Crystalline order and mechanical properties of as-electrospun and post-treated bundles of uniaxially aligned polyacrylonitrile nanofiber
This study investigates the crystalline order and mechanical properties of as-electrospun and posttreated polyacrylonitrile nanofibers. To keep the nanofibers under tension during the posttreatment, a modified method of preparing bundles such as multifilament yarn was used in which the alignment of the nanofibers and linear density of the bundles were controlled successfully. An increase in the nanofibers' diameter from 240 to 500 nm led to the E modulus, ultimate strength, and elongation at break of the bundles rising from 836 MPa, 45 MPa, and 38% to 1915 MPa, 98 MPa, and 120%, respectively. The crystallinity index (%) and coherence length of the nanofiber bundles were evaluated through wide-angle X-ray diffraction. The mechanical properties and crystalline order of the nanofiber bundles were both increased as a result of the posttreatment. Wide-angle X-ray diffraction patterns of annealed bundles showed equatorial diffraction from the (1010) reflection at ∼ 5.1 Å and from the (1120) reflection at ∼ 3 Å. The values of the coherence length, crystallinity index (%), ultimate strength, and E modulus of the bundles prepared from 240-nm nanofibers increased from negligible, 2%, 1109 MPa, and 48 MPa to 54 Å, 35%, 2235 MPa, and 95 MPa after annealing at 85°C in a mixture of water (95 wt %) and N,N-dimethylformamide (5 wt %), respectively.