# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Meta-Aramid
The m-aramid nanofibrous mats were washed in distilled water and dried at 70 \({}^{\circ}\)C. The Young's modulus increased from 170 mPa to 752 mPa after washing, confirming the significant effect of the presence of LiCl on the mechanical properties of the nanofibrous mat and showing the efficiency of the washing treatment at improving the performance of the nanofibrous mats. The annealing of m-aramid nanofibrous mats was also attempted [47]. However, a heat treatment below 200 \({}^{\circ}\)C showed little effect on the mechanical properties of the nanofibres. Above 200 \({}^{\circ}\)C, the properties of the membrane decreased. Conversely, microwave irradiation was able to remove excess solvent and salt present in the as-spun nanofibrous membrane, leading to an increase in tensile strength of 2.8 times compared to the untreated membrane [49]. However, this method was highly energy intensive. A lower-energy, solvent-assisted heat treatment was proposed as an alternative method [45]. Electrospun membranes were submersed in a solution of DMAc, water, and ethylene glycol before being heat-treated. It was found that a heat treatment of 120 \({}^{\circ}\)C was optimal to encourage crystallization within the nanofibres. The resultant membrane showed an increase in tensile strength and modulus compared to the untreated and heat-treated mats. The Young's modulus was 13.5 times and 1.7 times higher than that of the untreated and heat-treated membranes, respectively. It also showed improved chemical resistance. Due to the interest in m-aramid nanofibres as high-strength scaffolds for nanocomposites [7; 8] and coaxial nanofibres [46], finding efficient methods to increase the crystallinity and strength of the membranes is vital in ensuring their relevance to practical applications. Beyond optimizing the mechanical properties of m-aramid nanofibrous membranes, multiple studies have conducted other manipulations in membrane properties to make nanofibrous m-aramid mats more suitable for specific applications. One of the major advantages of working with the m-aramid polymer is its excellent resistance to heat and flame. Therefore, ensuring the heat and flame resistance of nanofibrous mats is important for many applications [10; 43]. The thermal degradation of m-aramid nanofibrous mats was found to be similar to that of the corresponding staple fibre, with weight loss occurring at 100 \({}^{\circ}\)C due to solvent evaporation and again at 430-450 \({}^{\circ}\)C due to chain scission [10]. In contrast, the heat shrinkage behaviour of the m-aramid nanofibrous membranes was much larger compared to microfibrous mats. In 30 min intervals, the membrane was exposed to temperatures between 200 and 350 \({}^{\circ}\)C. At 200 \({}^{\circ}\)C, the shrinkage was 7.5%, but increasing the temperature to 350 \({}^{\circ}\)C resulted in a heat shrinkage of 50.4%. Work was conducted to try reducing the thermal shrinkage of m-aramid nanofibrous membranes. In one study, m-aramid/polysulfone-amide (PSA) membranes with different ratios of the two polymers were electrospun to see if the blending would improve the dimensional stability of the resulting mats after heat exposure [50]. After 200 h in a 250 \({}^{\circ}\)C oven, the 100% m-aramidnanofibrous membranes had shrinkage of 10% and 6% in the warp and weft directions. In contrast, a 7/3 blend of m-aramid/PSA resulted in a 6% shrinkage in both directions, and blends of 5/5 and 3/7 displayed no shrinkage after heat exposure. PSA is a flame-resistant polymer, therefore its addition to the nanofibrous membrane should not affect the flammability of the membrane [51]. However, the flame resistance of the membrane was not tested in this study [50]. The addition of SiO\({}_{2}\) nanoparticles also reduced the shrinkage of the m-aramid nanofibrous membrane, providing thermal stability without shrinkage up to 260 \({}^{\circ}\)C [7].