The density changes of acrylic fibers by isothermal stabilization at temperatures from 221 to 281'C have been investigated. The density increased with increasing heat treatment time and reached independently of heat treatment temperature a value of about 1.58-x1.60 g(cm3 at longer heat treatment times. The molecular weight and molecular volume per repeat unit of polymer of the original and heat-treated fibers were calculated from elemental constitutions and density by assuming that a repeat unit contains three carbon atoms in average. By the heat treatment in air the molecular weight increased up to about 11%, and the molecular volume decreased up to about 17%. The deference in densities between the acrylonitrile copolymer fibers heat-treated in air and in a silicone oil bath was small at initial periods of the heat treatments. This indicates that with the copolymer fibers the density change at initial periods of the heat treatment in air is mainly due to a volumetric shrinkage accompanied with the cyclization of nitrile side groups from initiation centers already present in the fibers. Polyacrylonitrile homopolymer fibers showed a distinctive retardation in the relation between density and heat treatment time. This retardation is attributed to the fact that the formation of initiation centers of the cyclization in homopolymer fibers is rate-determined by the diffusion of oxygen from the atmosphere into the fibers.

The orientations of polyacrylonitrile crystallites and stacks formed with cyclized polymer chains on thermal stabilization have been investigated for acrylic fibers in relation with the length change during stabilization. There are two regions in the length change with heat treatment time. One is a change appearing in an initial period of heat treatment time, and the other is a change developed in the following period. The change in the initial period is mainly due to the relaxation on heating of the molecular orientation confined by the fiber-manufacturing process and shifts from shrinkage to extension with increasing applied stress during heat treatment. The length change appearing in the later period is observed mostly as shrinkage even under higher applied stresses during heat treat ment and is considered to arise as a directional component parallel to the fiber axis of the volumetric shrinkage due to the stacking of the cyclized segments. The orientation of polyacrylonitrile crystallites changes in corresponding directly with the length change in the initial period of heat treatment. The orientation of the stacks of cyclized segments changes dependently on the variation of the orientation of polyacrylonitrile crystallites in the initial periods of heat treatment if the fibers are heat-treated under constant applied stresses. That is, if the fibers are heat-treated by keeping the orientation of polyacrylonitrile crystallites in a higher degree, the orientation of the stacks of cyclized segments gives a high value. The effect of applied stress during heat treatment on the orientation of the stacks of cyclized segments has been studied at different heat treatment temperature and time conditions which can be assumed equivalent mutually in respect of the shrinkage in free state. The effect of various applied stress cycles during heat treatment on the orientation of the stacks of cyclized segments has been also investigated.

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