DYNAMIC LIGHT-SCATTERING FROM MIXTURES OF 2 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 invest igated binary solutions (where the solute is one of the samples) and t ernary 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 f or the translational diffusion coefficients, while the dynamic scatter ing correlation functions for the ternary solutions can be deconvolute d into two independent contributions. In the semidilute regime, the co rrelation functions corresponding to the binary solutions are monoexpo nentials, showing a cooperative diffusional mode, due to local fluctua tions in the polymer concentration. The dependency of hydrodynamic cor relation lengths vs concentration clearly approaches the universal cur ve obeyed by other previously obtained data for semidilute systems. Th e semidilute ternary solutions exhibit a clear bimodal behavior. We ob serve a fast mode that can also be described as a cooperative diffusio n coefficient (whose correlation length likewise falls into the univer sal curve). The slow mode is also diffusive, but the apparent diffusio n coefficient shows a dependence with concentration opposite to that e xpected for the cooperative motion, approaching zero with increasing c oncentration. It is, in fact, an interdiffusional mode, due to the dif ferent chain lengths in the two samples. These experimental data are i n quantitative agreement with the random phase approximation theory, d eveloped by Benmouna et al. for several types of semidilute systems. T he same theory is expected to also describe the dilute regime, if an a ppropriate modification is introduced to take into account hydrodynami c interactions between the chain segments.