smectic-A Phase Research Articles

8OCB and 8CB Liquid Crystals Confined in Nanoporous Alumina: Effect of Confinement on the Structure and Dynamics.

The effect of oxygen substitution is studied in two homologous compounds of n-cyanobiphenyls with n = 8 in the bulk and under confinement within self-ordered nanoporous alumina (AAO). Oxygen substitution in 8OCB increases the dipole moment and stabilizes the crystalline, smectic, and nematic phases to higher temperatures relative to 8CB. Within their smectic- A (SmA) phase both 8CB and 8OCB behave as weak viscoelastic solids with low shear moduli reflecting the underlying supramolecular defect structure. Dielectric spectroscopy assisted by DFT calculations identified strong dipolar associations within the isotropic phases characterized by a Kirkwood-Fröhlich interaction parameter, g ∼ 0.36. Dielectric spectroscopy further identified a slow process (∼ kHz) of low dielectric strength. The proximity of this process to the rheology time scale suggests as common origin a cooperative relaxation of the defect structure. Confinement alters the phase diagram by stabilizing certain crystalline phases and by reducing the N-I transition temperature in agreement with surface tension effects. However, the N-I transition seems to retain its first order character. Surface treatment with n-decyltrichlorosilane results in destabilization of the SmA phase at the expense of the N phase. This is consistent with a picture of surface anchored LC molecules at the pore walls that stabilize the nematic phase.

Corresponding states in the smectic-A phase

ABSTRACTIn the last few years, experimental data of many different sources has been gathered to show that order parameter curves of essentially all nematic liquid crystals (NLCs) compounds can be reduced to a single and universal curve of the corresponding states; experimental data of an impressive number of different nematic compounds has been linearly rescaled in such a way that all them coalesced in a single curve. The surprising novelty observed on NLCs is that this universal behaviour is not restricted to the neighbourhoods of a critical region, as is usually expected from critical phenomena, but it covers the entire domain of the nematic phase, from the nematic isotropic temperature to the nematic crystalline temperature. The aim of this paper is to study the statistical mechanics fundamentals of this behaviour and show that it is a consequence of the universality of the angular part of the potential of the interaction between the particles composing the liquid crystal (LC). We will illustrate this behaviour presenting evidences that it can be found on the smectic-A phase.

Fast Diffusion of Long Guest Rods in a Lamellar Phase of Short Host Particles.

We investigate the dynamic behavior of long guest rodlike particles immersed in liquid crystalline phases formed by shorter host rods, tracking both guest and host particles by fluorescence microscopy. Counterintuitively, we evidence that long rods diffuse faster than short rods forming the one-dimensional ordered smectic-A phase. This results from the larger and noncommensurate size of the guest particles as compared to the wavelength of the energy landscape set by the lamellar stack of liquid slabs. The long guest particles are also shown to be still mobile in the crystalline smectic-B phase, as they generate their own voids in the adjacent layers.

Proton conductive polymers obtained by in-situ polymerization of a mesomorphic benzimidazole monomer in smectic A, nematic and isotropic phases

Side-chain polymers with different microstructures have been prepared by in-situ ultra-violet (UV) photopolymerization of a liquid crystal monomer bearing a terminal benzimidazole moiety. The molecular weights of PBIS, PBIN, and PBII obtained from the smectic A (SA), nematic and isotropic liquid phases were 1.53, 1.40 and 1.38×104gmol−1, respectively. In-situ UV polymerization of the monomer aligned by mechanical shearing in the SA phase resulted in a macroscopically layered polymer PBIalS. PBIalS exhibited higher glass transition temperature than PBIS, PBIN, and PBII. Electrochemical impedance spectroscopy (EIS) measurements suggested the anhydrous proton conductivity of PBIalS parallel to the aligned layers was more than one order of magnitude higher than those of PBIS, PBIN and PBII. At the same temperature, proton conductivity of PBIS was higher than those of PBIN and PBII. The results suggested that the architecture of the benzimidazole polymer, which could be tuned by the mesomorphases of the monomer, had a profound impact on the anhydrous proton conduction.

