Berlin 2012 – wissenschaftliches Programm
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 13: Poster: Interfaces and Thin Films
CPP 13.7: Poster
Dienstag, 27. März 2012, 18:15–20:45, Poster A
Competing Ordering Processes at Liquid Crystal Surfaces Laden with Semifluorinated Alkane Molecules — •Xunda Feng1, Ahmed Mourran2, Martin Möller2, and Christian Bahr1 — 1Max-Planck-Institute for Dynamics and Self-Organization, Göttingen, Germany — 2Institute for Technical and Macromolecular Chemistry, RWTH Aachen, Germany
We present an ellipsometric study of the interface between isotropic liquid crystals (LCs) and air in the vicinity of the nematic – isotropic and smectic-A – isotropic transition. The LCs are doped with a small amount of the semifluorinated alkane C18H37–C12F25 (H18F12) which forms a Gibbs film (GF) at the LC/air interface. Our measurements elucidate the interplay between the nematic or smectic surface order at the isotropic LC/air interface and a structural phase transition in a GF on the same interface.
GFs of H18F12 on normal alkanes are known to exhibit a transition from a dilute state at higher temperatures to a dense state at lower temperatures. Our results indicate that the same transition takes place in GFs formed on the isotropic LCs. The transition temperature can be tuned by controlling the H18F12 concentration in the bulk LC phase. When the transition takes place in the temperature range in which a molecular thin smectic or nematic film exists at the isotropic LC/air interface, the smectic surface order is destroyed while the nematic surface order is affected by a change of the orientation of the LC molecules. The ellipsometric data indicate that both behaviors result from a change of the anchoring condition of the LC molecules in contact with the GF.