# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Meta-Aramid  
Meta-aramid (m-aramid) fibres are composed of poly(m-phenyleneisophthalamide) and were the first aramid fibres to be released onto the market [1]. Today, there are two commercial versions of m-aramid fibres: Nomex(r) from DuPont and Conex(r) from Teijin. Kermel(r) fibre, commercialized by Rhodia, is sometimes classified as an m-aramid fibre, but its structure is different, which affects its electrospinning process; therefore, it is covered in a separate section (Section 3.2). The structure of m-aramid is aromatic, containing a large number of double bonds and high C/H ratios [1]. This provides the polymer with inherent flame resistance. M-aramid is also highly temperature resistant. The polymer maintains its useful properties up to 370 \({}^{\circ}\)C and displays self-extinguishing properties upon ignition. As a result, m-aramid nanofibres are a popular choice in applications where flame resistance is required for a nanofibrous membrane. For example, [PERSON] et al. [43] studied the use of m-aramid nanowebs to act as a wood-surface flame retardant. Other applications of m-aramid nanofibres include antimicrobial water filters [8], lithium-ion batteries [7], and breathable waterproof membranes [21]. Multiple studies have confirmed the feasibility of electrospinning m-aramid. M-aramid is readily soluble in many polar aprotic solvents including dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP) [11]. However, due to the highly crystalline structure of the polymer, the use of solvent alone can only swell the polymer. To fully relax and dissolve m-aramid, the solvent is combined with alkaline salts such as lithium chloride and calcium chloride. In this process, the lithium and calcium ions link to the carbonyl group in the solvent, leaving the chloride ions free to interact with the m-aramid polymers. These ionic interactions allow for improved solubility of the polymer in the solvent. DMAc and LiCl is the most common solvent system reported for m-aramid polymers and was found to be the best solvent system for producing m-aramid nanofibrous webs with good mechanical properties (e.g., Figure 2) [11]. NMP is also an effective solvent for dissolving m-aramid with both LiCl and CaCl\({}_{2}\), but the solvent properties made electrospinning difficult. Notably, DMF and DMSO are less effective at dissolving m-aramid. Both solvents require high concentrations of salt to fully dissolve m-aramid at a 10 wt% concentration. The addition of salt changes the viscosity and conductivity of the solution, which impacts the diameter of the resultant nanofibres. With an increase in the concentration of LiCl salts, the average fibre diameter increases [11]. Generally, optimal concentrations of LiCl range from 0.5 wt% to 4 wt% for solutions containing between 10 wt% and 14 wt% m-aramid [11; 43]. As a starting point to produce m-aramid polymer solutions, a variety of polymer sources were used. In many cases, m-aramid polymer (presumably in powdered form) was purchased directly for use [7; 9; 12; 21; 43; 44]. However, there were instances in which m-aramid fibres were used as a polymer source for the solution [8; 11; 45; 46; 47]. Moreover, some studies chose to synthesize their own m-aramid prior to electrospinning [48; 49]. Although m-aramid has been electrospun by many different researchers, the exact parameters for electrospinning have varied widely between individual studies. Table 1 describes some of the different electrospinning conditions for producing m-aramid nanofibres. Voltage is one parameter that has a significant impact on fibre diameter. The electrospinning of m-aramid fibres using CaCl\({}_{2}\) and DMAc solvent at voltages of 12 kV and 15 kV produced average fibre diameters of 150 nm and 121 nm, respectively [12]; a difference of only 3 kV created a variation of 20% in the average fibre diameters. Other studies used  
Figure 2: SEM Image of m-aramid nanofibres spun from a solution of 14 wt% Nomex and 3 wt% LiCl in DMAc.