The thermal behavior of polyacrylonitrile (PAN) has been investigated using X-ray diffraction, differential scanning calorimetry (DSc), and ultrafast DSC. In conventional DSC, it is difficult to prevent the concurrent occurrence of the exothermic reactions of PAN with melting. However, in the ultrafast DSC curve, the exothermic peak due to these reactions disappears over the temperature range 0-400 degrees C at heating rates above 250 degrees C s(-1). Alternatively, the glass transition and the melting of PAN are observed over the temperature range 109-129 degrees C and 335-362 degrees C, respectively. Moreover, upon cooling from the molten state at a rate of -7500 degrees C s(-1), PAN crystallization is observed at 204 degrees C. These findings were observed repeatedly during heating and cooling measurements. From the extrapolation analysis, the zero-entropyproduction melting temperature of PAN is found to lie in the temperature range 320-350 degrees C. Finally, the equilibrium melting temperature of PAN is estimated to be ca. 465 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.