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

极小直径的纤维可以形成价格低廉、重量轻、超多孔的材料，这些材料对于包括储能、超高效空气/水过滤、药物输送、伤口愈合和人工组织工程在内的各种应用的持续发展至关重要。目前的方法使用溶剂来处理超小直径纤维，这些超小直径纤维的机械强度不足以充当独立的过滤器，并且在制造和/或生物医学应用期间无意中滤出有害的残留溶剂。该奖项支持基础研究，以显著增加可加工材料的数量，包括相对不溶的热塑性塑料，以及制造更高质量(即，更小直径)的纤维。新的开放式几何结构电纺工艺产生了许多紧密堆积的平行纤维，从而大幅提高了生产率，使商业制造成为可能。这种方法是“绿色”的(不含溶剂，与可回收塑料兼容)，可产生具有更好机械性能的纳米到微米级纤维，并允许操纵静电纺丝过程以产生比传统加工路线通常可以实现的更小的纤维直径。由这些纤维制成的基板可以用于上述许多不同的技术领域，从而使美国经济和社会受益。这项研究涉及多个学科，包括材料科学、聚合物和纤维加工与制造、流体物理和电气系统，将扩大未被充分代表的群体参与研究并对工程教育产生积极影响。这项工作的目的是加深对基础知识的了解，使稳健、绿色的制造方法能够生产热塑性中间纤维。由于加工问题(即频繁堵塞)，传统的针熔电纺很难实现。聚合物熔体自由表面熔体电纺是一种新的研究范式。这种变革性的方法消除了熔体静电纺丝和受限喂料几何形状之间的根本不兼容性，同时提高了商业可行性所需的制造速度。需要检验的科学假设是，通过电场、聚合物温度和熔体导电率(通过添加剂或受控放电调节)控制流动速度将产生以前无法达到的小喷射直径，以形成纳米纤维。无限制的几何形状使得能够控制低流量和局部导电率，这在受限方案中是不容易实现的。在熔体电纺中，导电性的明确作用以前还没有被探索过。这项工作涉及电导率对熔体电纺的机理影响，通过控制放电调节熔体电导率的能力，熔体电导率变化引起的流动速度的变化，发生这种变化的机理，以及使用熔体电纺的基本限制。

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.