Stable Liquid Crystalline Phase Research Articles

Liquid-Crystalline Order in the Phosphorus-Containing DenDrimers

The structure of phosphorus-containing dendrimers has been studied by IR spectroscopy and optical polarization microscopy. The repeating units of dendrimer molecules are mesogens. This property arises from the conjugation of the aromatic ring and the hydrazone group. An analysis of the IR spectra showed that, with an increase in the generation number, the width of the stretching vibration bands ν(PN) and ν(PO) increases. Difficulties in packing molecules of higher generations cause conformational diversity. The shape of the dendrimer molecules was determined by analyzing the increments of dipole moments. Additionally, the modeling of the stacking of repeating links was performed. The spherical model of molecules does not satisfy the experimental dipole moments of the dendrimers. The flat disk model is more suitable for explaining step changes in dipole moments. The liquid-crystalline ordering of dendrimers under the action of applied pressure was found. With simultaneous heating and uniaxial compression, optical anisotropy appears in dendrimers. It is associated with the formation of liquid-crystalline order. However, a thermodynamically stable liquid-crystalline phase is not formed in this case. Dendrimers most likely have disk-shaped molecules.

Phytosterols and γ-Oryzanol as Cholesterol Solid Phase Modifiers during Digestion.

Literature reports that ingestion of phytosterols and γ-oryzanol contributes to cholesterol lowering. Despite in vivo observations, thermodynamic phase equilibria could explain phenomena occurring during digestion leading to such effects. To advance the observations made by previous literature, this study was aimed at describing the complete solid-liquid phase equilibrium diagrams of cholesterol + phytosterol and γ-oryzanol systems by DSC, evaluating them by powder X-ray, microscopy, and thermodynamic modeling. Additionally, this study evaluated the phenomena observed by an in vitro digestibility method. Results confirmed the formation of solid solution in the cholesterol + phytosterols system at any concentration and that cholesterol + γ-oryzanol mixtures formed stable liquid crystalline phases with a significant melting temperature depression. The in vitro protocol supported the idea that the same phenomena can occur during digestion in which mechanochemical forces were probably the mechanisms promoting cholesterol solid phase changes in the presence of such phytocompounds. In this case, these changes could alter cholesterol solubility and possibly its absorption in the gastrointestinal lumen.

Supramolecular meso-Trick: Ambidextrous Mirror Symmetry Breaking in a Liquid Crystalline Network with Tetragonal Symmetry.

Bicontinuous and multicontinuous network phases are among nature's most complex structures in soft matter systems. Here, a chiral bicontinuous tetragonal phase is reported as a new stable liquid crystalline intermediate phase at the transition between two cubic phases, the achiral double gyroid and the chiral triple network cubic phase with an I23 space group, both formed by dynamic networks of helices. The mirror symmetry of the double gyroid, representing a meso-structure of two enantiomorphic networks, is broken at the transition to this tetragonal phase by retaining uniform helicity only along one network while losing it along the other one. This leads to a conglomerate of enantiomorphic tetragonal space groups, P41212 and P43212. Phase structures and chirality were analyzed by small-angle X-ray scattering (SAXS), grazing-incidence small-angle X-ray scattering (GISAXS), resonant soft X-ray scattering (RSoXS) at the carbon K-edge, and model-dependent SAXS/RSoXS simulation. Our findings not only lead to a new bicontinuous network-type three-dimensional mesophase but also reveal a mechanism of mirror symmetry breaking in soft matter by partial meso-structure racemization at the transition from enantiophilic to enantiophobic interhelical self-assembly.

Phase Behavior and Polymorphism of Saturated and Unsaturated Phytosterol Esters.

This study investigated how the physicochemical characteristics of phytosterol esters are influenced by the chain length and degree of unsaturation of the fatty acid ester moiety. Saturated and unsaturated phytosterol esters (PEs) were synthesized by the esterification of different types of fatty acids (stearic, palmitic, lauric, oleic, and linoleic acid) to β-sitosterol. The non-isothermal crystallization and melting behavior of the pure PEs were analyzed. It was proven by X-ray diffraction that saturated β-sitosteryl esters and β-sitosteryl oleate formed a bilayer crystal structure. The lamellar spacings of the bilayer structure decreased with decreasing fatty acid chain length and with an increasing degree in unsaturation. The degree of unsaturation of the fatty acid chain of the β-sitosteryl esters also influenced the type of subcell packing of the fatty acid moieties in the bilayer structure, whether or not a metastable or stable liquid crystalline phase was formed during cooling. Furthermore, it was found that the melting temperature and enthalpy of the β-sitosteryl esters increased with an increasing fatty acid chain length while they decreased with an increasing degree of unsaturation. The microscopic analyses demonstrated that β-sitosteryl oleate formed much smaller spherulites than their saturated β-sitosteryl analogues.

