Tilt-angle, polarization, and heat-capacity measurements near the smectic-A-chiral-smectic-C phase transition of p-(n-decyloxybenzylidene)-p-amino-(2-methylbutyl)cinnamate (DOBAMBC)

Twist elastic constant in the SmC* phase of an antiferroelectric liquid crystal with and without photo-polymer networks

Chiral liquid crystalline phases with pitches that are comparable to visible wavelengths, all Bragg-reflect visible light. We measured Bragg reflections from the chiral SmC* phase of the antiferroelectric liquid crystal (AFLC), AS612, and explored how applied electric fields and polymer networks modify these reflections. From the critical field that unwinds the helix and destroys Bragg reflections, the twist elastic constant of the SmC* phase is determined. We estimate from this result, the elastic constant of polymer network, which when introduced into the liquid crystal matrix, fixes the helical pitch (and hence, Bragg peaks). These parameters are crucial for designing both FLC and AFLC displays since networks enhance the mechanical properties of the liquid crystal host while not affecting their optical properties.

Smectic C to smectic A transition induced mechanically by the rupture of freely suspended liquid crystal films

The tilt angle of smectic C phases can be controlled by external forces of very different nature. In particular near a smectic A-smectic C transition, it is susceptible to temperature changes. It can be influenced with electric fields (electroclinic effect), and even mechanically by intra-layer stresses in elastomers. We show that capillary forces that act during the rupture of a free-standing smectic C film can trigger a smectic C to smectic A transition, which rapidly reduces the surface area of the films, concurrently increasing the film thickness. The effect occurs on the sub-millisecond scale, practically independent of film thickness and temperature. We propose that this mechanical effect could even trigger a ferroelectric to paraelectric transition in chiral phases.

Modifying optical Bragg reflections from an antiferroelectric liquid crystal with photopolymer networks

By using an achiral monomer and a photoinitiator, we introduced polymer networks into the Sm-C* phase of an antiferroelectric liquid crystal that forms ferro-, ferri-, and antiferroelectric phases. We then investigated the temperature dependence of Bragg wavelengths selectively reflected from these samples and found that the reflection bands shift to shorter wavelengths with increasing polymer concentration. The intermediate and the Sm-C*(A) phases dominate over the Sm-C* phase for polymer concentration ≥4% by weight and the Sm-C* phase disappears completely for a 6% polymer sample.

Direct observation of diffuse cone behavior in de Vries smectic-A and -C phases of organosiloxane mesogens

Simultaneous and direct x-ray measurements of the smectic layer spacing, molecular tilt, and orientational order in the de Vries smectic A (SmA) and C (SmC) phases of two organosiloxane mesogens reveal that (i) the SmC (tilt) order parameter exponent β=0.26±0.01 for 2nd order SmA-SmC transition--in excellent agreement with the tricritical behavior, (ii) the siloxane and hydrocarbon parts of the molecules are segregated and oriented parallel to the director with very different degree of orientational order, and (iii) thermal evolution of the effective molecular length is different in the two phases.

de Vries behavior of the electroclinic effect in the smectic-A* phase near a biaxiality-induced smectic-A*-smectic-C* tricritical point

Using a generalized Landau theory involving orientational, layering, tilt, and biaxial order parameters we analyze the smectic-A* and smectic-C* (Sm-A*-Sm-C*) transitions, showing that a combination of small orientational order and large layering order leads to Sm-A*-Sm-C* transitions that are either continuous and close to tricriticality or first order. The model predicts that in such systems the increase in birefringence upon entry to the Sm-C* phase will be especially rapid. It also predicts that the change in layer spacing at the Sm-A*-Sm-C* transition will be proportional to the orientational order. These are two hallmarks of Sm-A*-Sm-C* transitions in de Vries materials. We analyze the electroclinic effect in the Sm-A* phase and show that as a result of the zero-field Sm-A*-Sm-C* transition being either continuous and close to tricriticality or first order (i.e., for systems with a combination of weak orientational order and strong layering order), the electroclinic response of the tilt will be unusually strong. Additionally, we investigate the associated electrically induced change in birefringence and layer spacing, demonstrating de Vries behavior for each, i.e., an unusually large increase in birefringence and an unusually small layer contraction. Both the induced changes in birefringence and layer spacing are shown to scale quadratically with the induced tilt angle.

