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Shear induced orientation in nematic liquid crystal monodomains

David Michael Boudreau, University of Massachusetts Amherst


The flow behavior and orientational order of nematic liquid crystal monodomains in shear flow were studied using the optical interference technique of conoscopy. Planar monodomains of two small molecule liquid crystals (n-( p-methoxybenzylidine)-p-butyl aniline (MBBA), and n-pentyl-cyano-biphenylene (5CB)) were prepared at various orientations to the flow direction. Shear and relaxation of molecular orientation was observed at constant temperature with various shear rates. The transient splay and twist director alignment was found to be dependent upon initial alignment conditions. The steady state splay alignment angle, or Leslie angle, also showed a dependence on the initial conditions. The initial monodomain alignment also showed an effect on the twist relaxation but played no observable role in splay relaxation. Investigation of polymeric liquid crystals focused on two samples of a side-chain liquid crystalline polymer with laterally attached mesogens. The samples varied only by molecular weigh (1.68 × 105 g/Mol and 18.9 × 105 g/Mol respectively). Monodomains were prepared using a 7T magnetic field. Use of this field required the development of a non-magnetic, high temperature shearing device. The shear behavior of the polymers was observed at a constant rate of 5s−1, in a temperature range around the nematic to isotropic transition temperature (TNI ± 10K). Both materials showed flow-aligning behavior at all orientations and temperatures studied. Monodomain orientation perpendicular to flow was lost on shear start-up but reappeared parallel to flow within five strain units. The flow alignment angle of the higher molecular weight sample was approximately three times lower than for the low molecular weight sample. This has been explained in terms of the development of shear induced smectic fluctuations within the higher molecular weight sample.

Subject Area

Chemical engineering|Organic chemistry|Polymers

Recommended Citation

Boudreau, David Michael, "Shear induced orientation in nematic liquid crystal monodomains" (2001). Doctoral Dissertations Available from Proquest. AAI3027181.