Low Molecular Weight Liquid Crystals Research Articles

A Two‐Stage Polymerization Strategy for Preparing Polymer‐Network Liquid Crystals with Oxygen‐Sensing Property

Polymer‐network liquid crystal (PNLC) possesses both advantages of low‐molecular‐weight liquid crystal (LMWLC) and liquid crystal (LC) polymer. Herein, a two‐stage polymerization strategy for the formation of unique PNLC with oxygen‐sensing properties is reported. The reaction mixture consists of 6% diacrylate RM257, 93.5% LMWLC 4‐cyano‐4′‐pentylbiphenyl (5CB), and 0.5% photoinitiator dimethoxy‐2‐phenylacetophenone (DMPA). In the first stage, the mixture is exposed to UV light for 2 min to form a primary polymer network, which is highly uniform with a planar orientation. However, some free radicals are trapped inside the LC due to the short UV exposure time. Subsequently, the trapped free radicals are released by heating the PNLC sample into an isotropic state. Under this condition, the free radicals can move freely and react with surrounding monomers to form a secondary polymer network, which is highly disordered and scatters light strongly. Because oxygen can deactivate free radicals trapped inside the PNLC, the phenomenon to detect oxygen and monitor the diffusion of oxygen through the PNLC is exploited. The PNLC‐based oxygen sensor is potentially useful for the detection of oxygen and monitoring the exposure time to oxygen.

Thermoreversible Gel-Dispersed Liquid Crystals.

A simple model is introduced to describe phase behaviours of binary mixtures of a thermoreversible gel and a low-molecular-weight liquid crystal (LC). We predict novel phase diagrams on the temperature-concentration plane, including sol-gel transition, nematic-isotropic phase transition, and phase separation. At high temperatures, the phase separation between the isotropic sol and gel phases appears. As the temperature decreases, we have the phase separation between nematic sol and isotropic gel phases, in which the nematic domains are dispersed in the isotropic gel phase. We suggest that thermoreversible gelation of reactive molecules mixed with LCs will become one of the new classes of polymer-dispersed liquid crystals.

The relaxation dynamics and dielectric properties of cyanobiphenyl-based nematic tripod liquid crystals

We present a detailed investigation of the phase behaviour, molecular relaxation dynamics, rheology and dielectric properties of two cyanobiphenyl-based liquid crystal (LC) tripods, differing only in the length of their spacer units (6 or 9 carbons). These LC tripods combine properties of low molecular weight LCs and LC polymers, resulting in a range of advantageous properties including a wide nematic range (ΔT>90 K) and a large birefringence (0.3 at T−TNI=-80 K for the 6 spacer tripod). Using broadband dielectric relaxation spectroscopy, calorimetry, oscillatory and steady state shear rheology, we identified four molecular relaxation processes: the structural (α) relaxation (defining the glass transition temperature, Tg); the δ relaxation (reorientation of the mesogen unit around its short axis); the β relaxation (reorientation around its long axis); and the γ relaxation (internal tripod arm fluctuations). The β and γ relaxations follow Arrhenius temperature dependencies in the glass, whereas the α and δ relaxations merge above, but near Tg, where they follow a non-Arrhenius VFT behaviour. For higher temperatures, the two relaxations separate and for T>T⁎, where T⁎ marks a dynamic crossover, the α relaxation transitions from VFT to Arrhenius behaviour. For the two tripods, ratios of T⁎/Tg=1.14 and T⁎/Tg=1.13 were observed respectively, consistent with the ratio observed for many side-chain LC polymers (and other LC systems), and consistent with the ratio where a dynamic crossover is typically observed also for non-LC glass-formers. However, for non-LC systems, the transition to Arrhenius behaviour happens at significantly higher temperatures relative to Tg and the dynamic crossover at T⁎ is typically observed as a transition between two different VFT behaviours. We argue that for the tripods these differences arise from differences in the molecular relaxation mechanisms induced by the LC order. Finally, for temperatures where δ and α relaxations are separated, we find that ion conductivity decouples from the α relaxation, instead following the δ relaxation; this demonstrates that the ion transport properties for the tripods can be tuned by the design of the tripod mesogen arm.

