Vera Hamplova

New photosensitive polymer composites based on oriented porous polyethylene filled with azobenzene-containing LC mixture: reversible photomodulation of dichroism and birefringence

A new type of polymer–liquid crystal (LC) composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene (PE) films with pores size in the range 50–500 nm were used as polymer matrices. A photochromic cholesteric mixture based on a commercial nematic mixture doped with azobenzene and merocyanine dopants was prepared. Highly birefringent deeply coloured flexible films possessing strong dichroism were obtained by introduction of the photochromic LC mixture into the stretched PE films by capillary forces. UV irradiation leads to E–Z isomerisation of azobenzene‐containing molecules inside the composite films and results in a significant decrease of the dichroism almost down to zero due to isothermal transition from the mesophase to isotropic melt stimulated by the isomerisation process. This process is reversible and the visible light irradiation or annealing lead to a back transition to the uniaxial birefringent and dichroic state. The composite films show a high fatigue resistance during the alternating cycles of the UV–visible irradiation. These novel composites can be considered as promising materials for photonics and optical information processing.

New ferroelectric liquid crystalline materials with an azo group in the molecular core

Six series of new liquid crystalline materials with an azo group (N=N) located in different parts of the mesogenic core of the molecule have been synthesized and their physical properties studied. The chiral segments of these materials are based on alkoxypropionate or alkyllactate units. It has been found that lateral methyl substitution on a phenyl ring in the molecular core disturbs the packing of the molecules. As a result smectic phases disappear and the phase transition temperatures decrease. In addition, shifting the N=N group closer to the chiral centre of the molecule leads to the disappearance of the ferroelectric SmC* phase. For the compounds which show the ferroelectric SmC* phase, the temperature dependence of the spontaneous polarization, the spontaneous tilt angle, the helix pitch length and the complex permittivity has been studied. The effect of shifting the azo group in the molecular core on the physical properties is discussed.

New antiferroelectric liquid crystalline materials containing a keto group and two lactate groups

New chiral liquid crystalline materials containing a keto group and two lactate units attached to the mesogenic core have been synthesized and studied. All the new compounds show the paraelectric SmA, ferroelectric SmC* and antiferroelectric SmC*A phases each over rather broad temperature ranges. The properties of the phases were characterized by DSC, electro-optical and dielectric studies. The spontaneous polarization, tilt angle, helical pitch, and switching times were determined for the polar phases.

Conformational Properties and Orientational Order of a de Vries Liquid Crystal Investigated through NMR Spectroscopy

Solid-state and liquid-state NMR spectroscopic techniques are used to describe at molecular level the behaviour of a de Vries liquid crystal (namely the mesogen 9HL) at the SmA-SmC* transition, which is characterized by the absence of the layer shrinkage, typical of non-de Vries smectogens. Previous (2)H NMR studies on the same smectogen, performed at a different magnetic field (from 4.70 to 18.80 T), provided evidence of the occurrence of a tilt of one of the three phenyl rings, constituting the aromatic core of 9HL, at the SmA-SmC* phase transition. In this work, the study is extended to the whole rigid aromatic core of the 9HL. In particular, the variable temperature behavior of the mesogen studied by 1D (13)C NMR cross-polarization (CP) and 2D (1)H-(13)C PDLF (proton-encoded (13)C-detected, local field) NMR experiments made possible the characterization of the conformational and orientational properties in the two smectic phases. These results are compared with various proposed models invoked to describe the SmA-SmC* transition in de Vries smectogens at a molecular level.

1H NMR relaxometry study of a rod-like chiral liquid crystal in its isotropic, cholesteric, TGBA*, and TGBC* phases.

