Abstract
Polyacrylonitrile fiber was heated in the temperature range of 200 to 250°C in an inert atmosphere (in the vacuum of 10-4_??_-5mm Hg) and in oxidative atmosphere (in air). The change in the molecular structure with heating was examined by means of X-ray diffractometry, infra-red spectrop-hotometry, electron spin resonance absorption, chemical analysis and measurements of solubility and of amounts of hydrogen cyanide and ammonia formed.
From the results of infra-red and elementary analysis, following two types of cyclic systems are suggested to be formed: It seems that the system A, expected to have a plane, sheet-like configuration, is possible to be arranged to form crystallites while the system B, expected to have a kink, twisted configuration, is hardly possible to be arrange to form them. This supposition seems to be supported by the facts that, on the X-ray diffraction curves, a halo corresponding to the diffraction of the (002) plane of graphite apgears at 2θ=26° with heating in the oxidative atmosphere while the halo does not appear at the angle in the inert atmosphere.
It is further observed that polyacrylonitrile fiber heated at 250°C in the inert atmosphere is completely soluble in 85% formic acid while the fibers heated at any temperatures in the oxidative atmosphere are not soluble in the acid. This suggests that formation of intermolecular crosslinks occurs only in oxidative atmospheres.
Mechanisms on coloration, oxidation, dehydrogeneration and formations of ammonia and hydrogen cyanide are also discussed.
