Rheological Properties of ABA Telechelic Polyelectrolyte and ABA Polyampholyte Reversible Hydrogels : a Comparative Study

International audienc

Three different types of physical gels originate from a common triblock copolymer precursor: the case of an ionomer gel

International audienc

Physical Hydrogels via Charge Driven Self-Organization of a Triblock Polyampholyte - Rheological and Structural Investigations

We investigate the conformational properties of stimuli-responsive hydrogels from triblock polyelectrolytes PtBA-b-P2VP-b-PtBA (PtBA and P2VP are poly(tert-butyl acrylate) and poly(2-vinylpyridine)) and the corresponding polyampholytes PAA-b-P2VP-b-PAA (PAA is poly(acrylic acid)), the latter with nonquaternized or quaternized P2VP blocks. The block lengths are the same in all three polymers with relatively short end blocks and long middle blocks. The mechanical properties of the hydrogels have previously been found to depend strongly on the pH value and on the nature of the blocks ( Polymer 2008, 49, 1249). Here, we present results from rheological studies and small-angle neutron scattering revealing the underlying hydrogel structures. The hydrogel structure of the polyampholyte depends on the charge asymmetry, controlled by the pH value, and reveals several transitions with increasing charge ratio. A low charge asymmetry causes the collapse of the chains into large globular structures due to the fluctuation-induced attractions between oppositely charged moieties. In contrast, at higher charge asymmetry, a network is formed. The latter is also found for the polyelectrolyte system. These results demonstrate the origin of the strong changes in mechanical properties upon change of pH

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.