抄録
Polyacrylonitrile fiber was heat-treated for 1.5 hours in nitrogen at various temperatures mainly between 250 and 1000°C, and measurements of weight, density, solubility, X-ray, electron spin resonance and elementary analysis were carried out on the heat-treated samples. From the results of these measurements, following explanation may be given to the change in structure with the heat-treatment.
By heating in the range of 250 to 280°C, polyacrylonitrile fiber changes to a brittle fused solid due to rapid decomposition with following depression of molecular weight. Solubility for 85% formic acid develops by heating at above 250°C, but is lost by heating at above 400°C. The density increases fairly fast in the range of 230° to 250°C and continues to increase with rising of temperature. These changes may be attributed that low-molecular weight molecules having A type structure are formed and then the structure is transformed gradually to B type structure as dehydrogeneration reaction proceeds at higher temperatures. The results of X-ray and of elementary analysis seem to support the change in structure shown above. Moreover, insolubilization occures above 400°C suggests formation of intermolecular linkages.
Number of unpaired electrons tends to increase as temperature is elevated from 230 to 600°C. It is supposed that stable unpaired electrons detected as singlet spectra by electron spin resonance are mainly of the following types:
It is considered that as temperature is elevated, especially above 500_??_600°C, foreign atoms (including some carbon atoms) are eliminated markedly while carbon-carbon bonds increase in number, resulting in increase of aromatic planar fragments (condensed carbon rings) in size and in number.
