Improvement of the blue phase stability in chiral nematic liquid crystal mixtures

ABSTRACT

The method of widening of the blue phase (BP) temperature range is presented. By means of polarising optical microscopy, we demonstrate that the temperature regime in which BP is stable is greatly enhanced by the cell thickness changes in mixtures of nematic liquid crystal (LC) and chiral dopant.     

Near-infrared luminescence emitted by an electrically switched liquid crystal cell

Switching of near-infrared emission was observed for liquid crystal (LC) cells filled with a chiral nematic liquid-crystal mixture (mixture of E7 and cholesteryl nonanoate) doped with luminescent neodymium(III) or ytterbium(III) β-diketonate complexes. The operating principle of the LC cell is based on an electrically induced phase transition from a chiral nematic to a nematic phase. The chiral nematic phase scatters the excitation light more strongly, resulting in more efficient light absorption and intenser photoluminescence.     

Helical twisting power of antiferroelectric chiral terphenyls in different matrices

To determine the influence of chemical structure on the Helical Twisting Power (HTP), we tested four optically active dopants having a terphenyl rigid core and the same chiral centre but differing in the length of nonchiral terminal chain and the substitution of benzene rings with fluorine atoms. The compounds were added to different achiral liquid crystalline matrices: nematic and smectic C. It was found that HTP as well as its temperature variation depends on the kind of used matrices. It gives a conclusion that information about HTP obtained in one matrix cannot be uncritically transferred to another one.     

Quantitative experimental determination of the Landau-potential of chiral enantiomer doped ferroelectric liquid crystals

The full Landau potential was determined for a ferroelectric liquid crystal doped with varying concentrations of the chiral dopant R1011 and its enantiomer S1011. A multi-curve fitting procedure using temperature and electric field dependent tilt angle and polarization data was employed to the generalized Landau model of ferroelectric liquid crystals. From this analysis the three Landau coefficients as well as the polarization-tilt coupling parameters were obtained as a function of dopant concentration and configuration. It is shown that the two most varied parameters are those of the first Landau coefficient α and the (chiral) linear polarization-tilt coupling constant C. The effect on the first Landau term is equivalent for the two dopants of opposite handedness indicating its achiral nature, while the effect on the (chiral) bilinear coupling term differs for the R1011 and S1011 dopant, increasing and decreasing the coupling between tilt and polarization respectively. This difference in the bilinear coupling term quantifiably evidences that the R1011 dopant increases and S1011 dopant reduces the inherent chirality in this system.     

A fast anharmonic mode in electrooptical switching of liquid crystal structures based on chiral nematics

Polarization, spectral, and relaxation features of a new electrooptical effect in oriented layers of chiral nematic liquid crystals (LCs) are considered. The physics behind this electrooptical effect is the induction of higher order spatial harmonics in the helical distribution of the director field, which ensures the high speed of electrooptical response. It is shown that the spectral properties of the electrooptical response can be effectively controlled by varying the optical anisotropy of the LC and the pitch of the helical structure.     

Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic-isotropic phase transition temperature (T_NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.     

Light scattering study of a twist grain boundary liquid crystal

Dynamic light scattering is used to probe the fluctuation modes of a liquid crystal exhibiting twist grain boundary (TGB) structure. At the chiral nematic to proposed "chiral line liquid" phase transition, anomalous temperature dependence in the fluctuation spectrum and an instability in the helicoidal director structure signify developing TGB order. At lower temperatures, the behavior of the smectic layer-director fluctuations ("soft" mode) indicates that the previously identified commensurate TGB(A) phase may in fact be a TGB(C) phase with an unusually small tilt angle.     

染料掺杂聚合物分散胆甾相液晶薄膜激光特性研究

采用激光染料DCM、向列相液晶TEB30A、手性剂S-811、聚乙烯醇(PVA), 通过微胶囊法制备了聚合物分散胆甾相液晶薄膜, 测量激光辐射谱, 研究了其激光辐射机理和温度调谐特性. 利用正交偏光显微镜观察器件织构, 看到液晶微滴分散均匀, 尺寸较大, 约为80 μm, 并且微滴中液晶分子呈现平面态排列织构. 以532 nm的Nd:YAG固体激光器作为抽运源, 测得在634.5 nm和680.2 nm波长处出现了尖锐的激光辐射峰, 线宽分别约为0.25 nm, 0.29 nm. 并与染料掺杂胆甾相液晶激光器件进行比较. 升高器件温度, 其输出激光波长蓝移, 获得666.7 nm至643.9 nm共22.8 nm的调谐范围. 由实验结果分析得出, 激光辐射机理为光子禁带末端激光, 出射波长分别对应光子禁带的两个边沿.     

