# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 4 Discussion
### State of Progress
When compared to polymer manipulation before electrospinning, post-treatments may be more easily implemented at an industrial scale. However, crosslinking treatments can adversely affect the resultant nanofibrous membrane, particularly with respect to pore size [32]. Therefore, the ability to improve mechanical properties using both polymer manipulation and post-treatments may be useful in improving the properties of the nanofibrous membranes without affecting the membrane performance. Overall, there is a focus on improving the mechanical properties of high-performance nanofibres as high strength is one of the mainmotivations for using these polymers. However, the manipulation of other properties such as chemical resistance, hydrophilicity, hydrophobicity, porosity, and flame resistance has also been attempted for specific applications [21; 32; 43]. Since these properties are highly dependent on the application, the manipulation of the properties generally does not occur until research has progressed to stage 3, where a direct application is being pursued. The third stage of research progress covers polymers for which both an electrospinning process and methods to optimize the process and improve the properties of the fibres have been well established. At this stage, research on electrospinning tends to narrow and specify itself to specific applications. Here, researchers may look at variations in membrane orientation, fibre morphology, and physical properties. They may also investigate the use of nanofibres within composites. Within the category of high-performance fibres, m-aramid and PAI are the two polymers best established in electrospinning. Methods for the electrospinning of m-aramid and PAI are well-documented, and the optimization of the electrospinning process has been thoroughly studied; the polymer solution, fibre orientation, and post-treatments of the nanofibrous membranes were considered, among other parameters [9; 11; 43; 45]. Studies to improve the mechanical performance of these nanofibres so that they are closer to the properties of conventional high-performance fibres are ongoing (e.g., via the post-treatment of m-aramid) [45; 49]. However, the majority of the research into these fibres considers specific fibre properties for a large number of applications. Examples of electrospinning work with PAI include forming unique fibre morphologies, using it as a support in composite materials, and scaling up the nanofibre manufacturing [19; 20; 21]. Likewise, m-aramid is being used within composite materials and has been studied for various applications including surface flame retardants, antimicrobial water filters, lithium-ion batteries, and breathable waterproof membranes [7; 8; 9; 21; 44]. In terms of industrial scalability, nanofibrous PAI has been produced using a needleless electrospinning setup, providing a method for the mass-production of PAI nanofibrous membranes [20]. In contrast, no studies have been conducted that consider the scalability of m-aramid nanofibrous membranes. Future investigation into the use of scalable electrospinning processes, such as needleless or multi-needle electrospinning, will be a vital next step in determining the scalability of m-aramid nanofibres. Although PBI electrospinningning progress mainly falls within the second category, specific applications of PBI nanofibrous membranes have been considered, including lithium-ion battery separators, fuel cells, water electrolyzers, respiratory masks, and carbon nanofibre precursors [26; 28; 30; 31; 32]. However, only one study investigated the scale-up processing of PBI using a multimozzle setup [28] and another study produced PBI nanofibrous membranes using a needleless setup [65]. As the applications for PBI move toward mass production, further study of mass production methods becomes necessary. Interestingly, PBI nanofibrous membranes are the only high-performance membranes for which the washing and reuse of the nanofibrous membranes have been examined [30].