Temperature and concentration dependence of diffusion coefficient in dilute solutions

Temperature and molecular weight dependence of k D in D( C) = D(O) [1 + Ck D ], where D( C) is the diffusion coefficient for the density fluctuations in a dilute polymer solution, is investigated by first expressing D( C) as a function of the static structure factor S( q, C) within the framework of the Kirkwood-Riseman theory. The continuous transition of k D from negative values under theta conditions to positive values in good solvents is calculated using various models for the intermolecular interaction potential and the results are presented graphically as function of a reduced variables S ̄ R H that combines both molecular weight and temperature effects. It is shown that the negative value of k D at the theta temperature can be explained at least partially, in terms of an increase in the chain dimensions of two overlapping molecules. The concentration dependence of the self-diffusion coefficient is also discussed.