# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Pbo  
Poly(p-phenylene-2,6-benzobisoxazole) (PBO) is a rigid-rod polymer with excellent chemical, thermal, and mechanical properties [1; 59]. The conventional fibre is commercially produced by Toyobo under the trade name Zylon(r). The polymer has very high thermal resistance and will not degrade in air until 600 \({}^{\circ}\)C [1]. PBO is also flame-resistant and has self-extinguishing properties. Although the rigid rod structure imparts many desirable  
Figure 4: Electrospinning of p-aramid nanofibres in sulphuric acid: (**a**) electrospinningning setup; (**b**) resulting electrospray droplets from spinning with a 4 wt% p-aramid isotropic solution; and (**c**) electrospun fibres resulting from spinning with a 19 wt% p-aramid anisotropic solution Reprinted with permission from Ref. [15]. 2015, Wiley. characteristics to the polymer, such as excellent mechanical properties, it also makes the processing of PBO very difficult. PBO is only soluble in strong acids, making it difficult to electrospun directly [37; 59]. Instead, PBO nanofibres can be electrospun indirectly either from poly(o-hydroxy-amide)s (OH-PA) or poly(o-hydroxy-amic acid)s (OH-PAA) [25]. These precursors are soluble in organic solvents and can be transformed into PBO nanofibres via heat treatment processing after electrospinningning [25; 59]. The most common method of electrospinninging PBO from precursors involves the electrospinningning of an o-hydroxy aromatic monomer, such as poly(o-hydroxy-amide) (OH-PA) or poly(o-hydroxy-amic acid) (OH-PAA), which can be dissolved in common organic solvents, electrospun, and then thermally converted to PBO nanofibres [25]. However, using the OH-PA precursor can be problematic due to its low molecular weight, leading to poor mechanical properties of the resultant nanofibres. However, the lower molecular weight of the precursor can also lead to thinner nanofibres, which should be kept in mind while optimizing nanofibre diameters [23]. OH-PAA, having a higher molecular weight, can be used to electrospun PBO nanofibres with higher thermal resistance and improved mechanical properties. [PERSON] et al. [25] used a solution of 6 wt% OH-PAA in DMAc with cetyltrimethylammonium bromide (CTAB) to electrospun PBO precursor nanofibres. The solution was electrospun at 20 kV with a distance of 20 cm and collected on a rotating disc to form a nanofibre belt. The nanofibres were then heated under a nitrogen atmosphere and annealed at various temperatures to form PBO nanofibres. To assess the mechanical and thermal properties, a thermal-mechanical analyzer was used. The tensile testing of the aligned nanofibre belts was performed at various temperatures. It was observed that the modulus was well retained as the heat increased, with an 86% modulus retention at 350 \({}^{\circ}\)C. The test was performed up to 500 \({}^{\circ}\)C, where the modulus retention was still over 50%. The use of a high-molecular-weight precursor allowed for the formation of heat-resistant and high-strength PBO nanofibres with stability up to 643 \({}^{\circ}\)C.  
However, the use of the o-hydroxy monomer as a precursor to PBO can be problematic as it is difficult to synthesize, leading to high costs that limit the scalability of the process [29; 60; 61]. [PERSON] et al. [59] attempted to create PBO nanofibres using inexpensive precursor materials. Monomers of 4, 4\({}^{\prime}\)-diamino-3, 3\({}^{\prime}\)-dimethyloxydiphenyl (DMOBPA), and isophthaloyl chloride (IPC) were used in combination with DMAc, pyridinium, and LiCl to prepare a solution of methoxy-containing polyaramide (MeO-PA). The solution was electrospun on a needle-based electrospinning setup at a voltage of 30 kV, a flow rate of 0.8 mL/h, and a distance of 30 cm. The mat was then post-treated to remove LiCl and pyridinium and heat-treated under a nitrogen atmosphere. FTIR analysis indicated that a temperature of 350 \({}^{\circ}\)C was sufficient to transform the precursor to PBO. This method provides an inexpensive way to produce PBO nanofibres from an electrospun precursor.