Shear and elongational rheology of solutions of hydrophobically modified polymers with spherical and rodlike surfactant micelles

Aqueous solutions of hydrophobically modified polymers (hm-polymers) and surfactants were investigated by rheological and rheo-optical experiments. The objectives were to understand the interactions between hm-polymers and surfactants and to control their rheology by changing the surfactant morphology from spherical to rodlike micelles. Randomly grafted hm-hydroxyethylcellulose (hmHEC) with C{dollar}sb{lcub}12{rcub}{dollar} and C{dollar}sb{lcub}16{rcub}{dollar} hydrophobes and hm-polyacrylamide (hmPAM) with sulfonated C{dollar}sb{lcub}12{rcub}{dollar} hydrophobes were studied. The surfactants used were sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), and cetyldimethylamine oxide, along with sodium salicylate (NaSal) and hexanol cosurfactants and KBr salt.; Interactions between hm-polymers and spherical micelles were investigated by shear rheology. With increasing surfactant concentration, the viscosity of semidilute hm-polymer solution first increases and then decreases due to micellar aggregation around hydrophobe clusters and masking of hydrophobes by excess micelles, respectively. For hmHEC and CTAB systems, steady-state fluorescence quenching experiments showed that the number of hydrophobes per mixed micelle decreases from over two with no surfactant to around two at the viscosity maximum and to one when hydrophobes are masked. The strength of interactions increases with hydrophobe length and content and surfactant tail length. Hydrophobic interactions also depend on inherent electrostatic interactions. In shear and elongational flows, hmHEC solutions were shear- and elongational-thinning due to the breakup of hydrophobe associations. Elongational thickening was also observed and attributed to flow-enhanced associations. Interactions of hmHEC with spherical SDS and CTAB micelles merely affect the viscosity values and do not alter their inherent flow profiles, which can be collapsed and fitted with a rheological model for telechelic hm-polymers.; Interactions between hm-polymers and rodlike micelles were also studied by shear rheology. The viscosity of semidilute hm-polymer solution increases dramatically with surfactant concentration due to extensive bridging interactions between hm-polymers and micellar rods. For various polymer/surfactant systems, the viscosity at high surfactant concentrations can be tuned by controlling the extent of rod formation. Bridging of semidilute micellar rods with hm-polymers resulted in a similar viscosity enhancement. Small-angle neutron scattering (SANS) and flow birefringence experiments showed that flow-induced growth and structural changes of rodlike CTAB/NaSal micelles in shear (and elongational) flow were suppressed due to interactions with hmHEC, which hinder micellar alignment.