| Post-spinning modification of polyacrylonitrile carbon fiber precursors was performed using plasticization methods with the intention of removing surface defects (to improve tensile strength), attenuating fiber diameter (to promote more uniform heat treatment), and reducing intermolecular dipole interactions (to allow further molecular orientation). Among the various organic and inorganic solutions tested, treatment using aqueous dimethyl formamide (DMF) offers far and away the best properties. Tested individually, fibers exposed to 80% DMF for ten seconds gave the highest precursor values of elastic modulus (9.07 GPa) and tensile strength (675 MPa). The other solutions tested (i.e., DMSO, ZnCl2, and CuCl) produced fibers offering no improvement over drawn, untreated PAN fibers. While fibers treated in 80% DMF gave a 73% improvement in elastic modulus and a 53% improvement in tensile strength over as-received PAN, limitations in sample preparation and carbonization necessitated a reduction in DMF concentration (to 30%) to allow extraction of individual carbon fibers for tensile testing. Despite this compromise, results ultimately showed a 32% improvement in carbon fiber elastic modulus and a 14% improvement in carbon fiber tensile strength over regularly prepared carbon fibers. Analysis using additional characterization methods (e.g., WAXS, SEM) suggests that these improvements are due in part to improved lateral order, larger crystallite size, and prevention of skin-core formation. |
