Phase Separation Of Liquid Crystal Research Articles

Fast Switchable and Bistable Microlens Array Using Ferroelectric Liquid Crystals

Fast-switching ferroelectric liquid crystal microlens arrays have been fabricated using the anisotropic phase separation of liquid crystal from its solution in an ultra violet-curable prepolymer. The microlenses can be electrically controlled to modulate the transmitted light and change their focal length within a few microseconds, i.e., ∼1000 times faster than nematic liquid-crystal-based microlenses. These microlenses also exhibit the memory effect.

Phase Behaviour of Uncured and UV-cured (2-ethylhexylacrylate-1,6-hexanedioldiacrylate)/E7 Mixtures

The equilibrium phase diagram of UV-cured (2-ethylhexylacrylate-1,6-hexanedioldiacrylate) and the multicomponent nematic fluid E7 is established by polarized optical microscopy and differential scanning calorimetry. The enthalpy changes at the nematic-isotropic transition for both uncured and UV-cured samples are used to determine the liquid crystal (LC) solubility limit in the acrylate rich phase and to estimate the fractional amount of phase separated LC.

Improvement of holographic performance by novel photopolymer systems with siloxane-containing epoxides

Diffraction efficiency and morphology of holographic gratings were improved by combining ring-opening polymerizable epoxides with radical polymerizable conventional acrylate systems. By adding epoxide materials, the grating spacings were increased when compared with the case where only acrylate was used. The effects were especially notable for cyclohexane oxide derivatives. The diffraction efficiency was the highest, and nanoscale phase-separated liquid crystal domains were the most clearly formed using siloxane-containing cyclohexane oxide group, due to their incompatibility and fast curing.

PA30 ポリマーの分子量が液晶との相分離に及ぼす影響(物理・物性)

PA30 ポリマーの分子量が液晶との相分離に及ぼす影響(物理・物性)

2H-NMR investigation after a polymerisation-induced phase separation process*

Polymer-dispersed liquid crystals (PDLC) are composite materials consisting of micron-sized droplets of liquid crystal dispersed in a polymer matrix. The easiest method to obtain a PDLC film is the polymerisation-induced phase separation process (PIPS). The liquid crystal is mixed with a monomer of low molecular weight and polymerisation is induced by heat or UV light. The increasing molecular weight of the polymer causes the phase separation of liquid crystal from the polymer matrix as micron-sized droplets. In this work, we have studied the structural changes induced in the polymer matrix of a PDLC after the PIPS process by deuterium nuclear magnetic resonance. Two different selectively deuterated monomers have been synthesized and investigated: isobutyl methacrylate (IBMA-d2) and methyl methacrylate (MMA-d3). The main results were the disappearance of the characteristic two-site hop in poly-IBMA, due to liquid crystal molecules, and the lack of unreacted MMA molecules in the liquid crystal droplets. In this last case, we found that it is possible to confine temporarily the unreacted MMA molecules within liquid crystal droplets.

Effect of electric field on phase separation of polymer dispersed liquid crystal

The kinetics of the polymerization induced phase separation of liquid crystal (LC)/monomer mixture has been investigated by means of depolarized light intensity technique and polarized light microscope (PLM). To examine the effect of the electric field, a DC electric field was applied across the mixtures during the phase separation process. The kinetic study indicates that the phase separation process is accelerated when the electric field is applied. The morphologies of the formed polymer dispersed liquid crystal (PDLC) films were observed by PLM. The electric field applied during the phase separation process yields the PDLC with small LC domains and fine morphologies. The clearing temperature ( T NI) of the formed PDLC films was measured by the PLM and it is found that the T NI increases with the applied electric field intensity.

