Polystyrene-Poly(sodium methacrylate) Amphiphilic Block Copolymers by ATRP: Effect of Structure, pH, and Ionic Strength on Rheology of Aqueous Solutions

Three well-defined polystyrene-poly(sodium methacrylate) amphiphilic block copolymers characterized by different molecular architecture (diblock, triblock, and four-arm star) have been synthesized by ATRP. The rheology of their water solutions has been evaluated by measuring dynamic moduli and shear viscosity at different concentrations. All polymers show remarkable thickening properties and a sol-gel transition at low concentration (0.1 wt %). Above the gel concentration the solutions are shear thinning without an apparent Newtonian plateau. The observed viscosity profile can be interpreted in terms of percolation theory applied to highly stretched polymeric micelles, which start to contract, above the percolation threshold. An interesting correlation between solution viscosity and concentration of hydrophilic block (defined here as "arm concentration") has been observed, giving indirect evidence for the arrangement of the polymers into micelles. The influence of ionic strength and pH on the rheology of these systems has also been preliminary investigated.