DETERMINATION OF THE CRITICAL MICELLE, CONCENTRATION OF BLOCK-COPOLYMER MICELLES BY STATIC LIGHT-SCATTERING

A method is proposed for the determination of the critical micelle concentration (cmc) of block copolymer micelles from static light scattering measurements, which is based on a recent model of micellization of block copolymers. The method considers the polydispersity of the block copolymers, the variation of the total single chain concentration with total concentration, and the relationship between the cmc and the length of the insoluble block. One family of block ionomers, polystyrene(660)-b-poly(sodium acrylate) with a low polydispersity index and ionic block lengths varying from 2.6 to 14 units, was investigated by light scattering near the cmc in THF. For this system, which had a relatively narrow molecular weight distribution and which showed a weak dependence of the cmc on the insoluble block length, it was found that the cmc values could be evaluated both by the present proposed extrapolation method and by the Debye equation. However, for polystyrene(470)-b-poly(4-vinylpyridine) (52) in toluene, where the dependence of the cme on the block length is stronger, the Debye equation was not an adequate representation of the system while the proposed method was. Simulated light scattering curves for polystyrene-b-polyisoprene in n-hexadecane were calculated for various,distributions for the insoluble block length. It was found that for monodisperse samples the Debye equation gave a good fit, while for broad distributions the Debye equation did not describe the Kc/R(0) versus concentration curves and the cmc values,were found to depend on the range of points used in the cmc evaluation.