In situ ESR measurement of mesophase formation during the heat treatment and cooling processes of pitches

Abstract

Both heating and cooling processes for mesophase formation from various pitches have been followed by in situ ESR measurements. In addition to signal intensity and linewidth, the saturation phenomenon of the signal intensity with changing microwave power was measured during heat treatment and cooling processes. As a convenient measure of the saturation factor, l/ P max ( P max is the microwave power at which the signal shows maximum intensity) was employed. In general, 1/ P max decreased with an increase in heat-treatment temperature; this change seems to correspond to a decrease in the viscosity of pitch. When pitches were heat treated at 500°C for 2 h, the carbonization reactions and the rearrangement of molecules may occur, which irreversibly changes 1/ P max . Pronounced differences in the behavior of 1/ P max measured during cooling of the heat-treated pitches were found to depend upon pitch optical texture (isotropic, flow and mosaic).