# Mechanical and Dielectric Properties of Aligned Electrospun Fibers
## 3 Mechanical and Dielectric Properties of Nanofibers
### Drawing Process  
Hot drawing of polymers is achieved by drawing the precursors at temperature above the glass transition temperature. [PERSON] et al. (2017) successfully carried out hot-drawing of PAN electrospun ribbons to enhance molecular orientation. PAN nanofiber ribbons were hot-drawn to two times and four times their original length. These ribbons were further stabilized at 250-300 \({}^{\circ}\)C and then carbonized at 1100 \({}^{\circ}\)C for 1 h. The tensile modulus and strength of four times hot-drawn ribbons showed maximum values of 287 and 5.4 GPa, which are 71% and 111% increment respectively as compared to as-spun electrospun CNFs. [PERSON] and [PERSON] (2019) studied the templating effect of functionalized SWCNTs in CNFs and the contribution of that to mechanical properties of CNFs. To enhance the packing of polymer chains of the precursor around CNT's, PAN as-spun electrospun fibers were subjected to thermomechanical processing (hot-drawing). The MEMS-based single-nanofiber mechanical testing result showed a strong relationship between the modulus, strength improvement and hot-drawing process. The average tensile strength and modulus of CNF/SWCNT were measured to be 7.6 \(\pm\) 1.72 and 268 \(\pm\) 29 GPa respectively [146]. Polyamide (PA) fibers are undergone post-drawing process to obtain moderate molecular orientation and crystallinity. After the post-drawing process, PA is found to be with good mechanical strength and abrasion resistance [134]. [PERSON] et al. (2020) revealed enhanced effect of graphene oxide (GO) in PAN nanofiber yarns. The alignment of PAN chains and GO in nanofibers was enhanced by hot-drawing which resulted in increased orientation induced crystallization [147]. [PERSON] et al. (2019) experimentally found that hot-drawn polyethylene fibers with decreasing fiber diameter, Young's modulus increase rapidly. That is due to the fact that chain orientation parameter increases with increasing hot-drawing ratio [148]. [PERSON] et al. (2006) reported that higher rotational speed of fiber collecting disc and post-processing such as annealing and hot-drawing on molecular structure of PLLA nanofibers improved crystalline structure orientation and mechanical strength of fibers. Better mechanical properties were found at higher hot-drawing ratio [149]. Isotactic polypropylene (iPP) nanofibers with the diameter range of 75-375 nm were made from the blends of cellulose acetate butyrate (CAB) and iPP with a ratio of 97.5-2.5. The hot-drawn nanofibers with the ratio of 25 resulted in lower crystallinity than that of bulk iPP. The increase in the amount of CAB in blends gave rise to higher crystallinity in iPP fibers [150].  
Cold drawing of polymers is normally done below the glass transition temperature. Cold-drawn nanofibrillated cellulose nanopaper increases its modulus and strength from 10 GPa and 185 MPa to 24.6 GPa and 428 Mpa, at the draw ratio of 1.6 [151]. Cold-drawn blend fibers of PVA and PTFE increased the degree of crystallinity in PTFE/PVA fibers [152]. The addition of hydroxyethyl cellulose (HEC) in cellulose nanofiber improved mechanical property. An aqueous solution of low concentration cellulose nanofiber with HEC promoted nanofiber alignment which was further improved by cold drawing [153].