Smectic Liquid Crystalline Phase Research Articles

NMR Spectroscopic Studies of Cation Dynamics in Symmetrically-Substituted Imidazolium-Based Ionic Liquid Crystals.

Ionic liquid crystals (ILCs) present a new class of non-molecular soft materials with a unique combination of high ionic conductivity and anisotropy of physicochemical properties. Symmetrically-substituted long-chain imidazolium-based mesogenic ionic liquids exhibiting a smectic liquid crystalline phase were investigated by solid state NMR spectroscopy and computational methods. The aim of the study was to reveal the correlation between cation size and structure, local dynamics, and orientational order in the layered mesophase. The obtained experimental data are consistent with the model of a rod-shaped cation with the two chains aligned in opposite directions outward from the imidazolium core. The alignment of the core plane to the phase director and the restricted conformations of the chain segments were determined and compared to those in single-chain counterparts. The orientational order parameter S~0.5–0.6 of double-chain ionic liquid crystals is higher than that of corresponding single-chain analogues. This is compatible with the enhanced contribution of van der Waals forces to the stabilization of smectic layers. Increased orientational order for the material with Br− counterions, which exhibit a smaller ionic radius and higher ability to form hydrogen bonds as compared to that of BF4−, also indicated a non-negligible influence of electrostatic and hydrogen bonding interactions. The enhanced rod-shape character and higher orientational order of symmetrically-substituted ILCs can offer additional opportunities in the design of self-assembling non-molecular materials.

Y-shaped tricatenar azobenzenes – functional liquid crystals with synclinic–anticlinic transitions and spontaneous helix formation

The photoisomerizable functional azobenzene unit is organized in synclinic hexatic, anticlinic smectic and bicontinuous cubic liquid crystalline phases as well as in achiral or mirror symmetry broken isotropic network liquids.

Liquid-crystalline phase transitions in lipid droplets are related to cellular states and specific organelle association

Lipid droplets (LDs) are ubiquitous organelles comprising a central hub for cellular lipid metabolism and trafficking. This role is tightly associated with their interactions with several cellular organelles. Here, we provide a systematic and quantitative structural description of LDs in their native state in HeLa cells enabled by cellular cryoelectron microscopy. LDs consist of a hydrophobic neutral lipid mixture of triacylglycerols (TAG) and cholesteryl esters (CE), surrounded by a single monolayer of phospholipids. We show that under normal culture conditions, LDs are amorphous and that they transition into a smectic liquid-crystalline phase surrounding an amorphous core at physiological temperature under certain cell-cycle stages or metabolic scenarios. Following determination of the crystal lattice spacing of 3.5 nm and of a phase transition temperature below 43 °C, we attributed the liquid-crystalline phase to CE. We suggest that under mitotic arrest and starvation, relative CE levels increase, presumably due to the consumption of TAG metabolites for membrane synthesis and mitochondrial respiration, respectively, supported by direct visualization of LD-mitochondrial membrane contact sites. We hypothesize that the structural phase transition may have a major impact on the accessibility of lipids in LDs to enzymes or lipid transporters. These may become restricted in the smectic phase, affecting the exchange rate of lipids with surrounding membranes and lead to a different surface occupancy of LD-associated proteins. Therefore, the composition and the resulting internal structure of LDs is expected to play a key role in their function as hubs of cellular lipid flux.

Synthesis and improved mechanical properties of twin‐mesogenic epoxy thermosets using siloxane spacers with different lengths

ABSTRACTA series of twin‐mesogenic epoxy resins with siloxane (di‐, tri‐, and tetra‐siloxane) chain spacers of different lengths were successfully synthesized. Results showed that the melting point of the twin‐mesogenic epoxy monomers decrease by more than 50 °C due to the change of siloxane spacer length. In particular, the tri‐ and tetra‐incorporated siloxane twin‐mesogenic epoxy monomers showed a liquid crystalline (LC) phase below 100 °C during the cooling process. Epoxy thermosets with isotropic and smectic (Sm) LC phases were then prepared using an aromatic amine and the dependence on curing temperature was investigated. Furthermore, the relationship between the ordered network chain structure and mechanical properties was investigated. Results were compared with isotropic and Sm LC phases. The tensile strain and fracture energy in the Sm type LC thermosets increased without a decrease in the tensile modulus, and this occurred in all systems irrespective of siloxane spacer length. The maximum value of the fracture energy (283 kJ m−2) reached by LC thermosets with increased spacer lengths was twice as large as that using the alkyl chain spacer (144 kJ m−2). © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47891.

