smectic-A Phase Research Articles

Monodomain alignment of the smectic-A liquid crystalline phase from the isotropic phase

The liquid crystal alignment method described here provides uniform orientation of otherwise difficult-to-align smectic-A liquid crystal materials lacking a nematic phase. The smectic-A phase is grown in the presence of a 10–20 K/mm temperature gradient from an air bubble located within a cell by a photolithographically defined channel in the cell substrates. We obtain uniform layer alignment in millimeter-wide smectic regions at growth rates below about 0.05 μm/s even though there is a tendency for spontaneous nucleation of focal-conic defects at higher growth rates once the width of the smectic-A region exceeds the critical value of about 20 μm.

Synthesis, structure and properties of new chiral liquid crystalline monomers and homopolysiloxanes containing menthyl groups

The synthesis is described of four new chiral liquid crystalline monomers (M2–M5 ) and their corresponding side‐chain homopolysiloxanes (P2–P5 ) containing menthyl groups. Chemical structures were characterised using FT‐IR or 1H NMR spectra, and specific optical rotations were evaluated with a polarimeter. The phase behaviour and mesomorphic properties of the new compounds were investigated by differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy, UV/visible/NIR spectrocopy and X‐ray diffraction. The monomers and homopolymers with more aryl segments showed noticeably lower specific optical rotation value. The monomers M2–M5 formed a cholesteric or blue phase when a flexible spacer was inserted between the rigid mesogenic core and the terminal menthyl groups by reducing the steric effect. M2–M5 revealed enantiotropic cholesteric phase. Moreover, M2 also exhibited a monotropic smectic A (SmA) phase, and M4 also exhibited a cubic blue phase on cooling. The selective reflection of light shifted to the long wavelength region with increasing rigidity of the mesogenic core for M2–M5 . P2–P5 exhibited SmA phases, and the mesogenic moieties were ordered in smectic orientation with their centres of gravity in planes. Melting or glass transition temperature and the clearing temperature increased, and the mesophase temperature range widened with increasing rigidity of the mesogenic core.

Structure-Property Correlations of Novel S-Shaped Liquid Crystal Oligomers

Transition properties of a homologous series of novel S-shaped oligomers on varying the spacer length, core structure and linking group were investigated. All the S-shaped oligomers showed a nematic (N) phase on cooling. Formation of the N phase is interpreted in terms of the length to diameter (L/D) ratio, indicating that intramolecular core-core interaction plays an important role in stabilizing N phases. A non-symmetric S-shaped oligomer possessing cyanobiphenyl and phenylpyrimidine moieties exhibited enantiotropic N and smectic A (SmA) phases. The high stability of the SmA phase results from intermolecular interactions between the unlike mesogenic moieties. We discuss physical properties of the S shaped system that has structure within structure.

Molecular packing and phase transitions of side-chain liquid crystalline polymethacrylates based on p-methoxyazobenzene

The phase structures and transition behaviors of a series of side-chain liquid crystalline (LC) polymethacrylates based on p-methoxyazobenzene (PMnAzs, n=6, 8, 10, 12) were studied using differential scanning calorimetry, one- and two-dimensional (1D and 2D) wide-angle X-ray diffraction, and Fourier transform infrared spectroscopic experiments. The LC phase transition of PMnAz follows the sequence of smectic A (SmA)↔nematic (N)↔isotropic (I). For PM10Az and PM12Az, the transition of SmA-to-N is not complete upon heating. In the low-temperature SmA phase, the polymers adopt a fully interdigitated side-chain packing with the smectic layer period almost identical to the side-chain length. For all the four samples, the first-order diffraction of the SmA structure only renders when the temperature approaches the transition of SmA ↔ N, with the intensity much lower than that of the second-order diffraction. The absence of the first-order diffraction at low temperatures is ascribed to the possible matching of the electron densities between the center portion of the side-chain sublayer and the main-chain sublayer of the SmA structure. Since only the mesogens from the same main-chain sublayer can stack parallel together, the distribution of the azobenzene domains may cause some sort of density undulation within the smectic layer. Among the samples, PM6Az presents the strongest undulation with some additional orders. We also examined the annealing effect on the H-aggregation of PMnAzs. It is found that isothermal annealing at a temperature slightly higher than the Tg of PM8Az and PM10Az can significantly enhance the UV absorption at 326nm, indicating a further development of H-aggregation. However, for PM6Az and PM12Az, the UV–vis spectrum of the annealed sample is nearly identical to that without annealing.

