Achievement of High Surface Charge in Poly(vinylidene fluoride) Fiber Yarns through Dipole Orientation During Fabrication

Electrospun fiber materials are of scientific interest for use in multiple application areas due to the high surface area to volume ratio, high porosity, good mechanical strength, and flexibility. Charged fiber structures show enhanced properties as desired for some of these applications. It is suspected that fiber alignment may affect the charge capacity and mechanical strength of charged fiber structures, but it has not been explored. Materials such as piezoelectric Poly(vinylidene fluoride) (PVDF) can be charged by heating and stretching the fibers and by exposing the fibers to a strong electrical field. It is expected that the polarized fiber structures result in a higher charge with aligned fiber structures, than non-aligned fiber structures. Electrospun flat fiber membrane structures, typically consist of randomly oriented fibers. In this research, polarized and non-polarized randomly oriented electrospun membrane structures and aligned fiber yarn structures were compared by measuring the charge per unit mass on these structures.

The randomly oriented¹ (RFM) and aligned fiber yarn structures² (AFY) were used as non-polarized structures. Polarized fiber structures were produced by stretching, heating, and poling the RFM and AFY to produce RFM-P and AFY-P structures which were compared for a charge per unit mass using the Faraday bucket.  It was measured that the polarized samples had higher charge per unit mass than the non-polarized sample. The results show that stretching and orienting of fibers during the fiber yarn formation process endows more charges to the fibers compared to the polarized fiber membrane produced from the electrospinning process and post-polarization step. It was observed that the fraction of β [F(β)] phase was much higher in the aligned fiber yarns (AFY) (76%) compared to the 48% for RFM. From the FTIR, XRD, and DSC measurements², it can be observed that these higher charge values were caused by the electric field-induced dipole rotation of the amorphous phase during the fiber yarn formation for AFY samples. While the AFY samples were post-processed for polarization, there was a small drop in the F(β) value, which could be caused by the high temperature annealing during high temperature polarization.  This facile process of preparation of enhanced charge-functionalized aligned fiber yarn (AFY) structures is suitable for a wide range of applications. The higher charge held by the aligned fiber yarns may improve the performance of these materials in filtration and separation applications.

References

1. Gade, H., Bokka, S., Chase, G. G., Polymer, Volume 212, 2021, 123152, https://doi.org/10.1016/j.polymer.2020.123152.
2. Bokka, S., Li, Y., Reneker, D. H., Chase, G. G., J. Appl. Polym. Sci. 2023, 140, 1.