# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Pbi  
Polybenzimidazole (PBI) was developed in the 1960s; it has excellent mechanical, chemical, and thermal properties [62]. The polymer has a high glass transition temperature of 450 \({}^{\circ}\)C and a degradation temperature of 580 \({}^{\circ}\)C [1]. Due to its high thermal resistance, PBI lends itself to many applications for nanofibrous membranes. PBI nanofibrous membranes have been studied for applications including lithium-ion battery separators, fuel cells, water electrolyzers, respiratory masks, and carbon nanofibre precursors [26; 28; 30; 31; 32]. PBI is readily dissolved in DMAc alone and in the presence of aprotic salts such as LiCl and LiBr [26; 63; 64]. The earliest reported method for electrospinning PBI fibres involved a solution of 20 wt% PBI dissolved in DMAc and 4 wt% LiCl in a nitrogen atmosphere under high temperature [63]. A top-down electrospinning setup was used, where the polymer solution was contained in a glass pipette with a small opening, and the positive charge was supplied by a wire immersed directly in the polymer solution. A grounded rotating drum was used as a collector. After collection, nanofibrous mats were washed in methanol to remove excess salt and solvent, then washed again with 50% sulphuric acid to improve the thermal shrinkage resistance. Finally, the membranes were heat-treated above the glass transition temperature of PBI. This method was able to produce randomly oriented nanofibres with diameters around 260 nm. More recently, electrospun nanofibers mats using an altered form of PBI (m-es-PBInet) have been produced using needleless electrospinning on the Elmarco Nanospider [65]. The m-es-PBInet was dissolved at 15 wt% in a solution of DMAc and ethanol and electrospun at a voltage of 70 kV and a distance of 180 mm. This produced nanofibres with diameters between 520 and 250 nm. Optimizations of electrospinning processes and solution properties have been conducted for PBI nanofibres. However, there is a major focus on the application of PBI; therefore, studies looking at the effect of electrospinning parameters alone are not found in the literature. Instead, studies that examined PBI nanofibres for a specific application also performed optimization processes for the electrospinning process. [PERSON] et al. [28] observed the impact of the PBI concentration in the polymer solution on the nanofibre diameter. PBI was dissolved in DMAc at concentrations between 14 and 18 wt% and electrospun to determine the optimal polymer concentration. At PBI concentrations of 14 and 15 wt%, highly beaded nanofibres were observed [28]. Between 16 and 18 wt%, smooth and even fibres were formed via electrospinningning. It was found that 16 wt% was the optimal concentration for achieving even fibres with small diameters (90-150 nm). Higher concentrations of PBI (up to 35 wt%) have also been electrospun successfully; however, the diameter of these nanofibres was significantly higher, with a mean diameter of 729.9 nm [26]. [PERSON] et al. [27] used various solution concentrations, voltages, distances, and feed rates to optimize the electrospinning process for PBI nanofibres (Figure 5). The relationship between feed rate and polymer concentration was examined; it was determined that 15 wt% PBI and 100 \(\mu\)L/h was the most effective combination for stable electrospinning and bead-free fibreswith an average diameter of 170 nm. The effects of voltage were also characterized using voltages of 12, 15, and 18 kV. It was found that the combination of 15 wt% PBI, 15 kV, and 100 \(\mu\)L/h was the most effective for a reproducible electrospinning process with smaller fibre diameters. A variety of post-treatments have been examined to enhance the mechanical properties of the PBI nanofibrous membrane [26; 28; 32]. Chemical treatments are a common way to encourage the crosslinking of polymers after electrospinning. [PERSON] et al. [32] immersed an electrospun PBI mat into a solution of glutaraldehyde to encourage crosslinking and observed how the properties of the nanofibrous mat changed over time.