Photo-switchable liquid crystalline brush as an aligning surface for liquid crystals: modelling via mesoscopic computer simulations

We consider the mesoscopic model for the liquid crystalline brush that might serve as a photoswitchable aligning surface for preorientation of low molecular weight liquid crystals in a bulk. The brush is built by grafting the polymer chains of a side-chain molecular architecture, with the side chains terminated by a chromophore unit mimicking the azobenzene unit, to a substrate. When irradiated with ultraviolet light, the chromophores photoisomerize into a non-mesogenic cis state and the whole system turns into an ordinary polymer brush with no orientational order and two states: the collapsed and straightened one, depending on the grafting density. When irradiated with visible light, the chromophores photoisomerize into a mesogenic trans state, resulting in formation of a transient network between chains because of a strong attraction between chromophores. Spontaneous self-assembly of the brush in these conditions results in an orientationally isotropic polydomain structure. The desired uniaxial planar ordering of chromophores within a brush can be achieved at certain temperature and grafting density intervals, as the result of a two-stage preparation protocol. An external stimulus orients chromophores uniaxially at the first stage. The system is equilibrated at the second stage at a given temperature and with the external stimulus switched off. The preoriented chromophores either keep or loose their orientations depending on the strength of the memory effect inherent to a transient network of chains that are formed during the first stage, similarly to the case of the liquid crystalline elastomers, where such effects are caused by the covalent crosslinks.

Preferential Circularly Polarized Luminescence from a Nano-Segregated Liquid Crystalline Phase Using a Polymerized Twisted Nematic Platform.

In this study, a polymerized twisted nematic (TN) network was used as an extrinsic chiral platform to overcome the heterogeneity during spontaneous symmetry breaking in a mixed system comprising an achiral bent-core molecule and rod-like mesogen. The TN platform was prepared by photopolymerizing a reactive mesogen dispersed in a low molecular weight liquid crystal with TN orientation. The use of TN orientation to correct the degeneracy in bent-core molecular systems has been previously reported; however, to the best of our knowledge, this is the first study that uses an extrinsic chiral platform of a polymerized TN network. The heterogeneity in the nano-segregated phase of the achiral mixture was suppressed using the extrinsic TN platform with a twisted angle θ of ≥ |±30°|. When an achiral mixture doped with a luminescent guest molecule was refilled into the extrinsic chiral platform, preferential deracemization with one-handedness occurred, corresponding to the handedness of the TN platform. Therefore, circularly polarized luminescence with a preferential handedness can be achieved using this extrinsic chiral platform.

Wave propagation on a ferroelectric liquid crystal based acoustic metamaterial

In the present work, the acoustic band structure of a 2D phononic crystal (PnC) containing an organic ferroelectric (low molecular weight liquid crystal polimer in C-smectic phase - LMWLCS) and topological insulator (GeTe) were investigated by the finite element method and experimentally. 2D PnC with square lattices composed of GeTe cylindrical rods embedded in the LMWLCS matrix are studied to find the allowed and stop bands for the waves of certain energy. Phononic band diagram ω = ω(k) for a 2D PnC along the Г-X-M-Г path in the square Brillouin zone shows few stop bands in the frequency range between 10–110 kHz.

Responsive Liquid-Crystal-Clad Fibers for Advanced Textiles and Wearable Sensors.

A simple process to clad conventional monofilament fibers with low-molecular-weight liquid crystals (LCs) stabilized by an outer polymer sheath is demonstrated. The fibers retain the responsive properties of the LCs but in a highly flexible/drapable format. The monofilament core makes these fibers much more rugged with a magnified response to external stimuli when compared to previously reported LC-core fibers produced by electrospinning or airbrushing. The microscopic structure and the optical properties of round and flattened fibers are reported. The sensitivity of the response of individual fibers can be tuned over a broad range by varying the composition of the LCs. Complex fabrics can be easily woven from fibers that respond to different external stimuli, such as temperature variation, chemical concentrations, and pressure. The fabrics can be fashioned into garments that can sense and report the state of health of the wearer or the status of their environment.

