Photo-differential Scanning Calorimetry Study on Photopolymerization of Polyacrylate/E7 Liquid Crystal Blends

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics of a photo-initiated polymerization of tripropylene glycol diacrylate (TPGDA) monomers mixed with a eutectic liquid crystal mixture, E7, chosen due to its significant optical properties. The thermal photo-polymerization reactions were performed in isothermal mode in order to identify and evaluate the different thermal effects occurring during the photo-polymerization reaction under UV light. The results obtained help to better understand the curing mechanisms as well as the kinetics of the curing reactions. The isothermal photo-DSC provided complex thermograms showing the presence of two thermal effects: an exothermic heat (positive activation energy) generated during the conversion of monomers and an endothermic heat produced by the UV lamp. The latter phenomenon was observed in our previous work and was associated to the heating of the polymer dispersed liquid crystals (PDLC) material by the UV irradiation lamp. The dependence of photopolymerization on the time and intensities of the curing UV light were investigated. The results showed that the high intensity UV light heats the material and slows the process of phase separation which, in turn, affects the electro-optical properties of the material under study. The photo DSC results also revealed that the heating of the system was insignificant and can, therefore, be neglected, for materials exposed to UV radiation with an irradiation time of 5?seconds and a low intensity of 13.80?mW/cm².     

Increasing rewriting speed of optical rewritable e-paper by selecting proper liquid crystals

Effect of interaction between liquid crystal (LC) and photoalignment material on speed of optical rewriting process is investigated. The theoretical analysis shows that smaller frank elastic constant K₂₂ of liquid crystal corresponds to larger twist angle, which gives rise to larger rewriting speed. Six different LC cells with the same boundary conditions (one substrate is covered with rubbed polyimide (PI) and other with photo sensitive rewritable sulfuric dye 1(SD1)) are tested experimentally under the same illumination intensity (450 nm, 80 mW/cm²). The results demonstrate that with suitable liquid crystal, LC optical rewriting speed for e-paper application can be obviously improved. For two well known LC materials E7 (K₂₂ is larger) and 5CB (K₂₂ is smaller), they require 11 s and 6 s corresponding to change alignment direction for generating image information.     

Electro-optical characteristics of photoinduced isomerization in 2,2′-azobis[5,6-bis(dodecyloxy)quinoxaline]-azo-dye-doped nematic liquid crystals

The formation of photoinduced gratings was realized and dielectric properties were investigated in a nematic liquid crystal (LC) doped by 2,2′-azobis[5,6-bis(dodecyloxy)quinoxaline] azo dye (AD). The enhancement of the optical and electrical properties of the system was studied by dielectric spectroscopy and diffraction grating measurements. We report the electro-optical properties of LC cells containing pure LC (E7) and its doped form with 1% (wt/wt) 2,2′-azobis[5,6-bis(dodecyloxy)quinoxaline] AD. Diffraction efficiencies, η, of 441 nm pump and 632 nm probe beams were measured in two-wave-mixing experiment. Diffraction efficiency has a noteworthy increase in AD-doped nematic liquid crytals. A maximum diffraction efficiency of 14% was found for cell doped with AD, while cells without AD had a maximum efficiency of 1%. Photoinduced refractive index change Δn and nonlinear index coefficient n₂ were also determined by the two-wave-mixing method. The dielectric parameters of AD-doped E7 and pure E7 LC have been investigated in the frequency range 1 kHz-1 MHz by the dielectric-spectroscopy method. Dielectric anisotropy (Δε) property of the LCs changes from the positive type to negative type and dielectric anisotropy values decrease with AD.     

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.     

