Spectroscopic manifestations of nitrite group intermolecular repulsion in PAN

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Abstract
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 intermolecular repulsion in PAN, which leads to helical winding of the polymer chains. The distortion of the elementary unit geometry occurs on spiraiization, 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°C. This transition is a necessary prerequisite for initiation of cyclization.