(175b) Electrospinning Nafion Nanofibers

Nafion^® nanofiber electrospinning has received recent attention for applications in electrochemical/electromechanical devices including proton exchange membrane fuel cells, sensors, and polymer-based actuators.^1-3 Unfortunately, Nafion is notoriously difficult to electrospin.  Nafion does not form a true solution in solvent, but rather a micellar dispersion.  Such dispersions lack the required chain entanglements for polymer electrospinning and can only be electrosprayed as discrete droplets.  Thus, a carrier polymer such as poly(ethylene oxide) (PEO), poly(acrylic acid), or poly(vinyl alcohol) must be added to Nafion solutions to provide chain entanglements and permit electrospinning of nanofibers.  Previously, researchers have electrospun Nafion with high concentrations of carrier polymer (15-25 wt% carrier polymer). A low concentration of carrier is desirable for many end uses, particularly for fuel cell applications since one does not want to contaminate Nafion and compromise its proton conductivity properties by partial dilution with an inert carrier material. In this talk we will present the results from our recent studies demonstrating Nafion electrospinning with low (1 wt%) carrier polymer concentrations.⁴

The effects of relative humidity, electrospinning voltage, polymer solution flow rate, carrier polymer molecular weight, and solvent were investigated for their effect on the fiber diameter of Nafion/PEO nanofibers and the presence of bead or bead-on-fiber structures.  Relative humidity during was found to have a profound effect on Nafion electrospinning.  Electrospinning fibers into air at a high relative humidity increased the rate of Nafion precipitation, thus leading to the presence of bead-on-fiber structures and an increase in the diameter of electrospun nanofibers.  In general, higher voltages and lower flow rates led to fibers of smaller diameter.  Fiber diameter was a strong function of the carrier polymer’s molecular weight; higher molecular weights led to more viscous solutions and larger electrospun fiber diameters.  Furthermore, at carrier polymer high molecular weight, ribbon structures (i.e. flattened fibers) appeared in electrospun mats.  Ribbons form due to rapid precipitation of polymer at the fiber surface and subsequent collapse of the resulting tube-like structure upon contact with the electrospinning collector plate.  Solvent volatility also effects fiber diameter, where electrospinning with a more volatile solvent produced fibers of larger diameter.

Over the course of the electrospinning experiments, experimental conditions were identified to electrospin well-formed mats of Nafion nanofibers where the average fiber diameter could be controlled between 300 and 900 nm.  These conditions will be presented and discussed.

1.     Lee, K. M.; Choi, J.; Wycisk, R.; Pintauro, P. N.; Mather, P., Nafion nanofiber membranes. ECS Transactions: 2009; Vol. 25, pp 1451-1458.

2.     Nah, C.; Lee, Y. S.; Cho, B. H.; Yu, H. C.; Akle, B.; Leo, D. J., Preparation and properties of nanofibrous Nafion mats for ionic polymer metal composites. Composites Science and Technology 2008, 68, (14), 2960-2964.

3.     Zhou, C. S.; Liu, Z.; Dai, J. Y.; Xiao, D., Electrospun Ru(bpy)(3)(2+)-doped nafion nanofibers for electrochemiluminescence sensing. Analyst 2010, 135, (5), 1004-1009.

4.     Ballengee, J. B.; Pintauro, P. N., Morphological control of electrospun Nafion nanofiber mats. Journal of the Electrochemical Society 2011, 158, (5), B568-B572.