Synthesis and physico-chemical properties of 2-alkoxy- 6-[4-(2-perfluorohexyl)ethylthiophenyl]naphthalene

ABSTRACTThis paper describes the synthesis of and mesomorphic property and gelation ability for 2-Alkoxy- 6-[4-(2-perfluorohexyl)ethylthiophenyl]naphthalene (compounds 1-n). Compounds 1-n show exclusively a smectic A (SmA) phase, where the SmA–isotropic phase transition temperatures are 192°C for 1-1 and 171°C for 1-6. The SmA phase is characterized using a polarized microscope and DSC measurements. Interestingly, compounds 1-n form physical gels (thermo-reversible gels) in n-octane, ethanol, and aprotic polar solvents such as acetonitrile and γ-butyrolactone, where the sol-gel transition temperatures are 60°C for 1-1 and 80°C for 1-6. The propylene carbonate gel is characterized by scanning electron microscope (SEM) observation, showing nano-fibers assembled with compound 1-6.

Alignment and electro-optical properties of SmC* with direct transition to N* phases

ABSTRACTHighly first order silane end-group difluoroterphenyl liquid crystal was added to trifluoroterphenyl host with a nematic to smectic-C phase transition to increase its first order nature, achieve a higher cone angle and reduce its temperature dependence. Increasing concentration of the additive, caused a linear decrease and eventual disappearance of the nematic phase. A smectic-A phase was injected at 35% concentration, making the transition second order. Optical studies of the mixtures showed little dependence of the highest value of cone angle on concentration. The original host showed first order cone angle behavior and was used to fabricate a twisted FLC device.

Rheological studies on liquid-crystal colloids prepared by dispersing spherical microparticles with homeotropic surface anchoring

ABSTRACTWe report rheological studies on the liquid-crystal colloids prepared by dispersing silica microparticles with homeotropic surface anchoring in 8CB liquid crystal. In nematic colloids, a shear-thickening behaviour is observed in low shear rate region. The apparent yield stress of both the nematic and smectic-A (SmA) phases increases with increasing volume fraction of particles . The critical strain amplitude (, i.e., crossover of and ) in SmA colloids decreases significantly with increasing . The frequency-dependent storage modulus of SmA colloids show a power-law behaviour and the loss modulus exhibits a shallow minimum. The optical rheomicroscopy shows that the nematic colloids form a network structure which are stretched and broken at high shear rate. When the temperature is decreased to SmA phase, the networks collapse showing regions of high-density particles. The variation of storage modulus with indicates that the SmA colloids response is dominated by defects.

Thermotropic MIDA Boronates as a Case Study for the Role of Dipolar Interactions in Liquid Crystalline Self-Assembly.

A series of MIDA (N-methylimino diacetic acid) boronates carrying 4-alkoxy, 3,4-bisalkoxy, or 3,4,5-trisalkoxyphenyl substituents were synthesized and their mesomorphic properties characterized by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques such as small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). Most derivatives were liquid crystalline. In the case of mono- and bisalkoxy-substituted derivatives, C6 chains already induced smectic A (SmA) mesophases despite the bulky MIDA head group. With increasing chain length, columnar hexagonal (Colh ) phases replaced SmA phases in the disubstituted series. Quantum chemical calculations on a series of MIDA boronates show that the B-N bond is a dative bond with a positive charge on the boron atom and negative charges on the nitrogen and oxygen atoms. In addition, no π-interaction between the aryl moiety and B-N bond was found, thus the mesogenic unit is electronically decoupled from the MIDA head group. These theoretical findings were supported by IR and Raman spectra as well as by asingle crystal structure analysis of 4-ethoxyphenyl MIDA boronate. Calculations of the electrostatic potential of the MIDA boronate reveal a special polarity pattern that can support the formation of a two-dimensional network and is likely to explain the liquid crystalline self-assembly. The absence of any electronic cross-talk between the MIDA head group and B-aryl or B-alkyl substituents allows the efficient tailoring of the mesophase type through variation of the substituents.

New side chain cholesterol-functionalised aliphatic polycarbonate copolymer: synthesis and phase behaviour

ABSTRACTA new side cholesterol-functionalised liquid crystal (LC) copolymer based on aliphatic polycarbonate backbone was synthesised. The chemical structures of the block copolymers obtained in this study were characterised with Fourier Transform Infrared Spectroscopy (FT-IR) and Proton Nuclear Magnetic Resonance (1H NMR) spectra. Their thermal stability and phase behaviours were investigated with thermogravimetric analysis (TGA) measurements, differential scanning calorimetry, and polarising optical microscopy. The molecular organisation in the mesophase was studied by temperature-dependent X-ray diffraction (XRD). As a result, the block copolymer bearing side cholesteryl groups showed a glass transition at 15.8°C and a smectic A (SmA) to isotropic phase transition at 151.3°C on heating cycle. XRD indicated that the block LC copolymer showed an interdigitated molecular arrangement of the mesogenic units within the smectic layers. This partial bilayer structure was similar to the SmA phase formed by polar mesogens.