Anchoring behavior, orientational order, and reorientation dynamics of nematic liquid crystal droplets dispersed in cross-linked polymer networks

The orientational ordering and the electro-optical properties of nematic liquid crystal (LC) droplets confined to cross-linked polymer networks are investigated as a function of the anchoring conditions at the polymer-liquid crystal interface. Normal alignment (homeotropic) or parallel alignment (planar) inside LC droplets was controlled by using acrylate polymers with appropriate side chains. Drastic changes in the reorientation dynamics of the confined nematic liquid crystal phase are observed, as well as in the orientational ordering of the phase-separated LC which was investigated by 13C-NMR (nuclear magnetic resonance) spectroscopy. The cross-link density of the polymer network also affects the orientational ordering and the electro-optical properties of the confined LC phase. Faster switching times and higher-order parameters were found for samples with LC droplets exhibiting planar anchoring.

PA37 液晶とポリマー繊維の光重合相分離におけるポリマー分子量の硬化温度依存性(物理・物性)

PA37 液晶とポリマー繊維の光重合相分離におけるポリマー分子量の硬化温度依存性(物理・物性)

Switchable orthorhombic F photonic crystals formed by holographic polymerization-induced phase separation of liquid crystal

Electrically switchable photonic crystals are simply and rapidly formed by holographic polymerization-induced phase separation of liquid crystal from a monomer-liquid crystal mixture. We report the fabrication and electro-optical properties of liquid-crystal-filled polymer photonic crystals of orthorhombic F symmetry. Inverse opal and fcc structures can also be obtained. The crystals exhibit electrically switchable Bragg diffraction at ~8-10 V/microm with crystal structure in good agreement with theoretical expectations. These photonic crystals compare favorably with liquid-crystal-imbibed colloidal crystal arrays.

Effect of director distortions on morphologies of phase separation in liquid crystals.

We study phase separation from a nematic liquid crystal with spatially nonuniform director gradients. Particles of a phase-separated component, which is either an isotropic fluid (silicone oil) or a nonmesogenic photopolymer, accumulate in the regions with the strongest director distortions, thus reducing the overall energy of the system.

Evolution of anisotropic reflection gratings formed in holographic polymer-dispersed liquid crystals

The temporal evolution of an anisotropic reflection grating produced in a holographic polymer-dispersed liquid crystal film is investigated. We find that this type of grating is preceded in time by an isotropic concentration grating, and that the development of the anisotropic grating can be delayed until several seconds after laser exposure. The formation of an anisotropic grating is nearly coincident with the onset of phase separation of liquid crystal and implies a macroscopic ordering of liquid crystal droplet directors. Detailed knowledge of grating evolution may allow in situ control over the polarization sensitivity of the hologram.

Phase separation of liquid crystal–polymer mixtures

Phase separation in liquid crystal–polymer mixtures is studied by computer simulations in two dimensions. The domain morphology resulting from the phase separation either by temperature quench or by polymerization is investigated by solving the coupled set of equations for the local volume fraction and the nematic order parameter. In a temperature quench, it is found that transient concentric domains are constituted near the nucleation regime of nematic ordering. Phase separation induced by polymerization is modeled by taking into account the time dependence of the molecular weight of polymer chains. Assuming a strong concentration dependence of the mobility, a transient network-like domain of polymer-rich phase is formed. The morphology is compared with the experimental results.

Morphology Development and Dynamics of Photopolymerization-Induced Phase Separation in Mixtures of a Nematic Liquid Crystal and Photocuratives

Dynamics of photopolymerization-induced phase separation (PIPS) and morphology development in mixtures of low molar mass nematic liquid crystal (LC) 4-n-heptyl-4‘-cyanobiphenyl (designated K21) and photocuratives (designated NOA65) have been investigated by means of optical microscopy, light scattering, and differential scanning calorimetry. The equilibrium phase diagram of the LC/monomer (uncured NOA65) system was first established theoretically on the basis of the Flory−Huggins/Maier−Saupe theory. The calculated phase diagram displayed isotropic, liquid + liquid, liquid + nematic, and pure nematic coexistence regions in conformity with experiment. The effect of photopolymerization on the phase diagram was examined through comparison of the calculated phase diagrams of LC/linear polymer with LC/cross-linked polymer mixtures. Photopolymerization of the LC/monomer mixtures resulted in phase separation due to instability caused by an increase in molecular weight and subsequent network formation. Photopolymerization initiated in the isotropic phase gave uniformly distributed polygonal-shaped LC droplets in contrast to photopolymerization initiated in the two-phase region. Of particular interest is that the size of the phase-separated LC droplet increases with LC concentration. The network morphology is reminiscent of an interconnected thread seemingly running through the interstices of the polygonal droplets. Temporal evolution of structure factors was analyzed in the context of dynamical scaling law.

