Mesophase Properties Research Articles

Order and Dynamics of a Liquid Crystalline Dendrimer by Means of 2H NMR Spectroscopy

A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of (2)H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the (2)H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T(1) and T(2)) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T(1).

Lyotropic Liquid-Crystalline Mesophases of [Zn(H2O)6](NO3)2−C12EO10−CTAB−H2O and [Zn(H2O)6](NO3)2−C12EO10−SDS−H2O Systems

The mixture of two surfactants (C12EO10-CTAB and C12EO10-SDS) forms lyotropic liquid-crystalline (LLC) mesophases with [Zn(H2O)6](NO3)2 in the presence of a minimum concentration of 1.75 H2O per C12EO10. The metal ion/C12EO10 mole ratio can be increased up to 8.0, which is a record high metal ion density in an LLC mesophase. The metal ion concentration can be increased in the medium by increasing the CTAB/C12EO10 or SDS/C12EO10 mole ratio at the expense of the stability of the LLC mesophase. The structure and some thermal properties of the new mesophase have been investigated using XRD, POM, FTIR, and Raman techniques.

Synthesis of liquid crystalline materials based on 1,3,5-triphenylbenzene and 2,4,6-triphenyl-1,3,5- s-triazine

C 3-symmetric alkyloxy/aryloxy polyether dendrimers have been synthesized from 1,3,5-triphenylbenzene and 2,4,6-triphenyl-1,3,5- s-triazine and their liquid crystalline properties have been studied. C 3-symmetric derivatives with n-hexyl and n-dodecyl chains at the periphery show mesophase properties when they are complexed with trinitrofluorenone at different temperatures.

Super‐Fluorinated Liquid Crystals: Towards the Limits of Polarity

AbstractHighly fluorinated liquid crystals with unprecedented polarity were obtained by introducing a tetrahydropyran moiety into their mesogenic core structure. The new materials exhibit an unusually favourable combination of mesophase properties, excellent solubility and low rotational viscosity, which renders them highly attractive for application in fast‐switching active matrix LCDs.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

6- S-Alkyl-6-thiohexonolactones and 6- S-Alkyl-6-thiohexitols: Efficient synthesis and new smectic liquid crystal phases

The mesophasic properties of S-alkylthiohexonolactones ( d-galactono- and d-mannono-) and the corresponding itol derivatives with general formula Su- SR (R = C 11H 23) have been investigated. The observation that the synthesized compounds of this series has been studied using crossed polarized optical microscopy. This study involves the discovery of new chiral Smectic A liquid crystal phase through direct comparison with the defective texture of known Smectic A phase of available commercially biphenyls 8CB and 10CB.

2H NMR Studies on Two-Homopolypeptide Lyotropic Enantiodiscriminating Mesophases: Experimental Quantification of Solute-Fiber Affinities

The analytical potential and enantioselective properties of lyotropic mesophases made by mixing two chemically different chiral polypeptides are described. Here we examine the case of a mixture of poly-gamma-benzyl-L-glutamate (PBLG) and poly-epsilon-carbobenzyloxy-L-lysine (PCBLL). We demonstrate that 2H NMR spectroscopy on these chiral oriented mixtures can discriminate both enantiomers and enantiotopic directions in prochiral molecules. Moreover, in such systems, degree of enantiodiscrimination, resolution, and sensitivity can be conjointly optimized by changing the relative proportion of the two polypeptides. Therefore, these new enantiodiscriminating media provide a favorable alternative to single-polypeptide mesophases with respect to stereochemical applications. At a more fundamental level, the present work points out that solute distribution in the vicinity of each polypeptide partly governs the degree of enantiodiscrimination and NMR relaxation rates. To this end, the experimental trends of solute NMR observables (Delta nu Q, T1) versus the fraction of peptide units of each polymer were analyzed by using a "mean-field" model derived from that proposed for mixtures of thermotropic nematic solvents, and based on the separation of intermolecular interactions between the solute and both polypeptides. This approach allows the relative solute-fiber affinities in these lyotropic systems to be determined. To identify the factors controlling solute-polypeptide affinities, we investigated various solutes (polar/apolar, rigid/flexible, achiral/prochiral/chiral molecules) using 2H NMR at natural abundance or on isotopically enriched solutes.

