Thermodynamic approach to tailor porosity in piezoelectric polymer fibers for application in nanogenerators

Abstract

Low power density of polymer piezoelectric nanogenerators is a major hurdle for their application as a potential mode of powering wearable and portable electronic devices. To increase the efficiency, here we suggest use of porous piezoelectric poly (vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) nanofibers. However, designing a process that allows introduction of pores in the nanometric fibers with a diameter of only several 100 nm, is highly challenging due to the intricate physics of polymer/solvent/anti-solvent interactions. Realization of the porous nanofibers would be a breakthrough in the field of piezoelectric nanogenerators. We presents an elegant approach based on the thermodynamics of polymer solutions to tailor porosity in P(VDF-TrFE) nanofibers. By adding a conscious amount of water, carefully chosen as non-solvent based on the ternary phase diagram of P(VDF-TrFE)/water/solvent, we intentionally induce liquid-phase demixing, which leads to formation of nanopores in the electrospun nanofiber. By calculating the mean composition trajectories, we predict and explain formation of the pores in the nanofibers, and show how little variations in initial water content substantially influences fiber porosity. Nanogenerators based on the porous electrospun P(VDF-TrFE) nanofibers show output power that systematically increases with porosity (with 500 times increase in output power for 45% porous fibers). The enhanced output is due to the reduced effective dielectric permittivity of the nanofibers. We unambiguously show that the voltage generation in nanofibers is of the same origin as in neat piezoelectric P(VDF-TrFE) films and is due to the relaxation of segments within the restricted amorphous phase. Understanding how to form nanopores, would have a major contribution to other fields, ranging from nanoporous membranes, as well as porous polymer structures for triboelectric nanogenerators.