We present a self-consistent method for deriving moment equations for kinetic models of polymer dynamics. The Doi model [J. Polym. Sci., Polym. Phys. Ed. 19, 229 (1981)] of liquid-crystalline polymers with the Onsager excluded-volume potential is considered as an example. To lowest order, this method amounts to a simple effective potential different from the Maier-Saupe form. Analytical results are presented which indicate that this effective potential provides a better approximation to the Onsager potential than the Maier-Saupe potential. Corrections to the effective potential are obtained.

• Received 23 April 1999

DOI:https://doi.org/10.1103/PhysRevE.60.5783