Diffusion of Polystyrene Latex Spheres through Isotropic Rigid Rod Polymer Solutions

The diffusion of highly cross-linked polystyrene latex spheres of radii 0.152 and 0.208 μm through solutions of poly(γ-benzyl- l - glutamate) (PBLG) has been studied by dynamic light scattering (DLS). The solvent used was dimethylformamide (DMF), in which PBLG adopts a rigid rodlike α-helical conformation. Tracer diffusion coefficients of the spheres in solutions of long rods ( L ≈ 159 nm) were larger than predicted from the Stokes−Einstein equation. These positive deviations from the Stokes−Einstein equation are consistent with the theory of Auvray ( J. Phys. 1981, 42, 79), which predicts the presence of a rod monomer depletion layer locally surrounding the spheres that extends a distance ≈ L from the surface of the sphere. The results are also consistent with the experimental work of others concerning depletion of stiff polymers near surfaces. Sphere tracer diffusion coefficients in solutions of smaller rods ( L ≈ 48 nm) approximately follow the Stokes−Einstein equation. In this system, depletion effects are expected to be much smaller. Fits of the sphere diffusion data to the equation D / D 0 = exp(−α c υ ) yielded values of υ = 1−1.1, which exceed those predicted by theory. We were also able to estimate translational diffusion coefficients of the PBLG in the ternary rod/sphere/solvent systems by employing very short sample times in the DLS experiment.