast response and high sensitivity of pressure. As a result of the nanofiber web, it must be packaged to collect piezoelectric charge and bear strong mechanical behavior before industrial practice. The packaging of PVDF nanofiber web is usually sandwiched by incorporating a pair of flexible electrode. However, the effects of the surface and mechanical properties of electrodes such as morphology, roughness and compressibility have not been well investigated yet. This work will introduce three common types of packaging electrode materials (adhesive copper foil tape, indium tin oxide (ITO) thin film, adhesive conductive cloth.) in previously published literatures, compares the piezoelectric output of their sensor prototypes under a periodic impact, and discusses the effect of surface morphology, electrical resistance, and compressibility. The results showed that it has higher output of PVDF piezoelectric sensor packaged by electrode materials with the smooth surface and low mechanical compressibility. This result provides a guideline for designing the textile electrode material for the PVDF nanofiber web.