Dynamic light scattering from mixtures of two polystyrene samples in dilute and semidilute solutions

Static and dynamic light scattering experiments have been carried out for different solutions of two monodisperse polystyrene samples (with a ratio of molecular weights higher than 7) in toluene. We have investigated binary solutions (where the solute is one of the samples) and ternary solutions (where the solute is a mixture of the two samples) in relative concentrations so that both samples contribute similarly to the total scattered light. The total polymer concentrations range from the very dilute to the semidilute regimes. In the very dilute regime, the binary solutions yield the expected molecular weight dependence for the translational diffusion coefficients, while the dynamic scattering correlation functions for the ternary solutions can be deconvoluted into two independent contributions. In the semidilute regime, the correlation functions corresponding to the binary solutions are monoexponentials, showing a cooperative diffusional mode, due to local fluctuations in the polymer concentration. The dependency of hydrodynamic correlation lengths vs concentration clearly approaches the universal curve obeyed by other previously obtained data for semidilute systems. The semidilute ternary solutions exhibit a clear bimodal behavior. We observe a fast mode that can also be described as a cooperative diffusion coefficient (whose correlation length likewise falls into the universal curve). The slow mode is also diffusive, but the apparent diffusion coefficient shows a dependence with concentration opposite to that expected for the cooperative motion, approaching zero with increasing concentration. It is, in fact, an interdiffusional mode, due to the different chain lengths in the two samples. These experimental data are in quantitative agreement with the random phase approximation theory, developed by Benmouna et al. for several types of semidilute systems. The same theory is expected to also describe the dilute regime, if an appropriate modification is introduced to take into account hydrodynamic interactions between the chain segments.