Berlin 2008 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 2: DRG-DPG SYMPOSIUM Rheology II

CPP 2.2: Vortrag

Montag, 25. Februar 2008, 14:30–14:45, C 130

Linear to branched micelles transition: a rheometry and diffusive wave spectroscopy (DWS) study — •Norbert Willenbacher and Claude Oelschlager — Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe, 76128 Karlsruhe

The shear modulus G* of aqueous wormlike micellar solutions of cetylpyridinium chloride and the strongly binding counterion sodium salicylate has been measured as a function of temperature, surfactant and salt concentration by using DWS based tracer microrheology as well as mechanical techniques including rotational rheometry, oscillatory squeeze flow and torsional resonance. Good agreement between both approaches is found in the frequency range from 0.1 to 100.000 rad/s. Upon increasing the salt concentration, at fixed surfactant concentration, the solutions exhibit two maxima of the zero shear viscosity. The first one is attributed to a transition from linear to branched micelles. From G' and G'' at low and intermediate frequencies we obtain the mesh size of the entanglement network and the scission energy E_0. The latter passes through a maximum upon the transition from linear to branched micelles. In the frequency range above 10.000 rad/s a omega^3/4 scaling is observed for the loss modulus G'' as expected for semiflexible objects. The persistence length lp is determined from the moduli in this frequency range. For the linear micelles lp decreases with increasing salt content as expected, but increases significantly upon the transition to the branched structure. The structural changes at the second viscosity maximum are not resolved so far, we will discuss the variation of lp and E_0 in this concentration range.