Uniplanar smectic phases in free-standing films

Structures of various phases that appear in antiferroelectric liquid crystals can be described within a simple discrete phenomenological model of antiferroelectric liquid crystals, which for the smectic-C-alpha (SmC(alpha)) phase suggests a short-pitch helicoidal structure. In a paper presented here the same model is used to analyze theoretically free-standing films, formed by a finite number of smectic layers. In the bulk sample a transition from the smectic-A (SmA) phase is either to the ferroelectric SmC* phase, the SmC(*)(A) phase, or the antiferroelectric SmC(alpha) phase, as observed from experiments as well as comprehended in the model. However in free-standing films uniplanar structures, which in a bulk sample were not observed experimentally nor predicted by the model, are shown to appear immediately below the transition from the high-temperature SmA phase. In free-standing film uniplanar structure remains stable in a narrow temperature region between SmA and SmC(alpha) phase.