The molecular dynamics of a chiral liquid crystal showing a rich variety of frustrated mesophases has been investigated by means of 1H NMR relaxometry. The interest in this lactate derivative, HZL 7/*, is related to a large range of thermal stabilities of the twist grain boundary (TGB) phases. Dispersions of the 1H spin-lattice relaxation times, T1, in the frequency range from 300 MHz to 5 kHz were measured and consistently analyzed in the isotropic, chiral nematic, TGBA*, and two TGBC* phases. In the isotropic and N* phases, a three-exponential magnetization decay was observed and assigned to three specific molecular groups of the HZL 7/* (molecular core, methyl, and methylene groups). In the TGB phases, all T1 components merge into a single one. The analysis of the T1 dispersion in the TGBA* phase shows that the translational self-diffusion relaxation mechanism dominates over a broad frequency range and that layer undulations are less relevant than the relaxation contribution associated with the diffusion process across the TGB structure. In the TGBC1* phase, the T1 dispersion presents a strong contribution of in-layer tilt direction fluctuations (T1(-1) proportional to ν(-1/2)), while, in the TGBC2* phase, the linear frequency dependence of T1 could be associated with a much stronger contribution of layer undulations than for the other TGB phases. This is at present the first molecular dynamics investigation on several TGB phases by means of 1H NMR relaxometry.

New Series of Ferroelectric Liquid Crystals Incorporating Stilbene Unit in the Core

Abstract New liquid crystalline materials of the general formula have been synthesized (R1, R2 are alkyl chains). Mesomorphic properties were determined using polarizing microscopy. All compounds showed very broad smectic C* phase (about 70–90 K) and the cholesteric phase. No smectic A phase was observed. In the smectic C* phase the spontaneous polarization is very high, reaching values higher than 300 nC/cm2 at the low temperature limit of this phase for compounds with two chiral centres. The permitivity was measured showing a contribution of the Goldstone mode in the smectic C* phase.

Brief overview on 2H NMR studies of polysiloxane-based side-chain nematic elastomers.

This is a brief overview on recent studies on liquid crystalline elastomers (LCEs) based on polysiloxane chain, in the form of monodomain films, selectively 2H‐labeled in different parts of the LCE samples, i.e. on the crosslinker or mesogenic units. 2H NMR spectroscopic techniques were used to measure the temperature dependence of the quadrupolar splittings, line widths and relaxation times, T1 and T2. From these data, several information about the orientational order parameter, S, of various LCE fragments, thermodynamic features of the isotropic–nematic transition and main motional processes could be generalized for this type of elastomers. Copyright

Smectic A – Smectic C* Transition in a “de Vries” Liquid Crystal by 2H NMR

Solid State 2H Nuclear Magnetic Resonance (NMR) spectroscopy was applied to the structural and orientational studies of a selectively labelled ‘de Vries’ liquid crystal compound. Measurements were performed at five values of the magnetic field, ranging from 4.70 to 18.80 T. The trend of the orientational order S and tilt angle θ of the 2H-labeled moiety in the SmA and SmC* phases at different magnetic fields confirms previous results and allows us to better characterize the SmA-SmC* transition.

1H NMR relaxometry in the TGBA* and TGBC* phases

ABSTRACT Recent results obtained by 1H NMR relaxometry of liquid crystals having twist grain boundary (TGB) phases are here reviewed. In particular, three chiral rod-like lactate derivative mesogens were investigated. In the isotropic phase, three-exponential magnetization decay was observed in all cases and the three distinct spin-lattice relaxation times (T1) were assigned to three specific molecular groups of these molecules. In the TGBA* and TGBC* phases the magnetization decay is mono-exponential and the main features of the 1H NMR dispersion curves analyzed through a global target fitting procedure in terms of specific molecular and collective dynamics are discussed.

Properties of new polar liquid crystalline materials with the keto group and different number of lactate units

New series of chiral liquid crystalline materials with the same rigid core with keto group and a various number of (S)-lactate groups in the chiral part of the moelcule have been studid by DSC, X-ray, electro-optics and dielectric spectroscopy. All compounds contain branched terminal alkyl chain derived from (S)-2-methylbutanol. The influence of the lactate number on mesogenic behavior of those compounds has been studied. All compounds show the paraelectric SmA phase and the ferroelectric SmC* phase in a rather wide temperature range. The antiferroelectric SmC*A phase was detected for the compound possessing two lactate groups. A polar field-induced smectic phase have been observed by electro-optic studies.