Electrorheological Properties of Dual Frequency Liquid Crystal in Smectic A Phase

In this study, a dual frequency liquid crystal (DFLC) consisting of low molecular weight liquid crystals (LCs) was newly prepared, and the electrorheological (ER) properties were investigated. The ...

Anisotropy Enhanced Phase Separation in Polymer Dispersed Liquid Crystals

Phase separated blends of polymers and low molecular weight liquid crystals, commonly known as polymer dispersed liquid crystals in short PDLCs, are investigated. These materials offer a realm of applications in modern technologies, including sensors, commutable windows, display devices and telecommunication systems. A particular attention is given to the effects of anisotropy of the liquid crystal on the phase behavior under equilibrium and non equilibrium conditions. The theoretical formalism used is based on the lattice model of isotropic mixing, combined with standards theories of nematic and smectic-A orders. Considering the equilibrium phase behavior, we find that the nematic order enhances the polymer / solvent phase separation, and that the osmotic pressure shows substantial changes for relatively small polymer volume fractions. We find that the anisotropy enhanced phase separation is more pronounced for a smectic-A liquid crystal, and the miscibility gap is widened. The kinetics of swelling by nematic LCs is examined using a linear solvent diffusion process, with a rate of swelling directly related to the derivative of the osmotic pressure. An abrupt swelling / de-swelling transition is found, due to overwhelming effects of the anisotropic interaction beyond the threshold LC concentration. Anisotropy enhanced phase separation is also investigated in the method of synthesis based on the polymerization induced phase separation mechanism. We find that the kinetics of separation during early stages of polymerization is faster, due to the anisotropic interaction of the low molecular weight solvent. The kinetics speed up is favored by the long range viscous flow effects due to hydrodynamic interactions. A limited selection of experimental data in the literature is chosen to validate some theoretical predictions obtained from the present formalisms.

Homogenous photoalignment of nematic liquid crystals doped with photoalignable materials

We demonstrate the homogeneous photoalignment of low-molecular-weight liquid crystals (LC) using nematic LCs (NLCs) doped with photoalignable low-molecular-weight LC materials, achieved in the absence of substrate surface treatment; in-plane switching (IPS) is also shown using an IPS-mode LC cell. The exposure of the LC cells to linearly polarized (LP) light in the isotropic temperature range of the NLC composites generates homogenous NLC orientation, but exposure at room temperature results in the random orientation of the NLCs. Small amounts of photoalignable LC materials on the substrates, which have undergone axis-selective photoreaction, are found to control the NLC alignment.

PA31 液晶性金ナノ粒子の合成とその低分子液晶分散系におけるER効果(液晶化学材料,ポスター発表,2015年日本液晶学会討論会)

PA31 液晶性金ナノ粒子の合成とその低分子液晶分散系におけるER効果(液晶化学材料,ポスター発表,2015年日本液晶学会討論会)

Deformation-free switching of polymer-stabilized cholesteric liquid crystals by low-temperature polymerization

We investigate the fabrication of 'deformation-free' polymer/cholesteric liquid crystal (PChLC) nanocomposites by controlling the degree of phase separation in a precursor mixture containing mesogenic monomer and low molecular-weight LC molecules. Scanning electron microscopy investigations reveal that even in mixtures containing only 6.6 wt% of monomer, nano-sized LC domains can be formed by low-temperature polymerization of −20 °C that lead to the 'deformation-free' response. PChLC nanocomposites with reduced monomer concentrations exhibit improved tuning range of the refractive index and a short decay time of about 20 μs, making them more practical for real-life applications.

