Continuous carbon nanofibers prepared from electrospun polyacrylonitrile precursor fibers
The electrospinning process of fiber production is examined in regards to the preparation of continuous Polyacrylonitrile (PAN) nanofibers with the purpose of preparing carbon nanofibers for the reinforcement of thin films and nanocomposites. The mechanical properties and reinforcing behavior of nanofibers are expected to differ significantly from their conventional counterparts; the strength of a carbon filament increases as the diameter decreases.
Discrete lengths of partially aligned and oriented electrospun PAN fibers with diameters between 0.27μm and 0.29μm (FESEM) were prepared from solution with Dimethylformamide electrospun between 10-16kV and collected onto a take-up wheel rotating between 3.5m/s and 12.3m/s. A maximum chain orientation parameter of 0.23 is determined by dichroism (FTIR) and wide angle X-ray diffraction (WAXD) for fibers collected between 8.1m/s and 9.8m/s. Twisted yarns of aligned PAN nanofibers with an average denier of 446 and twist angles between 1.1° and 16.8° are prepared. The ultimate strength and modulus of the twisted yarns 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 9.3°. Drawing of the untwisted yarn in batch above the Tg of the material resulted in a further increase of the ultimate strength and modulus to 253±46 MPa and 4.8±0.5 GPa at a draw ratio of approximately 1:2; the ultimate strength of commercially prepared precursor fiber is 512MPa with post-treatment. A high temperature oxidation process is optimized and carbon nanofibers are prepared. The carbonized yarns are observed to exhibit graphitic structure by Raman scattering and wide angle X-ray diffraction (WAXD), although the mechanical properties are weak.
Continuous lengths of electrospun PAN yarn, provided through a collaboration, are characterized and evaluated for the production of carbon nanofiber yarn. The yarn is continuously processed and converted into carbon. Although, the yarn exhibits graphitic strucuture, the strength of the carbonized nanofiber yarn is only approximately 5% that of commercial carbon fiber prepared from the same precursor material. The poor mechanical properties are due to a loss of preferred orientation during an extended stabilization step.