Analyses of closed porosity of carbon fiber precursors using a robust thermoporosimetric method

We investigated a major fiber spinning process parameter, the coagulation bath concentration, which has a critical effect on the porosity of the polyacrylonitrile (PAN) fibers. Utilizing thermoporosimetry, a calorimetric method based on the Gibbs-Thomson effect, this work details the correlation between coagulation bath concentration, porosity and pore size distribution. Pore size, as well as the pore size distribution, of the fibers throughout the wet spinning process were determined in detail. Moreover, the changes in the number of porous zones, pore size and fiber morphology during the wet spinning process were also examined by changing the coagulation bath composition. Two coagulation bath compositions, 77.5 wt% and 80 wt% DMAc/deionized water were considered. The fibers spun at 80 wt% coagulation bath solvent concentration displayed skinless rough surfaces with high porosity values compared to the fibers spun at 77.5 wt%. Attempts to spin into slightly more concentrated DMAc baths were unstable due to reduced tow tenacity, leading to fiber failure. The study showed that even minor changes in the coagulation bath composition can have a pronounced effect on the fiber properties such as cross-sectional shape, the integrity of the external skin, and the number and size of pores.