M. Buivydas

The molecular aspect of the double absorption peak in the dielectric spectrum of the antiferroelectric liquid crystal phase

Abstract The dielectric spectrum of the antiferroelectric smectic C∗ phase exhibits a low and a high frequency absorption peak (Pl, Ph) which have been studied as a function of temperature and bias electric field. Measurements from 10 Hz to 10 MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A∗ phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C∗α (82·80° to 81·91°C) and C∗γ (80·10° to 79.17°C), and one broad antiferroelectric phase (79.17° to −10°C). On applying an increasing bias field, Δ∊ for both processes first increases by about a factor of two, then exhibits a maximum at a threshold field E c corresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, at E c the PH peak vanishes and the PL peak shows up at a frequency s...

The case of thresholdless antiferroelectricity: polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem., 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in the absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action.

Biaxial dielectric properties of phenyl ester ferroelectric liquid crystal mixture

Dielectric and electro-optic properties of phenyl ester FLC mixture have been studied in detail. Dielectric relatation spectroscopy was used to study the molecular dynamics and dipolar ordering. In the SmA* phase, this is accompanied by a sharp increase in the dielectric strength Δεs on approaching Tc. At such high frequency (2 MHz) Δε is usually negative in the SmC* and SmA* phase. In the N* phase, at lower frequencies, Δε had a weakly positive value. In the SmC* phase, δε is positive and its maximum value is less than 0.1. The dielectric biaxiality is discussed as an order parameter of the tilted smectic phase. It is verified that if the tilt angle increases according to θ ∝ (TC − T)β, the biaxiality increases according to δε ∝ (TC − T)2β. A critical exponent for θ and δε are 0.27 and 0.54, respectively. The rotational viscosity γθ for soft mode is of 10 Nsec/m2 in the SmA* phase. The activation energy for the rotational dynamics of Goldstone mode is estimated to be 1.4 eV from an Arrhenius plot. Optical response time is of about 50 μs at room temperature in SmC* phase, and is on the order of 10 μs in the N* phase and almost independent of the applied field.

Biaxial dielectric properties in the chiral smectic C phase of single compounds and their relation to molecular structure

Abstract In an attempt towards relating the dielectric biaxiality to the molecular structure, the dielectric anisotropy and biaxiality and their temperature dependence in the chiral smectic C phase have been determined for seven single compounds. It is found that, on a molecular level, compounds with a strong polar group connected with a cyclohexane ring exhibit a relatively large dielectric biaxiality. Among the substances with large dielectric biaxiality, it is noticed that attaching the polar group to the linking point of the cyclohexane ring and the alkyl chain containing the stereocenter results in the largest biaxiality. The studied compounds possess a smectic C phase preceded either by a smectic A or a nematic phase. As a general rule (with exceptions) we found that a large biaxiality is also coupled to a high value of the spontaneous polarisation. While the dielectric biaxiality for all studied materials is positive, the dielectric anisotropy (at 100 kHz) was found to be negative, especially at lo...

Anchoring conditions for different aligning materials from dielectric behavior of SSFLC cells

The dielectric response of thin surface stabilized ferroelectric liquid crystal cells with different aligning materials and filled with the same FLC mixture are studied and compared in the low frequency dielectric regime. For the measurements we adopted an improved cell structure with metallized pixel connections. In the FLC material in question, Merck SCE 13, only one absorption appears corresponding to a collective mode.It is ascribed to the Goldstone mode in the bulk FLC. From our measurements we calculate the surface anchoring energy density for the different aligning materials and discuss changes induced in dielectric spectra by a DC bias field.

The dependence of polarization and dielectric biaxiality on the enantiomeric excess in chiral dopant added to a smectic C host mixture

Abstract The dielectric stabilization of the field-on states is an important feature for all SSFLC devices using the so-called C2 chevron geometry. This requires a combination of positive dielectric biaxiality and negative dielectric anisotropy. In the present work we use the phenylpyrimidine base mixture M192/62.5 as a non-chiral host and dope it by 10%of a chiral oxirane derivative with varying enantiomeric excess χ of the R-compound. The results confirm our basic expectations: while the polarization increases with increasing χ, the dielectric coefficients seem to be essentially independent on the enantiomeric excess, therefore on the spontaneous polarization. This result is remarkably different from our previous results on pure compounds showing a clear correlation between the dielectric biaxiality and spontaneous polarization in different molecular structures.

Frequency dependence of the dielectric biaxiality in the chiral smectic C∗ phase and the observation of a compound with negative dielectric biaxiality

Abstract The principal components of the dielectric tensor in the isotropic, smectic A ∗ and smectic C ∗ phases have been measured, with emphasis on the smectic C ∗ phase, for a single ferroelectric liquid cystalline compound. The dispersion behaviour of the dielectric biaxiality in the chiral smectic C ∗ phase has been determined and it has been established that the dielectric biaxiality exhibits a sign reversal for the compound studied.

Molecular correlation in smectic phases measured by dielectric relaxation spectroscopy

In the present work the dielectric relaxation related to non-collective motion of liquid crystal molecules has been studied. In the first pace we observe the relaxation around the short axis for a choice of ferroelectric and antiferroelectric materials.Additionally, for some material the non-collective motion around the long axis has been investigated. We found that for all investigated compounds, except one, the relaxation obey an Arrhenius las f equals f=0) exp(-Ea/kT) and we were able to map the activation energy changes between different smectic phase. The changes of the activation energy Ea are related to the parallel correlation between the molecules. For most of the investigated ferroelectric material we observed that the correlation increase both along the long and short molecular axes at the phase transition to the smectic C* phase. For antiferroelectric materials the parallel correlation along the long axis weakens in the smectic Ca* phase. It thus seems that antitilt layer structure is related to the low degree of molecular correlation.

Dielectric biaxiality in smectic C* materials

The dielectric biaxiality and dielectric anisotropy have been determined for helical smectic C* materials of varying pitch. The dielectric biaxiality was found to be positive, the dielectric anisotropy to be negative at 100 kHz. Thus, the permittivity component in the direction of the spontaneous polarisation (C2 symmetry axis) has the largest value among the three principle components of the dielectric tensor, and Iµ2 > Iµ1 > Iµ3. The dielectric biaxiality is further discussed as an order parameter of the tilted smectic phase. The critical behaviour is compared to that of the primary order parameter, the tilt angle, for a number of smectic C* compounds. It is verified that if the tilt increases according to θ∼(TC–T)β, the biaxiality increases according to δIµ(TC–T)2β.

The spontaneous polarisation and rotational viscosity in host-dopant mixtures

Abstract In our search for systematic features we have studied the appearance of spontaneous polarisation in some simple cases of non-chiral smectic C mixtures with added chiral dopant. The electro-optic properties have been measured for each system and evaluated in the simplest possible dynamic model. In this way we determined the tilt angle, the polarisation and the rotational viscosity. Some methodological comments are made on the measurements, because their simplicity makes them well adapted for large scale screening of new compounds. If the host-dopant is a binary system we find that the coupling constant μ = ∂P/∂θ between polarisation and tilt behaves in a regular way. This is also true for the viscosity, which has an Arrhenius behavior with a dopant independent prefactor and an activation energy barrier increasing almost linearly with the dopant concentration. When the host is itself a binary mixture of fixed composition we find essentially the same behavior but with a lower value of pμ (i.e. the b...