Study on the electro-optical properties of polyimide-based polymer-dispersed liquid crystal films

The reflectivity control device, initially developed for attitude control, is utilised to control the solar sail orbit by switching the states between absorption and specular reflection. Actually, the major parts of the device are the polymer-dispersed liquid crystal (PDLC) films. Here, PDLC films based on polyimide (PI) as polymer matrix and a low molecular weight LC can be prepared by the thermally induced phase separation (TIPS) method. The influences of cooling rate and the content of LC on the size and uniformity of LC droplets dispersed in a polymer matrix by a TIPS process were investigated. It was found that a fast cooling rate gave smaller droplet sizes and hence a more uniform distribution as compared to the ones produced under a slow cooling rate. If the LC content was increased, the droplet size would be increased. Furthermore, the effect of LC droplet size on the electro-optical properties of the PI-based PDLC films was discussed, such as transmittance, threshold voltage, driving voltage and contrast ratio (CR).

Macroscopic behavior of ferrocholesteric liquid crystals and ferrocholesteric gels and elastomers.

We study the influence of macroscopic chirality on the macroscopic properties of superparamagnetic liquid crystals and gels. Specifically we derive macroscopic dynamic equations for ferrocholesteric low molecular weight (LMW) liquid crystals and for ferrocholesteric gels and elastomers in the local description using the director field as macroscopic variable. The magnetization is treated as a macroscopic dynamic degree of freedom and its coupling to all other macroscopic variables is examined in detail. We incorporate into our dynamic analysis terms that are linear in a magnetic field giving rise to a number of cross-coupling terms not possible otherwise. A number of properties that are unique to the class of systems studied arise. As an example for a static property we find a term in the generalized energy which is linear in the electric field and quadratic in the magnetic field. We find that applying a magnetic field to a ferrocholesteric can lead to reversible electric currents, heat currents and concentration currents, which change their sign with a sign change of macroscopic chirality. As an example of a rather intriguing dissipative dynamic contribution we point out that for ferrocholesterics and for ferrocholesteric gels and elastomers in a magnetic field extensional flow leads to electric and heat currents.

Photoalignment and resulting holographic vector grating formation in composites of low molecular weight liquid crystals and photoreactive liquid crystalline polymers

Polarization holographic gratings were formed in liquid crystal (LC) cells fabricated from a mixture of low molecular weight nematic LC and a photoreactive liquid crystalline polymer (PLCP) with 4-(4-methoxycinnamoyloxy)biphenyl side groups. The diffraction properties of the gratings were analyzed using theoretical models which were determined based on the polarization patterns of the polarization holography. The results demonstrated that vector gratings comprised of periodic orientation distributions of the LC molecule were induced in the cells based on the axis-selective photoreaction of the PLCP. The vector gratings were erased by applying a sufficiently high voltage to the cells and then were reformed with no hysteresis after the voltage was removed. This phenomenon suggested that the PLCP molecules were stabilized based on the axis-selective photocrosslink reaction and that the LC molecules were aligned by the photocrosslinked PLCP. This LC composite with axis-selective photoreactivity is useful for various optical applications, because of their stability, transparency, and response to applied voltage.

Water and sodium transport and liquid crystalline alignment in a sulfonated aramid membrane

Two sulfonated aramids, poly(2,2′-disulfonylbenzidine terephthalamide) (PBDT) and poly(2,2′-disulfonylbenzidine isophthalamide) (PBDI), were synthesized with the aim to explore their unique morphology for proton exchange membrane applications. Due to the different polymer structures, PBDT forms a nematic liquid crystal, whereas PBDI is isotropic. Both polymers show excellent thermal stabilities (Td5%>400°C), high storage moduli (E′=3–15GPa) and crosslinked films are flexible and easy to handle. Pulsed-field-gradient NMR diffusometry reveals that the in-plane water diffusion in the nematic PBDT membrane is as high as 3.3×10−10m2/s, whereas the diffusion in amorphous PBDI is only 2.5×10−10m2/s. Whereas neat and crosslinked PBDI shows isotropic diffusion, neat PBDT shows a high diffusion anisotropy (D∥/D⊥=3.0), which increases as a function of crosslink density (D∥/D⊥=4.6 at 80% crosslinking). This diffusion anisotropy is substantially higher than that typically observed for low molecular weight liquid crystals and for oriented polymeric conductors such as Nafion® (D∥/D⊥~2.0). The nematic order in the PBDT membrane also promotes directed ionic conductivity, i.e. Na+ conductivity in PBDT is 2.24×10−2S/cm and 1.67×10−2S/cm for PBDI, respectively. We propose that the rigid-rod PBDT chains form nano-scale hydrophilic channels, which act as pathways for transporting water molecules and ions.

