Phase Separation Of Liquid Crystal Research Articles

Low-threshold distributed feedback laser based on holographic polymer dispersed liquid crystals through the oriented organic semiconductor films

A specific optimized configuration for low threshold organic semiconductor laser based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating was demonstrated. Here the organic semiconductor films and phase separated liquid crystal (LC) molecules were oriented along the direction of the HPDLC grating grooves. The influence of the organic semiconductor chain orientation and the excitation polarization on the optical properties of the materials has been investigated. Especially, when polymer chain orientation, LC molecules and pump light polarization are consistent with the direction of the grating grooves, the performance of the outgoing laser is greatly improved. Up to 9.78% conversion efficiency with a threshold lower to 0.12 μJ/pulse can be obtained, indicating their potential for high-performance organic optoelectronics.

Thermodynamically controlled multiphase separation of heterogeneous liquid crystal colloids

Phase separation is a universal physical transition process whereby a homogeneous mixture splits into two distinct compartments that are driven by the component activity, elasticity, or compositions. In the current work, we develop a series of heterogeneous colloidal suspensions that exhibit both liquid-liquid phase separation of semiflexible binary polymers and liquid crystal phase separation of rigid, rod-like nanocellulose particles. The phase behavior of the multicomponent mixture is controlled by the trade-off between thermodynamics and kinetics during the two transition processes, displaying cholesteric self-assembly of nanocellulose within or across the compartmented aqueous phases. Upon thermodynamic control, two-, three-, and four-phase coexistence behaviors with rich liquid crystal stackings are realized. Among which, each relevant multiphase separation kinetics shows fundamentally different paths governed by nucleation and growth of polymer droplets and nanocellulose tactoids. Furthermore, a coupled multiphase transition can be realized by tuning the composition and the equilibrium temperature, which results in thermotropic behavior of polymers within a lyotropic liquid crystal matrix. Finally, upon drying, the multicomponent mixture undergoes a hierarchical self-assembly of nanocellulose and polymers into stratified cholesteric films, exhibiting compartmentalized polymer distribution and anisotropic microporous structure.

Dynamic light scattering analysis of size-selected graphene oxide 2D colloids fractioned via liquid crystal phase separation.

Exfoliated platelets of graphene oxide (GO) can be considered as polydisperse 2D colloids that form nematic colloidal liquid crystal phases in aqueous suspension even at very low concentrations thanks to their extremely high aspect ratios. However, with the rapidly emerging scientific interest in these GO-based liquid crystals, it became clear that the precise analysis and control of the GO sheet size distribution is essential, both for their scientific understanding and for potential applications, e.g., in optoelectronic devices, nanocomposites, or catalysis. In this work, we show that the mean effective (hydrodynamic) GO platelet width can be determined from the translational diffusion coefficient with depolarized dynamic light scattering by using a model for circular, infinitely thin disks. We further studied the phase separation process of biphasic isotropic-nematic GO dispersions and developed a simple fractionation protocol, which can be used to prepare relatively monodisperse fractions of GO sheets with widths ranging from 2.0-12.4 μm. Overall, we expect that the combined application of these relatively simple fractionation and analysis methods will advance the fabrication of well-defined and size-selected GO-based systems.

The Effect of Curing Temperature on the Morphology and Electro-Optical Properties of a Flexible UV-Cured Polymer Dispersed Liquid Crystal (PDLC)

In this work, the experimental data on flexible PDLC films at the industrial scale and the effect of cure temperature (Tc) on the morphology and electro-optical properties of a UV-cured PDLC system via polymer-induced phase separation (PIPS) method were studied. Under constant UV radiation intebsity and thickness, the morphological parameters such as dimension, number density, and volume fraction of the phase-separated liquid crystal droplets as well as the optical transmissions and switching voltages as a function of cure temperature were determined. The effect of cure temperature on the morphology and electro-optical properties of UV-cured PDLC films were analyzed.

Constructing the Bridge from Isotropic to Anisotropic Pitches for Preparing Pitch-Based Carbon Fibers with Tunable Structures and Properties.

