Wavy Graphene-Like Network Forming during Pyrolysis of Polyacrylonitrile into Carbon Fiber

Carbon fiber (CF) obtained by pyrolysis of polyacrylonitrile(PAN-CF)surpasses metals in properties suitable for diverse applications suchas aircraft manufacture and power turbine blades. PAN-CF obtainedby pyrolysis at 1200-1400 degrees C shows a remarkably high tensilestrength of 7 GPa, much higher than pitch-based CF (pb-CF) consistingof piles of pure graphene networks. However, little information hasbeen available on the atomistic structure of PAN-CF and on how itforms during pyrolysis. We pyrolyzed an acrylonitrile 9-mer in a carbonnanotube, monitored the course of the reaction using atomic-resolutionelectron microscopy and Raman spectroscopy, and found that this oligomerforms a thermally reactive wavy graphene-like network (WGN) at 1200-1400 degrees C during slow graphitization taking place between 900 and 1800 degrees C. Ptychographic microscopic analysis indicated that such materialconsists of 5-, 6-, and larger-membered rings; hence, it is not flatbut wavy. The experimental data suggest that, during PAN-CF manufacturing,many layers of WGN hierarchically pile up to form a chemically andphysically interdigitated noncrystalline phase that resists fractureand increases the tensile strength-the properties expectedfor high-entropy materials. pb-CF using nearly pure carbon startingmaterial, on the other hand, forms a crystalline graphene networkand is brittle.