Interactions between poly(ethylene oxide) and sodium dodecyl sulfate in elongational flows

This work investigates the elongational flow of aqueous solutions of mixtur es of a high-molecular-weight poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS). The formation of micellar aggregates of SDS along the PEO c hain results in an increase in the strength of the extension thickening of the PEO solutions. This is especially pronounced under conditions in which the PEO molecules form transient entanglements in the flow field. The minim um PEO concentration required to form intermolecular entanglements is subst antially reduced in the presence of micellar aggregates. This effect become s quantitatively less important in solutions with NaCl, which suggests PEO coil contraction due to electrostatic screening of micellar aggregates. How ever, once extension thickening starts in the presence of NaCl, the growth of pressure drop is more abrupt than without salt, which suggests stronger interactions between PEO coils with attached aggregates. The critical aggre gation concentrations of PEO/SDS and PEO/SDS/NaCl solutions agree with thos e reported in the literature, which were obtained by means of different exp erimental techniques. However, the saturation of the surfactant effect is a ttained at lower surfactant concentrations than the polymer saturation poin t previously reported. This might reflect a low sensitivity of the extensio n thickening effect to the amount of surfactant bound to the polymer chain as the saturation point is approached. (C) 2001 Academic Press.