Orientational phase transitions in the hexagonal phase of a diblock copolymer melt under shear flow

We generalize the earlier theory by Fredrickson [J. Rheol. 38, 1045 (1994)] to study the orientational behavior of the hexagonal phase of diblock copo lymer melt subjected to steady shear flow. We use symmetry arguments to sho w that the orientational ordering in the hexagonal phase is a much weaker e ffect than in the lamellae. We predict the parallel orientation to be stabl e at low and the perpendicular orientation at high shear rates. Our analysi s reproduces the experimental results by Tepe et al. [Macromolecules 28, 30 08 (1995)] and explains the difficulties in experimental observation of the different orientations in the hexagonal phase.