Smectic A Research Articles

Highly Oriented Liquid Crystalline Epoxy Film: Robust High Thermal-Conductive Ability.

The molecular orientation effect of a liquid crystalline (LC) epoxy resin (LCER) on thermal conductivity was investigated, with the thermal conductivity depending on the surface free energy of amorphous soda–lime–silica glass substrate surfaces modified using physical surface treatments. The LC epoxy monomer was revealed to form a smectic A (SmA) phase with homeotropic alignments on the surfaces of substrates that possess high surface free energies of 71.3 and 72.7 mN m–1, but forming a planar alignment on the surface of a substrate that possesses a relatively low surface free energy of 46.3 mN m–1. The optical microscopy observations and the X-ray analyses revealed that the LC epoxy monomer also induced a homeotropically aligned SmA structure due to cross-linking with a curing agent on the high-free-energy surface. The orientational order parameter of the resulting homeotropic SmA structure was calculated from the grazing incidence small-angle X-ray scattering patterns to be 0.73–0.75. The thermal conductivity of the cross-linked LCER forming a homeotropically aligned SmA structure was also estimated to be 2.0 and 5.8 W m–1 K–1 for the average and maximum in the direction of the Sm layer normal. The value of the thermal conductivity was remarkable among the thermosetting polymers and ceramic glass, and the LCER could be applied for high-thermal-conductive adhesives and packaging materials in electrical and electronic devices.

Heterocyclic pyridine-based liquid crystals: synthesis and mesomorphic properties

ABSTRACTA series of new calamitic liquid crystals, 4-{[(pyridin-4-yl)methylidene]amino}phenyl 4-alkoxybenzoates comprising a heterocyclic (pyridine) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were synthesised and characterised. This series consists of nine members wherein the members differ by the length of alkoxy chain (CnH2n+1O–, where n = 2, 4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviours were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first three members of the series (n = 2, 4 and 6). As the alkoxy chain increased to n = 8 and n = 10, the nematic phase appeared together with an additional smectic A (SmA) phase. When moving from n = 12 until the highest members (n = 18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).

Synthesis and liquid crystal behavior of new side chain aliphatic polycarbonates based on cholesterol

In this study, we synthesized a series of new liquid crystal aliphatic block polycarbonates copolymers mPEG43-b-P(MCC-Cn)51 (n=1–4) via the ring-opening polymerization, hydrogenation reduction and coupling reaction, which contained side functionalized cholesteryl groups and were different in the number of the flexible methylene groups. The chemical structures of the chiral compounds and copolymers obtained in this study were characterized by FT-IR and 1H NMR spectra, and the average molecular weights of the copolymers were investigated by average molecular weights gel permeation chromatographic (GPC). The polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were used to characterize the liquid crystal behavior. The relationship of the structure and phase behavior of the chiral compounds and copolymers influenced by spacer lengths was discussed. As a result, the chiral compounds with two methylene only showed a smectic A (SmA) phase, while those with more methylene showed a SmA phase and cholesteric phase. As the number of the flexible methylene groups increased, the corresponding melting temperature, the transition temperatures of LC phases and the isotropic temperature of chiral compounds all showed a decreasing trend, and mesophase temperature range narrowed. The polycarbonate copolymer with two methylene did not show mesomorphism, while the copolymers with longer spacer length seemed beneficial for the formation of mesophases at room temperature and revealed a smectic A phase with an interdigitated molecular arrangement on heating and cooling cycles. The results showed a decreased tendency toward the glass temperature, and an increased tendency toward isotropic temperature for the LC copolymers with an increase of the spacer length.

