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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 3: Liquid Crystals and Polar Fluids
CPP 3.4: Talk
Monday, February 25, 2008, 10:45–11:00, C 264
AFM Study of Surface-Induced Ordering in Liquid Crystals: Impurity Enhanced Ordering — •Doris Vollmer, Li-Juan Zhang, Michael Kappl, and Günter K. Auernhammer — MPI Polymerforschung, Mainz, Germany
We experimentally investigate surface-induced smectic-like ordering close to the isotropic-nematic phase transition of thermotropic liquid crystals of the n-CB series. Homeotropic alignment has been induced by depositing a lipid monolayer onto the silica surface. As shown by AFM the smectic like ordering alignment vanishes for temperatures a few degree above the phase transition.
Surprisingly, addition of a few percent of alkane to the liquid crystal enhances the presmectic ordering. In case of 4′-n-cyano-4-octylbiphenyl (8-CB) and octane we found maximal ordering at 4 percent of octane. This enhanced ordering depends on the length of the alkane, Ci, compared to length of the carbon tail of the liquid crystal, n-CB.
Probably impurity enhanced ordering is due to a combination of improved molecular packing and enhanced presmectic fluctuations, because addition of octane causes a decrease of the width of the nematic domain.