Quantitative experimental determination of the Landau-potential of chiral enantiomer doped ferroelectric liquid crystals

The full Landau potential was determined for a ferroelectric liquid crystal doped with varying concentrations of the chiral dopant R1011 and its enantiomer S1011. A multi-curve fitting procedure using temperature and electric field dependent tilt angle and polarization data was employed to the generalized Landau model of ferroelectric liquid crystals. From this analysis the three Landau coefficients as well as the polarization-tilt coupling parameters were obtained as a function of dopant concentration and configuration. It is shown that the two most varied parameters are those of the first Landau coefficient alpha and the (chiral) linear polarization-tilt coupling constant C. The effect on the first Landau term is equivalent for the two dopants of opposite handedness indicating its achiral nature, while the effect on the (chiral) bilinear coupling term differs for the R1011 and S1011 dopant, increasing and decreasing the coupling between tilt and polarization respectively. This difference in the bilinear coupling term quantifiably evidences that the R1011 dopant increases and S1011 dopant reduces the inherent chirality in this system.

Experimental determination of the full Landau potential of bent-core doped ferroelectric liquid crystals

The full Landau potential of a ferroelectric liquid crystal, doped with two different bent-core molecules at varying concentration, was determined experimentally. Using a multicurve fitting procedure, temperature and electric field dependent tilt angle and polarization measurements were analyzed according to the generalized Landau model of ferroelectric liquid crystals. From this analysis the three Landau coefficients as well as the polarization-tilt coupling parameters were obtained as a function of dopant concentration. It is shown that the two most varied parameters are those of the first Landau coefficient and the (chiral) linear polarization-tilt coupling constant. The results quantitatively verify the chiral induction capability of nonchiral bent-core shaped dopants and an observed increase in the electroclinic effect with increasing dopant concentration.

Frustration between syn- and anticlinicity in mixtures of chiral and non-chiral tilted smectic-C-type liquid crystals

We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmC(a)* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and SmC(a)*, transition via the SmC(beta)* and SmC(gamma)* subphases, or by "escaping" the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmC(a)* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected even in this case, suggesting that chirality--although necessary--does not have quite the importance that has previously been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order.

Optical studies on free-standing films of an achiral smectic liquid crystal

Employing null transmission ellipsometry and depolarized reflected light microscopy, we have studied two smectic phases, Sm-C1 and Sm-C2, of one achiral mesogen. Our results show that Sm-C1 and Sm-C2 are the synclinic Sm-C and anticlinic Sm-C(A) phases, respectively. We find no evidence to support recent claims that the Sm-C1 is chiral and ferroelectric [R. Stannarius et al., Phys. Rev. Lett. 90, 025502 (2003)].

Detailed optical studies of several banana-shaped compounds in liquid crystal B2 phase

Null-transmission ellipsometry has been performed on free-standing films of one compound in the liquid crystal B2 phase. We have studied films of thickness from 1 to 121 layers. One of the compounds used has an unusually wide 59 K window for the B2 phase. The tilt angle was investigated as a function of temperature and found to be constant over this temperature range to within our resolution of 1 degrees. The one-layer films studied exhibit the same structure as the thicker films, and have helped us to refine the optical model for the B2 phase. For thin films we find that modeling a single smectic layer as two uniaxial layers is a better description of the data than a single biaxial layer, but that for thick films the model used does not effect the simulated result appreciably. Preliminary results also find that the surface layers are less tilted than the interior layers, in contrast to rodlike liquid crystals, which show an enhanced surface tilt.

Ellipsometric studies of synclinic and anticlinic arrangements in liquid crystal films

The molecular arrangements in liquid crystal freestanding films are studied when smectic C* surface ordering occurs above the bulk smectic A to smectic C* transition temperature. Details of the resulting structures are resolved using ellipsometry. The behavior of the film is compound dependent. Below a critical electric field, either a synclinic or anticlinic structure may be present depending on the magnitude of the bulk spontaneous polarization. A model is presented in which the low-field anticlinic arrangement results from the interaction between polarization fluctuations at the two surfaces.

Critical fluctuations at the untilted-tilted phase transition in chiral smectic liquid crystals

The layer compression modulus B was measured near the Sm-A-Sm-C* and Sm-A-Sm-C(*)(A) transitions of chiral smectic liquid crystals. B shows a marked pretransitional softening due to the order parameter fluctuations above the phase transition. The width of the critical region of the Sm-A-Sm-C* and Sm-A-Sm-C(*)(A) transitions is smaller than that of the Sm-A-Sm-C(*)(alpha) transition. The critical exponents of B are in agreement with the theoretical description based on renormalization-group method. These results reveal that these transitions are not of the Landau mean-field type.

Temperature difference between bulk and surface transition in freely suspended smectic films

We report an ellipsometric study of freely suspended films of a chiral liquid-crystal compound possessing the phase sequence smectic-C-smectic-A-isotropic with a very broad (67 K) smectic-A phase range. We observe a smectic-C-smectic-A surface transition which is situated more than 60 K above the bulk smectic-C-smectic-A transition temperature. Possible reasons for this unusually large temperature shift are discussed.