Computer simulation study of a simple tetrahedratic mesogenic lattice model

Over the last 12 years, the possible existence of a tetrahedratic mesophase, involving a third-rank orientational order parameter and no positional order, has been addressed theoretically and predicted in some cases; no experimental realizations of a purely tetrahedratic phase are known at the time being, but various pieces of evidence suggest that interactions of tetrahedral symmetry do play a significant role in the macroscopic properties of mesophases resulting from banana-shaped (bent-core) mesogens. We address a very simple tetrahedratic mesogenic lattice model, involving continuous interactions; we consider particles possessing T(d) symmetry, whose centers of mass are associated with a three-dimensional simple-cubic lattice; the pair potential is taken to be isotropic in orientation space and restricted to nearest-neighboring sites; we let the two orthonormal triads {u alpha, alpha=1,2,3} and {v gamma, gamma=1,2,3} define the orientations of a pair of interacting particles; we let the unit vectors u alpha be combined to yield four unit vectors {e(j), j=1,2,3,4}, arranged in a tetrahedral fashion; we let the unit vectors v gamma be similarly combined to yield the four unit vectors {f(k), k=1,2,3,4}; and finally we let h(jk)=(e(j)f(k)). The interaction model studied here is defined by the simplest nontrivial (cubic) polynomial in the scalar products h(jk), consistent with the assumed symmetry and favoring orientational order; it is, so to speak, the tetrahedratic counterpart of the Lebwohl-Lasher model for uniaxial nematics. The model was investigated by molecular field (MF) theory and Monte Carlo simulations; MF theory predicts a low-temperature, tetrahedrically ordered phase, undergoing a second-order transition to the isotropic phase at higher temperature; on the other hand, available theoretical treatments point to the transition being driven first order by thermal fluctuations. Simulations showed evidence of a first-order transition.

Synthesis and mesogenic properties of β-tetrabrominated tetraalkyloxyporphyrins

A new class of 2,3,7,8-tetrabromo-5,10,15,20-tetrakis(4'-n-1-alkyloxyphenyl)porphyrins, H 2 T (4'- OC n P ) PBr 4 (n = 4,6,8,10,12,14,16 and 18) and their Cu(II) complexes have been synthesized regioselectively and their mesophase properties explored. The mesogenic properties of the porphyrins were examined by differential scanning calorimetry and optical microscopy. An increase in alkyl chain length of the mesogens, showed an increase in melting point temperature and lowering of their clearing point temperatures with the narrower mesophases. This has been ascribed to the decreased π-π interactions as evidenced from the crystal structure of H 2 T (4'- OC 6 P ) PBr 4· THF derivative. MT (4'- OC n P ) PBr 4 derivatives showed rectangular mesophases in contrast to discotic lamellar mesophases reported for the planar MT (4'- OC n P ) P mesogens.

Microstructure and Lubrication Properties of Lamellar Liquid Crystal in Brij30/[Bmim]PF6/H2O System

The microstructure of lamellar liquid crystal composed by nonionic surfactant polyoxyethylene laurylether (Brij30), room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6) and water is investigated by deuterium nuclear magnetic resonance (2H NMR) and rheological techniques. And the lubrication properties of the lamellar mesophase are determined to illustrate their relationship with the microstructure. The results show that the structure strength of the lamellar phase is enhanced with increasing amount of Brij30, so the anti-wear capacity of the lamellar phase is improved. But, both of the structure strength and lubrication properties are impaired with increasing water content in the system due to the increased interlayer space and the penetration of water into the amphiphile bilayer. However, though the structure of the lamellar phase is disturbed by [Bmim]PF6, the lubrication properties are still improved due to the inherent lubrication properties of [Bmim]PF6.

