Rotational dynamics of rodlike polymers in a rod/sphere mixture

Transient electric birefringence decay studies of the rotation of rodlike p olymers in solutions with spherical particles have been performed as a func tion of the sphere volume fraction. The rodlike polymers are helical poly-g amma-benzyl, alpha-L-glutamates of several lengths. The spheres are coated silica particles and the solvent is a mixture of dimethyl formamide and pyr idine that matches the refractive index of the spheres. The rotation of the relatively dilute rods is systematically studied as a function of the sphe re concentration. In the suspension where the rods are shorter than the ave rage sphere diameter, the rotational relaxation rate changes only slightly from the relaxation rate of pure rods in the same solvent. In the suspensio ns of longer rods, the rotation rate decreases rapidly as the sphere concen tration increases and scales with the rod length and sphere concentration. We develop a scaling model by combining our results on rod rotation in a ro d/solvent dispersion [J. K. Phalakornkul , Macromolecules 32, 3122 (1999)] and our experimental results on mutual diffusion of the spheres [J. K. Phal akornkul , Phys. Rev. E 54, 1(1996)], to determine the dependence of the ro d rotation rate on the sphere concentration. The scaling exponent of the ro d rotational time with rod length shows a weaker dependence on length than the scaling model prediction. We measured the viscosities of the sphere dis persions. The rotational relaxation time is not simply related to the backg round sphere suspension viscosity. (C) 2000 American Institute of Physics. [S0021-9606(00)51513-X].