Isotropic Liquid Crystalline Phases Research Articles

Liquid crystalline phase formation in suspensions of solid trimyristin nanoparticles.

The presence of liquid crystalline phases in suspensions of solid lipid nanoparticles can increase the risk of their gelling upon administration through fine needles. Here we study the formation of liquid crystalline phases in aqueous suspensions of platelet-like shaped solid lipid nanoparticles. A native lecithin-stabilized trimyristin (20 wt %) suspension was investigated at different dilution levels by small-angle X-ray scattering (SAXS) and visual inspection of their birefringence between two crossed polarizers. For trimyristin concentrations φMMM < 6 wt %, the dispersed platelets are well separated from each other whereas they start to self-assemble into stacked lamellae for 6 wt % ≤ φMMM < 12 wt %. For φMMM ≥ 12 wt %, the SAXS patterns become increasingly anisotropic, which is a signature of an evolving formation of a preferred orientation of the platelets on a microscopic scale. Simultaneously, the suspensions become birefringent, which proves the existence of an anisotropic liquid crystalline phase formed in the still low viscous liquid suspensions. Spatially resolved SAXS scans and polarization microscopy indicate rather small domains in the (sub)micrometer size range in the nematic liquid crystalline phase and the presence of birefringent droplets (tactoids). The observed critical concentrations for the formation of stacks and the liquid crystalline phase are significantly higher as for equivalent suspensions prepared from triglycerides with longer chains. This can be explained with the lower aspect ratio of trimyristin platelets. Special emphasis is put on the isotropic-liquid crystalline phase transition as a function of the ionic strength of the dispersion medium and φMMM. Higher salt concentrations allow shifting of the phase transition to higher trimyristin concentrations. This can be attributed to a partial screening of the repulsive forces between the platelets, which allows higher packing densities within the platelet stacks and of remaining isolated platelets.

Liquid crystal supermolecules stabilizing an optically isotropic phase with frustrated molecular organization

Cubic and blue phases have attracted a significant amount of attention because of their unusual phase structures with optical isotropy and because of their application to ion transfer in the cubic phase and to electrooptical switching in the blue phase. However, the driving forces for the frustrated liquid-crystalline phases differ from those for a conventional liquid-crystalline phase composed of rod-like molecules. We show that preprogrammed liquid-crystalline supermolecules can stabilize frustrated molecular organization in optically isotropic phases. Competition between steric effects caused by a taper-shaped oligomeric structure and segregation effects attributable to hydrocarbon/fluorocarbon amphiphilicity produces fluctuation of the layer structure, inducing the lamellar-to-cubic phase transition. Cooperation between inherent molecular chirality and the chirality-induced twist conformations of the mesogenic units stabilizes double-twist structures in blue phases. Furthermore, we demonstrate high-transmittance, sub-millisecond responses and hysteresis-free switching in a room temperature amorphous blue phase stabilized by a liquid crystal supermolecule.

Influence of Moieties and Chain Length on the Abundance of Orthogonal and Tilted Phases of Linear Hydrogen-Bonded Liquid Crystals, Py16BA:nOBAs

A novel series of linear hydrogen-bonded (HB) liquid crystalline (LC) complexes (HBLCs), namely Py16BA:noBAs, is reported. The series was prepared by the complexation of a novel non-mesogenic proton acceptor moiety, namely (4-pyridyl)-benzylidene-4′-n-hexadecyl aniline (Py16BA), with a mesogenic proton donor, p-n-alkoxy benzoic acid (noBAs for n = 3, 4, 5, 6, 7, 8, 9, and 12) moieties. Formation of HB complexes is confirmed through FTIR spectra. The LC phases exhibited by Py16BA:noBAs, the transition temperatures (Tc), and the heat of transition (enthalpy) (ΔH) are determined by polarizing optical microscope (POM) and differential scanning calorimetry (DSC) techniques. A comparative study of phase abundance is presented (with respect to the HB complexes of pure noBAs). An orthogonal smectic-A phase is induced by quenching the nematic phase in the lower homologs, while a quasi-2D tilted smectic-F phase is induced in the intermediate (or higher) homologs Py16BA:noBAs. The odd–even effect is observed across the isotropic–LC phase and LC–solid phase transitions. Overall mesogenic stability is enhanced. Optimization of the tilted smectic phase stability is discussed in the wake of the configuration of hydrogen bonding and the steric intrusion due to the increase in length of the flexible end-chain. Based on the extrapolation of the odd–even effect, an ACBF multi-critical point is predicted in the phase diagram in the vicinity of intermediate homologs, possibly in a binary of Py16BA:5oBA and Py16BA:6oBA.