Multiple bifurcations with signal enhancement in nonlinear mid-infrared thermal lens spectroscopy.

We report a novel nonlinear mid-infrared vibrational spectroscopy regime where multiple bifurcations and signal enhancement are observed in the photothermal spectrum of a 6 μm-thick layer of 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal. For increasing pump power values, the nonlinear evolution of the photothermal spectrum is studied in 8CB samples initially in the crystalline and smectic-A phase and their non-equilibrium transitions are characterized with pump-probe thermal lens spectroscopy. The nonlinear photothermal phenomena can be explained by the nucleation of localized non-equilibrium transitions that leads to the formation of bubbles, which modify the thermal lensing behavior. Analysis of the multiple bifurcations reveals a universal critical exponent for these non-equilibrium dynamics that can be linked to mean field theory. We report for the first time simultaneous measurement of the photothermal signal amplitude and phase behavior in the nonlinear regime. Due to the signal enhancement and spectral narrowing observed, nonlinear photothermal behavior shows promise for improvement in sensitivity and signal contrast in mid-infrared, attractive for sample characterization in the mid-infrared.

アゾベンゼン側鎖を有するポリマーブラシ膜の液晶相転移挙動

High density liquid crystalline (LC) polymer brushes with side chain LC azobenzene were synthesized via surface-initiated atom transfer radical polymerization. LC phase transition behavior was investigated by in situ UV-visible absorption spectroscopy and grazing incidence small angle X-ray scattering (GI-SAXS). The free LC polymer chains in a spincast film exhibited three distinct thermal transitions, glass phase-smectic C (SmC)-smectic A (SmA)-isotropic, which correspond to the phase change behavior in the bulk state. On the other hand, the brush film only exhibited the SmA phase in the LC temperature range and the SmA-SmC phase transition was inhibited. This fact indicates that the LC phase transition behavior in the LC polymer brush is different from that in the free LC polymer of the spincast film. The high density grafted polymer brush structure constrains the mobility of the main chains, which impedes the tilt of mesogenic side chains.

Investigation of liquid crystalline property of a new calamitic liquid crystalline system methyl 4-(4ʹ-(4ʹʹ-(decyloxy)benzyloxy) benzylideneamino)benzoate

ABSTRACTA new polar calamitic liquid crystal, methyl 4-(4ʹ-(4ʺ-(decyloxy)benzyloxy) benzylideneamino)benzoate (M3BA) containing ether and Schiff base groups as linker with terminal polar ester group, has been synthesised and mesomorphic properties are studied by differential scanning calorimetry, polarising optical microscopy, density functional theory, temperature-dependent X-ray diffraction and temperature-dependent micro- Raman study. The smectic A (SmA) mesophase with focal conic texture has been observed with wide mesomorphic range. Layer thickness in SmA phase is greater than the optimised length of the molecule, indicating partially bilayer SmA phase. Analysis of Raman marker bands of C–H in-plane bending of phenyl rings, C=C stretching of rings, C=N stretching and C=O stretching shows structural changes at molecular level at Cr → SmA phase transition and provides proper intermolecular interactions responsible for dimeric unit in partially bilayer SmA phase.

Influence of terminal substituent on non-linear S-shaped oligomers consisting of azobenzene moieties at the peripheral arm: Synthesis, characterisation and phase transition behaviour

ABSTRACTA series of symmetrical S-shaped mesogens based on 4,4′-bis-(6-bromohexyloxy)biphenyl as a central unit containing two 2-{6-[4-(4-substitutedphenylazo)phenoxy]hexyloxy}phenol as side-chain groups has been successfully synthesised. The terminal substituent was varied from halogen (X = F, Cl, Br and I) to non-halogen (X = C2H5 and OC2H5). The oligomers with C2H5 and OC2H5 substituents exhibit predominantly the monotropic nematic (N) phase. The OC2H5-containing derivatives possess long-range stability of N phase than its C2H5-containing analogue in which it has small range of N phase stability. As for halogen-containing analogues, oligomer with F exhibits monotropic N phase whilst oligomers with Cl and Br exhibit monotropic N and smectic A (SmA) phases. In addition, homologue with Br shows additional phase which is smectic B (SmB) phase upon further cooling. However, the oligomers in which F, Cl and Br were substituted by I exhibits purely monotropic SmA and SmB phases. X-ray diffraction measurements reveal that the smectic phase is inclined to the monolayer structure.

