Long‐Range Interactions in Thin Smectic Films on Substrates: X‐Ray Reflectivity Studies

Long‐range forces between interfaces of thin and ultrathin smectic films 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 possess a smectic C* phase at room temperature. We demonstrate spontaneous molecular self‐assembly after spin coating into a nearly perfect smectic layer structure on various substrates. However, annealing at room temperature is essential to achieve an equilibrium state. Measurements on ultrathin annealed films of the LC mixtures show dramatic variation of the smectic layer spacing, L, as a function of the number of smectic layers, n (or film thickness, d). The functional dependence of L(n) for all three different liquid crystal mixtures suggests a long‐range interaction between the interfaces that decays algebraically as 1/n^κ where κ = 2 ± 0.3. This decay is consistent with a van der Waals type of interaction, although its magnitude cannot be explained by the existing mechanisms. X‐ray studies of thick and thin annealed polysiloxane films allow determination of the phases, the phase transition temperatures, and the temperature dependence of the tilt angle in the smectic C* phase. Thin (300 Å to 600 Å) polysiloxane films far above the bulk smectic–isotropic phase transition temperature show the formation of smectic film at the film–substrate interface 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 κ = 1.5 ± 0.5.