Rheo-optical investigations of lyotropic mesophases of polymeric surfactants

The shear orientation of hexagonal and lamellar liquid crystalline phases o f polymeric surfactants was investigated by rheo-optical techniques (flow b irefringence (Delta n), small-angle light scattering) as well as by nuclear magnetic resonance and optical microscopy. The evolution of birefringence in the hexagonal phase is discussed for simple and oscillatory shear, and a n alignment of rodlike micelles along the flow direction was found. A shear induced formation of vesicles ("onions") is observed with the lamellar pha se. They displayed a characteristic four-lobe pattern in depolarized light scattering. Above a critical shear stress vesicles were degraded and perpen dicularly aligned lamellae (i.e. with their normal along the vorticity dire ction) were obtained. A comparison of experiments performed at constant str ess and constant rate revealed that the vesicle to planar lamellae transiti on occurred above a critical shear stress. The behavior of the polysoap lyo tropic mesophases under shear, i.e, the strain dependent alignment in the h exagonal phase, the sheer induced formation of vesicles, and a transition t o planar lamellae in the lamellar phase, is very similar to the behavior of lyotropic mesophases formed by low molar mass surfactants or amphiphilic b lock copolymers. The geometrical constraints that are introduced when amphi philic side groups are fixed to a polymer backbone do not significantly alt er the response of the mesophase to a shear deformation.