Induction of smectic behaviour in a carborane‐containing mesogen. Tail fluorination of a three‐ring nematogen and its miscibility with benzene analogues

Partial tail fluorination of a nematic three‐ring carborane mesogen, 1H, induced smectic A (SmA) and smectic C (SmC) phases, leaving a narrow range nematic phase in 1F. In contrast, fluorination of the benzene and bicyclo[2.2.2]octane analogues, 2H and 3H, eliminated their nematic behaviour, enhanced their respective SmC and SmA phases and induced SmA, smectic I (SmI) and smectic F (SmF) phases in 2F. A binary phase diagram of 1F with its non‐fluorinated analogue 1H shows nearly ideal miscibility. In the approximately equimolar mixture of 1F and 2H the SmC phase is expanded by 36 K, and in a mixture of 1F and 2F the SmA phase is stabilised by additional 37 K.

Synthesis and characterisation of novel rod–disc oligomers

Microwave‐promoted synthesis of novel rufigallol‐based liquid crystalline oligomers, containing six rod‐like cyanobiphenyl moieties connected to the rufigallol core via flexible alkyl spacers, is reported. The synthesis of the target compounds was challenging since classical reactions failed to produce these oligomers. Differential scanning calorimetry and polarising optical microscopy analysis revealed the existence of a nematic phase with a shorter spacer and a smectic A (SmA) phase with a longer spacer. The compound with medium alkyl spacer exhibits both nematic and SmA phases at higher temperature and a re‐entrant nematic phase at lower temperature.

Pressure-Induced Phase Transitions on a Liquid Crystalline Europium(III) Complex

The effect of pressure on the phase behavior of the liquid crystalline complex [Eu(bta)(3)L(2)] (bta is benzoyltrifluoroacetonate, and L is the Schiff base 2-hydroxy-N-octadecyl-4-tetradecyloxybenzaldimine) was studied by X-ray diffraction, Raman spectroscopy, and luminescence spectroscopy. The pressure was varied between ambient pressure and 8.0 GPa. [Eu(bta)(3)L(2)] exhibits a smectic A (SmA) phase at room temperature. The complex undergoes a transition from the SmA phase to a solid lamellar structure around 0.22 GPa and another transition from the solid lamellar phase to an amorphous state from 1.6 to 3.5 GPa. At low pressures, the smectic layer spacing increases, and the intermolecular distance decreases. Above 3.5 GPa, both the interlamellar and the intermolecular spacings hardly change, but the intensity of X-ray reflections exhibits a remarkable decrease and eventually vanishes. An interpretation of the changes in the molecular structure is given. It was found that less interdigitation of the alkyl chains situated in adjacent layers and/or a full extension of the alkyl chains occurred at low pressures and that the second phase transition was accompanied by a transfer of the hydrogen atom from the nitrogen atom of the imine group to the oxygen atom of the Schiff base ligand. The effect of applying pressure equals that of the lanthanide contraction on the phase behavior.

