The Study Of Mechanism Of Polyacrylonitrile During Preoxidation

Preoxidation is an indispensible step for fabricating PAN-based carbon fibers. In general, preoxidation is carried out under air condition in the temperature range of 200-300℃. During this step, a lot of chemistry reactions occur, such as cyclization, oxidation, dehydrogenation etc, which convert linear PAN macromolecules into conjugated ladder structures and improve significantly the thermal stability of PAN. Many reports have been documented on the preoxidation of PAN, but there are still many controversies about preoxidation of PAN, such as the sequence of preoxidation reactions etc. In the same time, there are few documents of influences of ester comonomer on the preoxidation reactions. In the present work, the structural evolution of polyacrylonitrile (PAN) and related copolymers were studied in detail to understand the roles and influences of comonomers during the preoxidation of PAN polymers.(1) The structure evolution and thermal behaviors of PAN homopolymer and terpolymer [P(AN/MMA/IA)] were studied. It was found that the presence of IA comonomer in PAN promoted the cyclization reactions significantly though initiating the cyclization reactions at lower temperature and proceeding at a faster rate than these of PAN homopolymer. Moreover, the dehydrogenation was based on the formation of cyclized structures. Meanwhile, MMA comonomer blocked the cyclization reactions to some extent. The sequence of reactions happened during the stabilization was oxidation> cyclization> dehydrogenation. The higher rate constant k implied that the ionic mechanism actually showed a kinetic advantage at low temperature over the free radical mechanism, even though the MMA comonomer inhibited cyclization reactions to some extent. The presence of IA comonomer promoted the formation of a large quantity of better ladder structures. In additional, the formed better ladder structures were easy to convert from ladder structures into graphite-like structures. The activation energy (Ea) of random scissions showed that the presence of IA comonomer inhibited the random scission reactions to some extent.(2) The structural evolution and thermal behaviors of poly(acrylonitrile-co-dimethyl itaconate) [P(AN/DMI)] copolymer was investigated in detail. DMI comonomer had the ability to promote the cyclization reactions significantly at the temperature of 240℃, compared to the PAN homopolymer. The results of quantitative FTIR analysis in the range of 1000-1800 cm-1 showed that the DMI comonomer not only promoted the cyclization reactions, but also facilitated the oxygen uptake reactions, especially the conjugated carbonyl group in an acridone ring in the ladder polymer chains, which proved DMI comonomer had the potential ability to make successful preoxidation process.