Influences of Ortho‐Fluoroazobenzenes on Liquid Crystalline Phase Stability and 2D (Planar) Actuation Properties of Liquid Crystalline Elastomers

AbstractIn this article, the influences of ortho‐fluoroazobenzenes (o‐Fbs) on liquid crystalline (LC) phase stability and actuation properties of liquid crystalline elastomers (LCEs) are presented. Such o‐Fbs find interest because they allow full photoswitching with visible light (no ultraviolet (UV) radiation). Novel o‐Fb monomers and crosslinkers are synthesized and characterized. Different amounts of an LC mixture are replaced with synthesized o‐Fbs successively and their influences on the LC phase stability in the cis and trans forms are observed through measuring the related clearing temperatures via polarized optical microscopy. LCE precursors containing 0, 1, 3, 5, 7, and 10 mol% o‐Fb monomer or crosslinker are aligned in a magnetic field and polymerized as films in the LC phase. Thermal and visible light–controlled photochemical 2D (planar) actuation properties of LCE samples are investigated and compared to LCEs containing a liquid crystalline non‐fluorinated azobenzene monomer as reference. It is shown that, while the favorable photochemistry is obtained, the integration of o‐Fbs is less effective for photoswitching of the LC phase, because i) less o‐Fbs can be integrated without destroying the LC phase and ii) the trans–cis isomerization is less effective in destroying the LC phase.

Hybrid Nanocomposites for 3D Optics: Using Interpolymer Complexes with Cellulose Nanocrystals.

Manipulation of optical paths by three-dimensional (3D) integrated optics with customized stacked building blocks has gained considerable attention. Herein, we present functional thin films with assembly ability for 3D integrated optics based on nanocomposites made of cellulose nanocrystals (CNCs) embedded in hydrogen-bonded (H-bonded) interpolymer complexes (IPCs). We selected H-bonded IPC poly(ethylene oxide) and neutralized poly(acrylic acid) to render films assembly ability without undesired interplay with charge distribution in CNCs. The CNCs can form a stable chiral nematic liquid crystalline phase with long-range orientational order and helical organization. The resulting nanocomposites are characterized with a high elastic modulus of 8.8 GPa and an adhesion strength of 1.35 MPa through reversible intermolecular interactions at the contact interface upon exposure to acidic vapor. Instead, simply stacked into 3D optics, these functional thin films serve as a facile material for providing a conceptually simple approach to assemble 3D integrated optics with different liquid crystalline orderings to manipulate the light polarization state.

Photoresponsive Halogen-Bonded Liquid Crystals: The Role of Aromatic Fluorine Substitution

A new strategy for controlling the liquid crystalline and photophysical properties of supramolecular mesogens assembled via halogen bonding is reported. Changing the degree of fluorination at the halogen-bond donor of the supramolecular liquid crystal allows for the fine-tuning of the halogen bond strength and thereby provides control over the temperature range of the mesophase. At least three fluorine atoms have to be present to ensure efficient polarization of the halogen-bond donor and the formation of a mesophase. In addition, it was found that stilbazole acceptors are superior to their azopyridine counterparts in promoting stable liquid crystalline phases. The halogen-bond-driven supramolecular liquid crystals between fluorinated azobenzenes and stilbazole/azopyridine acceptors show a rich variety of photoinduced processes driven by azobenzene photoisomerization, dictated not only by the photochemical properties of the molecular components but also by the difference between the operation temperature ...

Emerging assembly of ZnO-nanowires/graphene dispersed liquid crystal for switchable device modulation

Abstract The significant role of hydrogen-bonding (H ) interactions in the formation and/or stabilization of liquid crystalline phases have been recognized in recent years and excellent work has been conducted. Following the first and well-established examples of nanomaterials and graphene dispersed liquid crystal (NDLC/GDLC) matrix formation through the dimerization of aromatic carboxylic acids. ZnO-nanowires/graphene dispersed with liquid crystal composites based on H-bonding were explored using a model system. Several classes of hybrid compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behavior of which is crucially dependent on the structure of the resulting supramolecular systems. We present the nanosecond electro-optic response of liquid crystal (LC) in the nematic and isotropic phases. The results demonstrate that in the isotropic phase and both are dielectrically positive and negative liquid crystal belongs to a larger field induced birefringence than in the nematic phase. This also leads to the formation of supramolecular complexes that may exhibit thermotropic liquid crystalline character. Depending on the nature, number and position of the groups able to form hydrogen bonds (H-), a diversity of supramolecular structures, both dimeric and polymeric, have been obtained, affording in turn various liquid crystalline phases. Smart LC cells with electrodes made of graphene, attributes an excellent performance with a high contrast ratio. The feature article investigates the advantages of graphene compared to conventional use metal oxides in terms of low resistivity, high transparency, and chemical stability. Observed various textural phases due to nanostructures and stability of these HBLC hybrid complexes, may have benefits for the ultrafast novel electro-optic applications in smart switchable device modulation, optoelectronic and optical shutters those brand new features well established.