Structural and Rheological Investigation of Fd3m Inverse Micellar Cubic Phases

In the present study we demonstrate that a bulk inverse micellar cubic phase of Fd3m structure can be obtained by adding a hydrophobic component, such as the food-grade limonene, to the binary system monolinolein/water in a well-defined composition. The Fd3m structure studied in this work had a very slow kinetics of formation, as a consequence of partitioning of water into two types of micelle populations with different sizes. The Fd3m structure formed at a ratio of limonene oil to total lipids of alpha = 0.4 is stable in the bulk up to a maximum hydration of 12.68 wt % water, beyond which it starts to coexist with dispersed water. At full hydration, by combining small-angle X-ray scattering and available topological models, the inverse micellar cubic phase of Fd3m structure was shown to be formed by 16 small micelles and 8 larger micelles per cubic lattice cell (Q227 group), with radii of the micellar polar cores ranging between 1 and 3 nm and 149-168 monolinolein molecules per micelle depending on the water content. The temperature dependence of the structural and rheological properties of the Fd3m mesophase was investigated using SAXS, rheology, and turbidimetry. It appeared that the Fd3m phase underwent crystallization below 18 degrees C and began melting in an inverse microemulsion (L2 phase) coexisting with water above 28.5 degrees C with complete melting obtained at 40-45 degrees C, as evidenced by SAXS and rheology. Macroscopic phase separation between the L2 phase and excess water was observed with time at higher temperatures. The investigation of the viscoelastic properties of the Fd3m inverse discrete micellar cubic phase revealed a rheological signature similar to that of the bicontinuous cubic phases Pn3m and Ia3d observed in homologous binary systems. However, the Fd3m phase presented a complex set of slower relaxation mechanisms leading to a shift by 1 order of magnitude of the dominant relaxation times and whole relaxation spectrum, as compared to those of inverse bicontinuous cubic phases. These findings have been tentatively explained by (i) the multiple relaxation of micelles upon deformation, (ii) the small hydration level of the Fd3m phase, and (iii) the low temperature at which this phase can be observed.

How the Odd-Even Effects on the Inter-Molecular Potentials Propagate to the Order Parameter in the 4-Cyano-4′n-Alkylbiphenyl Series

We present a study on the dimers of the 4-cyano, 4′n-alkyl biphenyl (nCB) aimed at capturing information connected to the measured odd-even effects on the mesophase properties. The interaction energy of dimers of the 4-cyano, 4′n-alkyl biphenyl (nCB) is computed by a model atomistic potential. The latter has been derived for the 5CB molecule by ab initio calculations that include correlation energy, with an approach that has already provided accurate results. The analyzed inter-molecular energy profiles for selected parallel and antiparallel conformations along the nCB series (n = 5,6,7,8), present some features that might be at the root of the odd-even effect of this class of compounds. Preliminary molecular dynamics results yield a stable nematic phase for each homologue, whose computed order parameter show an evident odd even alternation, in good agreement with the experimental findings.

Order and dynamics of rod-like and banana-shaped liquid crystals by 2H NMR

Deuterium NMR spectroscopy is a very powerful technique for studying partially or totally ordered systems, such as liquid crystals (LCs) and polymers. LCs represent a branch of the most general class of soft materials, with peculiar physical and chemical properties which attracted scientific attention for their potentiality for technological applications. From a chemical point of view, there are three aspects in which2H NMR could provide significant insights: (i) the conformational and structural properties; (ii) the molecular dynamics and mobility; and (iii) the orientational order and aggregation/distribution of molecules in the different liquid-crystalline phases. In this work, some of the recent developments in this field are discussed, focusing on two main topics: (1) the molecular dynamics of the smectic liquid-crystalline phases formed by rod-like molecules and (2) the unusual orientational and dynamic properties of the new liquid-crystalline mesophases formed by banana-shaped molecules (BLCs).

