SAXS (Small Angle X-Ray Scattering) Study of Micelle Formation in Mixtures of Butadiene Homopolymer and Styrene-Butadiene Block Copolymer. 2. Effects of Block Lengths.

The small-angle X-ray scattering technique is utilized to determine the characteristics of micelles formed in the mixtures containing a low molecular weight polybutadiene and a styrene-butadiene diblock copolymer. Three different block copolymers of about the same overall molecular weight but with differing lengths of the two blocks are utilized. The observed scattering curve is fitted with a calculated one based on a model of polydisperse micelles interacting with each other according to the Percus-Yevick hard-sphere fluid approximation. The following quantities characterizing the structure of the micelle are evaluated as a function of temperature and copolymer concentration the average radius of the core, the polydispersity of the core radius, the apparent hard-sphere radius of interaction, the critical micelle concentration, the degree of swelling of the core, and the number density of the micelles. The critical micelle concentration decreases as the proportion of styrene in the copolymer increases. The size of the micelles of a given copolymer remains independent of concentration but shows a moderate variation with temperature, especially immediately before the final dissolution temperature.