High Electromechanical Deformation Based on Structural Beta-Phase Content and Electrostrictive Properties of Electrospun Poly(vinylidene fluoride- hexafluoropropylene) Nanofibers

Abstract

The poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) polymer based on electrostrictive polymers is essential in smart materials applications such as actuators, transducers, microelectromechanical systems, storage memory devices, energy harvesting, and biomedical sensors. The key factors for increasing the capability of electrostrictive materials are stronger dielectric properties and an increased electroactive β-phase and crystallinity of the material. In this work, the dielectric properties and microstructural β-phase in the P(VDF-HFP) polymer were improved by electrospinning conditions and thermal compression. The P(VDF-HFP) fibers from the single-step electrospinning process had a self-induced orientation and electrical poling which increased both the electroactive β-crystal phase and the spontaneous dipolar orientation simultaneously. Moreover, the P(VDF-HFP) fibers from the combined electrospinning and thermal compression achieved significantly enhanced dielectric properties and microstructural β-phase. Thermal compression clearly induced interfacial polarization by the accumulation of interfacial surface charges among two β-phase regions in the P(VDF-HFP) fibers. The grain boundaries of nanofibers frequently have high interfacial polarization, as they can trap charges migrating in an applied field. This work showed that the combination of electrospinning and thermal compression for electrostrictive P(VDF-HFP) polymers can potentially offer improved electrostriction behavior based on the dielectric permittivity and interfacial surface charge distributions for application in actuator devices, textile sensors, and nanogenerators.

Keywords: P(VDF-HFP) nanofibers; actuators; dielectric properties; electrospinning; electrostrictive properties; structural β-phase; thermal compression.

Figure 1

Schematic of the synthesis used with all samples. (a) Homogeneous poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP) solution, (b) solution casting, (c) electrospinning, and (d) fibrous film compression.

Figure 2

The electrostriction setup.

Figure 3

SEM images of the P(VDF-HFP) (a) film, (b) fiber, (c) 30 °C compressed fibers, (d) 60 °C compressed fibers, and (e) 80 °C compressed fibers.

Figure 4

X-ray diffractograms for P(VDF-HFP) film, fiber, and fiber mats compressed at 30°, 60°, and 80 °C.

Figure 5

IR spectra of film and fiber P(VDF-HFP) for wavenumbers from 400 to 1000 cm⁻¹.

Figure 6

The dynamic mechanical analysis curves of P(VDF-HFP) fiber and compressed fiber mats. (a) Storage modulus and (b) tan delta.

Figure 7

Variation of the dielectric constant (a), loss tangent (b) and AC conductivity (c) with frequencies from 10⁰ to 10⁵ Hz for the film, fibers, and compressed fiber mats.

Figure 7

Variation of the dielectric constant (a), loss tangent (b) and AC conductivity (c) with frequencies from 10⁰ to 10⁵ Hz for the film, fibers, and compressed fiber mats.

Figure 8

Schematic of the proposed β-phase transformation mechanism.

Figure 9

Strain behaviors of film, fibers, and compressed fibers as a function of the (a) electric field and (b) square of the electric field at 1 Hz. (c) The effect of the compression fibers on the electrostrictive coefficients and absolute β fraction.

Figure 9

Strain behaviors of film, fibers, and compressed fibers as a function of the (a) electric field and (b) square of the electric field at 1 Hz. (c) The effect of the compression fibers on the electrostrictive coefficients and absolute β fraction.