Intercalated Smectic A Phases in Banana‐Shaped Liquid Crystals with Carbonate End Groups

Novel, symmetrical, bent-core mesogens, namely substituted 1,3-phenylenebis{4'-[(ethoxycarbonyl)oxy]-1,1'-biphenyl}-4-carboxylates, are prepared with carbonate groups at both ends of the molecules. The mesophase properties are investigated for different electron-donating and -withdrawing substituents connected to the central ring at different positions. These compounds exhibit nematic and/or smectic A intercalated (SmA(int)) mesophases. Electro-optical studies show a very significant electric-field-induced biaxiality that is fast (<1 ms) and large (0.03) associated with a high birefringence (0.33 at 500 nm, 10 V mum(-1) field) for the unsubstituted analogue. For one substance this effect is observable from above 100 degrees C down to room temperature. Such electric-field-induced biaxiality may find practical applications in fast electro-optic modulation devices.

Multiscale theory and simulation for carbon fibre precursors based on carbonaceous mesophases

An integrated multiscale computational materials modelling toolbox developed to predict product and processing properties of carbonaceous mesophases is presented and used to simulate linear and non-linear rheology, processing flow induced textural transformation and carbon fibre textures. The spatial resolution of the predictions spans from the nanoscale to the microscale. The nanoscale processes that lead to defect nucleation are linked to process induced structuring. Modelling the linear rheology reveals the role of polydomain textures on the elasticity modulus and can be used to predict domain sizes from simple small amplitude oscillatory flows. Geometric modelling provides insights on fibre texture and elucidates the role of surface anchoring on the fibre structure. The shown ability of the integrated model to predict product structure, flow processing and rheology in a linked sequence demonstrates the utility of computational material science as an efficient and economical innovation tool.

Preparation and cholesteric mesophase properties of (Butyl-co-pentyl) propylcellulose

Butyl and pentyl ether derivatives of (2-hydroxypropyl) cellulose (HPC) and butyl/pentyl mixed ethers of HPC (BPPC) with different alkyl compositions were prepared in nonaqueous solution and their thermotropic cholesteric properties examined. The temperature dependence and the composition dependence of the optical pitch, nP, were then determined for all of the ether derivatives. The molecular conformation and chirality of BPPC appeared to be to be variably smooth with the side chain composition of the polymer. The response rate of cholesteric configurational change of the thermotropic mesophases arising from a temperature jump from 45 to 75 °C, was also determined. The transformation of the cholesteric mesophase formed by an equimolar ether derivative (BP-50) was faster than that of the cholesteric one formed by a single-alkyl (pentyl) ether derivative (BP-0). This seemed to be a general result reflecting a pseudo-copolymer effect of cellulose derivatives on the properties of their thermotropic cholesteric mesophases.

DFT and MD studies on the influence of the orientation of ester linkage groups in banana-shaped mesogens

The influence of the direction of ester linkage groups on the structural and electronic properties of five-ring banana-shaped molecules with a central 1,3-phenylene unit has been investigated including hexyloxy and dodecyloxy terminal chains. DFT studies on the B3LYP/6-31G(d) level were performed on the conformational behaviour of the ten isomers in a systematic way. The one- and two-fold potential energy scans show that the flexibility of the wings significantly depends on the orientation of the carboxyl linkage groups. Moreover, the different directions of the carboxyl groups between the aromatic rings cause remarkable changes on the dipole moment and its components of the molecules. These findings are supported by investigations on the global charge pattern of the molecules calculated from electrostatic potential group charges. The bending angle alpha obtained from a simple model for the five-ring bent-core molecules is a characteristic structural parameter which can be correlated with experimental findings. Calculations on the bent-core molecules in an external electric dipole field related to the direction of their polar axis show remarkable effects with respect to the flexibility and polarity of the isomers. First molecular dynamics simulations on the banana-shaped molecules were carried out within the AMBER 7 package. The trajectories of relevant structural parameters support the findings of the DFT studies. The results concerning the structure and polarity revealed from the DFT and MD calculations of the ten isomers can be correlated with data from dielectric measurements and mesophase properties.

