Tensile response of polyacrylonitrile fibers during air heating

Axial stresses generated by polyacrylonitrile filaments heated in air at constant length and length changes of filaments heated at constant load were measured. Fibers subjected to loads less than about 0.1 gpd shrank in the temperature range from about 40°C to 160°C. At about 160°C they began to stretch. Fibers that stretched out again to about their original lengths stiffened temporarily before undergoing a further elongation. At a temperature where the oxidation reaction begins to proceed with appreciable rate, elongation was retarded and finally reversed. Shrinkage was recorded during isothermal heating at 270°C, and a final length was approached when the oxygen content approached about 10 wt‐%. The tension generated when the fibers were restrained from shrinking increased as temperature increased to 160°C but dropped in the temperature range of 160° to 250°C. Tension again built up during isothermal oxidation at 270°C. In the case of one of the samples, the tension generated below 160°C exceeded the ultimate tensile strength of the fibers above 200°C. This condition leads to tensile failures when the filaments are heated in a steep thermal gradient. The tensile behavior of the filaments is discussed in terms of the helical molecular model.