LONG-RANGE INTERACTIONS IN THIN SMECTIC FILMS ON SUBSTRATES - X-RAY REFLECTIVITY STUDIES

Long-range forces between interfaces of thin and ultrathin smectic fil ms are responsible for their thickness and stability. We present here X-ray reflectivity studies of the forces in spin-coated thin films of three different commercial liquid crystal (LC) mixtures as well as in main-chain LC polymers based on polysiloxane. All these LC materials p ossess a smectic C phase at room temperature. We demonstrate spontane ous molecular self-assembly after spin coating into a nearly perfect s mectic layer structure on various substrates. However, annealing at ro om temperature is essential to achieve an equilibrium state. Measureme nts on ultrathin annealed films of the LC mixtures show dramatic varia tion of the smectic layer spacing, L, as a function of the number of s mectic layers, n (or film thickness, d). The functional dependence of L(n) for all three different liquid crystal mixtures suggests a long-r ange interaction between the interfaces that decays algebraically as 1 /n(kappa) where kappa = 2 +/- 0.3. This decay is consistent with a van der Waals type of interaction, although its magnitude cannot be expla ined by the existing mechanisms. X-ray studies of thick and thin annea led polysiloxane films allow determination of the phases, the phase tr ansition temperatures, and the temperature dependence of the tilt angl e in the smectic C phase. Thin (300 Angstrom to 600 Angstrom) polysil oxane films far above the bulk smectic-isotropic phase transition temp erature show the formation of smectic film at the film-substrate inter face due to surface freezing phenomena. Preliminary investigations of the temperature dependence of the smectic film thickness indicate that the interaction between the interfaces decays algebraically, with an exponent kappa = 1.5 +/- 0.5.