Spectroscopic manifestations of nitrile group intramolecular repulsion in PAN

It has been found that methylene stretching bands in polyacrylonitrile (PAN) occur at higher frequencies and have a lower intensity than in other hydrocarbon compounds. This is a spectroscopic manifestation of nitrile group intramolecular repulsion in PAN, which leads to helical winding of the polymer chains. The distortion of the elementary unit geometry occurs on spiralization, changing the methylene band parameters. On heating, the polymer spiral unwinds, which restores the normal geometry, band position, and intensity. The unwinding of a polymer chain represents a thermal transition, occurring in PAN at a temperature of about 180 degrees C. This transition is a necessary prerequisite for initiation of cyclization.