Tracer diffusion measurement in polymer solutions near the glass transition by forced rayleigh scattering

Forced Rayleigh scattering, a relatively recently developed optical technique, is used to measure tracer diffusion coefficients in polymer‐solvent mixtures near the system glass transition temperature, T_g. The technique has a wider range of potential application than has yet been realized, and so is presented in some detail. The objectives were to obtain data necessary to scrutinize free volume theory, and to understand so‐called anomalous, non‐Fickian diffusion effects observed by others in polymer‐solvent mixtures near T_g. Data on dye tracer diffusion coefficients in the systems polyvinyl acetate‐toluene, polystyrene‐toluene, and polystyrene‐tri‐m‐tolyl phosphate were obtained over a polymer concentration range from infinite dilution to 96 wt. %. Small molecule diffusion coefficients are seen to vary by as much as nine orders of magnitude (10⁻¹⁴ to 10⁻⁵ cm²/s) over this concentration range. The data are in reasonable accord with expectations based on the Duda‐Vrentas version of free‐volume theory.