Liquid-crystalline fork-like dendrons

ABSTRACTDendritic molecules having several rigid-rod moieties can be applied to induce liquid crystallinity for a variety of non-mesomorphic functional molecules such as metal complexes, nanoparticles, fullerenes and π-conjugated molecules when these dendritic molecules are covalently bonded to those non-mesomorphic molecules. These complex molecules are called supermolecular liquid crystals. Due to the cooperation of several mesogenic moieties, these dendritic molecules exhibit very stable liquid-crystalline (LC) phases. We have used fork-shaped LC dendrons having two or three rigid-rod moieties to induce liquid crystallinity for functional molecules such as interlocked molecules and π-conjugated molecules. In these fork-like molecules, the rigid-rod cores are attached to the 3,4,5-position of the phenyl moieties through flexible spacer, and these molecules are bonded to functional molecules through the 1-position. They basically form smectic LC phases, which induce the layered arrangement of functional moieties. Here we report on a new family of fork-like mesogens containing a hydrogen bonding moiety or an ionic group. They are designed to build supramolecular materials.

Unveiling the influence of inner spacer length of the non-linear S-shaped chiral oligomers on liquid crystalline phase

A series of S-shaped liquid crystal oligomers consisting of biphenylene, catechol and cholesterol moieties have been synthesized and characterized. The members in this series differ by the length of inner spacers –(CH2)n- where n are varied from 4 to 9 whereas the outer spacer of undecanoate chain is remained intact. The texture and phase transition temperatures with associated enthalpy changes are recorded. Increase in the number of carbon atom at the inner spacer affects the stability of liquid crystalline phase. The homologues with shorter inner spacers n=4 and 5 exhibit monotropic N* whilst the analogue with medium spacer n=6 exhibits monotropic N* and SmC* phase. However, the oligomers with longer spacers n=7, 8 and 9 exhibit enantiotropic N* and SmC* phase. The I-N* transition temperatures were found to depend on the parity of the inner spacers. An X-ray diffraction study reveals that the S-shaped oligomers exist as a monolayer chiral smectic C phase.

Liquid crystalline bilayers self-assembled from rod-coil diblock copolymers.

The structure and phase behaviour of bilayer membranes self-assembled from rod-coil diblock copolymers are studied using the self-consistent field theory, focusing on the occurrence and relative stability of liquid crystalline phases induced by the geometric shape and orientational interaction of the rod-blocks. A variety of liquid crystalline bilayers, corresponding to the smectic phases in bulk systems, are predicted to occur as equilibrium phases of the system. The ordered morphologies and phase behaviour of the system are analyzed. Phase diagrams of the self-assembled bilayers are constructed. The theoretical results provide an understanding of the formation mechanisms of these intricate phases.

Relationship between liquid crystalline phase stability and ingredient composition in liquid crystal oil–water emulsion

ABSTRACTIn this study, we postulated that the optical retardation magnitude could be used to evaluate the stability of the LC phase through the Mueller matrix polarimeter method in LC emulsion. In addition, we found that the increase of cetearyl alcohol concentration induced the more periodic dense lattice structure in crystalline phase of LC lamellae and the sorbitan olivate was relatively more responsible for higher ordering in LC phase compared to C12–20 alkyl glucoside in terms of molecular parallel packing on the basis of the optical retardation magnitude. Furthermore, it was confirmed that the LC emulsion had higher skin barrier function owing to lower TEWL and higher reflectance than ordinary emulsion on human skin and maintained a uniform phase without phase separation for 8 weeks.