Ionic liquid-crystalline network polymers formed by sulfonic acid-containing polysiloxanes and pyridinium compounds

Self-assembly of both ionic aggregates and mesogens in one polymer system represents the capability integrating intermolecular interactions and functions of materials. A series of ionic liquid-crystalline network polymers (ILCNs) containing cholesterol derivatives, siloxane backbone and ionic pyridinium sulfonic groups are synthesized, and the characterization, self-assembly behavior, and possible applications are discussed. The ILCNs are made from sulfonic acid-containing precursor polymers synthesized by use of cholest-5-en-3-ol-(3β)-4-(allyloxy)benzoate, 4-(allyloxy)benzenesulfonic acid and poly(methylhydro)siloxane. The ILCNs show a cubic mesophase and a smectic A (SmA) mesophase. The SmA layers develop saddle splay curvature and adopt the shape of infinite minimal surfaces, therefore self-assembly of bicontinuous cubic phases takes place due to ionic electrostatic attraction and asymmetry derived from the chiral cholesterol elements. Large ionic conductivity and remarkable thermal stability of the ILCNs provide promising opportunities for ion transport devices because they provides facilitated ion-transport pathways including smectic ordering and three-dimensional interconnected ionic channels.

Periodic structures on liquid-phase smectic A, nematic and isotropic free surfaces.

The free boundary of smectic A (SmA), nematic and isotropic liquid phases were studied using a polarized optical microscope, an interferometric surface structure analyzer (ISSA), an atomic force microscope (AFM) and a scanning near-field optical microscope (SNOM). Images of the SmA phase free surface obtained by the polarized microscope and ISSA are in good correlation and show a well-known focal domain structure. The new periodic stripe structure was observed by scanning near-field optical microscopy on the surface of the smectic A, nematic and isotropic phases. The properties of this periodic structure are similar to the charged liquid helium surface and can be explained by nonlinear electrostatic instabilities previously described.

The effect of molecular structure on the chiral random grain boundary phase

ABSTRACTA chiral random grain boundary (RGB) phase was recently observed in a hockey-stick biphenyl-substituted 1,3,4-oxadiazole derivative. In the RGB phase, achiral molecules self-assemble into randomly oriented blocks of chiral smectic layers and the motif of the unique microstructure is attributed to the similarity of hockey-stick molecules both to bent-core and to rod-like molecules. In order to explore the effect of molecular structure on the RGB phase in detail, we systematically change the molecular design. When the flexible tail decreases, the high-temperature Smectic A (SmA) phase is replaced by a nematic phase, showing a phase sequence of Iso-N-SmA-RGB-Cryst in 2-(4-dodecyloxyphenyl)-5-biphenyl-1,3,4-oxadiazole. However, when we replace the 1,3,4-oxadiazole group by the 1,3,4-thiadiazole group, the bending angle increases in the 1,3,4-thiadiazole derivatives and the RGB phase disappears. Or when the length of the arm becomes short in naphthalene-substituted 1,3,4-oxadiazole, these molecules exhibited only normal rod-like molecules’ mesomorphism. These results will provide deep insights on the relationship of molecular structure and mesophase structural property.

Theoretical studies of the influence of nanoparticle dopants on the ferroelectric properties of a ferroelectric liquid crystal

Recently, ferroelectric liquid crystals (FLCs) doped with nanoparticles have received extensive interest owing to their fascinating fundamental and technological perspectives. The purpose of this paper is to present our theoretical attempt to study the effects caused by ferroelectric nanoparticles on the properties of a ferroelectric liquid crystal. Our study is based on an extended thermodynamic model of the ferroelectric smectic C (SmC*) phase near the transition to the corresponding smectic A (SmA) phase. We have used the modified Landau free energy, developed for statistical mechanics of the spherical nanoparticles immersed in a FLC medium. The response time of the FLC dispersed ferroelectric nanoparticles is investigated under an applied electric field. The rotational viscosity is also calculated with the possible variation in temperature and the nanoparticle concentration. The obtained results, for the goldstone mode, show a faster response time for the nanodoped system as compared to the pure one. Moreover, a remarkable difference between the rotational viscosities of the pure and doped systems is observed. The variations of polarization and tilt angle are calculated as well. The obtained results for both pure and nanodoped FLC system, near the transition temperature, show the influence of dispersed ferroelectric nanoparticles on the FLC polarization and confirm that the tilt angle is not significantly affected by the presence of the particles. Also, slightly lower spontaneous polarization is revealed for a ferroelectric nanodoped system. Calculations based on this model show a good agreement with the recent experimental findings.

