Organisation and interactions in aqueous dispersions of polystyrene–polyethylene oxide block copolymer micelles

Abstract

Diblock copolymers of polystyrene (PS) and polyethylene oxide (PEO) with a total molecular weight of circa 7500gmol−1 and a PS weight fraction of circa 0.15 have been synthesised in all possible variants of hydrogenous and deuterated forms. The copolymers have been dispersed at concentration above the critical micelle concentration (cmc) in aqueous dispersion mediums of varying contents of light and heavy water to contrast match selected components of the micelles. Small angle neutron scattering has been used to determine the size of the micelles, the radius of the PS core and the thickness of the PEO corona using data for dispersions with concentrations less than or equal to 1×10−2gml−1. At higher concentrations, a structure factor peak becomes increasingly evident as the concentration increases to 6.5×10−2gml−1. The total scattering could not be fitted using the structure factor for an assembly of hard spheres; a structure factor originally developed for coulombically stabilised micelles was used and excellent fits to the data obtained. The structure factor is described in terms of a Yukawa potential between the micelles and the values of the repulsion potentials have been used to calculate the repulsive force between the micelles and compared to the predictions for two brush-like polymer layers approaching each other. There is partial agreement with the model, the disagreement being identical to that observed when the results from force balance experiments are compared with the model.