Dielectric anisotropy and relaxation studies of the homologous series of N -(4- n -alkyloxy-2-hydroxy-benzylidene)-4-carbethoxyaniline

Dielectric properties of the homologous series of newly synthesized nonchiral compounds N-(4-n-alkyloxy-2-hydroxy-benzylidene)-4-carbethoxyaniline, (n = 6, 8, 10, 12) having wide temperature range (∼60°C) smectic A (SmA) phase, have been studied by the impedance spectroscopy in the frequency range of 100 Hz to 1 MHz. Measurements have been carried out for two principal alignments (planar as well as homeotropic) of the SmA phase. Dielectric anisotropy (Δε′ = ϵ'∥ − ϵ'⊥) for all the members of the series has been found to be negative for the whole temperature range of SmA phase. Magnitude of the dielectric anisotropy (|Δε′|) has been found to decrease with the number of alkyl chains. Relaxation frequencies corresponding to the rotation of the individual molecules about their short axes, lie below 1 MHz and obey the Arrhenius law by which activation energies have been determined. However, the relaxation frequencies corresponding to the rotation of the molecules about their short axes apparently lie above 10 MHz.

Photoacoustic and Fluorescence Spectroscopy of Metallomesogens Containing Lanthanide Ions

Metallomesogens Ln(bta)3L2 (Ln3+: La3+, Eu3+, and Ho3+; bta: benzoyltrifluoroacetonate; L: Schiff base) were prepared. Photoacoustic (PA) spectroscopy was used to study physicochemical properties of the liquid crystalline metal complexes. In the region of ligand absorption, PA intensity increases for Eu(bta)3L2, La(bta)3L2, and Ho(bta)3L2, in that order. It is found that the PA intensity of the ligand bears a relation to the intramolecular energy transfer process. For the first time, phase transitions of Eu(bta)3L2 from glass state to smectic A (SmA) phase and SmA phase to isotropic liquid are monitored by PA and fluorescence (FL) spectroscopy from two aspects: nonradiative and radiative transitions. The results show that PA technique may serve as a new tool for investigating the spectral properties and phase transitions of liquid crystals containing metal ions.

Poly(ether imide)s from Diamines with Para-, Meta-, and Ortho-Arylene Substitutions: Synthesis, Characterization, and Liquid Crystalline Properties

We have synthesized homologous series of para-, meta-, and ortho-substituted aryl ether diamine monomers, with either 2, 3, or 4 ether linkages per monomer unit, and prepared their corresponding poly(ether imide)s with 3,3′,4,4′-biphenyl dianhydride (BPDA) and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA). All polymers were obtained in high molecular weights and gave good quality films with expected mechanical and thermal properties. The ortho- and meta-substituted diamines gave fully amorphous polymers, whereas the para-based diamines resulted in semicrystalline polymers. The glass-transition temperatures (Tg) drop in the order of para > ortho > meta, and the Tg drops considerably as a function of the aryl ether content and appears to level off at four ether linkages. The Tg values of our polymers were contrasted with a simple quantitative model and found to be in good agreement with the experimental results (±10 °C). BPDA in combination with an all para-substituted, aryl ether-based diamine (BPDA-P3) forms a thermotropic liquid crystalline phase. Optical microscopy experiments confirm the presence of a nematic melt. Highly aligned films could easily be obtained by stretching the films in the liquid crystal phase, and XRD analysis of quenched films confirmed the presence of a highly aligned smectic A phase (SmA) with an order parameter ⟨P2⟩ = 0.87, indicating a high degree of molecular alignment. To the best of our knowledge, this is the first example of an all-aromatic liquid crystalline poly(ether imide). Dynamic mechanical thermal analysis (DMTA) showed that the para-aryl ethers display broad β-transitions (25–160 °C), whereas the meta- and ortho-series do not show β-transitions. All PEIs with 3 or 4 aryl ether linkages become thermoplastic in nature. The purpose of this systematic study is to provided a basic set of design rules toward the design and synthesis of all-aromatic poly(ether imide) architectures.

