STATICS AND DYNAMICS OF SELF-ASSEMBLED DIPOLAR RODS

We study theoretically the effect of applied fields on self-assembled rods. The rods are assumed to be rigid, semidilute and to consist of repeat unit of variable number, with each repeat unit carrying a small dipole moment. The static nonlinear susceptibility of the system is calculated, and found to be quite similar to that of a system of monodisperse rigid-rod polymers of the same linear susceptibility. The dynamical response of the system is also studied. Our model couples the rotational diffusion of the rods with a description of their reversible exchange reactions. (There reactions are presumed to involve only collinear rods, and presumed to be frequent on the time scale of rotation of a rod of the average length.) In the linear response regime, we show that the relaxation time for the net polarization greatly exceeds that for the angular reorientation of any individual dipole in the system. For the nonlinear regime (strong fields) an equation of motion is presented and is solved numerically to determine the relaxation of the polarization under switch-on, switch-off, and field-reversal conditions. In this regime, highly nonexponential relaxation is found. The extension of the model to the case of rods with induced (rather than permanent) dipole moments is briefly described, and the structure factor S(q) of such rods calculated under various field conditions,