Monomer functionality effects in the anisotropic phase separation of liquid crystals

Holographic gratings formed through the anisotropic phase separation of liquid crystals show promise as switchable optical elements. In order to form useful elements, however, it is necessary to control the nanoscale morphologies within the grating films. In this manuscript, we evaluate the role of monomer functionality on the morphology and the electro-optical properties of both gratings and conventional scattering polymer-dispersed liquid crystal (PDLC) films. Both of these structures are formed using polymerization-induced phase separation (PIPS) of liquid crystals from a cross-linked polymer formed through free-radical photo-polymerization. Floodlit (uniform illumination) films and holographic gratings (from non-uniform illumination caused by the interference of two laser beams) were made using monomers with 2–5 acrylate groups, while keeping the LC concentration constant in the syrups. The morphologies of these films were examined using low voltage scanning electron microscopy (LVSEM). In all cases, very small LC domains were formed with little indication of growth or coalescence. Lowering monomer functionality reduced the volume fraction of phase-separated domains in the floodlit samples. For the grating samples, the local volume fraction and the LC domain sizes decreased substantially as the monomer functionality was decreased. Using detailed image analysis, differences in the anisotropy of the domains was also probed. A much stronger tendency to form anisotropically-shaped domains was observed for the higher functional syrups. These domain anisotropy differences are correlated with the number of reactive double bonds per monomer and are suggestive of local environmental differences exerted at the time of the domain formation.

Phase separated composite films of liquid crystals

Phase separation of liquid crystals from a solution with polymers has long been studied and used to prepare polymer stabilized and polymer dispersed structures. They are formed by spatially isotropic phase separation. A new mode, in which the phase separation proceeds anisotropically, has recently been discovered. Known as phase separated composite films (PSCOF), the resultant structures are made of adjacent parallel layers of liquid crystal and solidified polymer. PSCOFs have been made with nematic, ferroelectric (FLC), and antiferroelectric (AFLC) liquid crystals. Liquid crystals in PSCOFs exhibit electro-optical properties not observed in devices prepared by conventional methods, polymer dispersion, or polymer stabilization methods. Devices incorporating FLCs possess grey scale and switch 100 times faster at low fields than conventional surface stabilized devices. This method makes it possible to prepare very flexible devices and devices with liquid crystal film thickness comparable to optical wavelengths with great ease.

Spatio–temporal growth of nematic domains in liquid crystal polymer mixtures

Dynamics of phase separation and morphology development in mixtures of a low molar mass liquid crystal (LC) and a polymer have been investigated theoretically in comparison with experimental results. In the theoretical model, the combined free-energy densities of Flory–Huggins theory for isotropic mixing and Maier–Saupe theory for nematic ordering have been incorporated into the time-dependent Ginzburg–Landau equation (type C). The temporal evolution of the structure factor and the emergence of phase-separated liquid crystal domains have been simulated on the basis of an explicit central difference method based on a square lattice (128×128) with a periodic boundary condition. Of particular interest is the observed plateau (or inflection) region in the growth dynamic curve, which may be attributed to the breakdown of the interconnected domains caused by the nematic ordering. This unique behavior has been verified experimentally in terms of the growth of structure factor following several temperature quenches into a nematic+liquid region of the experimental phase diagram of an E7/poly(methyl methacrylate) mixture. Further, the emergence of LC domains in the metastable and unstable nematic–liquid spinodal regions has been investigated theoretically and compared with the reported experimental results.

