X. Py et al., ALPHA-RELAXATION OF AN ISOTROPIC PETROLEUM PITCH - A CONTROLLED STRESS AND STRAIN OSCILLATORY RHEOMETRY STUDY, Carbon, 35(7), 1997, pp. 1013-1021
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.