# Mechanical and Dielectric Properties of Aligned Electrospun Fibers
## 2 Molecular Orientation and System Configurations of Nanofibers
### System Configuration to Align Fibers  
In addition to molecular orientation, physical alignment of electrospun fibers contributes to the production of high strength/high toughness fiber reinforced composites [12]. Among the many ways to produce aligned fibers by using an electrospinning technique, drum collection and rotating disk collectors are the two most popular designs, as shown in Figure 5a,b [65; 107; 108; 109; 110; 111]. In the drum collector method, the drum collector rotates at high speed and the deposited fiber diameter can be controlled based on the rotational speed of the drum [112; 113; 114]. The linear speed at the surface of the rotational drum should match the evaporation rate of the solvent so that fibers are deposited and taken up on the surface of the drum. At a rotational speed less than the fiber take-up speed, randomly oriented fibers are obtained on the drum. At higher speed, the fiber take-up velocity breaks the fiber, and continuous fibers are not collected. Therefore, the optimal speed of the rotating drum that matches the evaporation rate is required for maximum alignment [115]. In the rotating disk collector, higher alignment is possible but the production rate is lower because fibers are effectively deposited at only a small area at the disk edge. [PERSON] et al. (2001) reported a conical and an inverted conical instability region of polyethylene-based polymer nanofibers. These finer fibers with diameters ranging from 100 to 300 nm got aligned and wound on a sharp edge disk wheel-like bobbin [116]. Other collection methods that are suitable for fiber alignment are the parallel conductor method, the wire drum collector method, and the wheel rotor collector method as shown in Figure 6a-d. In the parallel conductor method [117; 118; 119; 120], the length of aligned fibers is restricted by the distance between conductive stripes as shown in Figure 6a. [PERSON] et al. (2006) reported the fundamental parameters affecting the uniaxially aligned PAN nanofibers. The best alignment of nanofibers with a specific gap distance depends on concentration, voltage, and tip to collector distance. As shown in Figure 6b, a bundle collector is moved across the gap to another side for depositing bundle of nanofibers. The best alignment was formed between 10 and 15 wt.% solutions. Uniaxially aligned fibers formed had an aspect ratio (l/d) of higher than 5000 and these fibers are useful in composite reinforcement application. [PERSON] et al. (2018) studied the effect of alignment on fiber modulus, using the electrostatic gap method. Aligned polyethylene oxide (PEO) fibers have a higher modulus than the non-aligned fibers of similar diameter [121]. [PERSON] et al. (2017) provided an insight in to fabricating ultra-long polyvinylidene fluoride (PVDF) fibers. Here, parallel conducting U-shaped collectors are used to fabricate fibers [122]. According to [PERSON] et al. (2018), more than a meter long aligned PVDF nanofibers were fabricated by using gap electrospinning, where the needle is connected to positive power supply and the parallel plates are connected to the negative power supply [123]. [PERSON] et al. (2007) demonstrated a method that generates parallel fibers, using magnetic-particle-doped polymers in two parallel placed magnets. The magnetic field guides the magnetized electrospun polyvinyl alcohol (PVA) fibers to align in a parallel fashion [124]. [PERSON] and [PERSON] (2011) built uniaxial aligned PCL fibers by introducing an inclined gap into dual collectors that consisted of two conductive stripes which were arranged vertically and horizontally [125]. [PERSON] et al. (2009) used a hollow metallic cylinder with needle placed at the center of the cylinder. Fibers produced by this method are claimed to be well aligned and spread over large area [126]. Next, in the  
Figure 5: (**a**) Rotating drum collector. (**b**) Rotating disk collector [103]. wire drum collector method, shown in Figure 6c, fibers are deposited without the need for high speed rotation [126].