STUDIES OF THIN SMECTIC C-ASTERISK FILMS BY X-RAY REFLECTIVITY

The smectic order in thin and ultra thin films (150-600 angstrom) of t he chiral ferroelectric liquid crystal mixture ZLI-3654 is studied usi ng the X-ray reflectivity technique. The spin cast films on various su bstrates (float glass, Si wafer, polymer coated glass, etc.) order spo ntaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describ e well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We d emonstrate, for the first time, that it is possible to extract the mol ecular tilt angle, alpha, in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature depende nce of the tilt angle in the smectic C phase are almost independent o f the film thickness (down to approximately 200 angstrom) and are simi lar to those in the bulk.