Cooperative and self-diffusion of polymers in semidilute solutions by dynamic light scattering

The quasielastic light scattering from semidilute solutions of polystyrene in tetrahydrofuran has been measured and we observe two distinct exponential decays separated by several orders of magnitude. The angular dependence of the decay constants is indicative of diffusive processes which we identify with the cooperative diffusion coefficient, D_c and the self diffusion coefficient D_s. It is found that D_c, identified with the fast decay, increases with polystyrene concentration and is independent of molecular weight. However, D_s decreases sharply with concentration and molecular weight. An explanation is given for the light scattering detection of these two diffusion coefficients which is based only on the assumptions inherent in the reptation model. In a limited region of molecular weight and concentration the experimental results appear to be consistent with the predictions of scaling theory.