High temperature in situ 1H NMR study of mesophase formation in pitches

In-situ H-1 NMR has been used to follow the extent of mesophase development during isothermal heat-treatments at 470-480 degrees C for a petroleum pitch (Ashland A-240) and a selection of coal tar pitches. Two Gaussian signals with spin-spin relaxation times (T(2)s) of ca. 20 and 70 mu s have been identifed for mesophase with the fully rigid component (T-2 of 20 mu s) only arising at mesophase contents above ca. 30%. The less rigid component is only evident above ca.400 degrees C and is considered to be attributable to gel-like material observed previously by TEM with the rigid phase arising from mesophase spheres. The kinetics of mesophase development was very much feedstock dependent with the petroleum pitch giving first order kinetics while, in contrast, the coal tar pitches displayed more zero-order behaviour. Secondary QI formed by low temeprature heat-treatment of coal tar pitch increased the rate of mesophase development, but high temperature treatments to reduce volatiles retarded formation.