The molecular scaling of raw pitches by oscillatory rheometry

Abstract

Alpha relaxation and viscosity measurements performed above the glass transition region have been used for raw pitch molecular scale characterization. The characteristic time τ α of the alpha relaxation is known to be similar to the molecular mobility characteristic time τ m. Viscosity–temperature and τ m–temperature data allow molecular equivalent hydrodynamical diameter estimation through well known Stokes–Einstein relations. This procedure applied to two petroleum and two coal tar pitches led to a 9.5–20 Å hydrodynamical diameter range. Those results are in good agreement with basic structural units (BSU) theoretical molecular models or other experimental studies available in the literature. Pitch molecular scale seems similar to the coronene and ovalene monomers, dimers and trimer sizes. Moreover, an extrusion effect has been observed on coal tar pitch resulting in a possible edge-to-edge orientation. A correlation between α, β and γ resins and BSU diameter range is proposed.