Understanding self-assembly of rod-coil copolymer in nanoslits

Rod-coil diblock copolymers are a special kind of molecule containing a rigid rod and a flexible part. We present a systematic study on self-assembly of the rod-coil copolymers in nanoslits using a hybrid density functional theory. The self-assembly of the rod-coil molecule is driven by the bulk concentration, and there exists a critical bulk concentration beyond which the rod-coil molecule self-assembled into ordered lamellar structures in the slit, otherwise it is in a disordered state. By monitoring the effect of the interaction (epsilon(TT)(*)) of molecular tail on the self-assembly, we found that in the nanoslit of H=13sigma, it is at epsilon(TT)(*)=8 rather than epsilon(TT)(*)=10 or epsilon(TT)(*)=12 that the minimal critical bulk concentration occurs. It may be because the strong tail-tail interaction leads to aggregation of the copolymer molecules in bulk phase, and the resulting supramolecular structures are fairly difficult to enter the slit due to the depletion effect. At a fixed slit, the structural evolution of the self-assembled film with the bulk concentration is observed, including trilayer and five-layer lamellar structures, smectic-A, smectic-C, and a mixture of smectic-A and smectic-C liquid crystal phases and so on. We found that the critical bulk concentration, corresponding to the disordered-ordered phase transition, greatly depends on the separation between two walls, and it changes periodically with the increase of the slit width. In addition, it is also found that the molecular flexibility is one of key factors determining the self-assembled structure in the slit, and the critical bulk density increases with the molecular flexibility.

An investigation of soft-core potentials for the simulation of mesogenic molecules and molecules composed of rigid and flexible segments

The phase behaviour of three soft core spherocylinder models is investigated with a view to producing an effective potential for use in coarse-grained simulations of liquid crystal phases and polymers composed of rigid and flexible segments. Provided potentials are not made too soft, two of the soft core models are found to work well in terms of successfully reproducing mesophases and in providing considerable improvements in computational speed over other commonly used coarse-grained models. In Monte Carlo simulations a soft-core spherocylinder model in which a cut and shifted Lennard–Jones potential is truncated with a linear tangential potential is found to be particularly effective; while for molecular dynamics a better model is provided by a DPD-like quadratic potential. Here, computational speed-ups of 20 – 30 × are seen in equilibration times in comparison to the well-known soft repulsive spherocylinder (SRS) model. The quadratic potential is used in an additional set of coarse-grained simulations of a liquid crystal with a flexible chain, which exhibits spontaneous formation of a nematic phase. The use of different types of interaction sites is also illustrated by the simulation of a spherocylinder with two “tails” formed from spheres. Here, varying the hardness of the sphere-spherocylinder interaction potential allows the formation of a smectic-A phase which exhibits microphase separation.

Liquid‐Crystalline Ternary Rare‐Earth Complexes

AbstractThe thermal behavior of the metallomesogens [Ln(C12C16dbm)3(phen)] (Ln = La, Nd, Eu, Gd, Tb, Dy, Er, Yb; C12C16dbm = 1‐(4‐dodecyloxyphenyl)‐3‐(4‐hexadecyloxyphenyl)propane‐1,3‐dionate; phen = 1,10‐phenanthroline) was investigated. The complexes of the light lanthanides (Ln = La, Nd) are not mesomorphic, whereas the complexes of the heavy lanthanides (Ln = Eu, Gd, Tb, Dy, Er, Yb) exhibit a monotropic smectic A phase (SmA). A model for themolecular organization of the molecules in the smectic A phase is proposed. The luminescence properties of the complexes [Eu(C12C16dbm)3(phen)], [Er(C12C16dbm)3(phen)], and [Yb(C12C16dbm)3(phen)] were investigated. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Field-induced realignment of a smectic nanodroplet in an external magnetic field: A numerical investigation

The field-induced realignment of a smectic-A phase is in principle a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved X-ray scattering experiments have revealed major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process. In order to obtain a deeper insight into this process, we have carried out a dissipative particle dynamics study of the realignment kinetics of a nanodroplet of a smectic-A liquid crystal suspended in an isotropic fluid following a switch in the direction of an applied magnetic field. The strength of the mesogen-field interaction is small compared to the inter-molecular interactions. The reaction of the smectic configuration to the field switch was found to depend on the balance between the inter-molecular interactions stabilising the formation of the smectic layering and the interaction of the mesogens with the external field. It is found that the rotational behaviour of the smectic layers under the influence of an external magnetic field arises from a combination of stochastic translational displacements and rotational motions of the centres of mass of the mesogens in the nanodroplets. The simulations indicate that X-ray scattering and NMR experiments monitoring the orientational order are sensitive to different aspects of the realignment process.

