A study of diffusion in polymer solutions by the use of the polarization interferometer

The use of the polarization interferometer and the further perfection of the experimental technique makes possible the measurement of diffusion coefficients between solutions whose concentrations differ by less than 10 −4 g./cm. 3 These circumstances permit the direct measurement of the differential diffusion coefficient, D , corresponding to the given concentration c . The concentration dependence of the diffusion coefficient obtained for fractions of polystyrene in the region of not too small concentrations agrees well with the concentration dependence of the scattering of light in the same samples. In the region of low concentrations, the curve of the dependence D = D ( c ) has a horizontal section which cannot be described by the formulas usually used. In the region of concentrations, where the dependence D = D(c ) may be considered as a rectilinear one, it is shown experimentally that the integral coefficient of diffusion between solutions whose concentrations differ by c 1 – c 2 is equal to the mean of two differential diffusion coefficients, D 1 and D 2 , where D 1 = D(c 1 ) and D 2 = D(c 2 ). In that region of concentrations, the integral diffusion curve shows a marked asymmetry, which agrees with the concentration dependence of the differential diffusion coefficient. The measurement of the diffusion coefficient D in dilute solutions of fractions of a series of polymers shows that the value of D is inversely proportional to the radii of the high polymer molecules calculated from viscosity and from light scattering data. The value of the empirical constant was determined, which permits the determination of the radius of the molecules from the measured values of the diffusion coefficient.