STRUCTURAL FEATURES AND MESOPHASE FORMATION OF COAL-TAR PITCH FRACTIONS OBTAINED BY PREPARATIVE SIZE EXCLUSION CHROMATOGRAPHY

High-temperature coal-tar pitch was fractionated by preparative size exclusion chromatography on Sephadex LH-20 in pyridine at 60 °C. The fractions obtained were characterized by elemental analysis, molecular weight determination (vapour pressure osmometry), i.r. spectroscopy, and TGA. The linear correlation observed between log molecular weight (range 200-2500 Dalton) and elution volume shows that fractionation of the pitch was mainly due to size exclusion and that interfering polar adsorption on the gel was essentially avoided. This is supported by the similarity between the i.r. spectra and elemental analyses of the fractions, respectively. The molecular weight distribution of the nitrogen-containing pitch constituents is also discussed. Selected samples were heat-treated using hot-stage microscopy. The reactivity of the fractions towards polymerization is lower than that of the unfractionated pyridine-soluble portion of pitch. This leads to a slower increase in viscosity of the fractions, thus allowing the formation of larger mesophase spheres. Addition of a small amount of a low molecular weight fraction to a high molecular weight fraction enhanced polymerization. © 1990.