Synthetic naphthalene pitches (SNPs) with isotropy and anisotropy were prepared by a simple thermal polycondensation method to fabricate pitch-based carbon fibers. The structural characteristic, thermal stability, phase-separation behavior, and melt-spinnability of the SNPs and the structural properties of the derived carbon fibers were systematically investigated. The results show that spinnable SNPs with controllable mesophase contents ranging from 0 to 100 vol % and softening points (210–290 °C) could be easily obtained by a nitrogen-bubbling treatment to improve their thermal stability and melt-spinnability by avoiding the phase separation of liquid crystal (LC) in the pitch. An experimental phase diagram of spinnability and mesophase content is newly proposed for predicting the spinnability of a mesophase-containing pitch. The LC has a significant influence not only on the constituents, structure, and physical properties of the SNPs but also on the final structure and properties of the corresponding pitch-based carbon fibers. The low ash content (less than 0.15 wt %) in the pitch precursor is found to have no obvious effect on the pitch spinnability and the mechanical properties of derivative large-diameter carbon fibers.

The effect of curing temperature and time on morphology and Electro-Optical properties of flexible thermoset polymer dispersed liquid crystal (PDLC) films

In this work, we studied the effect of cure temperature (Tc) and cure time (tc) on the morphology and few electro-optical properties of flexible thermoset PDLC systems by Polymer Induced Phase Separation (PIPS) method. Under a constant thickness, we determined the morphological parameters including dimension, number density and volume fraction of phase separated liquid crystal droplets, as well as optical transmissions and switching voltages as a function of cure temperature and cure time. We analyzed the effect of cure temperature and time on morphology and electro-optical properties in the thermoset PDLC films.

Large Graphene Oxide Flakes: From Isotopic to Nematic Liquid Crystal Phase Transition

Herein, graphene oxide (GO) with large flakes of average size 38 μm is studied, following the isotropic–nematic phase transition over time and determining the critical concentration, i.e., the minimum value for the formation of the pure liquid crystal (LC) phase. The GO flakes in aqueous suspensions can form LC phases, which are attractive for various applications because of their exceptional properties. Stability over time is very important for their applications, especially, if the GO suspension is in the biphase concentration region in which a crucial change can occur—the isotropic LC phase separation. GO LCs have a wide biphase region, and even very low concentration suspensions appear purely nematic when prepared. However, over time, they appear to be biphasic and higher concentrations are needed for pure LC phase. The changes in the critical concentration were followed for over 4 months, and the findings indicate that it increases with time until it reaches a stable value that is, nevertheless, very low in concentration, 0.7 mg mL−1 (0.035 vol%). From the stable threshold volume fraction, the effective flake thickness, relevant in the LC phase formation behavior, is estimated. This value is almost double than what is measured by atomic force microscopy.

Nanoparticle-based hollow microstructures formed by two-stage nematic nucleation and phase separation

Rapid bulk assembly of nanoparticles into microstructures is challenging, but highly desirable for applications in controlled release, catalysis, and sensing. We report a method to form hollow microstructures via a two-stage nematic nucleation process, generating size-tunable closed-cell foams, spherical shells, and tubular networks composed of closely packed nanoparticles. Mesogen-modified nanoparticles are dispersed in liquid crystal above the nematic-isotropic transition temperature (TNI). On cooling through TNI, nanoparticles first segregate into shrinking isotropic domains where they locally depress the transition temperature. On further cooling, nematic domains nucleate inside the nanoparticle-rich isotropic domains, driving formation of hollow nanoparticle assemblies. Structural differentiation is controlled by nanoparticle density and cooling rate. Cahn-Hilliard simulations of phase separation in liquid crystal demonstrate qualitatively that partitioning of nanoparticles into isolated domains is strongly affected by cooling rate, supporting experimental observations that cooling rate controls aggregate size. Microscopy suggests the number and size of internal voids is controlled by second-stage nucleation.

Photomodulated Electro-optical Response in Self-Supporting Liquid Crystalline Physical Gels.

Photoresponsive liquid crystal (LC) physical gels have attracted more and more attention because of the nature of strong response via light stimulus. Although many efforts on the breaking and recovering of physical gels through photoisomerization have been focused, fast electro-optical response and high mechanical properties even upon light irradiations are difficult to achieve at the same time. In this work, two kinds of azobenzene-containing gelators (AG1 and AG2) with different terminal groups were designed and synthesized. Both gelators could induce the nematic LC P0616A self-assemble into anisotropic phase-separated LC physical gels at low contents. Their phase-transition behavior, thermal stability, microstructure, and mechanical strength were systematically studied. Compared with AG2 in P0616A, the P0616A/AG1 gels showed better mechanical property. When the gelator content was above 3 wt %, the P0616A/AG1 gels possessed good self-supporting ability with a storage modulus more than 104 Pa. Thus, the photoresponsive electro-optical properties and structures of P0616A/AG1 gels were focused in detail. It was surprising that the electro-optical response speed of the P0616A/AG1 gels could be promoted upon UV irradiation. In particular, the decay time (τoff) was only about half when compared with the initial state, whereas the gels still exhibited good self-supporting ability; also the network of the LC physical gels had no change at macro- and microstructural levels. These exciting results would open a door for the application of this material in electro-optical devices.

