Cross linking and carbonization processes in PAN films and nanofibers

Cast films and nanofibers of polyacrylonitrile (PAN) were considered in order to study the chemical reactions occurring during the stabilization and carbonization steps in the thermal conversion of PAN into a carbonaceous structure. Cast films were subjected to a thermal ramp from room temperature to 600 degrees C in nitrogen or in air, the chemical modifications were observed in situ with infrared spectroscopy in reflection absorption mode. The thermal process performed on fibers consisted instead in a stabilization step in air (half an hour at 250 degrees C), followed by a carbonization step in nitrogen up to 800 degrees C. In nitrogen the chemical modifications started around 300 degrees C; it is suggested that the majority of nitriles takes part in cyclization and trimerization reactions below 400 degrees C. During the treatment performed in air the chemical reactions occurred during the isothermal step. Oxygen causes the formation of oxidized species and a faster effective dehydrogenation; at the end of the treatment the samples resembled to a graphitized structure more than the samples heated under nitrogen. Also, the oxidation process was much more effective in char formation. The products obtained on the nanofibers were comparable with the products observed in air on film, but with a higher carbon content. In all thermal treatments a new IR signal around 800 cm(-1) appeared, which was attributed to substituted triazine rings formed in the trimerization of nitrile groups of different chains. The data supports the hypothesis of crosslink of the fibers at the beginning of the thermal conversion. The infrared and Raman spectra at 800 degrees C confirm the carbonization process on the nanofibers. (C) 2015 Elsevier Ltd. All rights reserved.