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Nelson S. Bell Sandia National Laboratories, Albuquerque, New Mexico 87123

Wolfgang Sigmunda) University of Florida, Department of Materials Science and Engineering, Gainesville, Florida 32611; and Hanyang University, Department of Energy Engineering, Seoul, South Korea (Received 15 January 2010; accepted 29 March 2010)

Superhydrophobic membranes have the potential to protect devices from incidental exposure to water. This paper reports on the processing of Teflon AF fluoropolymers through electrospinning. Teflon AF is difficult to electrospin due to its low dielectric constant and the low dielectric constants of the liquids in which it is soluble. The two approaches that have been utilized to produce fibers are direct electrospinning in Novec engineering liquids and core-shell electrospinning. Both methods produced superhydrophobic membranes. Fibers with an average diameter of 290 nm and average water contact angle of 151
were obtained by core-shell electrospinning. One suggested application for electrospun superhydrophobic membranes is the lithium-air battery. I. INTRODUCTION

Superhydrophobic surfaces are defined conventionally as having water contact angles greater than 150
and a low roll-off angle. The fibrous coatings found on lotus leaves and other plants and animals are found to be superhydrophobic and are thus water repellent and selfcleaning.1–3 The typical route in designing such a surface is to increase the roughness of a material with low surface energy.2,3 Vertical rods or roughened structures served as the initial model for creating these surfaces, but electrospun polymers have also demonstrated the ability to form water repellent films.3–9 Most of these methods have used polymers amenable to electrospinning such as polycaprolactone followed by surface modification using initiated chemical vapor deposition (CVD), or electrospinning of polystyrene or one of its copolymers.4–9 Nonwoven fiber mat morphologies with varying degrees of surface roughness are viable platforms for generating the desired solvent barrier morphology. Electrospinning is one processing technique used to produce materials of reduced dimensions, typically in the form of nonwoven fiber mats. A polymer solution is fed through a syringe needle to which a high voltage has been applied, typically between 5 and 30 kV. The fluid pumped out the end of the syringe needle forms a Taylor cone from which a fiber is drawn through a combination of electrohydrodynamic phenomena and polymer chain entanglement. Essentially, charge accumulates on the a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0205 J. Mater. Res., Vol. 25, No. 8, Aug 2010

electrospinning fluid until surface tension and viscous forces are overcome. Solution is ejected from the tip due to repulsion between the charges, and the entanglement of the polymer chains keeps the fluid from forming droplets during elongation. The fiber jet starts on a straight trajectory that breaks down into a spiraling, whipping motion. Particularly duri