Dynamics and Rheology of GlassyMicellar Block Polyelectrolytes
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P8.10.1
Dynamics and Rheology of Glassy Micellar Block Polyelectrolytes
Julien Grandjean and Ahmed Mourchid Complex Fluids Laboratory, Unité Mixte de Recherche 166 du CNRS-Rhodia, 259 Prospect Plains Rd, Cranbury, NJ 08512, USA ABSTRACT Micellar solutions of polystyrene-poly(acrylic acid) block copolymers are studied in the jammed regime. Increasing attraction between soft spheres, both the rheology and dynamic light scattering show a re-entrant liquid to glass transition. The dynamics was also investigated at an attraction strength above the re-entrant transition line. Increasing concentration of sticky soft spheres in this region, the particle dynamics shows a remarkable extended logarithmic decay. The theoretical predictions of mode coupling theory agree well with our results.
INTRODUCTION Amphiphilic diblock polymers consisting of a charged hydrophilic block and a hydrophobic block exhibit fascinating morphological behavior. One such system is polystyrene-poly(acrylic acid), PS-PAA, which can display morphologies including spheres, cylinders or lamellar structures [1]. The morphology can be controlled by varying the ionic strength, solvent, or degree of polymerization of the diblocks. These copolymers tend to self-assemble at low solution concentrations. This versatility with respect to the morphology and self-assembly, is associated with unusual rheological behavior that is important in many applications like viscosity modifier products or drilling fluids for oil field applications [2]. Thus, the relationship between the solution structure and the rheology is an important issue to understand. We have investigated diblock polyelectrolyte gels with tunable rheological properties. These systems are prepared from polystyrene-poly(ethyl acrylate) diblocks (PS-PEA). The poly(ethyl acrylate) is partially converted to poly(acrylic acid) by a hydrolysis reaction. The extent of the hydrolysis reaction, f, is directly related to the fraction of ethyl-acrylate groups in the second block. These polymers self-assemble in aqueous solutions to form spherical micelles where the core is comprised of the PS block and the corona consists of the poly(acrylic acid/ethyl acrylate) block [2, 3]. The elastic modulus is a strong function of f. It is thought that the hydrophobic ethyl acrylate groups act as stickers between micelles, causing attractive micellar associations. However, there is some evidence that elasticity is triggered by crowding of the micelles, rather than or in addition to formation of an attractive network structure. To gain insight into such effects, rheology and dynamics investigations were performed on micellar solutions of PS-P(AA/EA) of varying f. The data are compared with the most recent experimental and theoretical studies on attractive colloids.
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