The Effect of the Liquid-Crystalline Order on Releasing Dye Molecules from Liquid-Crystalline Nano-Emulsions

We fabricated dye-containing liquid-crystalline nano-emulsions (LCNEs) dispersed in water, and studied the controllability of releasingdye molecules from the coreof LCNEs into water. We estimated the concentration of the dye molecules held in the core of LCNEs from transmission spectrum. As a result, it was revealed that the dye molecules are released from the core of LCNEs below the characteristic release-temperature on cooling. Moreover, the depolarized static light scattering measurement was performed to estimate the Isotropic-Nematic(I-N) phase transition temperature of low molecular weight liquid crystals in the core of LCNEs. By controlling mixing ratio of binary liquid crystalline mixture, we tuned theI-N phase transition temperature. The release-temperature completely corresponds to the I-N phase transition temperature in the coreof LCNEswithin the whole mixing ratio. Thus, we confirmed that the emergence of the liquid-crystalline order in the core of LCNEs plays a principal role in releasing the dye molecules from LCNEs.

Isotropic-to-nematic phase transition of liquid crystals confined in nanoemulsion droplets

We fabricated liquid crystalline nanoemulsions (LCNEs) by introducing low molecular weight liquid crystals (LMWLCs) into the core of nanoemulsions, and investigated the phase transition behavior of LMWLCs in the core part with the various weight ratios of LMWLCs to surfactants. The polarized dynamic light scattering measurement was performed to estimate the radii of LCNEs, and it is found that their radii can be controlled by the weight ratio of LMLCs to surfactant polymers. In the depolarized light scattering, it was revealed that the order of the isotropic-nematic phase transition behavior changes from the first order to biased second order with decreasing radius of LCNEs because of the three-dimensional confinement effect surrounded by an anchoring surface.

Effect of cyanobiphenyl homologue molecules on electro-optical properties in liquid crystal blue phases and polymer-stabilized blue phases

The effects of cyanobiphenyl (CB) homologue molecules on the electro-optical properties of low-molecular-weight liquid crystal blue phase (BP) and polymer-stabilized BPs are investigated using chiral nematic liquid crystal mixtures that include CB molecules. The driving voltage and switching hysteresis of BPs were found to increase by polymer stabilization of the BP. It was also confirmed that the electro-optical rising time and Kerr coefficient of BPs vary with the molecular parity, as determined by the alkyl chain length of the n-CB homologue molecule dissolved in a chiral nematic liquid crystal mixture. The electro-optical properties of the low-molecular-weight liquid crystal (LC) blue phase and the polymer-stabilized blue phase are investigated using the cyanobiphenyl homologue chiral nematic LC mixtures. It was found that the driving voltage and the switching hysteresis of blue phase were increased by polymer stabilization. Also, it was confirmed that the electro-optical rising time of blue phase was varied with the molecular parity of the n-CB homologue chiral nematic LC mixtures.

Design of Functional Materials based on Liquid Crystalline Droplets.

This brief perspective focuses on recent advances in the design of functional soft materials that are based on confinement of low molecular weight liquid crystals (LCs) within micrometer-sized droplets. While the ordering of LCs within micrometer-sized domains has been explored extensively in polymer-dispersed LC materials, recent studies performed with LC domains with precisely defined size and interfacial chemistry have unmasked observations of confinement-induced ordering of LCs that do not follow previously reported theoretical predictions. These new findings, which are enabled in part by advances in the preparation of LCs encapsulated in polymeric shells, are opening up new opportunities for the design of soft responsive materials based on surface-induced ordering transitions. These materials are also providing new insights into the self-assembly of biomolecular and colloidal species at defects formed by LCs confined to micrometer-sized domains. The studies presented in this perspective serve additionally to highlight gaps in knowledge regarding the ordering of LCs in confined systems.