Improving Transverse Compressive Modulus of Carbon Fibers during Wet Spinning of Polyacrylonitrile

The performance of carbon fibers depends on the properties of the precursor polyacrylonitrile (PAN) fibers. Stretching of PAN fibers results in improved tensile properties, while potentially reducing its compressive properties. To determine optimization trade-offs, the effect of coagulation conditions and the stretching process on the compressive modulus in the transverse direction (E_T) was investigated. A method for accurately determining E_T from polymer fibers with non-circular cross-sectional shapes is presented. X-ray diffraction was used to measure the crystallite size, crystallinity, and crystallite orientation of the fibers. E_T was found to increase with decreasing crystallite orientation along the drawing direction, which decreases the tensile modulus in the longitudinal direction (E_L) proportionally to crystallite orientation. Stretching resulted in greater crystallite orientation along the drawing direction for fibers formed under the same coagulation conditions. Increasing the solvent concentration in the coagulation bath resulted in a higher average orientation, but reduced the impact of stretching on the orientation. The relationship between E_T and E_L observed in the precursor PAN fiber is retained after carbonization, with a 20% increase in E_T achieved for a 2% decrease in E_L. This indicates that controlled stretching of PAN fiber allows for highly efficient trading off of E_L for E_T in carbon fiber.