Electrical Conductivity of Electrospun Polyaniline and Polyaniline-Blend Fibers and Mats

Submicron fibers of polyaniline (PAni) doped with (+)-camphor-10-sulfonic acid (HCSA) and blended with poly(methyl methacrylate) (PMMA) or poly(ethylene oxide) were electrospun over a range of compositions. Continuous, pure PAni fibers doped with HCSA were also produced by co-axial electrospinning and subsequent removal of the PMMA shell polymer. The electrical conductivities of both the fibers and the mats were characterized. The electrical conductivities of the fibers were found to increase exponentially with the weight percent of doped PAni in the fibers, with values as high as 50 ± 30 S/cm for as-electrospun fibers of 100% doped PAni, and as high as 130 ± 40 S/cm upon further solid state drawing. These high electrical conductivities are attributed to the enhanced molecular orientation arising from extensional deformation in the electrospinning process and afterwards during solid state drawing. A model is proposed that permits the calculation of mat conductivity as a function of fiber conductivity, mat porosity and fiber orientation distribution; the results agree quantitatively with the independently measured mat conductivities. * Corresponding author. Tel.: +1 617 253 0171; fax: +1 617 258 5776. E-mail address: [email protected]. 2 INTRODUCTION Electrospinning is a convenient method to produce polymer nanofibers with controlled diameters on the order of tens of nanometers to microns. The resulting nonwoven fiber mats have high specific surface areas, around 1 to 100 m/g. Combined with the high electrical conductivity of intrinsically conductive polymers, conductive electrospun fiber mats are promising for a variety of applications, such as multifunctional textiles, resistance-based sensors, flexible reversibly hydrophobic surfaces, organic photovoltaics, scaffolds for tissue engineering, and conductive substrates for surface functionalization and modification. 9 Polyaniline (PAni) is one of the most studied electrically conductive polymers, yet it is relatively hard to process compared to most other polymers. As is common among intrinsically conductive polymers, it has a fairly rigid backbone due to the high aromaticity, and is available only in relatively low molecular weight forms, so that the elasticity of its solutions is generally insufficient for it to be electrospun directly into fibers. To circumvent this problem, several different approaches have been reported. Strategies include electrospinning from solutions in concentrated sulfuric acid, a corrosive solvent, coating polyaniline onto a non-conducting substrate, and blending with more flexible, high molecular weight polymers that serve as processing aids. 14 15 16 17 However, most of these reports demonstrate only the formation of nanofibers, but do not report their electrical properties. The highest conductivity reported for an electrospun PAni blend fiber is about 1 S/cm. By contrast, conductivities as high as 600 S/cm have been reported for pure polyaniline films and fibers with diameters on the order of hundreds of micrometers. The difference is attributed at least in part to the necessity of blending of PAni with non-conducting polymers in order to form submicron diameter fibers. While blending high molecular weight non-conducting polymers with the conductive polymers to make the solution electrospinnable remains one of the most effective ways to solve the problem of low 3 solution elasticity, the resulting fibers have much lower conductivity due to dilution of the conducting component. The co-axial (also known as “two-fluid”) electrospinning technique uses two spinnerets that are arranged concentrically so that a low-elasticity fluid introduced to the core of the jet can be elongated along with an electrospinnable fluid introduced to the shell of the jet. The result is a continuous filament with core-shell morphology. 21 With the selective removal of the shell component of the resulting fibers, pure component electrospun fibers can be formed from fluids like the pure PAni solutions that are otherwise non-electrospinnable. In this work, we report the systematic study of conductivities of electrospun fibers of doped PAni blended with different polymers over a range of electrospinnable compositions. Using the co-axial electrospinning method, we produce electrospun fibers of 100% doped PAni, and report their conductivities for the first time. We also report the conductivities of nonwoven mats comprising these fibers, and provide a means for rationalizing the mat conductivities in terms of the mat structure and fiber conductivities.