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.

Automated summary

Carbon fiber obtained by pyrolysis of polyacrylonitrile exhibits superior properties for various applications, such as aircraft manufacture and power turbine blades. The structure of polyacrylonitrile-derived carbon fiber consists of hierarchically piled up wavy graphene-like networks, which exhibit high tensile strength and resistance to fracture. Conversely, pitch-based carbon fiber, consisting of crystalline graphene networks, is brittle.