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Zusammenfassung:
Free-standing films of the liquid-crystal compound C7 [4-(3-methyl-2-chloropentanoyloxy)-4’-heptyloxybiphenyl], which possesses a first-order smectic-A–smectic-C transition, are studied. In various films with thicknesses between 2 and 27 molecular layers, the average tilt angle, i.e., the order parameter of the transition, is measured by ellipsometry. In thick films of more than about 15 layers, a first-order transition with a discontinuous jump of the average tilt value is observed at the same temperature at which the smectic-A–smectic-C transition in the bulk sample occurs; in contrast to the bulk, a finite average tilt remains in the high-temperature phase, indicating that the transition takes place in the interior layers, whereas the surface layers are tilted in the whole temperature range. Films thinner than about 15 layers exhibit instead of the first-order transition a steep but continuous variation of the tilt with temperature. This feature becomes less pronounced with decreasing film thickness and is not observable in the two-layer film, which shows large tilt values in the whole temperature range, indicating the influence of a strong ordering field resulting from surface interactions.
