Fabrication and Electromechanical Characterization of Piezoelectric Polymer fibers-based composite

Abstract

Over the past few decades, the use of functional materials such as piezoelectric polymer has extensively increased in a variety of fields such as (nano) energy harvesting, flexible vibration, force sensors, etc. Piezoelectric polymers processed to micro/nanofiber are attractive owing to their intrinsic properties, such as large surface area, low weight, high endurance, high flexibility, high piezoelectric voltage generation, and high capability to integrate with the flexible and complex substrates. Recent reports show that polypeptide Poly(γ-benzyl-α, L-glutamate) (PBLG) polymer fibers have shown notable electromechanical characteristics, such as the high piezoelectric charge constant d33 along the fibers’ axial, outstanding thermal stability, and stable dipole moment caused by the polarized α-helical structure. Among various methods used to produce continuous polymer micro/nanofibers, electrospinning (ES) is proved to be the most effective fabrication method to polarize the α-helical structure to form bundles of PBLG micro/nanofibers. However, the conventional ES method is unable to effectively control the fabrication process as well as the fibers’ structure. This poses several limitations on further studies on the correlation between the fibers’ morphology and piezoelectricity. In this study, a new fabrication technique has been invented to overcome the present challenge of controllability in the ES method. The new technique is named as the End-point Control Assembly Electrospinning (EpCA) method and can produce highly well-aligned fibers with high fabrication productivity and high precision of fibers’ deposited position. The Taguchi method, which is mainly used for robust experimental design, is used to select the optimum level of processing parameters and obtain bundles of electrospun fibers with high uniformity of fiber diameters and high fiber alignment. A study on the electromechanical properties of thin and flexible, unidirectional continuous fiber PBLG/Polydimethylsiloxane composite is also presented. By encasing the polarized electrospun PBLG microfibers in an elastomer — Polydimethylsiloxane (PDMS) — the unidirectional continuous fiber PBLG/PDMS composite has shown an improvement on the flexibility (Young’s modulus E = 0.4 MPa), while its piezoelectric constant, d33, has increased to 54 pC/N. The electromechanical coupling coefficient, d13, has shown a value of 10.2 pC/N for the first time and the force sensitivity in the bending test is more than 600 mV/N. This shows great potential for force and pressure sensors and electromechanical conversion applications. A new approach to increase the piezoelectricity of polarized polymer fiber by fabricating the electrospun coaxial fibers in which PBLG performs as the core material and Poly (vinyl fluoride) (PVDF) contributes as the shell material is presented. Combining the polarization in the fibers’ axial direction of the core PBLG fibers with and the dipole moment in the radial direction of the shell PVDF fibers raises the piezoelectric charge constant (d33) to a value of 68 pC/N. The core-shell PBLG/PVDF fibers can produce a maximum of 400 mV, peak to peak, under the axial load. This is substantially greater than the output voltage measured from the monolithic electrospun PBLG and PVDF fibers. The new type of electrospun coaxial PBLG/PVDF fibers shows great potential to apply the piezoelectric polymer fiber on flexible and high-performance piezoelectric-based applications, such as pressure sensor, underwater acoustic transducers, and lightweight energy harvesting devices.