Structure and Dynamics of Isotropic Order

“Isotropic order”. Soft condensed matter system can form the huge length scale order. SmBPIso is characterized by the simultaneous presence of the local order parameter of an helix and of a smectic layer, while being spontaneously isotropic without any characteristic discontinuity on a mesoscopic length scale. It is great advantage for the optical devices that the spectrum of the iridescent color of SmBPIso is completely equivalent by the changing the viewing angle because of the special isotropic symmetry and its wavelength can be successively controlled by changing temperature. Recently, we found the color can be tuned by shining the strong pulsed laser light. Then we can artificially design the special pattern of the “structural color” by scanning and on/off the laser light beam. Response time of the color shift is not so fast but not so slow (~200msec) in spite of the existence of the complicated inter-connected multi-lamellar structure. We also confirms experimentally that the existence of the collective fluctuation mode just around the sub second by the dynamic light scattering measurement. Relaxation time can be assigned as reorientation motion of the helical pitch, which must be correlated to the dynamics of the shrinkage/elongation of the inter-connected multi-lamellar structure through the coupling between two types of the liquid crystalline orders. Furthermore, we found that cubic SmBP reenter the SmBPIso at super cooling temperature (Tsc) in the twin poor region. Since Smectic A phase appear by applying the heating shock and is stable until Tsc where cubic SmBP reappear. So we can conclude that the re-entrant SmBPIso is super-cooled state. We also found the power law slow dynamics in the director orientation fluctuation as similar to the conven smectic-A phase, which is hidden by the super cooling of SmBP tional liquid-glass transition. However, it should be noted that

Reinvestigation of Carrier Transport Properties in Liquid Crystalline 2-Phenylbenzothiazole Derivatives

We have reinvestigated the charge carrier transport properties in a liquid crystal of 2-(4'-heptyloxyphenyl)-6-dodecylthiobenzothiazole (7O-PBT-S12), for which the electronic conduction was first established in rodlike liquid crystals and for which the highest hole mobility in the smectic A (SmA) phase ever achieved was reported. We found that 7O-PBT-S12 exhibited three crystal phases, one of which appeared in a limited temperature range of 10 degrees just below the phase transition temperature from the SmA phase. In this crystal phase, nondispersive transient photohole currents were observed in time-of-flight experiments, and its hole mobility was determined to be 8 x 10(-3) cm(2)/Vs, slightly higher than that reported previously in the SmA phase. For the SmA phase, however, the hole mobility was 1 x 10(-4) cm(2)/Vs. Furthermore, we established the electron transport in the SmA phase of purified 7O-PBT-S12, whose mobility was the same as the hole mobility in that phase. In order to confirm generality of the new findings in 7O-PBT-S12, we investigated the carrier transport properties of its derivative having a short hydrocarbon chain, 2-(4'-heptyloxyphenyl)-6-butylthiobenzothiazole (7O-PBT-S4), and obtained comparable results. The present results correct a mistake in the previous report and give an idea of what a typical mobility in the SmA phase is. On the basis of these results, we discuss what determines the charge carrier mobility in smectic mesophases.

