Study of structure–mechanical heterogeneity of polyacrylonitrile-based carbon fiber monofilament by plasma etching-assisted radius profiling

We present a feasible method, named as radius profiling, to study the structure–mechanical heterogeneity of polyacrylonitrile (PAN)-based carbon fibers (CFs). CF monofilaments were peeled layer by layer by plasma etching, and their electrical resistances and mechanical properties were measured by a multimeter and a single-filament tensile tester, respectively. The method allows the structure and mechanical properties of CF monofilament to be explored with high spatial resolution. The measurement results show that the modulus distribution along the fiber radius direction is highly dependent on the local crystallinity, and the maximums of both modulus and crystallinity appear at approximately 1.1–1.6 μm beneath the surface. The observation is used to explain the high modulus of PAN-based CFs.