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The internal fibre stresses govern the behaviour of the fibres during processing and in service. It was shown elsewhere /I/ that the magnitude of these stresses in the region of the first peak of the isometric heating (IH) diagram depends not only on the orientation of the macrochains but also on that of the macrostructure of the fibre. In the work reported here the relation was determined between the remaining parameters of the IH diagram, the conditions under which the polyacrylonitrile fibre was produced, and the composition of the copolymer.

A common method of studying fibre behaviour in heating is that based on isometric heating. It is well known /I/ that the stresses developing in the specimen in isometric heating depend on the mean orientation of the macromolecules but for fibres spun from a polymer solution other factors enter. The results of the investigation described here were obtained by isometric heating and are linked with the structure and properties of wet-spun polyacrylonitrile (PAN) fibres.