Quasistatic direct electromechanical responses from as-electrospun submicron/micron fiber mats of several polymers

Abstract

Electromechanically active submicron/micron polymer fibers are promising components of wearable pressure sensors because they are mechanically flexible and lightweight. Recently, an as-electrospun microfiber mat comprising atactic polystyrene (aPS) demonstrated significantly high quasistatic direct electromechanical responses, although aPS films are not normally piezoelectric in nature. In this study, we investigated the quasistatic direct electromechanical properties of as-electrospun submicron/micron fiber mats composed of several polymers, namely poly(methyl methacrylate) (PMMA), aPS, poly(vinyl alcohol) (PVA), poly(D,l-lactic acid) (PDLLA), poly(l-lactic acid) (PLLA), and poly[(R)-3 hydroxybutyric acid] (PHB). As-electrospun PMMA, aPS, PDLLA, PLLA, and PHB fiber mats demonstrated significantly high direct electromechanical responses 22 h after fabrication, despite the nonpiezoelectric nature of PMMA, aPS, and PDLLA, whose films normally do not exhibit electromechanical responses. In contrast, the as-electrospun PVA fiber mat hardly showed any direct electromechanical response. We examined the surface potentials of these fiber mats, which revealed that the surface potential of the PVA fiber mat decayed rapidly and was essentially quenched within 2 h after fabrication, demonstrating that electrospinning forms polymer fibers with different abilities to retain charge and, as a consequence, different electromechanical responses. The direct electromechanical properties of the PMMA and aPS fiber mats were stable, even 30 d after fabrication, which is mainly attributable to the lower volume conductivities of these polymers. Selective discharging of real charges, including surface charges and space charges, through spraying with isopropanol mist revealed that the direct electromechanical responses of the fiber mats are due to real charges stored in the fiber mats through electrospinning. These fundamental findings provide novel material-choice opportunities for the development of large-area, flexible, lightweight, and inexpensive pressure sensors.