A COMPARISON OF MESOPHASE FORMATION UNDER SPARGING AND VACUUM

A petroleum pitch was heat treated using nitrogen sparging or vacuum, and the constitution of samples was investigated as a function of heat-treatment time with microscopy, solid state C-13 NMR, and ESR spectroscopy. The samples were extracted with methylene chloride, and the soluble fractions were characterized by number-average molecular weight, H-1 and C-13 NMR, and fluorescence spectroscopy. The free radical concentration and aromaticity of the two final (ca 80% mesophase) pitch samples were similar. Microscopic examination of samples showed extensive mechanical deformation of the isotropic phase whenever the anisotropic phase was continuous. Nucleation of anisotropic spheres continued throughout mesophase formation. The major difference between the two runs appeared to be the higher amount of methylene chloride-soluble components in the vacuum run. After vacuum heat treatment, the average MW of the methylene chloride solubles was considerably higher than for the starting pitch while, for samples which had been sparged, the MW of these solubles was slightly lower. Fluorescence and NMR spectroscopy indicated that considerable changes in the distribution of aromatic ring systems and alkyl substituents occurred in the methylene chloride-soluble fraction during heat treatment.