The friction constant in a concentrated polymer–solvent system by Rayleigh scattering: Polystyrene in n-butyl acetate

Abstract

Using an optical heterodyne technique (homodyne, in the definition used by the Malvern group), the intensity of the light scattered by concentration fluctuations and their correlation time were measured for a series of solutions of polystyrene in n‐butyl acetate in the mass concentration range 0.18 to 0.57, between 0 and 70 °C. The elasticity deduced for the polymer matrix is found to be temperature independent, in contrast with polystyrene:cyclohexane and in conflict both with the Flory–Huggins theory and with recent scaling theories. This result is attributed to steric hindrances in the rotation of the solvent molecules. As in the case of polystyrene:cyclohexane, the diffusion constant D passes through a maximum as a function of concentration at a given temperature, indicating that the viscosity entering into the definition of D is a local viscosity and not that of the bulk solvent. On the assumption that the time constant T r , which is central to de Gennes’ theory of the macroscopic viscosity, is proportional to the local viscosity, the dependence of T r upon the coherence length ξ can be deduced without introducing explicitly the concentration dependence of these quantities. It is found that T r ∝1/ξ3.35±0.18, as compared with de Gennes’ theory, which predicts T r ∝1/ξ2 in the semidilute regime.