DIRECTOR DYNAMICS OF UNIFORMLY ALIGNED NEMATIC LIQUID-CRYSTALS IN TRANSIENT SHEAR-FLOW

We have developed a modular rheo-optical apparatus to study the flow p roperties of liquid crystals. Its main components are shearing device, strong magnetic field, and optical microscope. We performed experimen ts on well defined initial morphologies with uniform molecular alignme nt. The monodomains were achieved with strong magnetic fields (4.7 T). Time-resolved conoscopy is the primary optical technique in our inves tigation. We propose a simple relation between the distribution of ali gnment angles over the sample thickness and the conoscopically measure d angle, to quantitatively measure the alignment angle in shear flow. We followed the relaxation of a shear-induced splay deformation in sma ll molecule model systems (N-(p-methoxybenzylidene)-p-butylaniline (MB BA), pentyl-cyano-biphenyl (5 CB) and a commercially available mixture OMI4244). We define a rotational director diffusivity D(R) = K(s)/eta (s) (K(s) splay elastic constant, eta(s) splay viscosity) from the rel axation process and devised a model, based on the diffusion equation t o determine their values. The director alignment behavior of the small molecule liquid crystals (SMLC's) in shear flow is well described by the two-dimensional Leslie-Ericksen model. The effect of director elas ticity can clearly be seen in our experiments, resulting in a decrease of the steady state alignment angle at smaller Ericksen numbers. We f ound that there is no strain rate dependence of the director vorticity from 0.002/s to 2/s for poly-(gamma-benzyl-D/L-glutamate) (PBG). We d etermined alpha2/alpha3 = -44 for a 20% solution of 280 000 molecular weight PBG in m-cresol at 20-degrees-C. The conoscopic interference pa ttern vanished after 8 strain units from an initially planar alignment and shearing could be reversed up to 10 strain units to completely re cover the initial monodomain.