Effect of an electric field on the structural and optical properties of fluorinated freely suspended smectic films

Within the framework of the generalized mean-field model that takes into account the anisotropic interactions between the nearest neighbors of molecules forming freely suspended smectic films (FSSFs) and the stabilizing effects of the smectic-A (SmA)–air interface, a numerical study was performed of the structural, thermodynamic, and optical properties of these systems in the process of their layer-by-layer thinning. The results of calculating the disjoining pressure P, the average thickness of the smectic layers L, and the reflectivity index R of a FSSF formed by 5-n-alkyl-2-(4-n-(perfluoroalkyl-methylene oxide)-pentyl) (H10F5MOPP) molecules showed that these values undergo precipitous changes in the process of layer-bylayer thinning of the film. Calculations of R(T) as a function of temperature T exceeding the phase transition temperature of SmA into an isotropic state in the bulk of the liquid crystal material are in good agreement with the experimentally obtained data for the reflectivity of the FSSF formed by H10F5MOPP molecules.

New chiral liquid crystal materials based on menthol: Synthesis and phase behavior

ABSTRACTTo study structure-mesomorphism relationships of chiral materials based on menthol, a series of chiral compounds containing menthyl group were synthesized. Their chemical structures, formula, and the purity were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (1H NMR), and elemental analyses. The corresponding phase behavior was investigated with differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The effect of the mesogenic core, flexible spacer, and terminal groups on the mesophase formation and type was discussed. The compound 4-menthyloxyacetoxybenzoyloxy-4′- (6-bromohexyloxy)biphenyl showed the smectic A (SA) and cholesteric phases on heating cycle, and a cubic Blue Phase, chiral smectic C (SC*), SA and cholesteric phases on cooling cycle. However, the compound 4-menthyloxyacetoxybenzoyloxy-4′-(4-bromobutoxy)biphenyl showed enantiotropic SC*, SA and cholesteric phases, moreover, the platelet texture of a cubic Blue Phase was also observed on cooling cycle. The methacrylate monomer with six flexible methylene units and three phenyl rings as mesogenic core only showed cholesteric phase, while the methacrylate monomer with four methylene unit and three phenyl rings showed the SC* and cholesteric phases. However, when the terminal methacryloyloxy groups in the monomers were substituted by thiophenylacryloyloxy groups, the corresponding monomers all show no mesophase. With increasing the rigidity of mesogenic core or decreasing the flexible methylene spacer length, the corresponding melting temperature (Tm) or isotropic temperature (Ti) increased.

Ethyl 7-hydroxycoumarin-3-carboxylate derivatives: Synthesis, characterization and effect of liquid crystal properties

Two sets of new ethyl 7-hydroxycoumarin-3-carboxylate derivatives were synthesized and characterized to study the liquid crystalline properties. Chemical structures were confirmed by IR, NMR, CHN analysis techniques. Mesomarphic properties were accomplished by DSC, POM and X-ray studies. Density functional theory calculations and photophysical studies also performed. In the first set, smaller homologues of alkoxybenzoic acid derivatives exhibit monotropic smectic A (SmA) and higher homologous exhibit enantiotropic smectic A mesophase. The second set alkyl biphenyl derivatives exhibit stable SmA and nematic (N) mesophases. The well defined focal conic texture for SmA and threaded texture for nematic mesophases have been observed.

Smectic phases in ionic liquid crystals

Ionic liquid crystals (ILCs) are anisotropic mesogenic molecules which carry charges and therefore combine properties of liquid crystals, e.g. the formation of mesophases, and of ionic liquids, such as low melting temperatures and tiny triple-point pressures. Previous density functional calculations have revealed that the phase behavior of ILCs is strongly affected by their molecular properties, i.e. their aspect ratio, the loci of the charges, and their interaction strengths. Here, we report new findings concerning the phase behavior of ILCs as obtained by density functional theory and Monte Carlo simulations. The most important result is the occurrence of a novel, wide smectic-A phase , at low temperature, the layer spacing of which is larger than that of the ordinary high-temperature smectic-A phase . Unlike the ordinary smectic SA phase, the structure of the phase consists of alternating layers of particles oriented parallel to the layer normal and oriented perpendicular to it.

