Researchers at Cornell recently revitalized a process for creating polymer fibers called electrospinning. The process, first patented in 1939, is effective at creating thin but long fibers from small amounts of material.
Most polymer fibers spun today, such as nylon used in clothing, are produced through mechanical processes that result in large amounts of waste.
Electrospinning, however, uses electrically generated motion to spin fibers so that no waste is produced.
“Electrospinning is actually a fairly simple process,” said Prof. Margaret Frey, textiles and apparel. The process involves excreting a drop of a polar solution containing a dissolved polymer from the tip of a syringe and then applying a large voltage, somewhere between 15,000 and 30,000 volts. A grounded sheet, termed the collector, is then placed about six inches from the droplet. “Since the solvent is polar, we start getting negative charges building up at the surface [of the droplet],” Frey said.
The unequal charge distribution between the droplet and the grounded sheet eventually causes a spark which then drags a small amount of the droplet with it. The path taken by the substance is not straight. Instead, the fiber undergoes a whipping motion “causing the length of the fiber to become really long,” Frey said.
Because the solvent dissolves during the process, what remains is a unwoven mat atop the collector sheet. “[The] fibers are really tiny, [but] they have a huge surface area,” Frey noted.
Because they are so tiny, “you can also make materials that have very small pore sizes, but the overall porosity is still very high,” Frey said. Such materials would be ideal for the development of filters that may be able to trap things as small as viruses and still allow air to flow through freely.
Prof. Yong L. Joo, chemical engineering, envisions the development of an advanced filter using the electrospinning technique. “While we are making the fibers, we can add catalysts to the solution and spin fibers that include them,” Joo said. Once spun into a mat, the fibers could then be arranged into an advanced filtration device that would break down specific molecules passing through them but not others.
“This could be used to transform carbon monoxide to carbon dioxide” while not affecting other gases, Joo said.
Joo has been involved in studying the dynamics of the spinning process and developing mathematical models that allow scientists to predict the consequences of changing variables such as applied voltage, solution viscosity, polarity and salt concentration. For example, increasing the voltage applied, increases the amount of whipping motion the fibers undergo during formation.
“[The whipping] is like a hose of water,” said Joo, “as you increase the pressure through a hose, the water begins to force the hose to whip around.”
Another correlation exists between the diameter of the nanofibers and the properties of the solution. “Diameter of the fiber can be controlled by having higher charge density, lower viscosity and lower surface tension,” said Choowon Kim grad, a researcher collaborating with Frey and Joo.
Joo and Kim have also worked with Frey to perfect the electrospinning technique with cellulose, an organic polymer abundant in plant cells. “Cellulose is one of the toughest ones [to perfect] because it is hard to dissolve into the solvent,” Joo said.
Nonetheless, the use of cellulose nanofibers will have many future applications, Frey claimed. Among them is the creation of protective layers for agricultural workers that would effectively prevent pesticides from coming in contact with skin while also allowing moisture to move through freely so that workers could stay cool. Another is the development of sponges that would allow controlled release of pesticides so that only necessary amounts are used. Ideally, these sponges would also be biodegradable.
Interest in electrospinning as an effective technique for generating nanofibers has blossomed over the past few years. “It has, in a way, been rediscovered,” Frey said.
The renewed interest in electrospinning was evident at a conference of the American Chemical Society that Frey attended in New York City on Sept. 9.
“Two full days [were devoted to] electrospinning and the different things people are working on,” Frey said.
Funding for the research at Cornell has come from the New York Department of Agriculture, the USDA, National Science Foundation and one private company.
Archived article by David Andrade