Rheo-optical investigations of lyotropic mesophases of polymeric surfactants |
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Authors: | Gudrun Schmidt Stefan Müller Claudia Schmidt Walter Richtering |
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Institution: | Albert-Ludwigs-Universit?t Freiburg Institut für Makromolekulare Chemie Stefan-Meier-Strasse 31 D-79104 Freiburg, Germany email: rich@uni-freiburg.de, DE
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Abstract: | The shear orientation of hexagonal and lamellar liquid crystalline phases of polymeric surfactants was investigated by rheo-optical
techniques (flow birefringence (Δn), 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 an alignment of rodlike micelles along the flow
direction was found. A shear induced formation of vesicles (“onions”) is observed with the lamellar phase. They displayed
a characteristic 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
performed at constant stress and constant 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 transition to planar lamellae in the lamellar phase, is very similar
to the behavior of lyotropic mesophases formed by low molar mass surfactants or amphiphilic block copolymers. The geometrical
constraints that are introduced when amphiphilic side groups are fixed to a polymer backbone do not significantly alter the
response of the mesophase to a shear deformation.
Received: 4 May 1999 /Accepted: 19 July 1999 |
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Keywords: | Hexagonal Lamellar Vesicle Shear orientation Polymeric surfactant |
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