Low-frequency dielectric spectroscopy and distinct relaxation modes in smectic phases of liquid crystal dimers

Low-Frequency (LF~20 Hz–10 MHz) ac field response and relaxation modes are investigated in layered liquid crystalline (LC) smectic phases exhibited by Hydrogen bonded dimers, viz., nOnBA:MA:nOBA’s with sort spacer moiety and long flexible end chains for n = 10,11 and 12. Permittivity (ε′) and loss (ε″) determined from capacitance and loss factor. Phase transition temperatures (Tc) are determined. Intra-molecular aromatic interaction and storage capacity are evinced by hysteresis behavior. Data of loss ε″(ω) and permittivity ε′r(ω) recorded in smectic phases infers non-Debye type of asymmetric dispersion. Multiple loss peaks in ε″(ω) is argued to involve distinct time scaled relaxations. Relaxation analyzed by Cole-Davidson method. Re-orientation of longitudinal dipole moment μ to the field analyzed. μ is resolved by rigid core, intra-molecular repulsive and flexible end-chain components. Observed distinct time scale response confirms dipolar components. Trends of relaxation frequency, activation energy, loss maxima, dielectric strength and distribution parameter are analyzed in device perspective. Relaxation and order director fluctuations (ODF) analyzed. ODF suggest the device utility in high temperature smectic phases far away from critical fluctuations.

Shear alignment and 2D charge transport of tilted smectic liquid crystalline phases – XRD and FET studies

In the tilted smectic phases of terthiophenes, shear alignment macroscopically aligns the tilt direction of the director leading to a substantial in-plane anisotropy of charge carrier mobility in organic field effect transistor (OFET) experiments.

Light-Triggered Reversible Self-Engulfing of Janus Nanoparticles.

Block copolymers containing azobenzene liquid crystalline (LC) mesogen are used to prepare snowman-like Janus nanoparticles (NPs) by emulsion solvent evaporation. The azobenzene-containing poly(methacrylate) (PMAAz) head of the Janus NPs is in the smectic LC phase with ordered stripes, which becomes amorphous and enlarged due to trans/cis transformation under UV irradiation. The expanded PMAAz can consequently engulf the other head. The self-engulfed NPs can recover to their original state in both shape and LC state via visible-light irradiation. This strategy is promising for programmable load and release of different payloads by remote trigger using light.

Enthalpy of formation and disordering temperature of transient monotropic liquid crystals of poly(butylene 2,6-naphthalate)

Melt-crystallization of poly(butylene 2,6-naphthalate) (PBN) at temperatures lower than about 160 °C follows Ostwald's rule of stages via intermediate formation of a smectic liquid crystalline phase (LC-phase). The transient LC-phase has been isolated by interruption of the isothermal crystallization process at 140 °C at sub-second timescale, and then its disordering was analyzed on heating at a rate of 2000 K/s, which suppresses the transition into α-crystals. The disordering temperature of the LC-mesophase is slightly lower than 200 °C, and as such 20–30 K lower than the melting temperature of α-crystals formed from the LC-phase at 140 °C. Analysis of the bulk enthalpy of formation of the LC-phase revealed that it covers only 20–25% of the total bulk enthalpy of crystallization, which is considered further proof of its smectic nature.

Cold-crystallization of poly(butylene 2,6-naphthalate) following Ostwald’s rule of stages

Melt-crystallization of poly (butylene 2,6-naphthalate) (PBN) at temperatures lower than about 160 °C follows Ostwald’s rule of stages, leading first to formation of a transient smectic liquid crystalline phase (LC) which then may convert in a second step into crystals, controlled by kinetics. In the present work, the PBN melt was cooled at different rates in a fast scanning chip calorimeter to below the glass transition temperature, to obtain different structural states before analysis of the cold-crystallization behavior on heating. It was found that heating of fully amorphous PBN at 1000 K/s leads to a similar two-step crystallization process as on cooling the quiescent melt, with LC-formation occurring slightly above Tg and their transformation into crystals at their stability limit close to 200 °C. In-situ polarized-light optical microscopy provided information that the transition of the LC-phase into crystals on slow heating is not connected with a change of the micrometer-scale superstructure, as the recently found Schlieren texture remains unchanged.

