Probe diffusion in polystyrene/toluene solutions

The technique of forced Rayleigh scattering has been used to measure the tracer diffusion coefficient, D_t, for two photochromic dyes, azobenzene and Aberchrome 540, in polystyrene / toluene solutions, as a function of polymer concentration (79–90% by weight) and temperature (−15–+60°C). The dye diffusivity can be scaled quantitatively to the solvent diffusivity, permitting direct application of the Vrentas‐Duda free volume predictions for binary systems. However, the scaling factors are not in agreement with the model assumptions. The temperature and concentration dependence of D_t above the solution glass transition temperature, T_gs, is in good agreement with the model, without any freely adjustable parameters. The value of D_t at T_gs is a very strong function of concentration, increasing by a factor of 10³ as the polymer concentration is reduced from 90 to 80%; this is also in quantitative agreement with the model. It is demonstrated that the temperature dependence of D_t exhibits a distinct break in the vicinity of T_gs, with both the existence and the character of the break in qualitative agreement with the extension of the Vrentas‐Duda approach to glassy solutions. However, a fundamental difficulty in the calculation of the polymer contribution to the solution free volume at temperatures well below the bulk polymer T_g is exposed.