Chiroptical and Thermotropic Properties of Helical Styrenic Polymers: Effect of Achiral Group

Six novel chiral bulky styrenic monomers, (+)-2-[4′-((S)-2″-methylbutyloxy)phenyl]-5-phenylstyrene (A-1), (+)-2-[4′-((S)-2″-methylbutyloxy)phenyl]-5-(4′-fluorophenyl)styrene (A-2), (+)-2-[4′-((S)-2″-methylbutyloxy)phenyl]-5-(4′-tert-butylphenyl)styrene (A-3), (+)-2-phenyl-5-[4′-((S)-2″-methylbutyloxy)phenyl]styrene (B-1), (+)-2-(4′-fluorophenyl)-5-[4′-((S)-2″-methylbutyloxy)phenyl]styrene (B-2), and (+)-2-(4′-tert-butylphenyl)-5-[4′-((S)-2″-methylbutyloxy)phenyl]styrene (B-3), were synthesized and radically polymerized to yield the corresponding polymers, PA-1–PA-3 and PB-1–PB-3. All of them consisted of laterally attached p-terphenyl pendants terminated by an identical (+)-(S)-2-methylbutyloxy end and an achiral end with various size. The first three differed the others by the position of vinyl group relative to chiral motifs. Evidenced by the results of NMR, polarimetry, circular dichroism spectroscopy, computer simulation, thermal properties, and X-ray diffractions, the chiral p-alkoxyphenyl group ortho to the vinyl group induced the helical conformation of polymer backbone with an excess screw sense as in PA-1–PA-3, whereas that meta to the vinyl group failed to dictate the growth of polymer backbone. The achiral end of the side group had a great effect on the optical rotation of polymer. The specific optical rotation of PA-3 that bore tertiary butyl groups was over 3 times larger than PA-1 and PA-2 terminated with hydrogen and fluorine atoms. Accompanied by the existence of helical structure with a predominant screw sense, stable liquid crystalline phases were generated by PA-1–PA-3 at above glass transition temperatures but not by PB-1–PB-3. An unusual glass transition temperature and structure relationship was also revealed.

Variable temperature studies on mesogenic polythiophenes using 2D-IR and WAXS

We synthesized and investigated the molecular ordering of 4-{(E)-[(4-alkoxy phenyl imino] methyl} phenyl] thiophene-3-carboxylate (APTC) oligomers of varying alkoxy chain lengths using polarised Infrared spectroscopy (IR) and wide-angle X-ray scattering (WAXS). Infrared polarisation study shows maximum dichroic ratio for Phenyl CC stretching for Decyloxy (PDPTC) at room temperature. During heating C–O–C shows higher dichroic ratio in dodcecyloxy (PDDPTC) due to the restrains in the torsional angle and hindered rotation of it. By using temperature as perturbation, IR correlation (2D-IR) was performed. 2D-IR analysis results are classified into three parts. First, at phase temperature, CO splits into two peaks due to dipole–dipole interaction; second, chain bending in CH2 alkyl chain and variation in CH2 stretching with molecular environment and third, thiophene out of plane bending vibration shows main chain thiophene flipping with respect to alkoxy chain length. In addition to that, an intriguing evidence of liquid crystalline phase stability by CO dipole moment over alkyl chain is seen. 2D-IR analysis of thiophene region also shows “four-leaf clover pattern” on increasing alkoxy chain length and a change in phenyl group C–H in plane bending vibration along with it is evidenced. Hetero correlation between WAXS and IR shows interlayer packing variation with zigzag alignment for lower homologues and linear alignment for higher homologues with lower ‘d’ spacing value plotting with change in CH2 stretching vibration of alkoxy chain length.

SYNTHESIS AND PROPERTIES OF GELATORS DERIVED FROM TETRAPHENYLETHYLENE AND GALLIC ACID WITH AGGREGATION-INDUCED EMISSION

Two novel organogelators (TEG and TAG) based on tetraphenylethylene and 3,4,5-tris(dodecyloxy) benzoic acid were synthesized through ester bond and amido bond linkages, respectively. Compounds TEG and TAG were able to induce gelation in ethanol. Aggregation-induced enhanced emission was observed in these organogelator molecules, with increased fluorescence intensity from the solutions to the gels. The completely thermoreversible gelation occurred due to the aggregation of the organogelators. In the process, a fibrous network was formed by a combination of intermolecular hydrogen bonding, π–π stacking and van der Waals interactions. These phenomena were observed in the xerogels by field-emission scanning electron microscopy and Fourier-transform infrared spectroscopy. The results of differential scanning calorimetry and polarized optical microscopy indicated that compound TAG exhibited stable liquid crystalline phases over a wide temperature range. The linking groups have severe influence on the properties of the organogelators, which was mainly attributed to the hydrogen bonding interaction in compound TAG.

