Low Molecular Mass Liquid Crystals Research Articles

Rough surfaces for orientation control in reverse mode polymer dispersed liquid crystal films

Reverse mode operation shutters have been achieved with polymer dispersed liquid crystals by means of polymerization-induced phase separation of nematic mixtures consisting of a low molecular mass liquid crystal and a liquid crystalline monomer. Fluid mixtures were homeotropically aligned by rough surfaces and transparent films were obtained after polymerization. Transmittance in the OFF state can be larger than 80% and decreases to less than 1% when an electric field of about 2 V μm-1 at 1 kHz is applied (ON state). Both rise and decay times can be lower than 10 ms and the drop in the OFF state normal transmittance is drastically reduced with respect to conventional polymer dispersed liquid crystals since samples exhibit a reverse morphology. The role played by surface roughness is also discussed.

PDLC-like patterns at the isotropic to cholesteric transition entrapped by in situ photopolymerization

When investigated by optical microscopy between crossed polarizers, the isotropic to cholesteric transition may appear like fingerprint-patterned droplets embedded in a black isotropic matrix. In the present work, such PDLC-like (polymer dispersed liquid crystal) patterns, only occurring over 0.7 C, have been entrapped and stored at ambient temperature in a polymer film. We used a UV polymerization process with different sequences in which illumination time and UV power progressively vary. From a conceptual viewpoint, these PDLC-like patterns come solely from liquid crystalline material, whereas all the conventional PDLCs are binary mixtures of a macromolecular compound or 'prepolymer' with a conventional low molecular mass liquid crystal. The fact that isotropic matrix and cholesteric droplets differ only from the viewpoint of molecular order and not in their chemical nature, permits comparisons with the usual case for which the choice of polymer-forming material is crucial and the polymer/liquid crystal interface is an important factor for controlling PDLC electro-optic properties. The present system gives an opportunity to investigate by scanning electron microscopy (SEM) the droplet microstructure (isotropic-cholesteric interface, fingerprint patterns or defects), whereas previous SEM studies were focused on the shape and size of empty cavities, since the fluid liquid crystal was inevitably removed from the PDLC system.

Phase diagrams of phenyl benzoate side group liquid crystal polymers and similar low molecular mass liquid crystals

The miscibility of a phenyl benzoate side group liquid crystal polymer (LCP) with a similar low molecular mass liquid crystal (LC) has been studied as a function of temperature and molecular mass by calorimetry and optical microscopy. The nematic to isotropic transition temperatures of the pure polymer and the LC are 112 and 48.8°C, respectively. There is a broad miscibility gap for polymer weight fractions below 50% and temperatures below 72°C. A relatively small increase in miscibility is observed when the polymer molecular mass decreases. In the systems investigated here, the experimental temperature/weight fraction phase diagram is well reproduced by a mean field model combining the Flory-Huggins model for isotropic mixing and the Maier-Saupe theory for nematic ordering. Consequences of the miscibility gap with regard to the behaviour of LCP brushes swollen in a similar low molecular mass nematic liquid crystal are discussed.

Phase diagrams of phenyl benzoate side group liquid crystal polymers and similar low molecular mass liquid crystals

Phase diagrams of phenyl benzoate side group liquid crystal polymers and similar low molecular mass liquid crystals

Broad band dielectric relaxation spectroscopy of molecular reorientation in smectic liquid crystalline phases (SmA, SmB, and CrE)

Broad band dielectric measurements reveal that the reorientation of non-chiral rod-shaped low molecular mass liquid crystals is active around their molecular long axis and a short axis, in the smectic A and hexatic smectic B phase, respectively, as well as in the soft crystalline E phase of two isomeric stilbene compounds possessing an equal molecular length of their all-trans -conformations. One ('generalized') Arrhenius equation describes the temperature dependence of the reorientation around a molecular short axis for each of these phases of both compounds. A change of the activation energy related to the reorientation around a molecular short axis is accompanied by a slowing down of the reorientation around the molecular long axis in the soft crystalline E phase in one of these compounds, compelling evidence for a coupling of both reorientations. This result is discussed with respect to the biaxiality of the soft crystalline E phase.

