“Breaking High-Performance Fiber Development Paradigm: Continuous Supernanofibers for Structural and Biomedical Applications”

ABSTRACT: Advanced fibers produced a revolution in structural and functional materials in the 20th Century and are now used in a myriad of applications from aerospace to automotive, medical, and sporting goods. However, there was no major breakthrough in advanced fiber development since the last carbon fiber introduction by the Japanese more than three decades ago. Classical manufacturing techniques for ultrahigh-performance polymer fibers rely on combination of high polymer crystallinity and high degree of macromolecular alignment to achieve superior mechanical properties. As a consequence, advanced fibers such as Kevlar and Spectra possess extraordinary strength but low strain to failure (<3%) and therefore low toughness. Our recent analysis of electrospun polyacrylonitrile (PAN) nanofibers (NFs) in the ultrafine (100-250 nm) diameter range (~100 times thinner than conventional advanced fibers) showed extraordinary simultaneous increases in strength, modulus, AND toughness. Finest nanofilaments exhibited strength on the par with the best advanced fibers while exceeding their toughness by more than an order of magnitude. Structural investigations showed that this unique and highly desirable mechanical behavior may be due to high degree of macromolecular alignment in conjunction with low crystallinity. We have demonstrated that it is possible to further improve NF mechanical properties by changing nanomanufacturing parameters. Reduction in crystallinity of nanofibers achieved through modified processing resulted in further increases in strain to failure and toughness. Remarkably, it has also resulted in improvements in NF strength and modulus that were attributed to improved polymer chain alignment as a result of increased drawability. Notably, the major improvements in mechanical properties were observed in the intermediate (250-500 nm) diameter range. NFs with these larger diameters are easier to produce and handle, simplifying the upscaling of the nanomanufacturing process. Reported dramatic (2-3 orders of magnitude) simultaneous improvements in mechanical properties of NFs can lead to inexpensive, simultaneously strong and tough materials for safety critical applications. The proposed structural explanation of the newly discovered NF mechanical behavior challenges the prevailing paradigm in advanced fiber development calling for high polymer crystallinity and can lead to the entirely new class of advanced fibers with ultrahigh toughness, in addition to strength. Such fibers can ultimately result in ultralight structures with the strength higher than carbon-epoxy composites (the current state-of-the-art) but much higher toughness rivaling that of metals. Recent results on bionanofibers and carbon, as well as applications of these fibers in hierarchical supercomposites and biomedicine are also discussed.