Unconfined Melt Electrospinning with Control of Conductivity: A Green Processing Approach to Fabricate Small Diameter Fibers from Thermoplastics

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Extremely small diameter fibers can be formed into inexpensive, lightweight, ultra-porous materials which are crucial to the ongoing development of myriad, diverse applications including energy storage, ultra-high efficiency air/water filtration, drug delivery, wound healing, and artificial tissue engineering. Current approaches use solvents to process ultra-small diameter fibers that have insufficient mechanical strength to act as stand-alone filters and unintentionally leach harmful residual solvent during fabrication and/or biomedical applications. This award supports fundamental research to enable a significant increase in the number of processable materials including relatively insoluble thermoplastics, as well as the fabrication of higher quality (i.e, smaller diameter) fibers. The new open geometry electrospinning process creates many closely-packed parallel fibers resulting in a substantial increase in the production rate enabling commercial manufacturing. This method is "green" (solvent-free and compatible with recyclable plastics), results in nano- to micro-scale fibers having improved mechanical properties, and allows manipulation of the electrospinning process to create smaller fiber diameters than can typically be achieved using traditional processing routes. Substrates made from these fibers can be used in the many different technology fields described above, consequently benefitting the U.S. economy and society. Involving several disciplines including materials science, polymer and fiber processing and manufacturing, fluid physics, and electrical systems, this research will broaden participation of underrepresented groups in research and positively impact engineering education.The objective of this work is to develop fundamental understanding to enable a robust, green fabrication methodology to produce thermoplastic meso-fibers. Traditional needle melt electrospinning is difficult due to processing issues (i.e., frequent clogging). Melt electrospinning from an unconfined surface of molten polymer is a new paradigm. This transformative approach removes the fundamental incompatibility between melt electrospinning and confined feed geometries while promoting scale-up to fabrication rates necessary for commercial viability. The scientific hypothesis to be tested is that control of the flow rate via the electric field, polymer temperature, and melt conductivity (tuned via additives or by controlled electrical discharge) will produce previously unattainable small jet diameters to form nanofibers. The unconfined geometry enables control of low flow rate and local conductivity not readily achievable in confined schemes. The explicit role of conductivity has not been previously explored in melt electrospinning. This work addresses the mechanistic effects of conductivity on melt electrospinning, the ability to tune melt conductivity via controlled discharge, alteration of flow rate due to changes in melt conductivity, the mechanics by which this occurs, and the fundamental limits to electrospinning using melts.