LINEAR AND NONLINEAR RHEOLOGICAL BEHAVIOR OF SALT-FREE AQUEOUS CTAB SOLUTIONS

We report here experimental results of the linear and non-linear theol ogical properties of concentrated aqueous solutions of cetyltrimethyla mmonium bromide (CTAB) without addition of salt. The range of concentr ation studied in this work is just before a static liquid crystalline phase is obtained. The flow curves show three different and well chara cterized domains of evolution. In the first part of the curve, the New tonian part, we obtain as a function of concentration a very unusual p ower law exponent of nearly 12 for the zero shear viscosity eta(0) and 62 for a terminal relaxation time. The second region is characterized by a plateau value sigma(c) of the shear stress ranging from gamma(1c ) to gamma = gamma 2(c). This domain corresponds to the transition fro m an isotropic to a nematic state with an intermediate biphasic system . The evolution of sigma(c), gamma(1c) and gamma(2c), are given as a f unction of concentration and temperature to obtain dynamic phase diagr ams. The third region shows a new increase of the stress with shear ra te. This domain is well defined in our system and is practically indep endent of temperature and concentration, contrary to the situation in the first and second domains which show a great dependence on these pa rameters. Measurements of flow birefringence confirm the nature of the phase transition occurring in the second domain and shows the existen ce of a shear-banding structure for gamma(1c) < gamma < gamma(2c). At present, theory cannot explain all the results obtained in this work.