Smectic-like bâtonnets in nematic/twist-bend nematic biphasic samples

ABSTRACTWe have studied a mixture of the twist-bend nemogenic dimer CB7CB with rod-like nematic molecules, which exhibits nematic (N) and twist-bend nematic (NTB) phases and a very large biphasic coexistence range. At the N-NTB transition, we observe the nucleation of highly anisometric NTB droplets which are very similar to the classic smectic A (SmA) bâtonnets. These observations confirm the recently proposed close analogy between the NTB and SmA phases, on the basis of their identical macroscopic symmetry. As for their smectic analogues, the NTB bâtonnets are fluid in two dimensions; they easily merge when brought into contact and they are solid-like in that they did not flow along their optic axis. The observed fluidity and low viscosity show that the NTB phase is indeed a nematic phase, i.e. an anisotropic fluid, rather than a soft crystal or glassy state. Unlike their smectic analogues, the NTB bâtonnets have almost perfect symmetry of revolution and the axis of the NTB helix is uniformly aligned parallel to the long axis of the bâtonnet. The large aspect ratio of the bâtonnets, typically ≈ 10–30, indicates a very strong anisotropy of the N-NTB interfacial energy, W2/W0 ≈ 200–2000, and suggests that the anchoring energy differs from the usual Rapini–Papoular form.

Graphene and liquid crystal mediated interactions

ABSTRACTThe two-dimensional graphene-honeycomb structure can interact with the liquid crystal’s (LC) benzene rings through π–π electron stacking. This LC–graphene interaction gives rise to a number of interesting physical and optical phenomena in the LC. In this paper, we present a combination of a review and original research of the exploration of novel themes of LC ordering at the nanoscale graphene surface and its macroscopic effects on the LC’s nematic and smectic phases. We show that monolayer graphene films impose planar alignment on the LC, creating pseudo-nematic domains (PNDs) at the surface of graphene. In a graphene-nematic suspension, these PNDs enhance the orientational order parameter, exhibiting a giant enhancement in the dielectric anisotropy of the LC. These anisotropic domains interact with the external electric field, resulting in a non-zero dielectric anisotropy in the isotropic phase as well. We also show that graphene flakes in an LC reduce the free ion concentration in the nematic media by an ion-trapping process. The reduction of mobile ions in the LC is found to have subsequent impacts on the LC’s rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off. In a ferroelectric LC (smectic-C* phase), suspended graphene flakes enhance the spontaneous polarisation by improving the tilted smectic-C* ordering resulting from the π–π electron stacking. This effect accelerates the ferroelectric-switching phenomenon. Graphene can possess strain chirality due to a soft shear mode. This surface chirality of graphene can be transmitted into LC molecules exhibiting two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase, and a macroscopic helical twist of the LC director in the nematic phase. Finally, we show that a graphene-based LC cell can be fabricated without using any aligning layers and ITO electrodes. Graphene itself can be used as the electrodes as well as the aligning layers, obtaining an electro-optic effect of the LC inside the cell.

Synthesis, liquid crystal property and gelation ability of semi-fluorinated 4-alkoxyphenyl 4-alkoxybenzoates

ABSTRACTThis paper describes the synthesis and physico-chemical properties for 4-(4-perfluorohexylbutoxy)phenyl 4-alkoxybenzoate (compounds 1-n), and the results are compared with the corresponding hydrocarbon derivatives (compounds 2-n). Compounds 1-n show exclusively a smectic A (SmA) phase, where the SmA–isotropic phase transition temperatures are 142°C for 1-1 and 147°C for 1-6. The SmA phase is characterized using a polarized microscope and DSC measurements. Interestingly, compound 1-6 forms gels in n-octane, ethanol, and aprotic polar solvents such as acetonitrile and γ-butyrolactone, while compound 1-1 shows no-gelation ability. The n-octane gel is characterized by scanning electron microscope (SEM) observation, showing nano-fibers assembled with compound 1-6.