Polarization Transfer Solid-State NMR for Studying Surfactant Phase Behavior

The phase behavior of amphiphiles, e.g., lipids and surfactants, at low water content is of great interest for many technical and pharmaceutical applications. When put in contact with air having a moderate relative humidity, amphiphiles often exhibit coexistence between solid and liquid crystalline phases, making their complete characterization difficult. This study describes a (13)C solid-state NMR technique for the investigation of amphiphile phase behavior in the water-poor regime. While the (13)C chemical shift is an indicator of molecular conformation, the (13)C signal intensities obtained with the CP and INEPT polarization transfer schemes yield information on molecular dynamics. A theoretical analysis incorporating the effect of molecular segment reorientation, with the correlation time τ(c) and order parameter S, shows that INEPT is most efficient for mobile segments with τ(c) < 0.01 μs and S < 0.05, while CP yields maximal signal for rigid segments with τ(c) > 10 μs and/or S > 0.5 under typical solid-state NMR experimental conditions. For liquid crystalline phases, where τ(c) < 0.01 μs and 0 < S < 0.3, the observed CP and INEPT intensities serve as a gauge of S. The combination of information on molecular conformation and dynamics permits facile phase diagram determination for systems with solid crystalline, solid amorphous, anisotropic liquid crystalline, and isotropic liquid (crystalline) phases as demonstrated by experiments on a series of reference systems with known phase structure. Three solid phases (anhydrous crystal, dihydrate, gel), two anisotropic liquid crystalline phases (normal hexagonal, lamellar), and two isotropic liquid crystalline phases (micellar cubic, bicontinuous cubic) are identified in the temperature-composition phase diagram of the cetyltrimethylammonium succinate/water system. Replacing the succinate counterion with DNA prevents the formation of phases other than hexagonal and leads to a general increase of τ(c).

Aqueous-Phase Room-Temperature Synthesis of Gold Nanoribbons: Soft Template Effect of a Gemini Surfactant

Gold (Au) nanoribbons were synthesized in aqueous phase under ambient conditions by using dimethylene bis(tetradecyldimethyl-ammonium bromide) (14-2-14) as a capping agent as well as a soft template. A two steps seed-growth (S-G) method was used. The first step of S-G method mainly gave nanorods of high aspect ratio along with nanoparticles of other shapes, but the next step produced mainly fine Au nanoribbons of several micrometers long, 50–150 nm wide, and ≈5 nm thick. They were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray, and X-ray diffraction analysis. Isotropic liquid crystalline phase of 14-2-14 provided a soft template effect and derived nanorods of high aspect ratios toward nanoribbon formation at room temperature.

Effects of lipid composition on the membrane activity and lipid phase behaviour of Vibrio sp. DSM14379 cells grown at various NaCl concentrations

The membrane lipid composition of living cells generally adjusts to the prevailing environmental and physiological conditions. In this study, membrane activity and lipid composition of the Gram-negative bacterium Vibrio sp. DSM14379, grown aerobically in a peptone-yeast extract medium supplemented with 0.5, 1.76, 3, 5 or 10% (w/v) NaCl, was determined. The ability of the membrane to reduce a spin label was studied by EPR spectroscopy under different salt concentrations in cell suspensions labeled with TEMPON. For lipid composition studies, cells were harvested in a late exponential phase and lipids were extracted with chloroform–methanol–water, 1:2:0.8 (v/v). The lipid polar head group and acyl chain compositions were determined by thin-layer and gas–liquid chromatographies. 31P-NMR spectroscopy was used to study the phase behaviour of the cell lipid extracts with 20 wt.% water contents in a temperature range from −10 to 50 °C. The results indicate that the ability of the membrane to reduce the spin label was highest at optimal salt concentrations. The composition of both polar head groups and acyl chains changed markedly with increasing salinity. The fractions of 16:0, 16:1 and 18:0 acyl chains increased while the fraction of 18:1 acyl chains decreased with increasing salinity. The phosphatidylethanolamine fraction correlated inversely with the lysophosphatidylethanolamine fraction, with phosphatidylethanolamine exhibiting a minimum, and lysophosphatidylethanolamine a maximum, at the optimum growth rate. The fraction of lysophosphatidylethanolamine was surprisingly high in the lipid extracts. This lipid can form normal micellar and hexagonal phases and it was found that all lipid extracts form a mixture of lamellar and normal isotropic liquid crystalline phases. This is an anomalous behaviour since the nonlamellar phases formed by total lipid extracts are generally of the reversed type.