Physio- and chemo-dual crosslinking toward thermoand photo-response of azobenzene-containing liquid crystalline polyester

Combining the stability of chemical crosslinking and the processability of physical crosslinking is a well-established strategy to design new materials with desirable stimuli–responsive properties. Herein, a series of azobenzene-based thermotropic liquid crystalline polyesters were synthesized by introducing mesogenic dial named 4,4ʹ-bis(6-hydroxyhexyloxy)azobenzene (BHHAB), 2-phenylsuccinic acid (PSA), and different contents of 1,2,3-propanetricarboxylic acid (PTA) as the chemical crosslinker. All these polyesters showed good thermal stability and smectic liquid crystalline phase. Wide-angel X-ray diffraction (WAXD) and the fluorescence emission spectra confirmed the existence of π–π stacking interactions as the physical crosslinking in the polymer chains, particularly at the lower content of PTA. However, when the PTA content increased, the chemical crosslinking changed the chain conformation, and thus the intensity of physical crosslinking slackened gradually. Combining the physical and chemical crosslinking, these polyesters showed the thermoplastic processability, thermal shape memory, heat-assisted healing and photoresponsive behaviors. Taking advantages of these features, these multiple stimuli–responsive polymers can bring more chances for smart materials such as soft actuator.

Mesomorphic phase transitions of 3F7HPhF studied by complementary methods

ABSTRACTPhysical properties and the phase sequence of (S)-4′-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2-fluorobenzoate exhibiting the liquid crystalline paraelectric smectic A*, ferroelectric smectic C* and antiferroelectric smectic CA* phases were studied by complementary methods in the temperature range from −125 to 120 °C. Differential scanning calorimetry measurements together with polarizing optical microscopy provided the phase sequence, including the glass transition and a cold crystallization. X-ray diffraction was used to obtain the unit-cell parameters of the crystal phase, as well as the layer thickness and correlation length in the liquid crystalline smectic phases. The tilt angle was found to reach 45°, as determined from the measurements of the layer thickness and molecular modeling. Relaxation processes in the smectic phases and the fragility parameter were studied using frequency-domain dielectric spectroscopy.

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.

Micellization and Antimicrobial Properties of Surface-Active Ionic Liquids Containing Cleavable Carbonate Linkages.

Imidazolium-based ionic liquids (ILs) containing cleavable carbonate linkages, 1-alkyloxycarbonyloxyethyl-3-methylimidazolium chlorides with alkyl chains of 10, 12, and 14 carbon atoms, were synthesized, and their self-assembly behavior and antimicrobial activity were investigated. Differential scanning calorimetry and polarized optical microscopy studies reveal that carbonate-functionalized ILs form stable thermotropic smectic liquid-crystalline phases over a wide range of temperature. The surface activity and aggregation behavior of these new ILs were investigated by tensiometry, conductometry, potentiometry, and spectrofluorimetry. The size of aggregates was examined by dynamic light scattering (DLS). Carbonate-functionalized ILs display a higher adsorption efficiency and a lower critical micelle concentration (cmc) than simple alkyl-chain-substituted ILs. The insertion of a carbonate ester moiety in the alkyl side chain favors adsorption at the air-water interface and micellization in the bulk solution when compared to nonfunctionalized ILs. DLS measurements show that small micellelike aggregates are spontaneously formed above the cmc. Furthermore, carbonate-functionalized ILs were examined for their antimicrobial activity against a panel of clinically relevant microorganisms. Biological activity was found to increase with hydrophobicity. The presence of a carbonate ester moiety significantly enhances the antimicrobial efficiency as compared to nonfunctionalized ILs, with the susceptibility of Staphylococcus sp. toward the action of these compounds being particularly remarkable. It has been demonstrated that the functionalization of the alkyl side chain of the imidazolium salts can not only modify the aggregation behavior but also lead to differences in both efficiency and the spectrum of antimicrobial activity of amphiphilic ILs.

A fast linear electro-optical effect in a non-chiral bent-core liquid crystal

We report on the observation of an electro-optical effect for a linear rotation of the optical axis as a function of the electric field in a tilted liquid crystalline smectic phase of an achiral bent-core compound, attributed to a spontaneous helical state in its SmCSPF phase.

Assembly of porous smectic structures formed from interlocking high-symmetry planar nanorings

Materials comprising porous structures, often in the form of interconnected concave cavities, are typically assembled from convex molecular building blocks. The use of nanoparticles with a characteristic nonconvex shape provides a promising strategy to create new porous materials, an approach that has been recently used with cagelike molecules to form remarkable liquids with "scrabbled" porous cavities. Nonconvex mesogenic building blocks can be engineered to form unique self-assembled open structures with tunable porosity and long-range order that is intermediate between that of isotropic liquids and of crystalline solids. Here we propose the design of highly open liquid-crystalline structures from rigid nanorings with ellipsoidal and polygonal geometry. By exploiting the entropic ordering characteristics of athermal colloidal particles, we demonstrate that high-symmetry nonconvex rings with large internal cavities interlock within a 2D layered structure leading to the formation of distinctive liquid-crystalline smectic phases. We show that these smectic phases possess uniquely high free volumes of up to ∼95%, a value significantly larger than the 50% that is typically achievable with smectic phases formed by more conventional convex rod- or disklike mesogenic particles.

