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

Diblock copolymers of polystyrene (PS) and polyethylene oxide (PEO) with a total molecular weight of circa 7500 g mol(-1) and a PS weight fraction of circa 0.15 have been synthesised in all possible variants of hydrogenous an d deuterated forms. The copolymers have been dispersed at concentration abo ve the critical micelle concentration (cmc) in aqueous dispersion mediums o f varying contents of Light and heavy water to contrast match selected comp onents of the micelles. Small angle neutron scattering has been used to det ermine the size of the micelles, the radius of the PS core and the thicknes s of the PEO corona using data for dispersions with concentrations less tha n or equal to 1x10(-2) g ml(-1). At higher concentrations, a structure fact or peak becomes increasingly evident as the concentration increases to 6.5 x 10(-2) g ml(-1). The total scattering could not be fitted using the struc ture 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 Yuka wa potential between the micelles and the values of the repulsion potential s have been used to calculate the repulsive force between the micelles and compared to the predictions for two brush-lice polymer layers approaching e ach other. There is partial agreement with the model, the disagreement bein g identical to that observed when the results from force balance experiment s are compared with the model. (C) 2001 Elsevier Science Ltd. All rights re served.