Courtesy: NovaComp Inc. 2005

Electrospinning can be described as a process to produce fibers ranging from a few nanometers to several micrometers in diameter. This process utilizes the electrostatic attraction between a charged polymer and a grounded or oppositely charged collection plate within an electric field. When the electrostatic attraction overcomes the surface tension and viscoelastic components of the polymer, the polymer droplet will extend into a cone as described by Taylor before elongating into a fine jet. This jet may possess both stable and unstable regions before hitting the collection plate and becoming grounded. An example of a typical electrospinning set-up can be seen in Figure 1.

Figure 1: A conventional electrospinning set-up Electrospinning is not a novel technique

There is evidence dating back over 400 years when Gilbert showed that when a piece of rubbed amber is placed near a water droplet on a smooth surface, a cone can be formed. In 1745, Bose was the first to describe the process of electro-hyrdodynamic spraying of fluids and later in 1882, Rayleigh expanded on the field by studying thin liquid jets when placed in electric fields and their stability criterion. It was not, however, until 1934 when Formhals was issued the first patent on the formation of artificial threads, that electrospinning as it is known today, was born. Although there were a few papers trying to quantify and experimentally describe the electrospinning process, it was not until the early 1990s that a re-birth was witnessed. Through the work of Reneker and colleagues, electrospinning was experiencing rapid growth and many new scientists were emerging in the relatively unexplored field. During this time, nearly every polymer that was soluble was a potential candidate for the electrospinning process. Many of the early experiments were focused on materials that were soluble in water including Poly(ethylene oxide) due to their low cost and the overall availability of water.

Throughout this time, there was very little work being presented or being published on the subject of electrospinning of polymers from the molten state. The likely reason behind this is that the research was being driven to produce the smallest fiber diameter possible. The nanotechnology revolution was being born and the NSF defined nanomaterials as materials having at least one dimension smaller than 100 nanometers. In the case of polymeric fibers, this dimension would be the fiber diameter. Lorrando and Manley experimented with molten polypropylene at small distances. The polymers that they worked with possessed melt flow indexes ranging from 0.5-2.0. Lorrando and Manley experimented with a collection plate distance up to 3 centimeters and were able to get potentials as high as 7kV before discharge into the air began to occur. The fibers that were obtained from this process were in excess of 50 micrometers. This large fiber diameter was attributed to the viscosities that can be many orders of magnitude larger than viscosities experienced when electrospinning from solution.