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It has been well established that the excellent performance of carbon fibers is closely related to their microstructures resulted from the oxidative stabilization and the carbonization ofpolyacrylonitrile (PAN) precursor fibers.During these procedures,the skin-core difference in the fibers occurs due to the thermal transmission and limited oxygen diffusion.The non-uniformity of carbon fibers between the skin and core regions inherited from the stabilized fibers is of crucial importance since it can reduce the mechanical properties of the resultant carbon fibers.In this study,nanoscale dynamic mechanical imaging was used to map the storage modulus along the cross section of fibers during the evolution from polyacrylonitrile (PAN)precursors to carbon fibers.The quantitative characterization of skin-core difference in oxidative stabilized fibers indicated that the primary skin-core structure in the fibers forms at 265℃ and the skin-core difference of the final oxidative stabilized fibers in modulus was about 8%.The difference has been enhanced in modulus but reduced in spatial scale during the transition from stabilized PAN fibers to carbon fibers, as shown in Figure 1.