Shear Orientation of a Hexagonal Lyotropic Triblock Copolymer Phase As Probed by Flow Birefringence and Small-Angle Light and Neutron Scattering

Abstract

The shear orientation of the hexagonal liquid crystalline phase of a poloxamer-type triblock copolymer (poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic L64))/water mixture was investigated by means of different techniques, namely, flow birefringence (Δn), small-angle light scattering, and small-angle neutron scattering (SALS, SANS). The evolution of birefringence is discussed for different types of shear. Oscillatory shear showed that birefringence displayed small oscillations during the first cycles until the sample was shear aligned. Then the same Δn value was obtained as in creep experiments. The degree of orientation depended on the strain. On a microscopic length scale probed by SANS, the shear flow results in an alignment of rodlike micelles along the flow direction. SANS experiments with the incident beam along the flow direction revealed that the 10 plane of the hexagonal lattice was aligned parallel to the flow-vorticity plane. On a mesoscopic length scale, studied by microscopy and SALS, a stripe texture was observed. SALS showed that the texture was similar to that obtained in low molar mass surfactants.