The Correlation Between Diffusion During The Coagulation Step Of Wet-spinning And Radial Distribution In Pan Fibers

Carbon fiber shows the heterogeneity along the radial direction which skin layer has the apparently different structure compared to the core.The mechanical properties of carbon fibers are influenced by this existing defectives in the structure.From the viewpoint of structural inheritance,the radial heterogeneity in the structure of carbon fibers could be attributed to the evolution from Polyacrylonitrile(PAN)precursors through the process of the pre-oxidation and carbonization.Therefore,the radial structure in PAN precursors has become the major source of that in carbon fibers.According to the mechanism of fiber formation by the wet-spinning technique,the double diffusion which happened in coagulation process is the fundamental cause to the radial structure in PAN fibers.Hence,the coagulation process is the key stage which determines the radial structure in PAN fibers.In this study,SEM,EPMA,Micro-beam WAXD,Micro-Raman and Nano IR were determined to characterize the radial structure in PAN fibers from multiple sizes,morphological structure,aggregation structure and molecular chain structure and to establish the characterization system for hierarchical structure along the radial direction in PAN fibers.Based on kinetics of diffusion,the correlation between the diffusion during the coagulation process and radial distribution in PAN fibers has also been confirmed.Meanwhile,the controlling factors of diffusion like coagulating temperature and concentration acted on radial structure has been understood.The study results indicated that:(1)The established methods to characterize the radial structure from multiple sizes in PAN fibers were proved to be reasonable which revealed the regularities of distribution and built the radial distribution model.It was known that comparing the core region,the skin layer shows the higher density,preferable crystallinity and orientation,which the PAN chains tend to keep the shape of helical conformation.(2)The coagulation of the wet-spinning process had a decisive influence on the radial structure in PAN fibers,the bath temperature and concentration were both the main controlling factors.And the effects of these two factors on gradient of radial structure in PAN fibers were both reflected the positive correlation.(3)By using the Fick's second law,the diffusion coefficients of H2O and DMSO were calculated.The results indicated that the diffusion coefficients increase continuously with increasing temperature.And the value of diffusion coefficients of H2O and DMSO keep the same tendency when the bath concentration rises.After analyzing the functional relationship between distribution coefficient a of radial crystalline structure in PAN fibers and diffusion coefficient D,it can be proved that the double diffusion in coagulation process became more sufficient with the diffusion coefficient increases which resulted the value of a decreases.