Calorimetric study of phase transitions in nanocomposites of quantum dots and a liquid crystal

The complex specific heat is measured over a wide temperature range for the liquid crystal (LC) 4-cyano-4-octylbiphenyl (8CB) and cadmium sulfate quantum dots (QDs) composites as a function of QD concentration. The thermal scans were performed under near-equilibrium conditions for all samples having QDs weight percent (φ_w) from 0 to 3wt% over a wide range of temperature well above and below the two transitions in pure 8CB. Isotropic (I) to nematic (N) and nematic to smectic-A (SmA) phase transitions evolve in character and their transition temperatures offset by (～2.3 to 2.6 K) lower for all composite samples as compared to that in pure 8CB. The enthalpy change associated with I–N phase transitions shows slightly different behavior on heating and cooling and it also shows crossover behavior at lower and higher QD content. The enthalpy change associated with N–SmA phase transitions is independent of QD loading and thermal treatment. Given the homogeneous and random distribution of QD in these nanocomposites, we interpret that these results as arising that the nematic phase imposes self-assembly on QDs to form one-dimensional arrays leading to QDs and induces net local disordering effect in LC media.     

Electro-optical properties of polymer stabilized cholesteric liquid crystal film

Liquid crystals (LCs) and polymers are extensively used in various electro-optical applications.In this paper,normal mode polymer stabilized cholesteric LC film is prepared and studied.The effects of chiral dopant and monomer concentrations on the electro-optical properties,such as contrast ratio,driving voltage,hysteresis width and response time,are investigated.The reasons of electro-optical properties influenced by the concentrations of the materials are discussed.Through the proper material recipe,the electro-optical properties of polymer stabilized cholesteric LC film can be optimized.     

Origin of helical morphology of polyacetylene fibrils synthesized in chiral nematic liquid crystal solvents

Polyacetylene fibrils synthesized in chiral nematic (N*) liquid crystals (LCs) as a polymerization solvent show clockwise helical morphology when a clockwise N* LC is used as a solvent and vice versa. To explain the concordant result in terms of screw direction, a model is proposed in which the conjugated molecule makes a loose loop with a radius of micrometer order, twisting clockwise or counterclockwise. The observed twisted morphology of the fibril can be explained by the condensation of many loosely looped molecular chains grown in a twisted domain of the N* LC solvent.     

Electro-optical Kerr effect in the isotropic phase of the two antiferroelectric liquid crystal mixtures

We have experimentally studied for the first time the electro-optical Kerr effect and the pre-transitional behavior in the isotropic phase of two antiferroelectric liquid crystal mixtures, W-232 and W-204D, composed of rod-like ester molecules exhibiting the direct smectic-A to isotropic (SmA-I) phase transition. The Kerr law has been confirmed for the two compounds and the variation of inverse Kerr constant with temperature above the smectic–isotropic transition temperatures were determined. Both the mixtures with very broad antiferroelectric phase around room temperature have similar sequence of the phases (i.e., Cr-SmC*A-SmC*-SmA-I). Although, the pre-transitional behavior is usually complex in the isotropic phase of the chiral smectic liquid crystal compounds, the investigated compounds showed a similar behavior compared to that of nematic–isotropic behavior.     

Optically switchable, polarization-independent holographic polymer dispersed liquid crystal (H-PDLC) gratings

An optically switchable, polarization-independent holographic polymer dispersed liquid crystal (H-PDLC) transmission grating is demonstrated by adding azobenzene-LC and chiral molecules into the H-PDLC formulation. The optical switchable mechanism is from the trans-cis photoisomerization of the doped azobenzene-LC, which modulates the refractive index of the LC rich area. The dependence of the diffraction efficiency of the H-DPLC grating without chiral molecules on light polarization suggests that the orientation of LC directors within the droplet is ellipsoidal and uniaxial. However, the addition of chiral molecules into the H-PDLC formulation helps the formation of isotropic and non-uniaxial LC directors within the droplets. The polarization properties of the grating are investigated and analyzed by the coupled and modified coupled wave theory with a model of sinusoidal dielectric modulation. The results show that the addition of chiral molecules changes the LC phase from nematic to chiral-nematic, where the grating efficiency, which is modulated by the photoinduced phase transition, is independent of the polarization of incident light. Our findings may help improve optical systems that utilize non-polarized light.     