Insight into the liquid crystalline behavior of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene}: a typical thermotropic mesogen-jacketed liquid crystalline polymer

Based on the thermodynamic and kinetic analyses, the thermotropic hexagonal columnar liquid crystalline (LC) phase of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PBPCS) at high temperature is self-assembled by driving forces of the conformation entropy and macromolecular interaction, respectively. Because of the strong “coupling effect” between flexible backbone and rigid side chain, the molecular structure of PBPCS can be regarded as “dual chains” model that a flexible backbone and a rigid side chain are in parallel, the backbone and side chain are corresponded to different molecular thermal motion characteristics. When the orientation requirements of the side mesogenic chain are dominant, LC phase appears; when the random motions of flexible backbone are dominant, LC phase disappears. PBPCS demonstrates various types of metastable phases in the LC phase transition. Above LC phase transition temperature, molecular motion of rigid side chain is dominant; therefore, LC phase is formed. In the temperature range from glass transition to LC phase transition, molecular motion of flexible backbone is dominant; therefore, metastable LC phase may be relaxed partly and even disappear. The stability of LC phase can be determined by the kinetic factors, it is related to the residence time for LC phase formation and the interaction among the aggregated columnar macromolecular chains. Metastable LC phase disappeared in the subsequent cooling procedure, however, the stable LC phase was not.

Luminescent Columnar Liquid Crystals Based on Tristriazolotriazine

Five discotic molecules comprising a tris[1,2,4]triazolo[1,3,5] triazine core were designed and synthesized to obtain luminescent and charge-transporting columnar liquid crystalline materials. With the exception of one compound containing terminal hydroxyl groups all compounds presented a wide thermal range and stable columnar liquid crystalline phase, characterized by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction (XRD) techniques. The phase formation appeared to be associated to some extent with interdigitation of the alkoxy and benzylalkoxy portion, as suggested by the XRD results. All compounds have a strong blue luminescence in solution and solid phase. At the temperature at which the compounds enter in the mesophase the luminescence decreases significantly. This result suggests that entrance into the Col(h) phase is accompanied by a better π-stacking of the peripheral phenyl rings compared to the solid phase, consistent with the intramolecular distances (3.5 Å) observed in the XRD analysis. These compounds based on tristriazolotriazine are quite robust with good optical and thermal properties for application as solid state emitters, and we anticipate that they may provide an interesting alternative to other discotic molecules based on N-heterocycles, which generally present a high-temperature Col(h) phase.

Liquid Crystalline Order and Magnetocrystalline Anisotropy in Magnetically Doped Semiconducting ZnO Nanowires

Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.

Stability of liquid crystalline phases in the phase-field-crystal model

The phase-field-crystal model for liquid crystals is solved numerically in two spatial dimensions. This model is formulated with three position-dependent order parameters, namely the reduced translational density, the local nematic order parameter, and the mean local direction of the orientations. The equilibrium free-energy functional involves local powers of the order parameters up to fourth order, gradients of the order parameters up to fourth order, and different couplings between the order parameters. The stable phases of the equilibrium free-energy functional are calculated for various coupling parameters. Among the stable liquid crystalline states are the isotropic, nematic, columnar, smectic-A, and plastic crystalline phases. The plastic crystals can have triangular, square, and honeycomb lattices and exhibit orientational patterns with a complex topology involving a sublattice with topological defects. Phase diagrams were obtained by numerical minimization of the free-energy functional. Their main features are qualitatively in line with much simpler one-mode approximations for the order parameters.

Computer simulation of charged hard spherocylinders at low temperatures

In this work we report the stability of liquid crystalline phases of charged hard spherocylinders (CHSC) of aspect ratio L/σ=5 at low temperatures using NPT Monte Carlo computer simulations. Following the methodology used in previous work [C. Avendaño, A. Gil-Villegas, E. González-Tovar, J. Chem. Phys. 128, 044506 (2008); Chem. Phys. Lett. 470, 67 (2009)], long-range coulombic interactions are handled using the Wolf method. The supramolecular organization of CHSC is obtained by compression of a low-density isotropic state. The system under consideration exhibits the expected isotropic, nematic, smectic-A, and crystal phases. However two important phenomena emerge at low temperatures, namely the existence of an isotropic–nematic–smectic triple point, with the ending of the nematic phase for lower temperatures, and the apparent hexatic arrangement of the layers in the smectic phases. Assuming that the smectic-layers behave as quasi-bidimensional systems, lowering the temperatures is possible to observe the formation of hexatic phases, which are detected analysing the structure factor, order parameters and distribution functions. This hexatic ordering indicates that the CHSC phase diagram presents a smectic-B phase at low temperatures.