Dynamic light scattering from semidilute solutions of a styrene-acrylonitrile random copolymer

Dynamics of a random copolymer, poly(styrene-co-acrylonitrile), in dilute a nd semidilute solutions was studied by using dynamic light scattering. The solvent used was a mixture of dioxane and acetonitrile (ACN). The mixing ra tio and the temperature were changed to provide the copolymer with various solvent conditions that range from a good to a Theta solvent. At low ACN co ntents, the autocorrelation function of the light scattering intensity had a single relaxation mode. Its diffusion changed from the mutual diffusion t o the cooperative diffusion of entangled chains in the good solvent as the concentration c increased. At high ACN contents (near Theta), the diffusion coefficient decreased to 1/3 of the dilute solution value and the scatteri ng intensity increased almost linearly with c, before the diffusion coeffic ient turned to increase as similar toc and the scattering intensity to decr ease as similar toc(-1), indicating the cooperative diffusion of entangled Theta chains. In this high concentration range, the autocorrelation functio n exhibited a slow diffusion mode as well. A high friction coefficient and an extended near-linear range in the scattering intensity, especially at lo w temperatures, suggest dynamic clustering of chains upon increasing the co ncentration.