# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 4 Discussion  
### State of Progress  
The previous section discussed the current progress in the electrospinning of high-performance polymers. Each of the five polymers discussed in this review has experienced progress in electrospinning for the formation of high-performance nanofibrous membranes. However, the stage of progress for each polymer varies considerably. In general, three stages can be observed: (1) the polymer is used in preliminary electrospinning research to determine its viability for nanofibre formation using electrospinning; (2) the electrospinning process for the polymer has been determined, and variations are being made to ensure the nanofibres will share the properties of the corresponding high-performance fibre (e.g., its excellent mechanical properties); (3) the electrospinning and post-treatment of the polymer have been optimized and research is now focused on specific applications and/or industrial scale-up. Depending on the desired application, the second and third progress stages may overlap. For example, a nanofibrous membrane may have the porosity and strength to be applied in a filtration process but lack the necessary chemical stability if chemical resistance is required. Therefore, scale-up may be possible for the less demanding versions of the targeted application before all characteristics of the nanofibre are optimized. Progress on the electrospinning of p-aramid and PBO nanofibres mainly falls within the first stage, as the viability of electrospinning these polymers is still the major focus of the research at this time. P-aramid has faced considerable challenges in electrospinning due to its high crystallinity, as it can only be dissolved in concentrated sulphuric acid [14]. Likewise, PBO is a rigid-rod polymer and requires concentrated sulphuric acid to form a polymer solution [37; 59]. As a result, solution electrospinning is more difficult for these polymers compared to other nanofibre formation processes. For p-aramid, the most promising progress for solution electrospinning is side-chain substitutions on the polymer before electrospinning. This allows p-aramid to be dissolved directly in common organic solvents [3]. However, only one study has looked into this method, and it is still undetermined how side-chain substitutions may impact the properties of the p-aramid nanofibres. Further, the efficiency and scalability of the side-chain substitutions were not discussed in this study. For PBO, more progress has been made in solution electrospinning, with precursors being used to form a polymer solution. Precursors provide a method to produce a polymer solution in an organic solvent, as opposed to a concentrated sulphuric acid solution. Although, the research on electrospinning PBO from precursors is still quite limited [24; 25]. When compared to polymers that can be directly dissolved in solvents and electrospun, the processing of PBO nanofibres is more complex, which may not be as favourable for industrial applications. Further, it is uncertain how the post-treatment of the PBO precursor nanofibres could limit the combination with other materials (e.g., coaxial or composite nanofibres). Although research on the electrospinning of PBO predominantly falls within the first stage of electrospinning progress, it is beginning to move toward the second stage, for example with the use of the OH-PAA precursor instead of OH-PA to improve the molecular weight and mechanical strength of the PBO nanofibres [25]. The second stage of electrospinning progress considers polymers for which a viable electrospinning process has been established. In this stage, research is focused on improving the properties of the nanofibres to better align with the expected properties of conventional high-performance fibres. The electrospinning of PBI fits well within this stage. The mechanical strength of PBI nanofibrus membranes has been improved using crosslinking treatments and thermal calendering after electrospinning [26; 28; 32].