Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yams

The fiber spinning technique of electrospinning was optimized in order to prepare unidirectional aligned, structurally oriented, and mechanically useful carbon precursor fibers with diameters in the nanoscale range. The fiber spinning velocity and fiber draw ratio was measured to be between 140 and 160 m/s and 1:300,000, respectively, for fibers spun from 10 wt% polyacrylonitrile (PAN) solutions with dimethylformamide (DMF). A high-speed, rotating target was used to collect unidirectional tows of PAN fibers. Aligned and (+) birefringent fibers with diameters between 0.27 and 0.29 mum (FESEM) were collected from electrospinning 15 wt% PAN in DMF solutions at 16 kV onto a target rotating with a surface velocity between 3.5 and 12.3 m/s. Dichroism measurements (Polarized FTIR) of the nitrile-stretching vibration show an increase in the molecular orientation with take-up speed. Wide angle X-ray diffraction patterns (WAXD) show equatorial arcs from the (10 (1) over bar0) reflection at d similar to 5.3 Angstrom and (1120) reflection at d similar to 3.05 Angstrom. A maximum chain orientation parameter of 0.23 was determined for fibers collected between 8.1 and 9.8 m/s. Twisted yams of highly aligned PAN nanofibers with twist angles between 1.1 and 16.8degrees were prepared. The ultimate strength and modulus of the twisted yams increase with increasing angle of twist to a maximum of 162 +/- 8.5 MPa and 5.9 +/- 0.3 GPa, respectively, at an angle of 9.3degrees. (C) 2004 Elsevier Ltd. All rights reserved.