Phase diagrams of phenyl benzoate side group liquid crystal polymers and similar low molecular mass liquid crystals

The miscibility of a phenyl benzoate side group liquid crystal polymer (LCP) with a similar low molecular mass liquid crystal (LC) has been studied as a function of temperature and molecular mass by calorimetry and optical microscopy. The nematic to isotropic transition temperatures of the pure polymer and the LC are 112 and 48.8°C, respectively. There is a broad miscibility gap for polymer weight fractions below 50% and temperatures below 72°C. A relatively small increase in miscibility is observed when the polymer molecular mass decreases. In the systems investigated here, the experimental temperature/weight fraction phase diagram is well reproduced by a mean field model combining the Flory-Huggins model for isotropic mixing and the Maier-Saupe theory for nematic ordering. Consequences of the miscibility gap with regard to the behaviour of LCP brushes swollen in a similar low molecular mass nematic liquid crystal are discussed.

AFM investigations of a glassy cholesteric liquid crystal with hydrophobic aerosil particles

AFM investigations of a glassy heterogeneous system consisting of an oligomeric cholesteric liquid crystal and the hydrophobic aerosil R812 were carried out. With increasing aerosil concentration, a suppression of the characteristic cholesteric surface pattern was observed. Typical separated aerosil aggregates appear in the samples. Their size and form change from small lumps through bigger rod-like entities to large crystallite-like aggregates of aerosil particles. This matches with observations of light scattering of systems with low molecular mass liquid crystals and of the memory effect. The pitch of the cholesteric fingerprint pattern slightly decreases with increase in the aerosil concentration.

1H Multiple-Pulse Solid-State NMR Investigations of Electrically Oriented Liquid Crystals

For the discussion of orientional states in liquid crystals, quadrupolar (2H NMR) and dipolar (1H NMR) interactions have mainly been exploited. There have also been attempts to describe the individual (regarding different positions in a molecule) orientation and order by means of chemical shift information (13C NMR). Owing to the large chemical shift range of carbons, 13C NMR spectroscopy is suitable for providing resolved spectra of highly ordered systems such as low molecular mass liquid crystals. In the case of 1H NMR, the chemical shift range is much smaller than the dominating dipolar interactions. An attempt to use multiple-pulse techniques to extract chemical shift information for static samples is presented. The tensor character of the chemical shift is reflected in the orientational dependence of the line position. The orientation of the liquid crystals can be manipulated by the in situ application of electric fields. © 1997 John Wiley & Sons, Ltd.

Backbone effects on the anchoring of side-chain polymer liquid crystals

By comparing the anchoring behaviour of (end-on) side chain polymer liquid crystals with that of the corresponding low molecular mass liquid crystals, we have studied the effect of the backbone on the anchoring of side chain polymer liquid crystals. We can distinguish two different effects: the loss of rotational freedom of the side groups and the interaction of the polymer chain with the surface. In the case of free surfaces, we can formulate a general rule stating that compounds with side groups ending with an aliphatic chain (at least four carbon atoms long) exhibit a homeotropic anchoring, and compounds with side groups ending with a polar group exhibit a planar anchoring.

Liquid‐Crystalline, Substituted Octakis‐(dimethylsiloxy)octasilsesquioxanes: Oligomeric Supermolecular Materials with Defined Topology

Cuboid silsesquioxanes with appropriate side chains (see below; n = 4, 6, 11) provide a means of structuring soft matter with inorganic cores. These compounds combine material characteristics of polymer and low molecular mass liquid crystals.

Glass transition in semicrystalline systems

The investigation of amorphous layers between crystalline structures allows the estimation of glass transition length scales. Low molecular mass liquid crystals can form semicrystalline structures with amorphous regions smaller than 1 nm. In such structures we are able to investigate the breakdown of the co-operative molecular motions associated with the glass transition. The mixing of two liquid crystalline sulfur-bridged benzoates allowed the crystalline structure of the sample to vary. First calorimetric and dielectric investigations of different crystallized mixtures show significant differences in the relaxation behavior in the glass transition region.

