ORDERING EFFECTS IN THIN SMECTIC-C-ASTERISK FILMS - AN X-RAY-REFLECTIVITY STUDY

The x-ray-reflectivity technique is used to study the smectic order and the smectic-A (Sm-A) to smectic-C* (Sm-C*) phase transition in thin and ultrathin films (150-600 angstrom) of the chiral ferroelectric liquid-crystal mixture ZLI-3654 (produced by Merck), The films, which are spin cast on various substrates [very smooth (float) glass, Si Wafer, polymer-coated glass, etc.], order spontaneously with smectic layering parallel to the substrate surface; the film alignment is induced by anchoring forces at the film-air interface. The reflectivity profiles could be well described by a sinusoidal density modulation perpendicular to the film. We demonstrate that it is possible to extract the molecular tilt angle alpha in ferroelectric liquid crystals from x-ray-reflectivity measurements of thin films. The Sm- A - Sm-C* phase-transition temperature and the temperature dependence of the tilt angle in the smectic-C* phase are almost independent of the film thickness (down to approximately 200 angstrom) and are similar to those in the bulk. In all cases the dependence of the tilt angle alpha(T) can be described by a power law: alpha approximately t(gamma) '' (t = 1 - T/T(c)), where gamma = 0.31 +/- 0.04, in agreement with the de Gennes predicted superfluid helium analogy [Acad. Sci. Paris Ser. B 274, 785 (1972)]. In a film of about 200-angstrom thickness we observed a smectic layer spacing which is much larger than in thick films.