晶体电光和非线性光学效应的离子基团理论(Ⅰ)——利用氧八面体畸变模型计算BaTiO3晶体电光及倍频系数

本文对钙钛矿型晶体的电光及倍频效应的机理提出了一个(MO₆)离子基团模型。并从(TiO₆)离子基团的准分子轨道及晶格场位能作用下的(TiO₆)离子基团的离子键轨道出发,利用ABDP理论,分别计算了BaTiO₃的各个电光及倍频系数。计算结果表明,在没有引入任何可调整参量的情况下,用(TiO₆)离子基团的晶格场理论所得到的电光及倍频系数的计算值和实验值符合得相当好,而准分子轨道对这些效应的贡献很小。由     

溶液中向列型液晶有序度以及外场对它们的影响

根据Lebwohl-Lasher模型以及平均场近似得到了存在于外场中的溶液中向列型液晶分子有序度参数的自洽方程.无外场时,溶液中液晶发生一级相变,不同浓度相转变时2>均为0.324,T^N1与浓度成正比,T_φ^N1=T_φ=1^N1φ。而存在外场时,液晶发生二级相变,且都发生于2>=0.40处.液晶分子取向分布曲线为V型,均方偏差随2关键词：     

Phase transition behavior of hydrogen bonded liquid crystal (6BA)<Subscript>2</Subscript>-(BPy)<Subscript><Emphasis Type="Bold">x</Emphasis></Subscript> as studied by <Superscript>2</Superscript>H NMR

The thermal properties and the orientational order of hydrogen-bonded liquid crystals (6BA)₂-(BPy)_0.4 and (6BA)₂-(BPy)_0.3 (6BA: 4-n-hexylbenzoicacid, BPy: 4,4’-bipyridine) were investigated by DSC and ²H NMR. On cooling, isotropic liquid - liquid crystal phase transition temperatures were T _C= 409 and 405 K for (6BA)₂-(BPy)_0.4 and (6BA)₂-(BPy)_0.3, respectively. Thermal anomalies in the liquid crystal phase were observed at T _LC1= 402 and T _LC2= 375 K for (6BA)₂-(BPy)_0.4 and at T _LC1= 398 and T _LC2= 375 K for (6BA)₂-(BPy)_0.3. For (6BA)₂-(BPy)_0.4, only the smectic component was created above T _LC1. In addition, the nematic component was created below T _LC1. The nematic component gradually changed to the smectic component with decreasing temperature and only the smectic component was observed below T _LC2. For (6BA)₂-(BPy)_0.3, only the nematic component was created above T _LC2. The phase transition from the nematic phase to the smectic phase took place at around T _LC2.     

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.     

A study of the structural phase transition in strontium titanate single crystal by coherent and incoherent second optical harmonic generation

The surface phase transition in a SrTiO₃ crystal was studied by second optical harmonic generation. Nonlinear optical response singularities were observed at temperature T*=145 K, which was 40 K higher than the T_c structural phase transition temperature in the crystal volume. Nonlinear critical opalescence in the crystal volume caused by the presence of point defects was studied. The second harmonic field and the intensity of critical opalescence were calculated based on the phenomenological model of nonlinear optical processes with the use of the Landau theory of phase transitions.     

Orientational fluctuations and phase transitions in 8CB confined by cylindrical pores of the PET film

Abstract

Results of optical investigations of the isotropic-nematic and nematic-smectic A phase transitions in porous polyethyleneterephthalate (PET) films filled with octyl-cyanobihenyl (8CB) liquid crystal (LC) are reported. Samples of porous films of thickness 23 µm with normally oriented cylindrical pores of a radius R ranging from 10 nm to 1000 nm were prepared using the track-etched membrane technology. The dynamic light scattering method was used to probe the nematic orientational fluctuations of confined LC samples. The corresponding relaxation time τ was measured as a function of R and temperature T at slow enough cooling rates (0.3–0.6 K/h) to locate the phase transition temperatures. Changes in τ(T) dependencies relatively sensitivity fingerprint the LC phase transformations. Experimental results are analysed using the Landau-de Gennes-Ginzburg phenomenological approach.     

