Unusual thickness-dependent thermal behavior and anticlinic coupling in chiral smectic free-standing liquid-crystal films

Surface and finite-size effects on N-Sm-A-Sm-C phase transitions in free-standing films

The present study is devoted to the investigation of surface anchoring and finite-size effects on nematic-smectic-A-smectic-C (N-Sm-A-Sm-C) phase transitions in free-standing films. Using an extended version of the molecular theory for smectic-C liquid crystals, we analyze how surface anchoring and film thickness affect the thermal behavior of the order parameters in free-standing smectic films. In particular, we determine how the transition temperature depends on the surface ordering and film thickness. We show that the additional orientational order imposed by the surface anchoring may lead to a stabilization of order parameters in central layers, thus modifying the nature of the phase transitions. We compare our results with experimental findings for typical thermotropic compounds presenting a N-Sm-A-Sm-C phase sequence.

Surface-surface interaction in smectic liquid crystal films

Null transmission ellipsometry was employed to study the field induced transition of the surface arrangements in freestanding films of smectic liquid crystals. The interlayer interaction between the two surfaces obtained from the threshold voltage for the transition is found to be antiferroelectric and is quasilong ranged. The possible microscopic origins of the measured interaction and its relevance to the interlayer interaction in antiferroelectric liquid crystal materials are discussed.

Evolution of a rare sequence of surface transitions with temperature and film thickness

In free-standing smectic films, layers near the surfaces of the film often contain molecules tilted away from the layer normal, while in the bulk of the film the magnitude of the tilt decays exponentially with distance from the surface. We have identified the detailed molecular tilt orientations in the surface layers of films for one antiferroelectric liquid crystal compound. A series of five surface structures exists with different nonplanar tilt arrangements for each structure. The molecular orientations in the surface layers evolve with temperature. The polarization of the film also evolves with temperature, corresponding to the tilt arrangements. Using ellipsometric data, we reconstruct the changes in the magnitude and azimuthal direction of the tilt as functions of temperature. We have also studied films of several different thicknesses. We present a phase diagram for the five surface structures showing the dependence on temperature and film thickness.

Thickness dependent phase behavior of antiferroelectric liquid crystal films

Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-C(alpha)* phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-C(FI2)* phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-C(alpha)* phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems.

Nonplanar tilts in very thin smectic films of one liquid-crystal compound

Surface effects cause tilted molecular arrangements in smectic layers near the surface of a free-standing liquid-crystal film in which the bulk of the film is in the smectic- A phase. One recent work has shown that the tilt directions in adjacent surface layers may be nonplanar. In this paper we study films with thicknesses of two to six smectic layers. Surface effects dominate in these very thin films. We show that the molecular tilts are nonplanar even in these very thin films.

Effects of doping on an unusual smectic- C*alpha-smectic-C*FI2-smectic-C* phase sequence

The compound 10OHF has a partially inverted phase sequence, unique among the series of nOHF homologous compounds and all other known liquid crystals, with the smectic-C*FI2 (SmC*FI2) phase occurring at higher temperature than the smectic-C* (SmC*) phase. We present ellipsometric data to identify the phase sequences of 9OHF, 10OHF, 11OHF, and 12OHF. Binary mixtures of 10OHF with C11, a compound with the typical phase sequence among the smectic phases, show that the unusual phase sequence of 10OHF stabilizes upon mixing and that SmC*FI2 predominates over SmC* throughout the entire mixing phase diagram. In thin films of some mixtures, surface interactions induce a reentrant SmC*FI2-SmC*-SmC*FI2 transition in the rest of the film.

Inclusions in free standing smectic liquid crystal films

Colloidal inclusions in thin free standing liquid crystal films are ideal model systems for 2D anisotropic dispersions. Different types of self-organization in chain and lattice structures have been observed. The orientational elasticity of the anisotropic matrix and capillary forces are the dominating interaction mechanisms between solid or liquid inclusions, the director field, and dislocations of the films. We give an overview of the progress in this field, focussing on different inclusion types and their interactions in thermotropic smectic films.

Optical studies on the surface-induced tilted layers in freestanding films of two no-layer-shrinkage liquid crystal compounds

Null transmission ellipsometry has been employed to study the molecular arrangements in freestanding films of two no-layer-shrinkage liquid crystal homologous compounds above the bulk smectic A-smectic C* (Sm C*) transition temperature. An unusual nonplanar-parallel or nonplanar-antiparallel-parallel transition has been observed in both compounds under a proper electric field. With the addition of one CH2 group, while the SmC* phase is more stable thermally, the magnitude of the critical field needed to induce a parallel-antiparallel transition decreases dramatically.

Surface-induced multiple reentrant transitions

We have studied tilted surface layers in the bulk SmA temperature window of one liquid crystal using null transmission ellipsometry. Five distinct nonplanar surface structures were observed. We present analyses of the tilt and azimuthal profiles of the structures and examine the transitions between the structures. The transitions are identified as a double reentrant synclinic-anticlinic-synclinic-anticlinic transition. Meanwhile, the transitions also display reentrant ferroelectric-antiferroelectric-ferroelectric behavior.

Test of clock model in ellipsometric study of thin and thick free-standing films of an antiferroelectric liquid crystal

The temperature dependences of the ellipsometric parameters in a weak dc external field are studied in thin and thick free-standing films of MHPOBC. The results for thin films consisting of two, three, and four layers are analyzed within the discrete phenomenological model. We find very good quantitative agreement between the theory and experiment, which indicates an odd-even effect. We find that the XY structures are stable for an odd number of layers, whereas planar, Ising-like structures are stable for an even number of layers. The experiments on thick (several tens of layers) films show a combination of bulklike and free-surface behavior. This is most pronounced at high temperatures, where the interior of the film is not tilted, whereas the layers at the air interfaces show qualitatively similar temperature dependance of the ellipsometric parameters as in the four-layer film.