Side Chain Liquid Crystalline Thermoplastic Elastomers for Actuator and Electromechanical Applications

ABSTRACTThe synthesis of a polystyrene-b-polyvinylmethylsiloxane-b-polystyrene diblock and triblock copolymer functionalized with liquid crystals exhibiting a smectic C* phase on the PVMS central block is described. The synthetic route is based on the anionic polymerization of styrene and trimethyltrivinylsiloxane monomers and the functionalization of resulting triblock copolymers. The resulting polymer can self assemble into a thermoplastic elastomer where the high Tg styrene blocks serve as physical crosslinks for a low Tg siloxane block. The presence of a smectic liquid crystalline phase and the block copolymer mesophase are observed across various temperature ranges depending on the length of the spacer connecting the liquid crystalline moiety to the polymer backbone. The influence of mechanical deformation upon the morphologies of the liquid crystalline and block copolymer mesophases was investigated. The interactions between the smectic LC and the block copolymer morphologies and their influence upon their respective orientations in response to shear fields are detailed. The parallel-transverse orientation of the hexagonally close packed (HCP) cylinders of the block copolymer morphology and the smectic liquid crystalline phase, respectively, was observed for melt fiber drawn samples. However, the transverse-perpendicular orientation was observed for liquid crystalline block copolymers that experienced oscillatory shear. The transverse orientation of HCP cylinders was observed while shearing took place above the smectic to isotropic transition temperature, indicating that the presence of an isotropic liquid crystalline phase alters the orientation of the block copolymer morphology. Additionally, it was found that the spacer length was a key factor in the clearing points for the smectic liquid crystalline phase, as well as significantly influencing the nanophase segregation of the block copolymer.

Unusual Dynamic Behavior in the Isotropic Phase of Banana Mesogens Detected by 2H NMR Line Width and T2 Measurements

In this work the first experimental observation of a peculiar behavior in the isotropic phase of liquid crystals by means of 2H NMR is reported. In particular, two five-ring banana-shaped mesogens, the 1,3-phenylenebis{4,4'-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), selectively deuterium labeled on their central rings, are the subject of our investigation. The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line width and spin-spin relaxation time (T2) analysis. The results obtained reveal that the unusual line broadening observed in the 2H NMR spectra in the isotropic phase, even far above the isotropic phase-mesophase transition, has a homogeneous nature, thus indicating the presence of reorientational motions much slower than in conventional isotropic liquid-crystalline phases.

Isotropic-liquid crystalline phase diagram of a CdSe nanorod solution.

We report the isotropic-liquid crystalline phase diagram of 3.0 nm x 60 nm CdSe nanorods dispersed in anhydrous cyclohexane. The coexistence concentrations of both phases are found to be lower and the biphasic region wider than the results predicted by the hard rod model, indicating that the attractive interaction between the nanorods may be important in the formation of the liquid crystalline phase in this system.

Phase Behavior of the Lecithin/Water/Isooctane and Lecithin/Water/Decane Systems

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.

Structural Evolution During and After Cessation of the Shear Flow for a Side-Chain-Type Liquid Crystalline Polysiloxane Above the Isotropic-Liquid Crystalline Phase Transition Temperature

Evolution of the anisotropic texture during and after cessation of shear flow for a side-chain-type liquid crystalline polysiloxane (LCS) was studied by shearing microscopy at temperatures above the isotropic to liquid crystalline phase transition temperature (Tc=309.2 K if LCS is cooled from the isotropic phase in the quiescent state). The anisotropic texture of LCS during the shear flow was found at a temperature of 313 K and a shear rate of 50 s−1. After cessation of the shear flow, the anisotropic texture disappeared. At a temperature of 311 K and a shear rate of 50 s−1, LCS under shear flow also showed the anisotropic texture. It is noteworthy that the anisotropic texture was stable at this temperature, even after cessation of the shear flow. Furthermore, the evolution of the anisotropic texture was described quantitatively from the behavior of the birefringence as a function of time after cessation of the shear flow.

