Laser photolysis of zinc porphyrin dissolved in cyanohexylbiphenyl liquid crystal

A laser photolysis study of ZnTPP (P) oriented in nematic and isotropic cyanohexylbiphenyl (6CB) as a function of added 1.4-benzoquinone (Q) is reported. In the absence of Q, enhancement of triplet absorption below the clearing point (nematic phase) is observed. It is attributed to the improved alignment of the optical transition in the ordered matrix and also to the increase in the intersystem crossing efficiency. T−S₁. In the presence of Q in the nematic phase. An additional increase in triplet absorption is noticed. This result is interpreted in terms of a triplet radical pair [P^+√…Q^−√]^T,RP formation which is facilitated by the ordering in the liquid crystal, thus providing an additional channel for triplet formation. The triplet P^T in 6CB (nematic or isotropic) is quenched with a second-order rate of ≈10⁸ M⁻¹ s⁻¹ as compared to 2 × 10⁹ M⁻¹ s⁻¹ in toluene.     

Theory of the Nematic-Isotropic Transition in a Restricted Geometry

We discuss, using a Landaude Gennes formalism, the nematic-isotropic transition temperature for a system placed between two parallel plates, subject to identical homeotropic or homogeneous boundary conditions at each plate. The temperature at the phase transition may increase or decrease as the inverse sample thickness, D^-1, increases, depending on the nature of the boundary conditions. In all cases the transition terminates at a critical point for sufficiently large D^-1, beyond which the nematic and isotropic phases are no longer distinct. The phase transition temperature is well described by a liquid crystal analogy of the Kelvin equation which can be generalized to give an exact Clausius-Clapeyron relation. Under many circumstances the system behaves from a thermodynamic point of view as though it were in a bulk ordering field. The finite geometry restricts the growth of nematic or isotropic wetting films. We discuss the disjoining pressure experiment of Horn, Israelachvili and Perez [15]. Finally we place our work in the context of recent progress in the statistical mechanics of surfaces and systems in restricted geometries.     

Optical spectra of Yb and Yb–Nd interaction in Cs2NaNd0,4Yb0,6Cl6

Interaction between octahedrally coordinated Nd³⁺ and Yd³⁺ in Cs₂NaNd_0,4Yb_0,6Cl₆ reduces the Nd³⁺ luminescence lifetime by roughly two orders of magnitude with respect to that found in Cs₂NaNdCl_{6– · –} Analysis of low temperature absorption and emission spectra reveals that the nonradiative Nd³⁺–Yb³⁺ energy transfer has to be assisted by lattice phonon emission, nevertheless the rate of the transfer is high in the 4–300 K temperature region and attains 5.8×10⁵s^-1 at room temperature. A phase transition of Cs₂NaNd_0,4Yb_0,6Cl₆ between 12 and 13 K is evidenced by abrupt change of both the spectra and lifetimes of Yb³⁺. Reduction of Yb³⁺ lifetime from 5.3 ms to 150 μs is at the transition from low symmetry phase to high symmetry phase is supposed to be associated with a three ion interaction which occurs in ordered lattice and disappears in low temperature disordered structure.     

Mass transport in a chiral nematic/nematic liquid crystal system

The optical microscopic mass transport technique has been used to study diffusion phenomenon in a chiral nematic/nematic solute/solvent mixture. Analysis of the concentration-distance, concentration-time and distance-time of the diffusion profile gave the diffusion coefficient of the system as a function of time, distance and concentration, respectively. The mutual diffusion coefficient of the system was independent of the distance and time, showing an average value of 2.65 × 10^-7 cm² s^-1. In non-steady state diffusion, the diffusion coefficient was dependent on both distance and time. The diffusion coefficient exhibited an inverse relation with the local concentration of the chiral solute. The self-diffusion coefficient of the nematic solvent gave a value of 3.4 × 10^-7 cm² s^-1 via extrapolation to zero concentration of the solute.     

