Long-range correlations and yielding of diblock star-polyelectrolyte micelles in the diluted limit

We report a combined study of the synthesis, rheology, electron microscopy, and molecular dynamics of four-arm diblock poly(styrene) (PS) – poly(methacrylic acid) (PMAA) star polymers in solution. The stars assemble into spherical micelles with a ~10nm PS core and a ~100nm highly charged PMAA outer block which is almost completely stretched in the limit of low salt concentration. Linear rheology shows a liquid-to-solid transition at a mass concentration as low as 0.25 wt.%, while simulations show a threefold decrease of the effective radius of the micelles in the 0.1-1% concentration range. These results are consistent with a picture of ultrasoft colloids with large and highly tunable caging effects at extremely low concentrations. Additionally, the system presents a non-vanishing elastic modulus (from rheology) and a peak of the radial distribution function (from simulations) even in the dilute regime limit, caused by the long-range repulsion between the micelle corona. The micelle size and caging behavior are extremely sensitive to changes in pH or the addition of oppositely charged polymers forming the basis for a class of tunable soft colloidal glasses.