# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Polyamide-Imide  
Polyamide-imide (PAI) is commercialized under the trade name Kermel(r)[1]. The polymer combines the properties of polyimides and polyamides, providing excellent strength and thermal stability [17]. As a result, PAI nanofibres are a popular choice in high-temperature filtration applications [19]. In electrospinning, PAI is also considered a useful alternative to polyimide, as it displays similar properties but has enhanced solubility [19]. While PI nanofibres cannot be electrospun directly, PAI may be directly dissolved in solvent and electrospun. PAI is soluble in a variety of solvents including DMAc, DMF, DMSO, pyridine, cyclohexanone, and tetrahydrofuran [54]. The electrospinning of PAI has been performed via dissolution in DMF at 25 wt% [21], in a 34 wt% NMP/DMF mixture [19], and in 1,3 dimethyl-2-imidazolione [20]. The use of various solvents to produce PAI solutions allows enhanced customizability of the solutions and resultant nanofibres as well as the potential for polymer blend nanofibres, as other polymers may be soluble in the same solvents. One advantage of the excellent solubility of PAI in various solvents is that a mixed-solvent system can be used to alter the nanofibre morphology during electrospinning [19]. A mixed solvent system of NMP and DMF under high humidity produced biomimetic pine needle-like fibres with a grooved surface morphology (Figure 3). Upon exposure to high humidity, the DMF was replaced by water and formed a porous semi-solid structure. Then, the water gradually replaced the NMP, and the pores on the surface of the jet became stretched by the electrostatic repulsive forces, leading to a grooved fibre structure. The grooved structure increased the specific surface area to allow for greater adsorption and retention of particulate matter during air filtration. Electrospinning within a humid environment also led to the formation of curled fibres, which produced a highly porous membrane. Increased specific surface area and membrane porosity contributed to improved filtration and decreased pressure drop, respectively. PAI may also be electrospun from a precursor and then converted to PAI via post-treatment [16]. Traditionally, PAI is difficult to synthesize. However, [PERSON] et al. [25] considered a simplified method to synthesize PAI nanofibres inspired by the two-step process for polyimide fabrication. Here, an amide-containing diamine was reacted with a dianhydride to produce a precursor solution of amide-containing polyamic acid. The precursor solution was diluted with DMAc and DMSO and electrospun at 20 kV with a distance of 25 cm and a flow rate of 0.70 mL/h. They used a disk-shaped rotating collector and a stationary collector to produce aligned and randomly oriented fibres, respectively. The membrane was then thermally treated under a nitrogen atmosphere to produce PAI. Thermal treatment was successful up to 450 \({}^{\circ}\)C. Above this temperature, the fibres displayed poor thermal stability. PAI nanofibres produced via electrospinning showed improved tensile strength and modulus compared to PAI films produced in the same way. The strength of electrospun PAI nanofibres is significantly higher than that of other polymeric nanofibres including nylon, PBO, and polyimide [16]. Further improvements in the mechanical properties of PAI nanofibres may be achieved by thermal annealing at high temperatures. Aligned nanofibrous PAI membranes produced from a precursor and annealed at a temperature of 400 \({}^{\circ}\)C had a strength of 892 \(\pm\) 15 MPa. This was a significant improvement compared to the strength of a membrane which was annealed at 350 \({}^{\circ}\)C, with a strength of 504 \(\pm\) 4 MPa. However, further increasing the annealing temperature to 430 \({}^{\circ}\)C decreased the tensile strength of the nanofibrous membrane to 702 \(\pm\) 6 MPa. Fibre alignment also altered the mechanical strength of the fibres, with aligned fibres showing much greater tensile strength and modulus compared to randomly aligned fibres.