CUBIC AND BLUE PHASES IN A FLUORINE-CONTAINING DICHIRAL COMPOUND

The structure and dynamics in an optically isotropic liquid crystalline phase exhibited in a dichiral compound with fluorines have been investigated using X-ray, 19F-NMR and dielectric measurements. Results in the dielectric measurement and 19F-NMR measurements clearly show that molecular motion in this phase is just like in the isotropic phase as observed by texture. However, several X-ray diffraction peaks observed in a small angle range indicate akind of cubic phase (Cub). Based on the X-ray diffraction patterns together with the existence of circular dichroism, the phase is assigned to a chiral cubic structure belonging to crystallographic spacegroupI432. Moreover, the exotic phase with blue colored and foggy texture was found in the lower temperature range of isotropic liquid. X-ray diffraction in this phase clearly shows a diffraction ring corresponding to a layer structure, indicating that this blue-colored phase is assignable to a smectic blue phase (BPsm). It was also found that the layer correlation length is remarkably long compared with the previously reported BPsm phases. We want to emphasize that the present material is quite unique in the sense that both cubic phases, BPsm and Cub, appear. The influence of cell thickness and incubation time on the emergence of the smectic C* phase is also discussed based on dielectric measurements and AC calorimetry.

Shearing microscopy using polarized optical microscope with shear stage and spectral analyser to study liquid crystalline polymers.

In-situ polarized optical microscopy using a shear stage and a spectral analyser as well as a CCD camera were applied to study the phase transition under shear flow for a thermotropic and side-chain-type liquid crystalline polysiloxane. The onset of the appearance of anisotropic texture of the polysiloxane was observed under shear flow using the CCD camera at temperatures much higher than the isotropic-liquid crystalline phase transition temperature if the polysiloxane was cooled from the isotropic phase in the quiescent state. Both the onset temperature and the temperature for full development of the anisotropic texture across the field of view became higher as the shear rate increased. The transmitted light intensity was also measured using a spectral analyser with crossed polarisers at wavelengths from 300 nm to 800 nm, and the integrated intensity of the spectrum was calculated. Changes in the spectrum and the integrated intensity against temperature in the cooling process were compared with observation using the CCD camera. Temperature dependence of the integrated intensity showed that the onset of the appearance of the anisotropic texture under high shear rates was detected at temperatures slightly higher than that observed using the CCD camera.

New mesogens with cubic phases: hydrogen-bonded bipyridines and siloxane-containing benzoic acids I. Preparation and phase behaviour

Several 4-(oligodimethylsiloxyl)alkoxybenzoic acids and their hydrogen-bonded complexes with 4,4-dipyridyl or 1,2-bis(4-pyridyl)ethane were prepared and their phase behaviour studied by DSC and polarized optical microscopy. The neat acids showed no liquid crystalline phases. The 4,4-dipyridyl complex of 4-(n-heptamethyltrisiloxyl)hexyloxybenzoic acid (Si3C6BA) exhibits an optically isotropic, highly viscous liquid crystalline phase below a smectic A phase. The 1,2-bis(4-pyridyl)ethane complex of Si3C6BA also shows an optically isotropic liquid crystalline phase above its smectic C phase. Its behaviour is similar to that of the well known cubic D phase found in 4-n-alkoxy-3-nitrobiphenyl-4-carboxylic acids. In the hydrogenbonded mesogens studied herein, the cubic phase appears to assemble spontaneously in order to take account of the chemical incompatibility between the siloxane moiety terminating the H-bonded complex and the stiff aromatic cores. The transition temperatures of the cubic phases in these materials is around 100oC, hence they are amenable to a variety of physical measurements.