The microscopic theory for BPI and BPII of the blue phase

A statistical theory of the blue phase of chiral liquid crystal is proposed. The form of the interaction potential between molecules is assumed to be the same as that used in the theory for cholesteric phase. Using the cell model of the liquid statistical theory, we deduced the distribution function of single molecule. The free energy of the system obtained has the same form as that of the Ginzberg－Landau phenomenology theory. The order-parameter tensors of BPI and BPII phases we calculated can be compared with the experimental results.     

Theoretical study of reorientation and torque of liquid crystal molecules under influence of external electric field and experimentally generation of spatial optical soliton beam and getting a sharp switching in chiral nematic liquid crystal

Liquid crystals (LC) are anisotropic materials which experience a torque if an electric field is present. This field can be due to an external voltage or to the presence of a light beam. Reorientation due to light leads to non-linear behavior in the optical behavior. Due to this kind of nonlinearity therefore it is possible to generate optical spatial soliton beam in LC by bias voltage or without it and interestingly chiral nematic liquid crystals has a opportunity to generate spatial optical solitons without the need for a bias voltage. In this paper we also demonstrate that a sharp switching of the helix structure occurs when the spatial soliton is launched in the middle of two regions where soliton generation is favorable. Due to the optical nonlinearity, the helical structure becomes asymmetric and a sharp switching in one direction can be obtained. Moreover, in this paper, the torque and reorientation of the liquid crystal and the change in angular momentum of the light are discussed.     

铁电液晶螺旋结构的理论近似研究

由铁电液晶（FLC）手性近晶C相下螺旋结构的理论近似计算得出FLC一个螺距内平均折射率的表达式,根据表达式可得出一个螺距内的FLC分子作为一个整体可以看成一个向列相液晶分子模型的结论.当FLC沿螺旋轴方向的厚度等于FLC螺距的整数倍时,液晶盒内垂直取向的FLC分子可看作向列相液晶模型组成的集合.ZLI-3654型FLC与5CB型向列相液晶的实验结果验证了上述结论,理论结果和实验结果一致.这一理论可为畸变螺旋FLC和垂直排列畸变螺旋FLC器件的制备以及FLC的应用提供理论指导和更深的认识. 关键词： 铁电液晶 螺距 平均折射率 向列相液晶     

Molecular structure and physical parameters of some ferroelectric liquid crystal systems

We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC^*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC^*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC^*LC properties.     

Effect of nonmesogenic impurities on the order of the nematic to smectic-A phase transition in liquid crystals

By means of adiabatic scanning calorimetry, we have studied the effect of the nonmesogenic solutes cyclohexane (CH) and biphenyl (BP) on the nematic-smectic-A phase transition in the liquid crystal octylcyanobiphenyl (8CB). For all concentrations of BP studied, the transition remains second-order. For 8CB+CH, however, crossover from second-order to first-order is observed at a tricritical point of the mole fraction x of CH around 0.0460. The difference between the two systems and the crossover is explained in terms of a mean-field free energy density expression including coupling terms of x with the nematic and smectic-A order parameters.     

Inhomogeneous nematic-isotropic phase transition of a thermotropic liquid crystal doped with iron oxide nanoparticles

We elucidate the local distribution and aggregated structure of iron oxide nanoparticles (IONPs; 6 nm in diameter) doped in the matrix of a nematic liquid crystal (LC), 4-cyano-4'-pentylbiphenyl, utilizing in situ cryogenic transmission electron microscopy and polarized light microscopy. We show that tens of IONPs aggregate into a sphere-like morphology, and the aggregates combine into elongated clusters with a length of hundreds of nm. With the IONP-doped LC matrix confined to a thin glass cell, we study the nematic-isotropic (N-I) phase transition, and suggest that local heterogeneity of LC textures as seen in polarized microscopy is caused by the existence of IONP aggregate clusters. These clusters act also as nuclei for the formation of isotropic domains upon heating.     

Structure,elasticity and phase transitions in liquid crystals with deformations

ABSTRACT

A molecular-statistical theory describing the nematic liquid crystals (LCs) with spherical inclusions (or point defects) is proposed. At given size of inclusions and nematic order parameters at the surfaces of inclusions (zero in the case of point defects) and far from inclusions (where the nematic LC is almost uniform), the distribution of nematic order parameters in the bulk of LC with inclusions was found to be fully determined by the elastic constants of LC. We have found and explained the two-step heat-driven transformation from the nematic phase into the isotropic phase, with the intermediate phase in between. The nematic order parameters and the elastic constants are evaluated in the framework of a unified approach based on the features of pair interaction potentials of the individual LC molecules. It is shown that, in the case of K₃₃ < K₁₁, the point defects should destroy the conventional nematic phase.