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 p olymers show very similar aspects. They are composed of two main parts , a long streak perpendicular to the flow direction, with a strong int ensity modulation, and four asymmetric lobes. These general features s uggest a common origin for this pattern. A simplified model of the def ects created in the low shear rate region of a thermotropic polymer (D e'Neve, T.; Navard, P.; Kleman, M. Macromolecules 1995, 28, 1541), was used as the basic scattering unit. It consists of a twist loop, fluct uating around the shearing plane, with a director varying from the flo w direction to the vorticity axis. The light scattering by a collectio n of such objects is theoretically calculated and compared to the expe rimental behavior of the vertical streak.