Shear Thinning and Electro-Rheological Effect for Neat Liquid Crystalline Polysiloxane in the Vicinity of Isotropic-Liquid Crystalline Phase Transition

The phase transition behavior and the electro-rheological (ER) effect for a neat sample of the side chain type liquid crystalline polysiloxane were investigated in the vicinity of the isotropic-liquid crystalline phase transition temperature. The dependence of the dynamic viscoelastic properties upon temperature under no electric field changed at 308.1 K which was consistent with the isotropic-liquid crystalline phase transition temperature measured by the polarized optical microscopy. The shear thinning was observed under steady shear flow and no electric field at temperatures of 305 K as well as 313 K. The ER effect was also observed at the same temperatures and the shear rates at which the shear thinning was observed. It was thought that the ER effect is closely related to the shear-induced alignment of anisotropic domains as well as mesogenic side groups under high shear fields before application of external electric fields.

Thermotropic properties and molecular dynamics of cholesteryl ester rich very low density lipoproteins: effect of hydrophobic core on polar surface.

Cholesteryl ester rich very low density lipoproteins (CER-VLDL), isolated from the plasma of rabbits fed a hypercholesterolemic diet, have been studied by differential scanning calorimetry (DSC), 13C nuclear magnetic resonance (NMR), and spin-label electron paramagnetic resonance (EPR) to determine the temperature-dependent dynamics of cholesteryl esters in the hydrophobic core and of phospholipids on the polar surface. Intact CER-VLDL exhibit two DSC heating endotherms; these occur at 40-42 and 45-48 degrees C. Cholesteryl esters isolated from CER-VLDL also exhibit two DSC endotherms; these occur at 50.0 and 55.1 degrees C and correspond to the smectic----cholesteric and cholesteric----isotropic liquid-crystalline phase transitions. A model mixture containing cholesteryl linoleate, oleate, and palmitate in a ratio (0.21, 0.51, and 0.28 mol fraction) similar to that in CER-VLDL exhibited comparable DSC endotherms at 45.2 and 51.5 degrees C. CER-VLDL at 37 degrees C gave 13C NMR spectra that contained no resonances assignable to cholesteryl ring carbons but detectable broad resonances for some fatty acyl chain carbons, suggesting the cholesteryl esters were in a liquid-crystalline state. When the mixture was heated to 42 degrees C, broad ring carbon resonances became detectable; at 48 degrees C, they became narrow, indicating the cholesteryl esters were in an isotropic, liquid-like state. With increasing temperature over the range 38-60 degrees C, the resonances for cholesteryl ring carbons C3 and C6 in CER-VLDL narrowed differentially. Similar spectral changes were observed for the synthetic cholesteryl ester mixture, except they occurred at temperatures about 10 degrees C higher. These results indicate that the two DSC transitions in CER-VLDL do not directly correlate with the smectic----cholesteric and cholesteric----isotropic transitions exhibited by pure cholesteryl esters. (5-Doxylpalmitoyl)-phosphatidylcholine (5-DP-PC) and (12-doxylstearoyl)phosphatidylcholine (12-DS-PC) were used to probe the polar surface monolayer of CER-VLDL; the corresponding cholesteryl esters (5-DP-CE and 12-DS-CE) were used to probe the hydrophobic core. None of these probes in CER-VLDL detected an abrupt change in EPR order parameters, S, or maximum splitting, 2T max, over the temperature range 20-58 degrees C even though 12-DS-PC and 5-DP-PC can detect phase transitions in phospholipid bilayers and 12-DS-CE and 5-DP-CE can detect phase transitions in neat cholesteryl esters. However, 12-DS-CE and 5-DP-CE did detect a much greater acyl chain order for the neutral lipids of CER-VLDL than for those of normal triglyceride-rich VLDL.(ABSTRACT TRUNCATED AT 400 WORDS)

Liquid Crystal Formation in Aqueous Solutions of a Polysaccharide Schizophyllan

Abstract Concentrated aqueous solutions of a triple helical polysaccharide schizophyllan with molecular weight 4 * 105 were investigated by polarizing microscopy and laser light diffraction at 30[ddot]C. The solutions were birefringent above a polymer weight fraction w of about 0.098, indicating the formation of a liquid crystalline phase. This liquid crystalline phase resembled in many respects cholesteric mesophases reported for polypeptide liquid crystals, and was found to be also cholesteric. It was characterized by a cholesteric pitch of the order of several μm varying approximately in proportion to w −1.9. The system studied was completely liquid crystalline above w = 0.127 but biphasic in the range of w between 0.098 and 0.127, and underwent an isotropic-liquid crystalline phase transition when the temperature was changed.