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

The interactions between well-defined hydrophobically modified polyacrylami des (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 o f hydrophobic monomers (N,N-dihexylacrylamide) in the polyacrylamide backbo ne. A thorough investigation of the rheological behavior of HMPAM/surfactan t mixtures as a function of polymer concentration and of different molecula r parameters (molecular weight, hydrophobe content, and hydrophobic block l ength) provides some novel insight into the surfactant binding properties o f multisticker chains. The experiments were performed on series of HMPAM lo cated both in unentangled and entangled semidilute regimes. In both regimes , the rheological behavior strongly depends on the level of surfactant addi tion, 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 sig nificant change in the plateau modulus Go. The latter observation that diff ers from most of the other reported conclusions suggests that the viscosity enhancement is not due to the formation of additional mixed plurifunctiona l aggregates, but to an increase in the lifetime of the preexisting cross-l inks resulting from surfactant binding. At high surfactant concentrations ( greater than or similar to0.05 M) the solubilization of the individual hydr ophobic 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 a lso suggest that the maximum of viscosity corresponds to an optimum surfact ant binding which is about the same, whatever the concentration of hydropho bic 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 aggr egates.