ALPHA-RELAXATION OF AN ISOTROPIC PETROLEUM PITCH - A CONTROLLED STRESS AND STRAIN OSCILLATORY RHEOMETRY STUDY

摘要

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 alpha-relaxation step at T-alpha=114 degrees C followed by a corresponding 11 units peak of tan(delta) at T-delta=124 degrees C. Such alpha-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 tau(x) similar to tau(mol) 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 T-delta. 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 alpha-relaxation phenomenon, the applicability of the WLF procedure to an isotropic pitch is only possible at a temperature below T-delta. (C) 1997 Elsevier Science Ltd. References: 18