Correlation of chemical shrinkage of polyacrylonitrile fibres with kinetics of cyclization

It has been established that the most important step in the production of carbon fibres from polyacrylonitrile (PAN) precursor fibres is the oxidative thermal treatment applied. During this treatment, physical phenomena and chemical reactions take place accompanied by the shrinkage of the fibres, which has a physical or chemical origin, depending on the nitrile cyclization reactions. The aim of the present study is to establish a correlation between the chemical shrinkage of PAN fibres and the kinetics of cyclization reactions. Based on the isothermal treatment of PAN fibres, we developed a method in order to distinguish between physical and chemical shrinkages. The onset time for the chemical shrinkage follows a relationship with temperature. Chemical shrinkage versus cyclization time data were fitted with the exponential rise to the maximum of the curves. Furthermore, the cyclization kinetics was studied using differential scanning calorimetry and the kinetic parameters determined were identical to those calculated from the chemical shrinkage, demonstrating that the latter is directly related to the kinetics of the cyclization reactions. It was therefore concluded that according to the method developed to distinguish the physical from the chemical shrinkages: (1) there exists a certain onset time for a given treatment temperature to trigger the chemical shrinkage; (2) cyclization reactions do not start below a limiting temperature of 168 degrees C; (3) at 340 degrees C, the temperature where the cyclization reactions are completed, the maximum shrinkage is 24%; and (4) the oxidized PAN fibres contain mainly ladder polymer structures with approximately symmetrical sequences connected in angled positions. (c) 2007 Society of Chemical Industry.