Thermotropic liquid–crystalline properties of 4,4′-dialkoxy-3,3′-diaminobiphenyl compounds and their precursors

ABSTRACTA series of 4,4ʹ-dialkoxy-3,3ʹ-diaminobiphenyl compounds were synthesised by three-step procedure that involves alkylation, nitration and reduction reactions. Their chemical structures were characterised by FTIR, 1H and 13C spectroscopy and elemental analysis. Their thermotropic liquid–crystalline (LC) properties were examined by a number of experimental techniques including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VT-XRD). The 4,4ʹ-dialkoxy-3,3ʹ-dinitrobipheyl compounds, precursors to the diamine compounds, were also examined for their thermotropic LC properties. POM studies of focal conic textures and VT-XRD of the 3,3ʹ-diaminobiphenyl derivatives having flexible alkyl chains (C6–C12) exhibited the smectic A (SmA) phase independent of the length of alkyl chains. Similarly, the 3,3ʹ-dinitrobiphenyl derivatives containing alkyl chains C7, and C9–C11 exhibit the SmA phase, those containing C8 formed the smectic C (SmC) phase and C12 formed both the SmA and smectic B (SmB) phases, respectively. The 3,3ʹ-diaminobiphenyl derivatives had excellent thermal stability in the temperature range of 237–329°C, while those of 3,3ʹ-dinitrobiphenyl derivatives were in the temperature range of 270–321°C. The 3,3ʹ-diaminobiphenyl derivatives emitted UV light both in chloroform and acetonitrile.

Twist-grain-boundary-A* phase stabilisation in confined geometry by the interfaces

ABSTRACTWe report on the appearance and stabilisation of twist-grain-boundary-A (TGB) phase in confined geometry samples of the liquid-crystalline compound CE8, by means of polarised optical microscopy. The chiral liquid crystal (LC) compound was chosen such as to undergo a cholesteric–smectic-A transition with temperature (type-I smectic), and to be close to the triple point between the cholesteric, TGB, and smectic-A phases. We used several types of anchoring conditions and various effective anchoring strengths. The results reveal that (i) the TGB phase exists as a metastable phase with strong fluctuations in the bulk, and (ii) it can be stabilised by the sample boundaries for adequate anchoring conditions. This means that we probably observed a type-I to type-II smectic transition induced by the substrate. Some dependence of the stabilisation on the surface-anchoring conditions and on the cell thickness is reported.

Landau theory for uniaxial nematic, biaxial nematic, uniaxial smectic-A, and biaxial smectic-A phases

ABSTRACTWe use the Landau theory of phase transitions to obtain the global phase diagram concerning the uniaxial nematic, biaxial nematic, uniaxial smectic-A and biaxial smectic-A phases. The transition between the biaxial nematic and biaxial smectic is continuous as well as the transition between the nematic phases and the transition between the smectic phases. The transition from uniaxial nematic and uniaxial smectic is continuous with a tricritical point. The tricritical point may be absent and the entire transition becomes continuous. The four phases meet at a tetracritical point.

Smectic-A and Hexatic-B Liquid Crystal Phases of Sanidic Alkyl-Substituted Dibenzo[fg,op]naphthacenes

Despite longstanding interest in liquid crystalline compounds with simple rod- or disc-like shapes (calamitics or discotics), very few examples of the analogous board-shaped, or "sanidic", liquid crystals exist. A new series of alkyl-substituted dibenzo[fg,op]naphthacenes have been prepared by planarization of o-phenylene precursors through dehydrohalogenation. Their photophysical properties have been studied in dichloromethane. Liquid crystal phase behavior was characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. All of the compounds exhibit monotropic liquid crystal phases on cooling from the isotropic phase. The compounds with shorter alkyl (pentyl and heptyl) chains exhibit the uniaxial smectic-A phase analogous to that of simple calamitic mesogens. The compounds with longer alkyl (nonyl, undecyl, and tridecyl) chains exhibit a new smectic liquid crystal phase featuring short-range positional order with an apparent rectangular lattice in the smectic layers, that is, an orthogonal biaxial hexatic-B. The molecular arrangement in this phase likely corresponds to a distorted herringbone packing of the board-shaped structures. Further, the compound with nonyl chains exhibits an underlying smectic-B phase. DFT calculations show that the cores of the mesogens are twisted into C2-symmetric saddle-shaped geometries because of steric interactions along their rims. The liquid crystal phases and their structures are discussed in the context of the compounds' board-like shapes and intercore interactions.