High electromechanical response from bipolarly charged as-electrospun polystyrene fiber mat

Abstract

Electromechanical micrometer/submicrometer polymer fibers are promising components for wearable pressure sensors because the fibers are mechanically flexible, lightweight, and breathable. Although piezoelectric polymers are normally used as their materials, electrospun fibers of some nonpiezoelectric polymers have exhibited high electrical actuation, similar to the inverse-piezoelectric effect of piezoelectric materials. However, the pressure sensing properties and the origin of the behavior of the fibers have been unclear. This study demonstrates the electromechanical properties of an as-electrospun fiber mat composed of a nonpiezoelectric polymer, atactic polystyrene, and analyzes the origin of the properties. The fiber mat demonstrates a high apparent piezoelectric constant (d ) of 950-1400 pC/N with an applied load of 0.05-0.28 N, and a Young's modulus of 6.40 kPa indicates a soft nature. The surface potential of the stacked fiber mats with forward and opposite stacking confirms that the mat was charged with bipolar and uneven electric charges. This unique charge distribution is the likely origin of the fiber mat's electromechanical properties. The generated electric charge increased linearly with increasing indentation depth, which can be explained using a theoretical model of bipolarly charged polymer layers and air gaps, similar to the theoretical model of cellular ferroelectrets. This result also supports the bipolarly charged model, and the theoretical model implies that the high d results from the soft and modestly charged nature of the fiber mat. This finding will pave the way for the development of soft, lightweight, and breathable wearable pressure sensors from a variety of materials with high electromechanical performance.

Electromechanically Active As‐Electrospun Polystyrene Fiber Mat: Significantly High Quasistatic/Dynamic Electromechanical Response and Theoretical Modeling

“…Quasistatic direct electromechanical characteristics of the fiber mat were evaluated with a piezoelectric constant measurement apparatus (PF‐02B, Lead Techno Co., Ltd) equipped with a laser confocal displacement meter (LT‐9010M/LT‐9500, Keyence Corp.), as depicted in Figure 5a. [ 28 ] An upper disk‐shaped metallic probe with 8.0 mm diameter was contacted with the aPS fiber mat at a prepressure ( P Pre ) of 200 Pa; next, the probe was indented in a direction perpendicular to the fiber mat with a controlled load using a linear motor, and the load applied to the fiber mat was measured with a load cell. Here, the pressure applied to the fiber mat was calculated by dividing the load value measured with the load cell with the area of the disk‐shaped metallic probe (5.0 × 10 −5 m 2 ).…”

“…Recently, we have demonstrated high quasistatic direct electromechanical characteristics of an as‐electrospun fiber mat composed of atactic polystyrene (aPS), which is an inexpensive and widely used nonpiezoelectric polymer. [ 28 ] However, the converse electromechanical characteristics and direct dynamic electromechanical characteristics of such a fiber mat have not yet been reported. Therefore, in this study, we experimentally studied the converse electromechanical response and direct dynamic electromechanical response of an aPS fiber mat; in addition, we propose theoretical models to explain the results.…”

“…This ferroelectricization can be attributed to the uniquely charged nature of the as‐electrospun aPS fiber mat; the positive charges are generally held in the upper part of the fiber mat, while the negative charges are held in the lower part, as reported recently. [ 28 ] Further, Fourier‐transform infrared (FTIR) spectra of the as‐electrospun aPS fiber mat revealed that the aPS fibers hardly contain any residual solvent, N , N ‐dimethylformamide (see Figure S1, Supporting Information). The relative permittivity of the fiber mat (ε F ) was measured to be 1.19 at 1 kHz.…”

“…Further, quasistatic direct electromechanical characteristics of the aPS fiber mat were evaluated with the setup shown in Figure a. [ 28 ] In this evaluation, different external pressures ( P D ) were applied to the fiber mat after the application of a fixed prepressure ( P Pre = 200 Pa); subsequently, the amount of charge flowing from the upper electrode to the bottom electrode [Δ Q ( h ′)] and the position of the upper disk probe ( h ′) were determined. The apparent piezoelectric d constant ( d app2 ) was determined using Equation () shown later and plotted as a function of P D (Figure 5b).…”

“…Our recent report demonstrated that the as‐electrospun aPS fiber mat uniquely holds electric charges of both positive and negative polarities, and that the positive charges are generally distributed in the upper part of the fiber mat, while the negative charges are distributed in the lower part of the fiber mat. [ 28 ] Based on this finding, here the upper and lower surfaces of the as‐electrospun aPS fiber mat are assumed to be charged with effective surface charge densities of +σ Eff0 and −σ Eff0 , respectively, as shown in Figure 6a. The decrease/increase in the thickness of the fiber mat induces an increase/decrease in the charge density of the fiber mat.…”