Spacer length dependence of TGBC* and SmA phases in cholesteryl and dihydrocholesteryl‐containing trimer liquid crystals

Two series of trimer liquid crystals were investigated that contain a biphenylyl central group and two cholesteryl or dihydrocholesteryl terminal mesogenic groups. Only compounds with even spacers were investigated. The dihydrocholesteryl‐containing trimers show a triply intercalated smectic A (SmA) phase when the spacer lengths are greater than 8, whereas the cholesteryl‐containing trimers exhibit this triply intercalated SmA phase when the spacer lengths are more than 6. With shorter spacers, a twist grain boundary C* (TGBC*) phase was found. This is revealed by the formation of a typical dotted square grid pattern upon cooling from the chiral nematic (N*) phase in the planar texture. The dots are spaced by a distance of about 1.5–1.8 µm. Upon cooling from the N* phase in the focal conic texture a striped pattern is observed with the same spacing. X‐ray diffraction revealed a repeat distance for the TGBC* phase that corresponds with a monolayer ordering. The results show that the weaker interaction between the dihydrocholesteryl groups compared with cholesteryl groups or longer spacers destabilize the monolayer TGBC* phase.

Spatial Organization of GaSe Quantum Dots: Organic/Semiconductor Liquid Crystals

Mixed liquid crystals consisting of the smectic-A phase of 8CB and 8 nm GaSe nanoparticles have been formed and studied using static and time-resolved optical polarization spectroscopy. The mixture...

High-resolution x-ray scattering study of the effect of quenched random disorder on the nematic–smectic-Atransition

Using high-resolution x-ray scattering, the effect of quenched random disorder (QRD) on the second-order nematic-smectic-A (N-SmA) phase transition in butyloxybenzilidene-octylaniline (4O.8) has been studied. 4O.8 is a nonpolar liquid crystal (LC) with a monomeric smectic-A phase. The QRD is created by aerosil nanoparticles which gelate to form a three-dimensional network, confining the LC. The QRD caused by the aerosil gel generates quenched random fields acting on both the nematic and smectic-A order parameters. This results in the destruction of the quasi-long-range order of the smectic-A phase. The x-ray scattering data are modeled with a structure factor composed of two terms, one thermal and one static, corresponding to the connected and disconnected susceptibilities, respectively. Unlike previous studies, the two parts of the structure factor are decoupled by allowing different thermal and static correlation lengths. Our fitting procedure involves temperature-dependent and temperature-independent (global) variables. The amplitude and the parallel correlation length for the thermal part of the line-shape show critical-like behavior both above and below the transition temperature. Detailed analysis reveals that the thermal correlation length does not truly diverge at the phase transition. This effect is discussed on the basis of a cutoff for the divergence caused by the random fields generated by the aerosil network confining the liquid crystal. The intensity of the static term in the line-shape behaves like the order parameter squared at a conventional second-order phase transition. The effective order parameter critical exponent shows an evolution with increasing aerosil gel density ranging from the Gaussian tricritical value to the 3D- XY value. The results of a pseudocritical scaling analysis are compared to an analysis of 4O.8+aerosil heat capacity data and discussed using a phenomenological correlation between the nematic range of pure liquid crystals and the aerosil mass density, rho{s}.

Computer simulations of wetting of solid surfaces by liquid crystals

Atomistic computer simulations are presented, consisting of droplets of model liquid crystal molecules wetting a generic crystalline surface. It is shown that the type of wetting that occurs is highly dependent on the value of the surface interaction parameter epsilon{fs} , switching between partial wetting (no spreading) for epsilon{fs}=2.0 to complete wetting for epsilon{fs}=2.5 . epsilon{fs} is the multiplicative factor which scales the well depth of the fluid-surface interaction energy. During complete wetting, the spreading occurs through the growth of a precursor layer, and a set of secondary terraces. The temperature affects the structure of the droplet, as might be expected when different phases are sampled, and also the shape of the spreading droplet, and the rate of spreading. In particular, in the smectic-A phase at 350K, for values of epsilon{fs} just large enough to induce complete wetting, the precursor film assumes a diamond shape, with edges normal to the [110] directions in the crystal surface. It is shown that spreading occurs through diffusion across the surface, with the radius of the precursor layer increasing with the square root of time. Mass flow studies indicate that the spreading occurs by molecules cascading over the top of the droplet to feed the growing precursor layer. Calculations of contact angle relaxation are in qualitative agreement with experimental findings.