Quantitative Analysis of Electrospun Jets Instabilities: In Pursuit of Perfect Continuous Nanofiber Alignment

Ultrafine continuous nanofibers (NFs) are at the forefront of structural materials and advanced composites research and development due to their recently discovered unique properties, including simultaneously ultrahigh strength and stiffness. Their continuity differentiates them from other discontinuous nanomaterials, and their capacity to exhibit simultaneous strength and toughness gives them an advantage over commercial high-performance fibers that are strong, but brittle. Because of their exceptional mechanical properties, electrospun NFs can be used in a number of multi-functional applications, including structural composites, biomedicine, energy storage, electronics, agriculture, filtration, and textiles. The electrospinning process produces continuous NFs through polymer-based solutions when a high electric field causes the polymer to overcome surface tension, ejecting a fine jet that bends and whips unstably, thinning the jet, and randomly depositing nanoscale fibers on a conductive substrate. The electrospinning manufacturing process provides a cost-efficient, top-down method to manufacture uniform-diameter polymer nanofibers. However, pervasive electrospinning jet instabilities are responsible for the resulting random orientation of the NFs. Further analysis and quantification of these instabilities are necessary to manufacture highly-aligned NFs, which are critically required for many applications. In addition, to further improve their strength and stiffness, nanofibers are often carbonized to produce carbon nanofibers (CNFs). However, polyacrylonitrile (PAN)-based CNFs do not possess the expected mechanical improvements due to poor graphitic structure and intrinsic orientation. One way to improve the graphitic structure of CNFs is to increase carbonization temperatures. However, ultrahigh temperature carbonization process is extremely expensive. Here, we investigate an alternative approach where CNF structure is templated with one- or two-dimensional nanomaterials, such as carbon nanotubes (CNTs), graphene nanoribbons (GNRs), and MXenes. For this research, PAN nanofibers were electrospun onto a rotating cylinder to obtain aligned NFs. Extensive parametric study was performed to determine how electrospinning process parameters, including substrate speed and width, collection time, applied voltage, and polymer concentration influenced the degree of fiber alignment and the fiber diameter. Quantitative analysis of the polymer jet was performed through direct high-speed videography. Resulting structure-processing relationships between process parameters and fiber alignment, fiber diameter, and jet characteristics form the basis for controlled nanofabrication of nanofiber assemblies. In addition, electrospun NFs were templated with nanoreinforcements, stabilized in air, and carbonized in nitrogen. Characterization was performed through Raman spectroscopy, SEM, TEM, XRD, and electron diffraction. Preliminary results show potential for uniform dispersion of nanoreinforcements and significant improvements in graphitic structure and intrinsic orientation, which would pave the way for the development of next-generation structural nanomaterials with perfect orientation for demanding load-bearing applications. This research was supported by NSF, Nebraska Center for Energy Sciences Research, and NU RC Interdisciplinary Grant.