A new technique called gel electrospinning, which has been developed by two scientists at MIT, could offer strong and resilient ultrafine polymer fibers that have a range of applications, including in protective armor and nanocomposites. The process, which adds electrical forces to traditional gel spinning, can produce fibers with a diameter in nanometers, combining stiffness and strength comparable to the best commercially available fibers while being much tougher.
The study, reported in the Journal of Materials Science [Park, J. H., Rutledge, G. C. J. Mater. Sci. (2017) DOI: 10.1007/s10853-017-1724-z], demonstrated ultrafine polyethylene fibers that match or exceed the properties of some of the strongest fiber materials, such as Kevlar and Dyneema. The fibers are also of similar strength as the carbon fibers and ceramic fibers commonly used in composite materials, but are much tougher and have lower density.
“What really sets those apart is what we call specific modulus and specific strength, which means that on a per-weight basis they outperform just about everything”Greg Rutledge
The team have been researching the formation, properties and applications of electrospun fibers for many years, as unexpected behaviors can be identified when their diameter is reduced below 1 micron. Although their test materials had a modulus not quite as good as the best fibers currently used, they were found to be near enough to be competitive. As professor of chemical engineering, Greg Rutledge, said: “What really sets those apart is what we call specific modulus and specific strength, which means that on a per-weight basis they outperform just about everything”.
The process used is similar to the conventional gel spinning process in terms of materials; however, the use of electrical forces to draw the fibers out and a single-stage process instead of multiple stages means the team can achieve much more highly drawn fibers with a diameter of a few hundred nanometers, rather than the typical 15 micrometers. The charged fibers induce a "whipping" instability process that produces their ultrafine dimensions.
As Rutledge explains, “In materials science, one is often faced with trade-offs in properties. Strength and toughness are one such trade-off”. For the fibers produced by this new process, many such trade-offs are eliminated, as the toughness was shown to increase through the reduction of fiber diameter without compromising strength or stiffness.
The findings could lead to protective materials that are as strong but less bulky than those used in the automotive, aerospace and military industries. Although they are at an early stage of research, the team now hope to better understand the origin of the materials’ toughness, and well as its relation to fiber diameter, with a view to improving the reliability of the process and producing new materials cost-effectively and at scale.