High-temperature vapor deposition polymerization polyimide coating for elimination of surface nano-flaws in high-strength carbon fiber

The effect of polyimide coatings on the filament tensile strength of high-strength polyacrylonitrile-based carbon fiber was studied by using dip and high-temperature vapor deposition polymerization (VDP) coating processes. Unlike a VDP on a cold substrate, high-temperature VDP has the potential to directly synthesize and isotropically deposit a polyimide, from diamine and dihydride monomers without any by-products, on a substrate heated up to 200 degrees C. The average filament tensile strength of the flaw-sensitive carbon fiber improved with all the polyimide coatings used. Nevertheless, for the same monomers, the high-temperature VDP coating process was advantageous for high-efficiency surface flaw healing compared to the dip-coating process, resulting in a 25% increase in the average tensile strength of the carbon fiber. These results were evident not only for the carbon fibers without artificial nano-notches but also for those with artificial notches less than 30 nm in depth. Thus, we clearly showed the potential for the VDP polyimide coating to heal surface nano-flaws of the carbon fiber. The different infiltrations of the coating into nano-notches and its effect on the filament tensile properties were characterized, as well as discussing the impact of the VDP coating with an interlayer between the coating and the fiber. (C) 2011 Elsevier Ltd. All rights reserved.