Although it has been half a century since polyacrylonitrile (PAN)-based carbon fibers were first developed, the exact formation mechanism of skin-core structure of PAN-based carbon fibers, especially the stabilized PAN fibers, was still not well clarified from the viewpoint of the chemical structure. In order to address this aforementioned challenge, a powerful tool with nanoscale resolution named photo-induced force microscopy was applied to map the chemical group distribution in the cross section of stabilized PAN fibers and reveal the evolution mechanism of skin-core structure throughout the whole stabilization process. The results indicated that the formation of skin-core structure of stabilized PAN fiber was attributed to the complex and overlapped chemical reactions caused by gradient of oxygen along radial direction and the formation of dense crystal layer at the interface between the skin and core part. Finally, the crystal layer was destroyed and the monofilaments tended to be homogeneous with further oxidation.
Polymer dielectrics with a high energy density, an outstanding breakdown strength, and a low dielectric loss are currently in great demand in the field of film capacitors. Here, on the basis of fundamental understanding of dielectric relaxation in cellulose, environmentally friendly regenerated cellulose-based dielectrics were fabricated by manipulating their intrinsic hydrogen bonding network, in which hydroxyl groups of cellulose were reacted with epichlorohydrin (ECH) to simultaneously reduce the density of the intra- and intermolecular hydrogen bonding networks and the crystallinity for activating the movement of polar groups. As a result, when the molar ratio of ECH to glucose units of cellulose is 1:1, the regenerated cellulose-based dielectrics exhibited the highest dielectric constant of 9.7 at 103 Hz with the dielectric loss in an order of 10−2 and an energy density of 2.16 J/cm3 with a high charge–discharge efficiency of >85% at 200 MV/m. This methodology presented here provides a promising avenue for designing and improving the dielectric properties of cellulose-based dielectrics.
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