Phase-separated composite films for liquid crystal displays

A method of preparing liquid crystal devices by phase separation of liquid crystal from its solution in a prepolymer, which results in adjacent layers of liquid crystal and polymer, is described. Liquid crystals in these phase-separated composite films exhibit electro-optical properties not observed in devices prepared by conventional methods, polymer dispersion, or polymer-stabilization methods. Devices incorporating ferroelectric liquid crystals have gray scale and switch 100 times faster at low fields than conventional surface-stabilized devices. This method makes it possible to prepare devices with liquid crystal film thickness comparable to optical wavelengths.

Electrically switchable reflection gratings in polymer dispersed liquid crystals

ABSTRACTElectrically switchable volume reflection holograms were written by inhomogeneous illumination of a prepolymer syrup containing a nematic liquid crystal and a multifunctional acrylate monomer. Switchable holograms are diffractive optics structures and the diffraction efficiency can be controlled by the application of an electric field. Reflection gratings with grating spacing varying between 0.16-0.27 µm were made during the phase separation of liquid crystals from the fast curing prepolymer syrup. The reflection efficiency of the holograms were electrically modulated with the applied field of ∼10-15V/µm. Real time study of the grating formation revealed that the maximum efficiency is reached in ∼15 seconds. The shrinkage of the host polymer during grating formation resulted in the blue shift of the reflection notch. The response time of the grating in an electric field is ∼50 µs. Low voltage scanning electron microscope studies showed the presence of discrete nematic droplet domains of sizes 30-60 nm in liquid crystal rich region.

Phenotypic characterization of Lith genes that determine susceptibility to cholesterol cholelithiasis in inbred mice: physical-chemistry of gallbladder bile

Lith genes control susceptibility to cholesterol gallstone formation in inbred strains of mice on a lithogenic diet containing high fat, high cholesterol and 0.5% cholic acid. Our study defines the physical-chemical phenotypes of C57L, AKR, and (C57L x AKR) F1 mouse gallbladder biles during 56 days on the lithogenic diet. We found enhanced cholesterol supersaturation, accumulation of mucin gel, and larger gallbladders in all C57L and F1 mice, as well as more frequent gallstone formation in male C57L and F1 mice (80%) compared to females (40%) or AKR mice (15%). In male C57L and F1 mice, mucin gel accumulated at 3 days, followed by cholesterol supersaturation and phase separation of liquid crystals, solid monohydrate crystals, and, in 43% of mice, anhydrous cholesterol crystals; whereas, in females, phase separations were delayed 2 to 9 days, and anhydrous crystals did not form. In AKR mice, cholesterol supersaturation and phase separations were infrequent and delayed, and gender did not influence the phenotype. Taurocholate invariably replaced endogenous bile salts, especially tauro-beta-muricholate, with crystallization sequences matching taurocholate-containing model bile systems. We conclude: i) Lith genes determine biliary cholesterol supersaturation, mucin gel accumulation, gallbladder size, phase-separation, and prevalence of cholesterol gallstones. ii) Identical phenotypes in C57L and F1 mice indicate susceptibility to cholesterol gallstones is genetically dominant, favoring males 2:1. iii) Mucin gel accumulation, crystallization, and stone formation are rare in AKR mice. This definition of the physical chemistry of lithogenesis should aid in further elucidation of the Lith genes and the proteins they encode.

Stability Studies on Polymer Dispersed Liquid Crystal Films

Polymer Dispersed Liquid Crystal(PDLC) is a class of electroscattering materials exhibiting very promising properties for display device applications. For the last few years, the electrooptical properties of PDLC have improved drastically to use projection device with high brightness. We have studied stability of PDLC films which formed by ultraviolet initiated photopolymerization and liquid crystal phase separation. We evaluated several stability items in PDLC films such as running and environment test, accelerated life time at low and elevated temperature, voltage holding ratio(VHR) variations according to UV exposing time and temperature.