Modification of polyacrylonitrile (PAN) carbon fiber precursor via post-spinning plasticization and stretching in dimethyl formamide (DMF)

Abstract

This study investigates the possibility of using a post-spinning plasticization and stretching process to eliminate suspected property-limiting factors in polyacrylonitrile-based carbon fibers. This process was performed with the intention of removing surface defects (to improve tensile strength), attenuating fiber diameter (to promote more uniform heat treatment), and reducing molecular dipole interactions (to facilitate further molecular orientation). Among the various organic and inorganic solutions tested, treatment using aqueous dimethyl formamide (DMF) offered far and away the best properties and was therefore selected for further testing. Tested individually (as single filaments), fibers exposed to 80% DMF for 10 s gave the highest precursor values of elastic modulus (9.07 GPa) and tensile strength (675 MPa). 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 for fibers carbonized at 1000°C ultimately showed a 32% improvement in carbon fiber elastic modulus and a 14% improvement in carbon fiber tensile strength over regularly prepared carbon fibers. These results show that, to a certain extent, improvements in PAN precursor properties can translate to corresponding improvements in subsequently produced carbon fibers. Additional characterization using wide angle X-ray scattering (WAXS) and scanning electron microscopy (SEM) suggests that these improvements are due in part to improved lateral order as well as the successful elimination of surface defects and prevention of skin-core formation.