Constitution of coexisting phases in mesophase pitch during heat treatment: Mechanism of mesophase formation

Abstract The molecular constitutions of the coexisting anisotropic and isotropic phases of a heat-treated petroleum pitch were studied as a function of heat treatment time. The results show that the coexisting phases, which were isolated by high temperature centrifuging, contain different proportions of similar size molecules. The molecular weight distribution (MWD) of the mesophase did not change during the transformation, but the average MW of the isotropic phase increased significantly. Although the smaller molecules (400–1100 amu) present in the coexisting mesophase polymerize during the transformation, the constant MWD of the mesophase is explained by the stability of the larger pitch molecules (>1100 amu) and by the selective transfer of the smaller molecues (400–1100 amu) from the isotropic phase to the mesophase via stirring and generated gas percolation mechanisms. These transferred smaller molecules, which had been enriched in the isotropic phase, restore molecular equilibrium to the mesophase and result in the growth of mesophase regions. The nucleation of mesophase spheres in the coexisting isotropic phase is due to polymerization of the 400 to 1100 MW molecules in the isotropic phase and to the selective transfer of the 400 to 1100 MW molecules from the isotropic phase to the mesophase.