Mechanisms of transformation of acrylic fibres during thermooxidative stabilization

The shrinkage of polyacrylonitrile fibres comprising 93% acrylonitrile, 5.7% methyl acrylate, and 1.3% itaconic acid was investigated by a thermomechanical method in heating them from room temperature to 300 degreesC. It was shown that the irreversible shrinkage of the fibres during their first heating is due to a decrease in the stresses accumulated during spinning. The shrinkage observed in repeated heating of the fibres is caused by two processes: 1) by unfolding of segments of PAN macromolecules in the amorphous constituent of the structure of the polymer with an increase in the temperature (thermoelastic effect); this part of the shrinkage is reversible in heating-cooling cycles; 2) by cyclization of the polymer units due to polymerization of CN groups; this is the irreversible part of the shrinkage. In the 200-300 degreesC temperature region, relaxation processes caused by irreversible unfolding of the macromolecules of the "melted" ordered structure of the fibre also contribute to the shrinkage. Mechanisms of the shrinkage processes in a wide range of temperatures are proposed. Shrinkage caused by physical processes of transformation of the structure of the fibre and chemical processes of formation of heterocycles is distinguished. It was shown that the kinetics of cyclization of PAN fibres can be investigated during their thermooxidative stabilization as one of the stages of fabrication of carbon fibres from PAN based on thermomechanical data.