Surface electroclinic effect on the layer structure of a ferroelectric liquid crystal

Electroclinic effect in a chiral paranematic liquid-crystal layer above the bulk nematic-to-isotropic transition temperature

Electroclinic measurements are reported for two chiral liquid crystals above their bulk chiral isotropic-nematic phase transition temperatures. It is found that an applied electric field E induces a rotation θ [∝Ε] of the director in the very thin paranematic layers that are induced by the cell's two planar-aligning substrates. The magnitude of the electroclinic coefficient dθ/dE close to the transition temperature is comparable to that of a bulk chiral nematic, as well as to that of a parasmectic region above a bulk isotropic-to-chiral smectic-A phase. However, dθ/dE in the paranematic layer varies much more slowly with temperature than in the parasmectic phase, and its relaxation time is slower by more than three orders of magnitude than that of the bulk chiral nematic electroclinic effect.

Modeling the field control of the surface electroclinic effect near continuous and first-order smectic-A* to smectic-C* transitions

We present and analyze a model for the combination of bulk and surface electroclinic effects in the smectic-A* (Sm-A*) phase near a Sm-A*-Sm-C* transition. As part of our analysis we calculate the dependence of the surface tilt on external electric field and show that it can be eliminated, or even reversed from its zero-field value, as demonstrated in previous experimental work on a system (W415) with a continuous Sm-A*-Sm-C* transition. We also analyze, for the first time, the combination of bulk and surface electroclinic effects in systems with a first-order Sm-A*-Sm-C* transition. The variation of surface tilt with electric field in this case is much more dramatic, with discontinuities and hysteresis. With regard to technological, e.g., display, applications, this could be a feature to be avoided or potentially exploited. Near each type of Sm-A*-Sm-C* transition we obtain the temperature dependence of the field required to eliminate surface tilt. Additionally, we analyze the effect of varying the system's enantiomeric excess, showing that it strongly affects the field dependence of surface tilt, in particular, near a first-order Sm-A*-Sm-C* transition. In this case, increasing enantiomeric excess can change the field dependence of surface tilt from continuous to discontinuous. Our model also allows us to calculate the variation of layer spacing in going from surface to bulk, which in turn allows us to estimate the strain resulting from the difference between the surface and bulk layer spacing. We show that for certain ranges of applied electric field, this strain can result in layer buckling, which reduces the overall quality of the liquid crystal cell. For de Vries materials, with small tilt-induced change in layer spacing, the induced strain for a given surface tilt should be smaller. However, we argue that this may be offset by the fact that de Vries materials, which typically have Sm-A*-Sm-C* transitions near a tricritical point, will generally have larger surface tilt.

Surface electroclinic effect near the first-order smectic-A*-smectic-C* transition

We analyze the surface electroclinic effect (SECE) in a material that exhibits a first-order bulk smectic-A* (Sm-A*)-smectic-C* (Sm-C*) transition. The effect of a continuously varying degree of enantiomeric excess on the SECE is also investigated. We show that due to the first-order nature of the bulk Sm-A*-Sm-C* transition, the SECE can be unusually strong and that as enantiomeric excess is varied, a jump in surface induced tilt is expected. A theoretical state map, in enantiomeric excess-temperature space, features a critical point which terminates a line of first-order discontinuities in the surface induced tilt. This critical point is analogous to that found for the phase diagram (in electric field-temperature space) for the bulk electroclinic effect. Analysis of the decay of the surface induced tilt, as one moves from surface into bulk, shows that for sufficiently high-surface tilt the decay will exhibit a well-defined spatial kink within which it becomes especially rapid. We also propose that the SECE is additionally enhanced by the de Vries nature (i.e., small layer shrinkage at the bulk Sm-A*-Sm-C* transition) of the material. As such, the SECE provides a new means to characterize the de Vries nature of a material. We discuss the implications for using these materials in device applications and propose ways to investigate the predicted features experimentally.

