Effect of methyl acrylate composition on the microstructure changes of high molecular weight polyacrylonitrile for heat treatment

The acrylate comonomer in polyacrylonitrile (PAN) precursor markedly influences the microstructure of PAN fiber and its resulting carbon fiber. In this study, the change occurring during the heat treatment of PAN copolymers (acrylonitrile‐co‐methyl acrylate) in the presence of air up to 260°C has been examined using both physical and chemical techniques. Considering the effect of the methyl acrylate content of PAN copolymers on the cyclization mechanism and fine structural changes in polymer chains under various heat treatment conditions, the composition of acrylonitrile and methyl acrylate = 98 : 2 to 97 : 3 mol % was suitable for the precursor preparation because the cyclization mechanism and fine structural changes in the boundary of this composition of copolymers were significantly different. With increasing MA content, the exothermic onset and peak in the thermogram was found to shift toward higher temperatures, and the aromaticity index decreased. This evidence indicates that MA was believed not to activate at all but to interfere with the cyclization. Additionally, by means of 13C nuclear magnetic resonance (13C‐NMR) analysis, the tacticities of copolymers were nearly same, regardless of the copolymer composition, and the cyclization occurred nonstereospecifically during the initial heat treatment at 240°C for 1 h. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 2205–2213, 1998