Polyacrylonitrile fiber behavior. I. Mechanisms of tensile response

The behavior of polyacrylonitrile fibers has been studied by a number of methods over a wide range of temperatures. At the glass transition temperature Tg, in the vicinity of 100°C. by linear thermal expansion, creep, and dye sorption experiments, the break extension was found to rise abruptly, while an initial, stiff response disappeared. The tensile recovery as normally measured was incomplete at all temperatures; below Tg, however, all extensions were completely reversible when the kinetic conditions for retraction were provided. The main extension below Tg is attributed to the straightening of twisted molecules against the intramolecular repulsion of adjacent dipoles, the small initial extension up to the “yield” knee to an increase in volume required for the operation of the main response. The additional extension(s) above Tg appear(s) to involve permanent relative displacement of fiber elements. Water depressed Tg by 30–35°C.