Light scattering by shear-induced defects of nematic liquid crystalline polymers

The depolarized light scattering patterns observed at low shear rates for lyotropic and thermotropic nematic main-chain liquid crystalline polymers show very similar aspects. They are composed of two main parts, a long streak perpendicular to the flow direction, with a strong intensity modulation, and four asymmetric lobes. These general features suggest a common origin for this pattern. A simplified model of the defects created in the low shear rate region of a thermotropic polymer (De'Neve, T.; Navard, P.; Kleman, M. Macromolecules 1995, 28, 1541), was used as the basic scattering unit. It consists of a twist loop, fluctuating around the shearing plane, with a director varying from the flow direction to the vorticity axis. The light scattering by a collection of such objects is theoretically calculated and compared to the experimental behavior of the vertical streak.