The changes in the phase composition, the average coherent-scattering domain (CSD) sizes, and the texture of a polyacrylonitrile (PAN) filament during thermomechanical treatment are studied by X-ray diffraction. At the initial stage of treatment, the CSD sizes and the filament material texture increase. An increase in the tensile load improves the texture. The phase transformation of the structure of a PAN filament into the structure of a thermostabilized fiber proceeds via the formation of a new fine ~1-nm phase in local microvolumes. An increase in the number of such local microvolumes is accompanied by gradual fragmentation of CSDs and a break in the texture of noninteracting polyacrylonitrile.
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