Rheology of Pitches

Abstract

Rheological behavior of optically isotropic pitches derived from petroleum crude oils and coal tars were reviewed in relation to their dependence on molecular weight and temperature. Studied were the hydrodynamic characteristics (intrinsic viscosity, [η], and diffusion coefficient) of dilute pitch solutions, glass transition behavior, viscoelastic properties, and dielectric dispersion-electric conductivity characteristics of fractionated pitches with different molecular weights, which were prepared using the difference in solubility of the pitches in n-hexane and benzene. The hydrodynamic studies yielded an important information on the shape and dimension of constituent pitch molecules, i. e., oblate ellipsoid with the axial ratio of 1-3, and 10-20Å in equatorial diameter. The glass transition temperature, Tg, which decreased linearly with the inverse of number averaged molecular weight, was found to play an important role on the molecular weight dependence of steady-state shear viscosity, η, and steady-state shear compliance, Je0. The dependence of Je0 on temperature suggests that the elastic process in pitches is enthalpic, not entropic as observed in usual organic polymers. The dielectric behavior of pitches were described well by so-called Cole-Cole relation except prominent energy dissipation due to electrically conductive carriers in the region of low electrical frequency at high temperature. The importance of the rheological studies of mesophase pitches was emphasized in relation to the technical development of anisotropic carbon fibers with high elastic modulus and high strength