Comparison of the structural and rheological consequences of micelle formation in solutions of a model di-block copolymer

Abstract

In the selective solvent, dodecane, the di-block copolymer polystyrene- b-(ethylene- co-propylene), with a narrow molecular-weight distribution, forms micelles with polystyrene cores. Small-angle neutron scattering experiments were used to investigate both core shape and dimensions and the intercore structure factors as temperature or concentration were varied. Monodisperse spherical structures with radii around 120 Å were observed. Above a critical concentration these were arranged in relatively ordered structures in which preferred orientations could easily be induced. The core diameter and the intercore spacing were dependent on sample thermal history. During shear the intercore structure became less ordered. The structural results correlate well with measurements of the dynamic viscosity measured in oscillatory shear, which also show a sharp change from gel-like to liquid behaviour at this critical concentration. Data are compared to model calculations in the regions where the particle form factor or where the interparticle structure factor dominate. In the latter case a hard core potential with a soft tail is found to give reasonable agreement with the data, and to allow changes with shear rate, with concentration or with temperature to be interpreted.