Rheological Properties of Associative Star Polymers in Aqueous Solutions: Effect of Hydrophobe Length and Polymer Topology

Rheological properties of aqueous solutions of well-defined four-armed amphiphilic star block copolymers, poly(acrylic acid)-block-polystyrene (PAA-b-PS)(4), were investigated as a function of the length of the PS blocks, polymer concentration, and temperature and compared with a corresponding triblock copolymer. These polymers, synthesized by atom transfer radical polymerization (ATRP), were found to form hydrogels due to intermolecular association originating from the PS blocks. The increasing length of the PS block was observed to lead to more elastic networks due to increased hydrophobic interaction. Polymers bearing shorter PS blocks gave gels with relatively long linear response followed by strain hardening before shear thinning while the longer PS blocks lead to formation of elastic but brittle gels with limited linear regime before shear thinning. Star-block copolymers showed more elastic behavior compared with a triblock copolymer of comparable molar mass and composition. In small-angle X-ray scattering measurements the increasing lengths of the PS blocks were observed to lead to a shift in the scattering maxima toward lower q-values. Both rheological and X-ray characterization showed that the thermal properties of the gels are changed by increasing the PS block lengths. Gels with short PS blocks soften upon heating at lower temperatures compared with the gels with longer PS blocks.