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

Dynamics of a random copolymer, poly(styrene-co-acrylonitrile), in dilute and semidilute solutions was studied by using dynamic light scattering. The solvent used was a mixture of dioxane and acetonitrile (ACN). The mixing ratio and the temperature were changed to provide the copolymer with various solvent conditions that range from a good to a ⊖ solvent. At low ACN contents, the autocorrelation function of the light scattering intensity had a single relaxation mode. Its diffusion changed from the mutual diffusion to the cooperative diffusion of entangled chains in the good solvent as the concentration c increased. At high ACN contents (near ⊖), the diffusion coefficient decreased to 1/3 of the dilute solution value and the scattering intensity increased almost linearly with c, before the diffusion coefficient turned to increase as ∼c and the scattering intensity to decrease as ∼c^-1, indicating the cooperative diffusion of entangled ⊖ chains. In this high concentration range, the autocorrelation function exhibited a slow diffusion mode as well. A high friction coefficient and an extended near-linear range in the scattering intensity, especially at low temperatures, suggest dynamic clustering of chains upon increasing the concentration.