Chronological effects of oxygen on the structural transformations of polyacrylonitrile fibers during the rapid thermal stabilization process.

This study investigates the chronological effects of oxygen on the structural transformations of PAN fibers during the rapid thermal stabilization process. In the shorter-time period, PAN fibers undergo a few thermal-driven dehydrogenation reactions alone, while the initiation of both oxidative dehydrogenation and oxygen uptake reactions requires a longer stabilization time. Interestingly, both the amorphous regions and the lateral crystallite sizes increase simultaneously during these periods, resulting in greater exothermic enthalpy. In the longer time period, cyclization reactions begin, and the chain structures gradually become ladder-like and aromatized. In contrast to PAN fibers exposed to nitrogen, another type of cyclization reaction involving the nitrile groups occurs with negligible growth in the degree of reaction. The extent of tensile property loss in PAN fibers during this process strongly depends on the type of chemically thermal stabilization reactions. A more significant decrease in tensile strength occurs later in the longer time period, suggesting that oxygen may cause greater deterioration of the tensile strength of the PAN fibers'.