Introduction The in¯uence of shear on complex ¯uids with anisotro- pic structure has attracted growing interest during the last few years see, for example, Hadziioannou et al. (1979), Roux et al. (1993), Winter et al. (1993), Berret et al. (1994), Fredrickson and Bates (1996), Honglada- rom et al. (1996), Nakatani et al. (1996), Walker and Wagner (1996), Chen et al. (1997), Richtering (1997), Wiesner (1997), and Larson (1998). Samples with hexagonal and lamellar morphology have been studied intensely. These kinds of structures can be found in very dierent types of materials, e.g., in block copolymer melts and solutions, as well as in aqueous mixtures of low molecular weight surfactants. Shear ¯ow can have a manifold in¯uence on the structure of liquid crystalline phases. A homogeneous alignment of rodlike micelles or lamellae can be obtained and their orientation can be described by the director orientation with respect to directions of the ¯ow ®eld. Experiments on sheared hexagonal phases by Tepe et al. (1995), Lukaschek et al. (1995), Fairhurst et al. Rheol Acta 38: 486±494 (1999) Ó Springer-Verlag 1999 ORIGINAL CONTRIBUTION Gudrun Schmidt Stefan MuÈller Claudia Schmidt Walter Richtering Rheo-optical investigations of lyotropic mesophases of polymeric surfactants Received: 4 May 1999 Accepted: 19 July 1999 G. Schmidt á S. MuÈller á C. Schmidt W. Richtering (&) Albert-Ludwigs-UniversitaÈt Freiburg Institut fuÈr Makromolekulare Chemie Stefan-Meier-Strasse 31 D-79104 Freiburg, Germany email: rich@uni-freiburg.de Presented at EuroRheo 99-1 May 3±7, 1999, Sophia-Antipolis, France Abstract The shear orientation of hexagonal and lamellar liquid crys- talline phases of polymeric surfact- ants was investigated by rheo-optical techniques (¯ow birefringence (Dn), small-angle light scattering) as well as by nuclear magnetic resonance and optical microscopy. The evolu- tion of birefringence in the hexago- nal phase is discussed for simple and oscillatory shear, and an alignment of rodlike micelles along the ¯ow direction was found. A shear in- duced formation of vesicles (``on- ions'') is observed with the lamellar phase. They displayed a character- istic four-lobe pattern in depolarized light scattering. Above a critical shear stress vesicles were degraded and perpendicularly aligned lamellae (i.e. with their normal along the vorticity direction) were obtained. A comparison of experiments per- formed at constant stress and con- stant rate revealed that the vesicle to planar lamellae transition occurred above a critical shear stress. The behavior of the polysoap lyotropic mesophases under shear, i.e. the strain dependent alignment in the hexagonal phase, the shear induced formation of vesicles, and a transi- tion to planar lamellae in the lamel- lar phase, is very similar to the behavior of lyotropic mesophases formed by low molar mass surfact- ants or amphiphilic block copoly- mers. The geometrical constraints that are introduced when amphi- philic side groups are ®xed to a polymer backbone do not signi®- cantly alter the response of the mesophase to a shear deformation. Key words Hexagonal á Lamellar á Vesicle á Shear orientation á Polymeric surfactant