Behaviour of the Electroclinic Effect and the Soft Mode Rotational Viscosity in the Smectic A Phase of Ferroelectric Liquid Crystalline Polymer/Monomer Mixtures

AbstractMeasurements of the electroclinic effect in the smectic A phase of a chiral side‐chain polyacrylate, of a low molecular mass liquid crystal with very similar structure and of mixtures between these two compounds are reported. The ratio of the induced tilt angle and the electric field strength is found to be independent of the concentration, whereas the coupling constant between induced tilt and induced polarization decreases with increasing amount of polymer. This behaviour of the electroclinic effect implies a considerably lower tilt controlling susceptibility coefficient for the chiral polyacrylate than for the low molecular mass compound. The values of the soft mode rotational viscosity of the polyacrylate, which are obtained from the electrooptical measurements in combination with dielectric spectroscopy, are found to be three orders of magnitude larger than for the monomer.

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

AbstractMeasurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius‐like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.

Liquid crystals in polymeric membranes. Review

Recently the search for new types of membranes with specified properties for use in various branches of engineering, technology, medicine and pharmacology has widened. This review examines new membranes based on liquid crystals in a polymeric matrix through which thermoregulated selective transport of the components to be separated comes about (gases, liquids, ions). In the composite membranes containing low molecular mass liquid crystals in the polymeric matrix diffusion of the components to be extracted operates over the continuous LC phase and the selectivity of permeability is determined either by the structural orientation of the LC phase on exposure to external fields or by modification of the membrane by mobile carriers. Solid separating membranes are described obtained from LC polymers and preserving the structure of the LC phase. The prospects for LC membranes of different types are evaluated.

Thermotropic liquid-crystalline systems containing methylated rings

Abstract The thermal properties of low molecular mass and polymeric liquid crystals of similar chemical structure have been investigated in order to see if a parallel structure-properties relationship exists between the two groups of compounds. Two series of models and of the corresponding polymers were prepared, bearing the same mesogenic moiety obtained by reacting 4-hydroxybenzoic acid with different methylated hydroquinones. Ether or ester linkages have been used to bridge the rigid core to the flexible polymethylene spacer. The thermal stability and the nature of the mesophases were examined by different techniques. An interpretation of the results on the basis of the axial ratio and the strength of the orientation-dependent attractions, is attempted for the model compounds. As far as polymers are concerned thermodynamic parameters follow the expected trend if compared with those of low molecular mass analogues. The dependence of the liquid crystal to isotropic transition temperature on the degree of substitution shows the same trend as for the model compounds. Geometric and electronic effects must be taken into account to understand the experimental results.

Prospects for the use of functional liquid crystal polymers and composites. Review

A review is presented of the current state in the field of research associated with the exploration of the possibilities of practical uses of functional LC polymers and composites of low molecular mass liquid crystals with amorphous polymers. The molecular structure of the LC polymers is briefly described and the main features of their physicochemical properties analysed. From the structural features of LC polymers the authors systematize and critically analyse the results of work relating to the use of functional LC polymers as materials for passive optical elements, controlled optical and photo-optical media, for producing informational memorizing devices, polymeric LC ferroelectrics, thermo-indicator films with regulatable colour and optical characteristics, LC polymers with non-linear optical properties and other applications. The prospects for the possible use of LC elastomers, metal-containing and electroconducting LC polymers are discussed.

Liquid crystalline side-chain polymers

Liquid crystalline side-chain polymers are polymers that have linked conventional low molecular mass liquid crystals (I.e.) as side chains to a polymer backbone. Their properties are experimentally compared with the corresponding low molecular mass I.e. (i) Beginning with a monomeric I.e., with increasing degree of polymerization an increase of the phase transformation temperature of I.e. to isotropic is observed. (ii) The nematic order parameter of the monomers is about 10 % higher than the order parameter of the polymers. Owing to the linkage of the rigid mesogenic molecules to the polymer backbone, their rotation around the long molecular axis differs strongly from that of the monomers, (hi) Owing to the high viscosity of the polymers their response times in the electric field are much larger than the response times of the monomers. If a sufficient length of the flexible spacer that connects the mesogenic molecule to the polymer main chain is assumed, the threshold voltages of the monomers and polymers are of comparable magnitude.

Phase separations in the mixtures of a mesomorphic polymer with a low molecular mass liquid crystal

We have studied the phase diagram of a binary system, constituted with a side-chain mesomorphic polymer and a monomer liquid crystal, very similar to the mesogenic groups in the polymer. The main results is the existence of a gap of miscibility in the nematic state, leading to the coexistence, at a same temperature, of two nematic phases.