Parallel-Layer Orientation and Its Instability in Side-Chain Polymer Smectic Liquid Crystals under Shear Flow

Abstract

Smectic layer orientation was investigated for the side-chain PM10ME liquid crystal polymer to which steady shear and large amplitude oscillatory shear (LAOS) flows were applied. Two types of orientation are achieved depending on the shear rate γ̇ and the frequency ω, both of which are presumed to be inversely proportional to the time scale of shear flow. Parallel orientation with the layer normal in the velocity gradient direction is achieved by steady shear flow at γ̇ < 10−1 s−1, but the layers undulate along the neutral direction. Undulation is eliminated under LAOS flow at ω > 101 rad s−1, which gives a transparent sample with perfect parallel orientation. Shear flows under other conditions (steady shear at γ̇ > 10−1 s−1 and LAOS at ω < 101 rad s−1) tend to induce perpendicular orientation in which the layer normal points in the neutral direction. These results show that layer orientation changes from parallel to perpendicular and back to parallel with shortening time scale of shear flow. These changes in orientation are explainable if fluctuations, which couple to shear flow to induce perpendicular orientation, are averaged out and suppressed under the longer- and shorter-time scale shear flows.