Rheological characterization of the dilatant flow behavior of highly substituted hydroxypropylmethylcellulose solutions in the presence of sodium lauryl sulfate

The flow behavior of aqueous solutions of three highly substituted, hydrophobic hydroxypropylmethylcelluloses (HPMC) in mixtures containing the anionic surfactant sodium lauryl sulfate (SLS) was investigated both rheo-mechanically and rheo-optically. For the first time it was possible to demonstrate dilatant flow in these systems, a phenomenon which is otherwise only known of some suspensions and associative thickening solutions. Without addition of SLS, the aqueous HPMC solutions showed the predicted flow behavior of polymer solutions, and the Cox-Merz rule was fulfilled. With the addition of SLS to these HPMC solutions, the least hydrophobic HPMC displayed no dilatancy. The solutions of a more hydrophobic HPMC with SLS exhibited on the one hand an increase in viscosity, and on the other hand shear thinning as well as shear thickening. The most hydrophobic HPMC displayed more clearly the effects of an SLS-dependent viscosity increase and the appearance of dilatant flow. At constant HPMC concentration (0.5% w/w), a maximum increase in viscosity (factor 15) was observed in the critical micelle concentration range for SLS. By rheo-optical measurements it was possible to detect an unusually pronounced alignment of the polymer segments as well as a sharp increase in the birefringence values, even before the macroscopic occurrence of dilatant flow. According to the existing network theories, this behavior of the aqueous solutions of highly substituted HPMCs in mixture with SLS has been interpreted as a shear-induced transition from intramolecular to intermolecular interactions.