Alpha-relaxation of an isotropic petroleum pitch: A controlled stress and strain oscillatory rheometry study Original Research Article

Oscillatory measurements under controlled stress and controlled strain have been performed on the Ashland A240 isotropic petroleum pitch using a cone/plate rheometer. The storage and loss components of the modulus of rigidity have been analysed by means of time-temperature superposition coupled with the Williams, Landel and Ferry (WLF) procedure. Loss modulus results obtained under controlled stress are in good agreement with those previously published on the same material but using a plate/plate geometry (Turpin et al., Carbon, 1994, 32, 225). The temperature-dependence of the storage modulus presents a three decades α-relaxation step at Tga = 114 °C followed by a corresponding 11 units peak of tan (δ) at Tδ = 124 °C. Such a-relaxation has been extensively studied for amorphous polymers and other glassy materials within the glass transition temperature range but, up to now, has not yet been studied for pitches. Characteristic relaxation time τga similar to τmol has been calculated within the investigated experimental temperature range and correlated by the Tammann-Vogel-Fulcher (TVF) equation. The corresponding TVF parameters are within the same order of magnitude of those encountered in amorphous polymers. A detailed analysis of the applicability of the WLF method is presented. Same values for the storage and loss modulus shift factors are obtained at low temperature up to Tδ. At higher temperature, storage modulus shift factor presents a change of regime and a strong deviation from loss shift factor. Therefore, with respect to the α-relaxation phenomenon, the applicability of the WLF procedure to an isotropic pitch is only possible at a temperature below Tδ.