The kinetics of poly(acrylonitrile) homopolymer and co-polymer cyclization

Stabilization is the rate limiting step in the poly(acrylonitrile) (PAN)-based carbon fiber process, because long processing times are required to achieve adequate oxidation in PAN fibers. Since oxidation is thought to be diffusion limited, achieving uniform radial oxidation in a fiber may be difficult without a better understanding of how process variables affect oxidation. The development of a process model for isothermal filament stabilization is dependent on experimentally based expressions for the kinetics of cyclization and oxidation and for the diffusion of oxygen through the reacting polymer.; This research focuses on the elucidation of the cyclization kinetics of homopolymer PAN and itaconic acid (ITA)/PAN co-polymers. The effect of polymer composition, temperature and reaction environment on cyclization is experimentally studied. The results of this study show that cyclization in nitrogen tends to yield sigmoidally shaped curves, but an increase in the initial itaconic acid concentration minimizes the induction period found at short reaction times. Cyclization in 10.5 and 21% oxygen also minimizes the induction period. The sensitivity of cyclization to the presence of oxygen is decreased as the initial itaconic acid concentration in the polymer increases.; A nucleation equation is used to describe cyclization for both homopolymer and co-polymer PAN reacted in nitrogen, 10.5% oxygen and 21% oxygen. For all cases, an Arrhenius form can be used to describe the relationship between the apparent rate constant and temperature. Linear Arrhenius plots yield apparent activation energies of 23,000-44,000 cal/mol and apparent pre-exponential factors of {dollar}2 times 10sp9 - 7 times 10sp{lcub}18{rcub}{dollar} hr{dollar}sp{lcub}-1{rcub}{dollar}, depending on the polymer and reaction environment. For cyclization, the apparent activation energy and the apparent pre-exponential factor tend to increase with an increase in initial itaconic acid concentration and with a decrease in bulk oxygen concentration.; The kinetics for cyclization were used to investigate intrafiber nonuniformity through the development of an approximate isothermal filament stabilization model. The model is able to estimate experimentally determined average oxygen uptake data for acidic and non-acidic co-polymer fibers by assuming oxidation to be controlled by cyclization rate and oxygen diffusion. Predicted oxidation gradients were presented for cases where the model results for average oxygen uptake were in agreement with the experimental oxidation data.