Photoplasticization Effect of an Azobenzene-doped Liquid Crystal Depending on Phase Structures

A photoresponsive plasticizer for a photo-inactive polymer was developed using an azobenzene-doped liquid crystal (LC). The azo-doped LC was miscible with poly(methyl methacrylate) up to a critical concentration, above which it phase-separated from the polymer. A photoinduced decrease in the glass transition temperature (Tg) of the polymer was accomplished by photochemically changing the phase structures of the phase-separated LC; the large decrease in Tg (ΔTg = −23 °C) was obtained with the addition of 70 wt % azo-doped LC.

Study of Langmuir-Blodgett films of chiral nematic liquid crystals prepared by a templating method

The ordered alignment of liquid crystal molecules plays a critical role in the applications of their optical properties. Langmuir-Blodgett (LB) monolayer films of chiral nematic liquid crystal, 4-cyano-4′-pentylbiphenyl(5CB), doped with a proprietary left-handed chiral additive were prepared using stearic acid (SA) as template. The Langmuir isotherms of the chiral nematic liquid crystal at the CdCl2 interface with SA were studied. In the surface pressure curves, there was one sharp peak at the transition from gas phase to liquid phase. The phenomenon is considered to be the phase-separation of liquid crystals from SA. The chiral nematic liquid crystal multilayers LB films were characterized by ultraviolet-visible spectroscopy, X-ray diffraction, and atomic force microscopy. The results show that the chiral nematic liquid crystals were homogeneously and regularly dispersed in the SA template.

Light-addressable liquid crystal polymer dispersed liquid crystal

Scattering-free liquid crystal polymer-dispersed liquid crystal polymer (LCPDLC) films are fabricated by combining a room temperature polymerizable liquid crystal (LC) monomer with a mesogenic photosensitive LC. The morphological and photosensitive properties of the system are analysed with polarized optical microscopy and high resolution scanning and transmission electron microscopy. A two-phase morphology comprised of oriented fibril-like polymeric structures interwoven with nanoscale domains of phase separated LC exists. The nanoscale of the structures enables an absence of scattering which allows imaging through the LCPDLC sample without optical distortion. The use of a mesogenic monomer enables much smaller phase separated domains as compared to non-mesogenic systems. All-optical experiments show that the transmitted intensity, measured through parallel polarizers, can be modulated by the low power density radiation (31 mW/cm2) of a suitable wavelength (532 nm). The reversible and repeatable transmission change is due to the photoinduced trans-cis photoisomerization process. The birefringence variation (0.01) obtained by optically pumping the LCPDLC films allow their use as an all-optical phase modulator.

Morphology Tuning of Electrospun Liquid Crystal/Polymer Fibers.

This paper elucidates the means to control precisely the morphology of electrospun liquid crystal/polymer fibers formed by phase separation. The relative humidity, solution parameters (concentration, solvent), and the process parameter (feed rate) were varied systematically. We show that the morphology of the phase-separated liquid crystal can be continuously tuned from capsules to uniform fibers with systematic formation of beads-on-a-string structured fibers in the intermediate ranges. In all cases, the polymer forms a sheath around a liquid-crystal (LC) core. The width of the polymer sheath and the diameter of the LC core increase with increasing feed rates. This is similar to the results obtained by coaxial electrospinning. Because these fibers retain the responsive properties of liquid crystals and because of their large surface area, they have potential applications as thermo-, chemo-, and biosensors. Because the size and shape of the liquid-crystal domains will have a profound effect on the performance of the fibers, our ability to precisely control morphology will be crucial in developing these applications.

