Mutual diffusion in binary polymer mixtures as measured by dynamic light scattering

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Abstract
We report light scattering measurements of the static structure factor and mutual diffusion coefficient for the compatible polymer mixture poly (phenylmethylsiloxane)/poly(styrene). The measurements were made for a series of molecular weights of the poly(styrene) of 2000, 4000, and 9000 for different volume fractions of the poly(styrene) over a range of temperatures from 350 to 520 K. The principal aim was to try and distinguish between the various theoretical approaches which have appeared in the literature and which give radically different results for the concentration dependence of the mutual diffusion coefficient in a binary polymer mixture. Since the static structure factor was measured in the same light scattering experiment as the diffusion coefficient we have also been able to obtain the concentration, molecular weight, and temperature dependence of the Onsager coefficient directly from experiment. A concentration-dependent glass transition temperature complicates the comparison with theory and we propose a method for the data analysis to deal with this problem. Our results are consistent with the expression for diffusion coefficient based on the dynamic random phase approximation.