Effect Of Coagulation Conditions On Radial Properties Of Polyacrylonitrile Nascent Fibers

The radial homogenization structure of polyacrylonitrile(PAN)-based carbon fibers has an important effect on their performance.Attempts to reduce or eliminate differences in radial structures during the spinning,pre-oxidation,carbonization,and graphitization of PAN-based carbon fibers are currently receiving considerable attention in the carbon fiber industry.Coagulation is an important stage in the formation of nascent fibers from a PAN solution.During this process,substances are exchanged between a trickle of the PAN solution and a coagulation bath.The coagulation environment,and the path length of double diffusion at various points within the trickle are highly likely to cause differences in the radial structures of the nascent PAN fibers.Radial structural defects formed during the coagulation stage are inherited by the carbon fibers.This affects their performance.The coagulation conditions for the formation of PAN fibers with circular or nearly circular cross sections were investigated,and the results are reported herein.The radial properties of PAN nascent fibers were investigated by scanning electron microscopy(SEM),atomic force microscopy(AFM),nanoindentation testing,and optical density analysis.These techniques helped detect differences in the radial structures of the fibers,the reduction or elimination of which greatly affects their properties.(1)The deterioration of the cross-sectional circularity of the nascent fibers was partly due to the solubility of PAN in the aqueous solution of dimethyl sulfoxide at high temperatures or high concentrations,which resulted in the deformation of the fibers.As the coagulation bath concentration increased,the cross section of the fibers changed from waistshaped to circular-shaped.As the temperature of the coagulation bath increased,the overall compactness of the fibers decreases when the temperature of the coagulation bath is too high.However,there was no significant difference in the compactness of the structure between the edges and centers of the fibers obtained under various coagulation conditions in the present study,and there was also no difference in the radial structure of the fibers through morphology.(2)The concentration difference between the trickle of PAN solution and the coagulation bath,the thermal movement of molecules caused by the temperature,and the solubility of PAN in the aqueous solution of dimethyl sulfoxide affected the orderly stacking and arrangement of the PAN molecules.Before the maximum,the coagulation process was mainly driven by the concentration difference,and was controlled by the thermal movement of the molecules.The material exchange between the trickle and the coagulation bath was uniform and adequate,and the double diffusion was conducive to the orderly stacking and arrangement of PAN molecules.This improved the crystallinity of the nascent fiber;after the maximum point,the nascent fiber became disordered and the crystallinity decreased owing to the influence of the high coagulation bath concentration or the high coagulation bath temperature on the dissolution of PAN.(3)The mechanical properties,such as the radial modulus,and the distribution of PAN nascent fibers at various points can be determined by AFM analysis of a micro-area(1 × 1 μm).The change in the average radial modulus coincided with a change in the overall crystallinity of the PAN nascent fibers.The radial modulus of the fibers produced under various coagulation conditions differed depending on the position,but the core modulus was always smaller than the skin modulus.Nanoindentation was used to study the radial modulus distribution of nascent fibers produced using the same coagulation bath concentration and different coagulation bath temperatures.The difference between the core and skin modulus of the PAN nascent fibers formed in the coagulation bath at 35-45°C was small,indicating that AFM micro-area analysis was an effective method of determining the radial mechanical properties of the fibers.(4)A combination of ultra-thin sectioning technology and optical density analysis was used to study the effect of coagulation conditions on the radial light transmittance of the PAN nascent fibers.The light transmittance at a constant light intensity differed depending on the compactness of the fibers: the denser the material,the lower the optical density.In the present study,the radial density structures of PAN fibers derived from precursors of various densities was determined by measuring their optical densities.Using ultra-thin sectioning technology,it was possible to assess the radial optical densities of the nascent fibers according to their poor light transmittance.The change in the optical densities of the PAN nascent fibers coincided with the change in their bulk densities,which indicates that radial structure differences in PAN fibers can be quantitatively characterized by measuring their optical densities.