# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 3 Electrospinning of High-Performance Polymers
### Pbi
The initial PBI nanofibrous membranes showed smooth and even fibres, free of beading. However, these membranes displayed low mechanical properties (7 MPa of tensile stress at break). The use of glutaraldehyde caused the swelling of the fibre; the diameter increased until 12 h of treatment. After 12 h, the diameter began to decrease again, indicating the occurrence of the crosslinking reactions. The tensile strength of the nanofibrous membranes increased after crosslinking, with significant increases seen up until 24 h of glutaraldehyde treatment (37 MPa of tensile stress at break). However, after this point, the glutaraldehyde bonded to PBI and acted as side-chain plasticizers for the polymer, causing a slight decrease in tensile strength for membranes that had been treated beyond 24 h.  
A crosslinking treatment using \(\alpha\),\(\alpha^{\prime}\)-dibromo-m-xylene (DBX) was also performed to improve the chemical and thermal stability of PBI nanofibrous mats for application as a battery separator [28]. In addition, the mats were treated using 4 (chloromethyl) benzoic acid (CMBA) to improve electrolyte wettability. The DBX and CMBA treatments resulted in the formation of rough and curvy nanofibres. In terms of thermal resistance, TGA found weight loss starting as low as 200 \({}^{\circ}\)C, but this was likely due to the loss of residual solvent in the fibre. Polymeric weight loss began at around 450 \({}^{\circ}\)C. Thermal shrinkage was also measured at 150 \({}^{\circ}\)C for 30 min. The membranes displayed excellent thermal resistance at this temperature: no changes in the colour or size of the membrane were observed. The porosity of the PBI membrane was 87% before crosslinking but decreased to 70% following  
Figure 5: Optimization of PBI nanofibres by varying the polymer concentration and applied voltage at a flow rate of 100 \(\mu\)L/h and a distance of 10 cm. Reprinted with permission from Ref. [27]. 2017, Wiley. crosslinking with DBX. However, after CMBA treatment, the porosity increased again to 78%. The mechanical properties of the PBI membrane were also tested, and it was found that after crosslinking, the strength of the membrane increased, but it decreased after the CMBA treatment due to the increase in porosity. At its strongest, the crosslinked membrane had an average tensile strength of 11 mPa. Therefore, this membrane was almost three times weaker than the conventional polyethylene (PE) battery separator, indicating that additional research is needed before the PBI electrospun nanofibrous membrane has the strength to rival the conventional PE membrane. Physical post-treatments to improve the mechanical properties of PBI nanofibrous membranes have also been tested. [PERSON] et al. (2016) used a thermal calendering process and temperatures between 30 and 180 \({}^{\circ}\)C to encourage bonding between fibres and increase the strength of the membrane for battery application. As the temperature increased, interbonding between fibres increased, which led to an improvement in the mechanical properties of the nanofibrous membranes. However, this interbonding also led to a decrease in porosity, pore size, and electrolyte uptake as the three-dimensional pore structure was significantly reduced. Moreover, the durability and reusability of PBI membranes for filtration applications were enhanced by laminating the PBI membrane between two layers of polyethylene mesh at 140 \({}^{\circ}\)C ([PERSON] et al., 2017). This improved the durability by immobilizing the nanofibres and allowing for bend and flex abrasion without significant damage. The membrane was able to be cleaned in acetic acid and deionized water to remove both organic and inorganic compounds, then reused for several cycles. Other applications of electrospun PBI nanofibres involved post-treatments to improve the specific adsorption or absorption properties of the membrane. One study used molecular imprinting to make the nanofibres selective for the adsorption of sulfonic compounds for application in the oil and gas industry ([PERSON] et al., 2017).