Increasing the rewriting speed of optical rewritable e-paper by selecting proper liquid crystals

The effect of interaction between liquid crystal(LC) and photoalignment material on the speed of optical rewriting process is investigated.The theoretical analysis shows that a smaller frank elastic constant K 22 of liquid crystal corresponds to a larger twist angle,which gives rise to a larger rewriting speed.Six different LC cells with the same boundary conditions(one substrate is covered with rubbed polyimide(PI) and the other with photo sensitive rewritable sulfuric dye 1(SD1)) are tested experimentally under the same illumination intensity(450 nm,80 mW/cm 2).The results demonstrate that with a suitable liquid crystal,the LC optical rewriting speed for e-paper application can be obviously improved.For two well known LC materials E7(K22 is larger) and 5CB(K22 is smaller),they require 11 s and 6 s corresponding to change alignment direction for generating image information.     

Electro-optical properties of high birefringence liquid crystal compounds with isothiocyanate and naphthyl group

Liquid crystal(LC) compound with isothiocyanate and naphthyl group is an attractive high birefringence LC material,and can be used in optical devices. In this paper, the electro-optical properties of a series of this type of LC compounds were investigated. The melting points and enthalpy values of these LC compounds were higher than those of corresponding compounds with the phenyl group. These compounds exhibited high birefringence with a maximum value of 0.66. Fluorine substitution in the molecular almost does not affect the birefringence value. When these LC compounds with the naphthyl group were dissolved in a commercial LC mixture, the electro-optical properties depending on temperature were investigated. In the low-temperature region, LC mixtures with the naphthyl-group LC compounds exhibited higher viscosity than pure commercial LCs. In the high-temperature region, viscosity values very closely approached each other. When response performance was investigated, figure-of-merit(Fo M) values were measured. The Fo M values of LC mixtures containing LC compounds with naphthyl group were lower than those of reference benzene LCs in the low-temperature region. However, in the high-temperature region, the results were reversed. These isothiocyanate LC compounds with naphthyl group can be applied in special fast-response LC device, particularly the ones used under high-temperature conditions.     

Effect of the la concentration on the dielectric and optical properties of the transparent ferroelectric ceramics 75PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-25PbTiO<Subscript>3</Subscript>

The dielectric, optical, and electro-optical properties of transparent ferroelectric ceramics 75PbMg_1/3Nb_2/3O₃-25PbTiO₃ (75PMN-25PT) with different La concentrations (2, 3, and 4 at %) have been studied. It has been shown that all the samples under study undergo a diffuse phase transition, and the degree of diffuseness increases with an increase in the La concentration. The temperature dependences of the optical transmission in different regimes of applying an electric field and the quadratic electro-optical effect have been investigated. It has been found that, at room temperature, the maximum electro-optical effect is observed in the ceramics with a La concentration of 3 at %. This is explained by the fact that the measurement temperature is close to the Vogel-Fulcher temperature at which the minimum electric field is required to induce ferroelectric states from the relaxor phase.     

Electro-tunable one-dimensional photonic crystal structures based on grooved silicon infiltrated with liquid crystal

The paper reports on composite periodic structures fabricated by means of wet anisotropic etching of (1 1 0)-oriented Si on a SOI platform and infiltrated with liquid crystal E7. The electro-optical effect under low voltages was registered for inter-digital structures by both optical microscopy and micro-Raman spectroscopy.     

A.C. specific heat of terbium and dysprosium

The dependence of a.c. specific heat measurements upon the temperature modulation amplitude ΔT has confirmed the first-order nature of the antiferromagnetic-ferromagnetic transition in single crystal terbium (T_C～221K) and polycrystalline dysprosium (T_C～85K). No peak in a.c. specific heat is obtained for 2ΔT less than the temperature hysteresis at the transition. By comparison, the peaks expected in a.c. specific heat were observed at the higher-order antiferromagnetic-paramagnetic transition in both materials for ΔT amplitudes as low as～40 mK.     