Frustrated smectic liquid crystalline phases in lactic acid derivatives

ABSTRACTWe have prepared and studied a series of compounds with different types of molecular core and lactate unit in the chiral terminal chain. We draw a survey and comparison of their mesomorphic properties with respect to the occurrence of twist grain boundary (TGB) phases. The materials exhibit extremely wide TGBA phase more than 60K broad, unique TGBA-TGBC-SmC*-SmCA* phase sequence and unique re-entrant TGBA phase below the SmA phase. TGB phases have been induced in binary mixtures of molecules with different molecular shape and chirality (chiral lactic acid derivative and non-chiral hockey-stick mesogen). Unique effect is observed for compounds with TGBA phase, where the applied electric field transforms the planar texture into the homeotropic one, homogeneously dark in crossed polarizers. The process is analogy of the Frederiks transition so far known only for nematics. This effect, changing the bright state to the dark one, is promising for applications.

Fast linear electrooptic effect in non-chiral bent-core liquid crystal

abstractBent-core liquid crystal was studied by polarizing microscopy, and electro-optical measurements. This compound forms a sequence of smectic liquid crystalline phases (SmA, SmCSPR, SmCSPF) with growing tilt, polar coherence length and increasing layer coupling on decreasing temperature. The paper is focused on the first observation of a helix in achiral SmCSPF phase. The helix is parallel to the layer normal and the pitch is much shorter than the optical wavelength. This new phase shows fast (∼30 μs) grey-scale switching due to the deformation of the helix, providing potential use in devices with fast linear and thresholdless electro-optical response.

High hole mobility of benzo-2,1,3-thiadiazole polycrystals formed by utilizing self-assembly property of liquid crystalline mesophase

4,7-Di(p-alkoxyphenyl)-2,1,3-benzothiadiazoles having linear alkyl chains were prepared by the Suzuki–Miyaura coupling reaction of 4,7-dibromo-2,1,3-benzothiadiazole with the corresponding p-alkoxyphenylboronic acids. All compounds exhibited stable crystalline phase 1 (Cr1), quasi-stable polycrystalline phase 2 (Cr2), smectic liquid crystalline phase (A and C), and isotropic liquid. By conventional time-of-flight measurement using N2 laser, the hole mobility of benzothiadiazole derivatives was evaluated to be about 1 × 10−1 cm2 V−1 s−1 in the Cr1 phase, while the hole mobility of these in the smectic phases was on the order of 10−4–10−5 cm2 V−1 s−1. From a powder X-ray diffraction analysis, it was demonstrated that the interlayer spacing in the Cr1 phase was smaller than those in the smectic A and C phases. Microscopy observation of the polycrystalline films showed that the films in the Cr1 phase had less grain boundary structure. The high hole mobility of the benzothiadiazole derivatives in the Cr1 phase may be due to their small interlayer spacing and less grain boundary structure.

Studies on Anisotropic Molecular Orientation and Mesophase Stability in a Ternary Mixture of Liquid Crystalline Materials

ABSTRACTThe multicomponent system of cholesteryl linolenate (CL), sodium oleate (Naol), and orthophosphoric acid (H3PO4) exhibits liquid crystalline mesophases, like cholesteric and smectic phases, such as SmA, SmC, SmB, and SmG sequentially when the specimen is cooled from its isotropic phase. These phases have been observed by using microscopic technique. Helfrich potential and elastic moduli have also been estimated in the smectic phase using Helfrich model. Optical transmittance and electrical conductivity have also been discussed.

Surface and liquid–crystalline properties of FmHnFm triblock semifluorinated n-alkanes

A series of triblock semifluorinated n-alkanes of the general formula: F(CF2)m(CH2)n(CF2)mF, (in short FmHnFm), where m=10, 12, and n=6, 8, and 12 have been synthesized and employed for liquid crystalline studies and Langmuir monolayer characterization. Differential scanning calorimetry (DSC) measurements together with texture observation with polarizing microscope (POM) revealed the presence of liquid crystalline smectic phases for all the investigated homologs. The behavior of the studied molecules spread at the free water surface has also been investigated. Our results show for the first time that these unusual film-forming materials, which are completely hydrophobic in nature and do not possess any polar group in their structure, are surface active and form insoluble (Langmuir) monolayers at the air/water interface. Due to the fact that these molecules are chemically inert and, similar to the semifluorinated diblocks, are not toxic, they may be destined for biomedical uses as gas carriers and contrast agents, as well as in drug delivery systems.