# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 4 Discussion
### Challenges, Limitations, and Avenues of Solutions
#### 4.2.3 Mechanical Properties  
Another challenge documented in the electrospinningning of high-performance nanofibres is the loss in mechanical properties observed when producing fibres at the nanoscale [15]. A drastic decrease in strength is often observed due to poor chain orientation and chain extension within the nanofibres. This is particularly an issue for flexible chain polymers, which have very short relaxation times. Although the jet produced during electrospinning can help orient the polymer, a short relaxation time allows the polymers to relax before solidification on the collector, leading to decreased strength. This issue is also observed in conventional spinning methods and is often remediated with drawing processes to improve polymer alignment. However, at the nanoscale, drawing abilities are increasingly limited and often cannot significantly improve crystallinity or orientation.  
In most instances, a direct comparison between the strength of a nanofibre and the strength of a conventional fibre is unavailable, as the majority of research that characterizes the mechanical properties of high-performance nanofibres considers the strength of the overall membrane, as opposed to the strength of a single fibre. One study measured the strength of a single electrospun p-aramid fibre [15]. The 2.1 um diameter fibre had a tensile strength of approximately 1.0 GPa and a Young's modulus of 58 GPa. A conventional p-aramid fibre typically has a tensile strength greater than 2 MPa and a Young's modulus greater than 60 GPa. In this instance, the strength of the electrospun p-aramid fibre was in the same range as a conventional p-aramid fibre. However, the diameter of the electrospun fibre was not at the nanoscale. Therefore, more data are needed on the mechanical properties of single nanofibres in order to compare their mechanical properties to conventional microfibers.  
The mechanical properties of the nanofibre may also be affected by other factors, such as the use of aprotic solvents and salts, which inhibit the recrystallization of the polymer [45]. The use of rigid-rod polymers can improve mechanical strength during electrospinningning, as these polymers have a built-in chain extension and can be easily oriented during the spinning process using elongational flow [15]. Other methods have also been explored to improve the mechanical properties of nanofibres, including strengthening intermolecular interactions from imide groups and hydrogen interactions from amide groups [16].  
In terms of the mechanical performance at the membrane level, for non-woven structures, the strength of the membrane is strongly affected by the consolidation of the fibres, as opposed to the inherent strength of the fibres [69; 70]. Thus, membrane-level treatments may be more effective at improving the overall strength of the nanofibrous mat. For example, the post-treatment of non-woven membranes is a common option to improve their mechanical strength [69; 70]. By joining fibres together within the nanofibrous mat, either by chemical treatment or thermal calendering, for instance, the overall strength of the membrane can be increased [26; 28]. However, these treatments typically lead to undesirable side effects including fibre swelling, decreased porosity, increased brittleness, and decreased pore size. Another strategy relies on improving fibre alignment, which increases mat strength in one direction [16]. At the moment, methods to increase the strength of nanofibrous membranes without reducing other membrane properties have not been observed. Moving forward, the use of composites and laminates to improve the durability of nanofibrous membranes where high porosity and low pore size is needed may be a promising path to improve the durability of the membranes [7; 30].