Rotational dynamics of rodlike polymers in a rod/sphere mixture

Abstract

Transient electric birefringence decay studies of the rotation of rodlike polymers in solutions with spherical particles have been performed as a function of the sphere volume fraction. The rodlike polymers are helical poly-γ-benzyl, α-L-glutamates of several lengths. The spheres are coated silica particles and the solvent is a mixture of dimethyl formamide and pyridine that matches the refractive index of the spheres. The rotation of the relatively dilute rods is systematically studied as a function of the sphere concentration. In the suspension where the rods are shorter than the average sphere diameter, the rotational relaxation rate changes only slightly from the relaxation rate of pure rods in the same solvent. In the suspensions of longer rods, the rotation rate decreases rapidly as the sphere concentration increases and scales with the rod length and sphere concentration. We develop a scaling model by combining our results on rod rotation in a rod/solvent dispersion [J. K. Phalakornkul et al., Macromolecules 32, 3122 (1999)] and our experimental results on mutual diffusion of the spheres [J. K. Phalakornkul et al., Phys. Rev. E 54, 1(1996)], to determine the dependence of the rod rotation rate on the sphere concentration. The scaling exponent of the rod rotational time with rod length shows a weaker dependence on length than the scaling model prediction. We measured the viscosities of the sphere dispersions. The rotational relaxation time is not simply related to the background sphere suspension viscosity.