# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Para-Aramid  
Para-aramid (p-aramid) is a class of highly crystalline high-performance fibres with superior mechanical properties and excellent chemical resistance [1]. The most popular p-aramid is poly-paraphenylene terephthalalamide (PPTA), which has been commercialized by DuPont as Kevlar(r) and by Teijin as Twaron(r). Due to its high crystallinity, the fibre is especially useful for applications requiring ballistic resistance [3]. P-aramid has high thermal resistance, with a continuous operating temperature of 190 \({}^{\circ}\)C and a melting temperature of 400 \({}^{\circ}\)C [1]. It is also inherently flame-resistant. Unfortunately, the high crystallinity and lack of sidechain groups in the p-aramid polymer make it very difficult to dissolve [3]. As a result, electrospinning from solution is very difficult to achieve. The earliest record of electrospinningning p-aramid involved dissolution in a concentrated solution of sulphuric acid at a high temperature and with a grounded water bath as a collector [57]. However, this method was unable to allow a continuous electrospinningning process. Moreover, the use of concentrated sulphuric acid as a solvent for p-aramid makes industrial scale-up difficult, as the solvent is corrosive. Methods to produce p-aramid nanofibres without the use of sulphuric acid have also been explored. For the most part, p-aramid nanofibres are synthesized using alternative methods such as top-down dispersal [58], polymerization-induced self-assembly [14], and centrifugal spinning [13]. While bottom-up methods showed some promise, they still resulted in nanofibres with large diameters and low tensile strength [14]. Likewise, centrifugal spinning suffers from high energy consumption and faces the issue of equipment corrosion. Therefore, investigation into the electrospinningning process is still useful. Within electrospinningning, two major discoveries have encouraged the use of this method to produce p-aramid nanofibres. [PERSON] et al. [15] determined that anisotropic solutions of p-aramid could be more readily electrospun at higher concentrations compared to the traditional isotropic solutions (Figure 4). The p-aramid polymer was dissolved in sulphuric acid to form both isotropic p-aramid solutions at 2, 5, and 7 wt% of solid content and anisotropic solutions at 15, 17, and 19 wt%. The solution was electrospun using a needle-based electrospinningning setup. Fibres were collected into a water bath for coagulation, which was necessary due to the non-volatility of sulphuric acid. A heating band was also used to maintain an electrospinninging solution temperature between 80 and 90 \({}^{\circ}\)C. Below 80 \({}^{\circ}\)C, anisotropic solutions of p-aramid would begin to solidify, and above 90 \({}^{\circ}\)C, the solution would begin to degrade. After electrospinningning, fibres had to go through an additional drying step. It was observed that the jet had minimal whipping due to high voltage and low distances (2-8 cm), which formed a very strong electric field. At lower concentrations of isotropic p-aramid solutions, only droplets were observed. At 7 wt%, some very short fibres were seen, but breakages of the solution jet prevented continuous spinning. The entanglement of the polymer at higher p-aramid concentrations in anisotropic solutions allowed for the formation of continuous fibres. However, these fibres showed large distributions in diameters, between 16 and 300 \(\upmu\)m, which was attributed to branching during the electrospinningning process. Although some nanoscale p-aramid fibres were observed, the large diameter distribution, high voltages, use of corrosive solvent, and jet breakage inhibited the continuous production of large amounts of nanofibres. Therefore, although using higher polymer concentrations in anisotropic solutions was successful in the formation of fibres via electrospinningning, a continuous and scalable process could not be achieved.