(597b) Electrospinning Nafion® Nanofibers

Polyelectrolytes have been explored in a variety of applications, including drug delivery, antimicrobial agents, chemical and biological protective clothing, and biomimetic actuators. More specifically, the anionic polyelectrolyte Nafion^® has been widely used as a proton exchange membrane in fuel cells. In addition, applications for Nafion^® include electrochemical devices, chlori-alkali production, metal-ion recovery, water electrolysis, batteries, sensors, and super-acid catalysts. Commercially, Nafion^® is available in several forms, such as extruded films of various thicknesses and equivalent weights, as dispersions in water/alcohol solutions, and in pellet form. Scaling down Nafion^® on the size scale of nanometers is expected to increase its efficacy for many of the applications listed above by providing ultra-high specific surface areas. Recent results in our laboratory demonstrate the impact of Nafion® in nanofibrous form on improving fuel cell performance.

In this study, the electrospinning performance and solution properties of Nafion^® and its blend with another polyelectrolyte, poly(acrylic acid) (PAA), were investigated. Attempts to electrospin pure Nafion^® at various polymer concentrations, solvents, neutralization, and electrospinning conditions resulted in electrospraying rather than electrospinning. However, a polymer solution blend of Nafion^® and PAA resulted in beaded fibers at 8 wt% PAA and smooth electrospinning above 12 wt% PAA. Fiber sizes of the blend increased from 90 to 600 nm with increasing PAA content. Dynamic light scattering on pure Nafion^® solutions in various solvents reveals large aggregates (i.e., dispersion) of various sizes due to polymer backbone and ionic interactions. Low viscosity and the aggregate formation in Nafion^® solutions prohibit fiber formation during electrospinning due to the lack of sufficient polymer chain entanglement. The addition of PAA to Nafion^® results in increased viscosity, suppressed aggregate formation, and ionic interactions between the sulfonic acid groups in Nafion^® and the carboxylic acid groups in PAA (infrared spectroscopy) suggesting enhanced polymer chain entanglement. Coincidentally, the onset of suppressed aggregate formation in the blend solution coincides with the electrospraying-electrospinning transition (8 wt% PAA). In addition, thermal treatment of the hydrophilic electrospun blend fibers enhanced water stability significantly.