Diffusion in rod/sphere composite liquids.

The synthesis, characterization, and some studies of the dynamics of a new rod/sphere composite liquid are discussed here. The composite liquid consists of silica spheres (39.4 and 60.4 nm radius) and poly(γ-benzyl-α,L-glutamate) rods (102 000 and 249 700 g/mol molecular weight) dispersed without aggregation in dimethylformamide (DMF) or a mixture of DMF and pyridine. The solvent mixture was used to refractive-index-match dilute solutions of spheres. Dynamic light scattering (DLS) was used to measure the diffusion constants of both the rods and the spheres simultaneously in the solution. Two dynamical regimes were observed for the diffusion of the rods with or without the spheres. The transition point between these two regimes was attributed to the change from dilute to semidilute dynamics. Sphere diffusion constants up to a value two times larger than expected from the Stokes-Einstein equation were measured for the composite liquid containing the large rod (249 700 g/mol) and the small sphere (39.4 nm) in accordance with the argument of Langevin and Rondelez [17]. The sphere diffusion constants measured for the liquid containing the large sphere (60.4 nm) and the small rod (102 000 g/mol) followed a stretched exponential decay with increasing polymer concentration: D/D0=exp (−0.16 c0.81). The c0.81 dependence compares well to experimental results using coil polymers but not to the theories for rod solutions which predict a c0.5 dependence. The similar concentration dependence found for rod and coil solutions suggests that polymer flexibility alone does not account for the deviation from the theoretical c0.5 predictions. Instead, rod-rod hydrodynamic interactions may play an important role in this deviation. [ABSTRACT FROM AUTHOR]