Effect of Dielectric Titanium, Yttrium, and Silicon Oxide Nanoparticles on Electro-Optical Characteristics of Polymer-Dispersed Liquid Crystals

Electro-optical characteristics of composite polymer-dispersed liquid crystals doped (implanted) with inorganic SiO2, TiO2, and Y2O3 nanoparticles in strong electric fields are studied. The composites were obtained by the method of phase separation of liquid crystals (5CB) and polymer (polyvinyl acetate). It is revealed that implantation of up to 1 wt.% of nanoparticles does not noticeably affect the morphology of the composites. The implanted particles change the mismatch between the refractive indices of the polymer and the liquid crystal, and the strong particle aggregation increases the light scattering that can improve the electrooptical contrast of the composites. Changes of the dielectric permittivity with the field are correlated with the light transmittance. It is found that the yttrium and silicon oxides decrease and the titanium oxides increase the Fredericks threshold field. The titanium oxides and to a greater extent the silicon oxides decrease the dependence of the light transmittance on the changes in the dielectric permittivity.

Ultra-fast solid state electro-optical modulator based on liquid crystal polymer and liquid crystal composites

A different generation of polymer-dispersed liquid crystals (PDLCs) based on a liquid crystalline polymer host is reported wherein the fluid behavior of the reactive mesogenic monomer is an enabler to concentration windows (liquid crystal polymer/liquid crystal) (and subsequent morphologies) not previously explored. These liquid crystal (LC) polymer/LC composites, LCPDLCs, exhibit excellent optical and electro-optical properties with negligible scattering losses in both the ON and OFF states. These systems thus have application in systems where fast phase modulation of optical signal instead of amplitude control is needed. Polarized optical microscopy and high resolution scanning electron microscopy confirm a bicontinuous morphology composed of aligned LC polymer coexisting with a phase separated LC fluid. Operating voltages, switching times, and spectra of LCPDLCs compare favourably to conventional PDLC films. The LCPDLCs exhibit a low switching voltage (4–5 V/μm), symmetric and submillisecond (200 μs) on/off response times, and high transmission in both the as formed and switched state in a phase modulation geometry.

Isothermal electrically and optically induced phase separation of liquid crystal and poly(N-vinylcarbazole) films

This paper reports the electro- and photo-isomerization-induced isothermal phase separation of liquid crystals (LCs) and poly(N-vinylcarbazole) (PVK). The proposed phase separation process determines reformed PVK films on substrates to obtain switchable LC light valves. UV illumination induces simultaneous isothermal phase transition of the mixture and dissolution of PVK into the LCs. Phase separation of PVK and LCs occurs by the reversed phase transition via rapid electro-isomerization and slow dark-relaxation. During rapid phase separation, micron-sized LC domains (branch-like PVK structures) are generated to develop stable light scattering; during slow dark-relaxation, a uniform PVK film is obtained, thereby providing stable transparency.

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

ABSTRACTA simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41137.

Carbon Nanotubes Induced Gelation of Unmodified Hyaluronic Acid

This work reports an experimental study of the kinetics and mechanisms of gelation of carbon nanotubes (CNTs)-hyaluronic acid (HA) mixtures. These materials are of great interest as functional biogels for future medical applications and tissue engineering. We show that CNTs can induce the gelation of noncovalently modified HA in water. This gelation is associated with a dynamical arrest of a liquid crystal phase separation, as shown by small-angle light scattering and polarized optical microscopy. This phenomenon is reminiscent of arrested phase separations in other colloidal systems in the presence of attractive interactions. The gelation time is found to strongly vary with the concentrations of both HA and CNTs. Near-infrared photoluminescence reveals that the CNTs remain individualized both in fluid and in gel states. It is concluded that the attractive forces interplay are likely weak depletion interactions and not strong van der Waals interactions which could promote CNT rebundling, as observed in other biopolymer-CNT mixtures. The present results clarify the remarkable efficiency of CNT at inducing the gelation of HA, by considering that CNTs easily phase separate as liquid crystals because of their giant aspect ratio.

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70)wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20Hz to 20MHz and over the temperature range from 24°C to 100°C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.

Particular thermally induced phase separation of liquid crystal and poly(N-vinyl carbazole) films and its application

In the current study, a method of particular thermally induced phase separation (TIPS) of liquid crystals (LCs) and polymers is presented. The method involves a combination of dissolution process and TIPS. The LCs and poly(N-vinyl carbazole) (PVK) play the roles of solvent and solute, respectively, during the processes of particular TIPS. The nematic LC sample fabricated by two substrates coated with uniform PVK films is heated and then cooled, generating the rough PVK layers onto the surfaces of the substrates. The LC sample having rough PVK layers produces micron-sized, multiple domains of disordered LCs that can scatter incident light. Additionally, an application of a scattering mode light shutter, having the advantages of low driving voltage, polarization-independent scattering, fast response, high contrast ratio, and being polarizer free, is reported.