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

Oscillatory measurements under controlled stress and controlled strain have been performed on the Ashland A240 isotropic petroleum pitch usi ng a cone/plate rheometer. The storage and loss components of the modu lus of rigidity have been analysed by means of time-temperature superp osition coupled with the Williams, Landel and Ferry (WLF) procedure. L oss modulus results obtained under controlled stress are in good agree ment with those previously published on the same material but using a plate/plate geometry (Turpin et al., Carbon, 1994, 32, 225). The tempe rature-dependence of the storage modulus presents a three decades alph a-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-rela xation has been extensively studied for amorphous polymers and other g lassy materials within the glass transition temperature range but, up to now, has not yet been studied for pitches. Characteristic relaxatio n time tau(x) similar to tau(mol) has been calculated within the inves tigated experimental temperature range and correlated by the Tammann-V ogel-Fulcher (TVF) equation. The corresponding TVF parameters are with in the same order of magnitude of those encountered in amorphous polym ers. A detailed analysis of the applicability of the WLF method is pre sented. Same values for the storage and loss modulus shift factors are obtained at low temperature up to T-delta. At higher temperature, sto rage modulus shift factor presents a change of regime and a strong dev iation from loss shift factor. Therefore, with respect to the alpha-re laxation phenomenon, the applicability of the WLF procedure to an isot ropic pitch is only possible at a temperature below T-delta. (C) 1997 Elsevier Science Ltd.