Thermotropic liquid crystalline drugs

Crystalline solids are characterized by long-range positional and orientational order in three dimensions, whereas amorphous liquids lack long-range order in any dimension. Liquid crystals (mesophases) show structural, mechanical and optical properties intermediate to those of crystalline solids and the amorphous, liquid state of matter. There are two principle types of liquid crystals: thermotropic liquid crystals (TLCs) and lyotropic liquid crystals (LLCs). TLCs can be formed by heating a crystalline solid or by cooling an isotropic melt of a TLC-forming molecule (mesogen). In the first part of this review the types of liquid crystals are defined and classified and the structural properties of mesogens are explained. In the second part, ten case studies of thermotropic mesomorphous drugs and pharmaceutically relevant molecules (arsphenamine, nafoxidine hydrochloride, L-660711, palmitoyl propranolol hydrochloride, penbutolol sulfate, itraconazole hydrochloride, fenoprofen sodium, fenoprofen calcium, ciclosporin and cholesteryl esters) are presented and their thermotropic mesomorphism is described. The review closes with a brief discussion of the unusual properties of drug mesophases and a potential use of drugs and excipients in this fourth state of matter.

Rheology of the lamellar liquid-crystalline phase in polyethoxylated alcohol/water/heptane systems.

Linear viscoelastic tests as well as transient and steady flow experiments were carried out on lamellar liquid crystalline samples of poly (oxyethylene) alcohol/water/heptane systems. The effect of surfactant and heptane concentrations on the rheological properties of the lamellar mesophase was investigated. The mechanical spectrum inside the linear viscoelastic regime shows, in all cases, a well-developed plateau region in the whole frequency range studied. The values of the dynamic functions were higher for intermediate surfactant or heptane concentrations indicative of a major development of the elastic network in the midrange of existence of the lamellar phase. Transient and steady flow experiments point out a shear-induced evolution of the lamellar microstructure. Different power law regions with different values of the flow index were detected in the viscosity versus shear rate plots. These shear-induced structural modifications were confirmed by using polarizing microscopy in an optical shearing cell. Structural modifications appear to be highly influenced by shear rate. In general, applying relatively high constant shear rates, the alignment of the bilayers followed by the appearance of the “oily streaks” structure was observed. Appearance of shear-induced vesicles occurs at high heptane content, as indicates the texture of close-packed monodisperse spherulites detected by polarizing microscopy.

Synthesis and characterization of novel imidazolium-based ionic discotic liquid crystals with a triphenylene moiety

Two novel triphenylene-tethered imidazolium salts were synthesized either by the quaternization of 1-methylimidazole with an ω-bromo-substituted triphenylene or by the quaternization of a triphenylene-substituted imidazole with methyl iodide. Their chemical structures were determined by 1H NMR, 13C NMR, IR, UV spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of these salts were investigated by polarizing optical microscopy and differential scanning calorimetry. These triphenylene-based imidazolium salts with bromide or iodide as counterion show columnar mesophase properties over a wide temperature range.

Effect of oxidative stabilization on the electrochemical performance of carbon mesophases as electrode materials for lithium batteries

The effect of oxidative stabilization as a mean to modify the carbon texture was essayed in a group of mesophases previous to carbonization at 900 °C with the aim of evaluating the influence on electrochemical performance when used as electrode materials in lithium test cells. X-ray diffraction, optical microscopy and chemical analysis, Fourier-transform infrared spectroscopy have been used to describe the compositional and textural properties of the as-produced parent mesophases, the samples were further treated under air current to stabilize their microstructures and the corresponding carbonized samples at 900 °C. The electrochemical performance was determined by the galvanostatic method and further correlated to the physical–chemical properties and interface resistance of the materials. In all cases, the stabilization process has demonstrated a beneficial effect on the capacity retention in the measured range.