Anisotropic rotational diffusion of entangled rodlike polymers under elongational flow

Quantitative treatment of rotational diffusion of entangled rodlike polymer molecules in semidilute solution is extended to include anisotropy in the orientational distribution of rods. Noting that anisotropy has a similar effect to changing the polymer concentration of the size and form of a tube confining a rodlike molecule, we apply the mean-field argument to the hindered diffusion of entangled rods. The rotational diffusion constant, which depends not only on the orientation but also on the diffusion direction, is derived as a functional of the orientational distribution function. As an example of external force which causes anisotropy, we study stationary elongational flow and obtain the rotational diffusion constant self-consistently as a function of the strain rate. It is found the tube is not a round but an oblate cylinder.