Thermal, optical and Photophysical behaviour of some Mesogenic Benzimidazole-based Schiff-bases

A series of luminescent benzimidazole-based Schiff-bases, 5-(alkyloxy)-2-(((2-(4-(alkyloxy)phenyl)-1H–benzimidazol-5-yl)imino)methyl)phenol, with different alkyl chains was synthesized and characterized by 1H &13C NMR, UV–Vis, fluorescence and mass spectrometry. All the compounds were found to be mesogenic (smectic A1), as studied by POM and DSC techniques, and their green emission was implied by fluorescence studies. Electrochemical study of one of the compounds, 3d, shows a band gap of 2.56eV with HOMO and LUMO energy levels at −5.22eV and −2.67eV, respectively.

Critical behavior in the vicinity of nematic to smectic A (N–SmA) phase transition of two polar–polar binary liquid crystalline systems

ABSTRACTPhase diagram, critical behavior and order of the nematic (N)–smectic A (SmA) phase transition of two polar–polar binary systems (i) 4-n-heptyloxy-4′-cyanobiphenyl (7OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB); (ii) 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-n-nonyloxy-4′-cyanobiphenyl (9OCB) by means of a high-resolution temperature scanning measurement of birefringence have been reported in this work. A simple power law analysis has been adopted to extract the specific heat critical exponent (α′) at N–SmA transition from birefringence data. The α′ for N–SmA transition indicates a uniform crossover behavior and has appeared to be non-universal in nature. With increasing concentration of the higher homologues for both the binary systems, the N–SmA transition reveals a strong tendency to be driven towards the tricritical nature. The 3D-XY limit (i.e. α′ = −0.007) for N–SmA transition reaches at the concentration x8OCB = 0.28 corresponding to the McMillan ratio 0.914, whereas the tricritical point has been found to appear near x9OCB = 1.0 corresponding to McMillan ratio 0.992.

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.

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.

Synthesis of Biocompatible Liquid Crystal Elastomer Foams as Cell Scaffolds for 3D Spatial Cell Cultures.

Here, we present a step-by-step preparation of a 3D, biodegradable, foam-like cell scaffold. These scaffolds were prepared by cross-linking star block co-polymers featuring cholesterol units as side-chain pendant groups, resulting in smectic-A (SmA) liquid crystal elastomers (LCEs). Foam-like scaffolds, prepared using metal templates, feature interconnected microchannels, making them suitable as 3D cell culture scaffolds. The combined properties of the regular structure of the metal foam and of the elastomer result in a 3D cell scaffold that promotes not only higher cell proliferation compared to conventional porous templated films, but also better management of mass transport (i.e., nutrients, gases, waste, etc.). The nature of the metal template allows for the easy manipulation of foam shapes (i.e., rolls or films) and for the preparation of scaffolds of different pore sizes for different cell studies while preserving the interconnected porous nature of the template. The etching process does not affect the chemistry of the elastomers, preserving their biocompatible and biodegradable nature. We show that these smectic LCEs, when grown for extensive time periods, enable the study of clinically relevant and complex tissue constructs while promoting the growth and proliferation of cells.

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.

Synthesis and spectral studies of some rare earth metal complexes of a heterocyclic mesogenic Schiff-base

ABSTRACTA heterocyclic liquid crystalline Schiff's base, N-(4′-(4″-hexyl-1H-1″,2″,3″-triazol))-4-dodecyloxysalicylaldimine (HL), and a series of lanthanide(III) complexes of the type [Ln(LH)3(NO3)3] (LnIII = La, Pr, Nd, Sm, Eu, Tb, and Dy) have been synthesized and characterized by elemental analyses, mass spectrometry, FTIR, NMR, and UV-Vis spectral techniques. All the complexes involve binding of HL (through phenolate oxygen in zwitter-ionic form) and bidentate chelation of nitrato groups. The ligand exhibits smectic-A (SmA) mesophase over a wide temperature range as evidenced by DSC and polarizing optical microscopic studies. Fluorescence studies show emissions of HL and TbIII complex.

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.