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, hy drophobic hydroxypropylmethylcelluloses (HPMC) in mixtures containing the anionic surfactant sodium lauryl sulfate (SLS) was investigated bo th rheo-mechanically and rheo-optically. For the first time it was pos sible to demonstrate dilatant flow in these systems, a phenomenon whic h is otherwise only known of some suspensions and associative thickeni ng solutions. Without addition of SLS, the aqueous HPMC solutions show ed 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 i n viscosity, and on the other hand shear thinning as well as shear thi ckening. 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 concentrat ion range for SLS. By rheo-optical measurements it was possible to det ect an unusually pronounced alignment of the polymer segments as well as a sharp increase in the birefringence values, even before the macro scopic 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 tra nsition from intramolecular to intermolecular interactions.