Molecular orientational properties of a high-tilt chiral smectic liquid crystal determined from its infrared dichroism

The orientational characterisitics and the temperature dependencies of the molecular apparent tilt angle of a partly fluorinated chiral smectic liquid crystal (/S/)-4-(1-methylheptyloxycarbonyl)phenyl 4'-[6-(3,4,4,4-tetrafluoro-3-trifluoromethylbutylcarbonyloxy)hexyloxy] biphenyl-4-carboxylate (acronym MHPHFHHOBC) are studied using the polarized Fourier transform infrared (FTIR) spectroscopy. The molecular orientational distributions and the orientational order parameters for a homogeneously aligned liquid crystalline sample at various temperatures and external electric fields are examined. The analysis uses the dichroic parameters of the phenyl and the carbonyl bands. For a temperature range of 65-80 degrees C corresponding to the antiferroelectric SmCA* phase, the molecular apparent tilt angle lies within the range 43 degrees -44 degrees; antiferroelectric smectic structure being rather close to the orthoconic SmCA* phase. An application of sufficiently high dc field across the cell in its SmA* phase surprisingly shows that the dichroism first increases slowly and then rapidly in two stages and finally a saturated apparent tilt angle of approximately 30 degrees is reached. The IR dichroic data is used to estimate the polar angles and the degree of rotational biasing of the carbonyl groups with respect to the molecular long axis. In the SmA* phase, the sample appears to demonstrate some of the typical properties of a de Vries material.

Evidence for de Vries structure in a smectic-A liquid crystal observed by polarized Raman scattering

The second- and fourth-order apparent orientational order parameters of the core part of the molecule P2 (app) and P4 (app) , have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries-type phenomena, i.e., very large electroclinic effect in the smectic- A (Sm-A) phase and a negligible layer contraction at the phase transition between the Sm-A and Sm- C(*) phases. The apparent orientational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in Sm-A . These results provide evidence for the existence of the de Vries Sm-A phase, where the local molecular director is tilted at a large angle.

Field control of the surface electroclinic effect in chiral smectic-A liquid crystals

The surface electroclinic effect, which causes an azimuthal deviation of the layer normal from the surface rubbing direction in cells of chiral smectic- A liquid crystals, can be eliminated (and even reversed) by applying an electric field during cooling from the isotropic phase. The observed dependence of layer orientation on field strength leads to a model in which the surface electroclinic tilt results from an effective surface electric field. The experiements suggest a general method for controlling the azimuthal layer alignment of chiral smectic cells.

Ferroelectric ordering and electroclinic effect in chiral smectic liquid crystals

Ferroelectric ordering, the electroclinic effect, and chiral smectic C(SmC*)-smectic A phase transitions in thin planar ferroelectric liquid crystal (FLC) cells are studied by means of linear electro-optic and second harmonic generation (SHG) techniques. The ferroelectric switching is detected in biased FLC cells by measuring azimuthal dependences of linear and nonlinear responses. The applied dc electric field rotates the FLC symmetry axis with initial and final orientations in the cell plane. Comparative studies of the SHG switching behavior in reflection and transmission geometries allows one to distinguish the contributions from the bulk and the subsurface layers of the cell. The analysis of SHG temperature dependences shows the existence of a strong surface coupling. The temperature-dependent nonlinear polarization shows a critical behavior with the exponent approximately 0.3 in SmC* phase.

Intrinsic aspect of V-shaped switching in ferroelectric liquid crystals: biaxial anchoring arising from peculiar short axis biasing in the molecular rotation around the long axis

To clarify the intrinsic aspect of practically usable thresholdless V-shaped switching in ferroelectric liquid crystals, we have observed textures and measured polarized Raman scattering as well as optical transmittance in a thin homogeneous cell of a single compound by applying an electric field. The results indicate that the so-called surface stabilized ferroelectric states are destabilized, and that there exist rather stable two domains with broad and narrow molecular orientational distributions, both of which show the almost ideal V-shaped switching with considerably low transmittance at the tip of the V. We have concluded that the main cause of the V-shaped switching is the biaxial anchoring on the substrates coated with polyimide, which makes the most polarizable short axis normal to the substrates. It is in competition with the ordinary anchoring that favors the director parallel to the substrates, when the material has such a bulk intrinsic property that this short axis is parallel to the tilt plane. The competition makes the total anchoring energy almost independent of the azimuthal angle and gives rise to the V-shaped switching.

Chiral twisting of a smectic-A liquid crystal

Chiral twisting of the molecular orientation within the layer of a smectic-A liquid crystal has been investigated using circular dichroism spectroscopy. The results indicate that a rotation of the layers away from the alignment direction is induced by the surface electroclinic effect. This leads to an interfacial region where the molecular director twists from the alignment direction until it reaches the layer normal direction. A theory is presented to explain the observed field and temperature dependence of the circular dichroism.