Structural, magnetic and magnetotransport behavior of some Nd-based perovskite manganites

A systematic investigation of Neodymium based colossal magnetoresistive manganites with general formula Nd_0.67A_0.33MnO₃, (A=Ca, Sr, Pb and Ba) has been undertaken mainly to understand their structural, magnetic as well as electrical behavior. The materials were prepared by the sol-gel route sintering at 900 °C. After usual characterization of the materials structurally by XRD, their metal-insulator transition (T_P) as well as magnetic transition (T_C) temperatures were determined and the reasons for the occurrence of ΔT_T(T_C-T_P) values have been explained. X-ray data have been analyzed by using Rietveld analysis and the variations of various parameters are explained. It has been concluded that not only A-site cation radius, 〈r_A〉 but also the size variance factor (σ²) influence both the metal-insulator as well as ferro to para magnetic transition temperatures. A systematic study of electrical conductivity of all the four materials was undertaken as a function of magnetic field upto 7 T mainly to understand the conduction mechanism in the presence of magnetic field. On analyzing the electrical resistivity data, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below T_P) can be explained by electron-electron scattering processes (∼T²) and two magnon scattering processes (∼T^4.5), while in the high temperature (T>T_P) paramagnetic insulating regime, the adiabatic small polaron and variable range hopping models are found to fit well.     

晶体电光和非线性光学效应的离子基团理论(Ⅲ)——利用畸变氧八面体的离子基团模型计算LiNbO3,LiTaO3,KNbO3,BNN晶体的电光和倍频系数

本文用文献[1]中所提出的(MO₆)离子基团模型,计算LiNbO₃,LiTaO₃,KNbO₃,BNN等晶体的电光和倍频系数。假设在O_h对称时,这些晶体中的氧八面体具有相同的能级和对称波函数,则通过群表示理论就可得到在C_3ν,C_2ν对称性时氧八面体的能级和对称波函数,并进而用ABDP理论计算它们的电光和倍频系数。计算结果和实验符合得相当好。本文并从理论上解释了这些晶体的倍频系数大小、符号和氧八面体畸变间的关系,由此可以得到以下两个结论:(1)畸变氧八面体的离子基团模型不但适用于钙钛矿型材料,同时也适用于钨青铜型、LiNbO₃型材料。(2)在钨青铜型、LiNbO₃型的材料中,仍是“离子键”对电光和倍频效应作出主要贡献,同时由于LiTaO₃的共价键成份比LiNbO₃大,因而LiTaO₃的非线性光学效应比LiNbO₃小。     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Entropy-driven aggregation of adhesion sites of supported membranes

A large number of interesting phenomena related to the insertion of colloidal particles in liquid crystals (LC) have recently been reported. Here, we investigate effects caused by the addition of spherically shaped ferroelectric nanoparticles to a nematic liquid crystal. Using molecular dynamics (MD) simulations, the density of LC molecules, the orientational order parameter, and the polar and azimuthal angle profiles are calculated as functions of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the assembly of ferroelectric nanoparticles enhances the nematic order in the LC medium changing many properties of its host above the nematic-isotropic transition temperature T ^* _NI .     

Influence of Chemical Environment on the Optical Properties in Transition Metal Ions Doped Materials

The sensibility of luminescent properties in transition metal doped materials to the matrices' chemical environment is explained in this paper, this is because of their strong phonon-electron coupling which are caused by the 3d electrons exposed nature. The influence of the chemical environments on the Mn²⁺-doped materials' optical properties, including the structure type of coordinate polyhedron, the polyhedral bridge linking manner and the lattice parameter, was illustrated in detail in this work. The impact of crystal field strength parameter (10 Dq) on the maximum energy differentiae in spontaneous emission band of Cr³⁺:⁴T_2g→⁴A_2g and in excited state absorption band ⁴T_2g→⁴T_1g (⁴F), and covalent bond intension's impact on the optical properties of Os⁴⁺ were also analyzed. This work's purpose is to discover the principle of the sensibility character, then we can use it to optimal the design of materials in order to find the excellent luminescent materials for practical utilization.