Thermal and collective diffusion in polymer solutions: A small angle light scattering study

Two experimental arrangements are described whereby the thermal and collective diffusion coefficients X and D_c of semi-dilute or concentrated polymer solutions may be measured using small angle light scattering. The first is achieved by measuring the characteristic time τ of the decay in the angular spread of the nonlinear diffraction pattern following an abrupt reduction of the incident laser power, together with a knowledge of the beam diameter. The second is obtained by allowing the solution to develop a slight refractive index gradient through surface evaporation of the solvent, and then following the decay in intensity of the light scattered through a small angle after the solution has been shaken.