Bent-core dopant in a liquid crystal having a reentrant synclinic phase

Small quantities of the bent-core mesogen P-7PIMB were dissolved in an anticlinic liquid crystal consisting of a mixture of left- and right-handed TFMHPOBC, with enantiomer excess X=0.2. The bent-core dopant promotes anticlinic order at higher temperatures, but becomes less effective in suppressing the synclinic phase at the reentrant synclinic transition due to an orientational transition of the dopant within the calamitic TFMHPOBC matrix. Measurements of the anticlinic-synclinic electric-field switching threshold as a function of temperature and dopant concentration facilitate a determination of the component of the anticlinic interaction coefficient U that is due to the bent-core dopant. It is found that the value of U per bent-core molecule is much larger than the corresponding value for a pair of TFMHPOBC molecules in adjacent smectic layers.

Thickness-dependent smectic-A–smectic-C*transition in chiral smectic free-standing liquid-crystal films

We have conducted heat-capacity measurements near the smectic-A -smectic-C* phase transition in free-standing films of a chiral liquid-crystal compound. The heat-capacity behavior confirms the evolution of the transition in the interior layers from first to second order as a function of film thickness suggested earlier in tilt angle measurements. The asymmetry in the heat-capacity anomaly exhibits an interesting crossover in thinner films. We also observed the phase transitions leading to the crystallization to the tilted crystal-G phase in these films at lower temperatures. The surface ordering strength in these films is found to be much larger than that in other orthogonal-smectic films, which we speculate is due to the molecular tilt.

Unusual behavior of the surface-induced tilted layers in free-standing films of a non-layer-shrinkage liquid crystal compound

Null-transmission ellipsometry has been conducted to study the molecular arrangements in free-standing films of one chiral compound above the bulk smectic-A-smectic-C* transition temperature. Upon cooling under a proper electric field, a nonplanar-anticlinic-synclinic or a nonplanar-synclinic transition has been observed. The nonplanar structure continuously evolves into the anticlinic or synclinic structures. Increasing electric field can induce a rare transition from a synclinic to an anticlinic structure.

Double reentrance of surface arrangements in freestanding liquid crystal films

The molecular arrangement of tilted surface layers on freestanding films above the bulk smectic-A-smectic-C(*) transition has been studied by null transmission ellipsometry. In the presence of an applied in-plane electric field, a synclinic-anticlinic-synclinic-anticlinic double reentrance has been observed. Furthermore, we have characterized a novel transition from the synclinic arrangement to a structure in which the interior layers tilt in the same orientation and the outermost surface layers tilt oppositely with the interior layers.

Reentrant synclinic phase in an electric-field-temperature-phase diagram for enantiomeric mixtures of an antiferroelectric liquid crystal

The threshold electric field E(th) for a transition from the anticlinic to the synclinic phase of enantiomeric mixtures of the liquid crystal TFMHPOBC was measured as a function of temperature T and enantiomeric excess X. For small X the phase boundary curve on a temperature-electric-field phase diagram exhibits the phase sequence synclinic-anticlinic-reentrant synclinic on decreasing the temperature. At one point along the curve the quantity dT/dE--> infinity. For large values of enantiomeric excess a reentrant phase is not observed. The results are discussed using a simple phenomenological theory that accounts for layer-layer interactions, such that the electric-field-induced transition to the synclinic phase, although completed by solitary-wave propagation, is facilitated by a percolation mechanism.

Electric-field-induced transition between the anticlinic and the synclinic smectic-C surfaces in free-standing films

Anticlinic smectic-C surfaces were found experimentally as ground state structures in free-standing films made of smectic liquid crystals with no anticlinic bulk phases. A mean-field interpretation of this observation is given within a discrete phenomenological model of antiferroelectric liquid crystals, which additionally considers the enhanced order present at the surfaces of the free-standing films. The temperature dependence of the critical electric field that drives the transition between the anticlinic and synclinic smectic-C surfaces is evaluated, and fair agreement with the experimental data is found.

Theory of the anticlinic pretilted surface phase in tilted chiral smectic films

In freely suspended chiral smectic films, the molecules tilt at the surfaces above the bulk ferroelectric transition temperature. The tilt angle at the two surfaces can be either equal (synclinic surface phase) or opposite (anticlinic surface phase) on each surface of the film. We propose a realistic theoretical model accounting for the stability and properties of these two structures, based on competing surface interactions. The anticlinic phase is stable for sufficiently thin samples and high temperatures. We predict a reentrant SmA phase merging with the surface phases at a three-phase point.

Nonplanar structure of molecular tilt planes in the surface layers of smectic-A free-standing liquid crystal films

We present ellipsometric results from thin free-standing films of one chiral liquid crystal compound. In the bulk SmA range with surface-induced molecular tilt, a nonplanar arrangement of the molecular orientations of the tilted surface layers is found under a small applied electric field.

Structural transitions in thin free-standing films of an antiferroelectric liquid crystal exhibiting the smectic-C(*)(alpha) phase in the bulk sample

Optical reflectivity studies have been conducted upon thin free-standing films of an antiferroelectric liquid crystal possessing the bulk phase sequence Sm C(*)(A)-Sm C(*)(alpha)-Sm A. Electrically induced field independent and field dependent structural transformations were observed above the bulk Sm C(*)(A)-Sm C(*)(alpha) transition temperature. The anisotropy of the reflectivity was measured for different states in thin films. Our data provide direct evidence of structures without the short-pitched helix in the temperature region between the antiferroelectric Sm C(*)(A) and untilted Sm A structures.