Effect of surfactant on the viscoelastic behavior of semidilute solutions of multisticker associating polyacrylamides

The interactions between well-defined hydrophobically modified polyacrylamides (HMPAM) and the anionic surfactant SDS (sodium dodecyl sulfate) or the cationic surfactant DTAB (dodecyltrimethylammonium bromide) were studied in aqueous solution using steady-flow and oscillatory rheological experiments. The structure of the HMPAM used consists of randomly distributed blocks of hydrophobic monomers (N,N-dihexylacrylamide) in the polyacrylamide backbone. A thorough investigation of the rheological behavior of HMPAM/surfactant mixtures as a function of polymer concentration and of different molecular parameters (molecular weight, hydrophobe content, and hydrophobic block length) provides some novel insight into the surfactant binding properties of multisticker chains. The experiments were performed on series of HMPAM located both in unentangled and entangled semidilute regimes. In both regimes, the rheological behavior strongly depends on the level of surfactant addition, with first an increase and then a decrease in the values of the zero-shear viscosity eta (0) and of the terminal relaxation time T-R without significant change in the plateau modulus Go. The latter observation that differs from most of the other reported conclusions suggests that the viscosity enhancement is not due to the formation of additional mixed plurifunctional aggregates, but to an increase in the lifetime of the preexisting cross-links resulting from surfactant binding. At high surfactant concentrations (greater than or similar to0.05 M) the solubilization of the individual hydrophobic blocks by the surfactant micelles leads to a decrease of G(0), eta (0), and T-R and the HMPAM lose their associative properties. The results also suggest that the maximum of viscosity corresponds to an optimum surfactant binding which is about the same, whatever the concentration of hydrophobic units in the HMPAM solution. SDS has a much larger effect on the eta (0) variation than DTAB, likely due to a different lifetime of the mixed aggregates.