Low-temperature heat capacity and thermodynamic properties of crystalline [Re2(Ala)4(H2O)8](ClO4)6 (Re = Eu, Er; Ala = alanine)

[Re₂(Ala)₄(H₂O)₈](ClO₄)₆ (Re=Eu, Er; Ala=alanine) were synthesized, and the low-temperature heat capacities of the two complexes were measured with a high-precision adiabatic calorimeter over the temperature range from 80 to 370 K. For [Eu₂(Ala)₄(H₂O)₈](ClO₄)₆, two solid–solid phase transitions were found, one in the temperature range from 234.403 to 249.960 K, with peak temperature 243.050 K, the other in the range from 249.960 to 278.881 K, with peak temperature 270.155 K. For [Er₂(Ala)₄(H₂O)₈](ClO₄)₆, one solid–solid phase transition was observed in the range from 270.696 to 282.156 K, with peak temperature 278.970 K. The molar enthalpy increments, ΔH_m, and entropy increments,ΔS_m, of these phase transitions, were determined to be 455.6 J mol⁻¹, 1.87 J K⁻¹ mol⁻¹ at 243.050 K; 2277 J mol⁻¹, 8.43 J K⁻¹ mol⁻¹ at 270.155 K for [Eu₂(Ala)₄(H₂O)₈](ClO₄)₆; and 4442 J mol⁻¹, 15.92 J K⁻¹ mol⁻¹ at 278.970 K for [Er₂(Ala)₄(H₂O)₈](ClO₄)₆. Thermal decompositions of the two complexes were investigated by use of the thermogravimetric (TG) analysis. A possible mechanism for the thermal decomposition is suggested.     

Dielectric studies of a laterally-linked siloxane ester dimer

This paper reports measurements of the dielectric response over the frequency range 10² to 10⁹ Hz of a liquid crystal dimer consisting of two ester mesogens laterally linked by an alkoxy chain containing a siloxane group. The synthesis and phase behaviour of the siloxane dimer are also reported. Results show that there are two relaxations in the isotropic phase and four in the nematic phase of the material. The possible molecular origins for these modes are given. It is found that there is a coupling between internal and external modes which gives rise to a cooperative mode as the temperature in the nematic phase is lowered towards a glass transition.     

Kinetics of the nematic ordered phase growth during a temperature quench of an isotropic siloxane-azomethine polymer

Kinetics of the nucleus growth during a deep temperature quench across the isotropic to nematic phase transition was experimentally investigated for a siloxane-azomethine polyether at cooling rates of 10 and 20°C min^-1. Nematic droplets revealed in the optical images during the phase separation were treated statistically and the resulting statistical size distributions were described using the model of reversible aggregation. Analysis of the time-dependent distribution parameters allowed two processes involved in liquid crystal phase ordering to be identified: nucleus growth and nucleus coarsening. Both regimes are quantitatively described using the universal growth law.     

Observation of textures of nematic polymers and estimation of the elastic constants

The texture of a copolyesteramide, Vectra® B950, has been studied by optical microscopy. At rest, the defects are typical of a biaxial nematic phase, but under the influence of a magnetic field or under shear, the nematic transforms into a uniaxial phase. The Freedericksz geometry yields a K₁ elastic constant of the order of 10^-8 N. The diffusivities K_III/_n and K_II/_n are obtained by measuring the light intensity transmitted through a sample during shear start-up and relaxation, and from this we estimate K_III ≅ 10 ^-9N and K_II ≅ 10 ^-10N. Since the biaxiality is small, these uniaxial constants adequately describe the phase at rest.     

Decylammonium chloride/ammonium chloride aqueous solutions A Raman scattering study

Aqueous solutions of decylammonium chloride and decylammonium chloride/ammonium chloride between 20 and 50 per cent of DACl have been studied by Raman scattering. The spectral behaviour was followed as a function of temperature and phase transitions were observed for all samples. We have focused our study on the intensity variations of both the symmetric and the antisymmetric stretching vibration modes of CH₂ groups situated in the region 2800-3000cm^-1. The study of the high frequency part of the spectrum showed the characteristics of order disorder transitions already studied for other materials. We have observed that the existence of a nematic phase in the ternary systems is caused by the addition of the salt.     

High precision density studies near the smectic A-nematic tricritical point

We have carried out very accurate density measurements (with a precision of ±5 × 10^-5g cm^-3) near the smectic A-nematic transition in binary mixtures of 4-n-nonyl-4'-cyanobiphenyl (9CB) and 4-n-decyl-4'-cyanobiphenyl (10CB). The transition crosses over from second to first order as the temperature range of the nematic phase decreases. For mixtures with the shortest nematic range the data deviate noticeably from a single power law behaviour. Such a deviation is an indication of the first order nature of the transition. Very good fits to a single power law have been obtained for pure 9CB and the x = 0·04 mixture where x is the mole fraction of 10CB in 9CB. The critical exponent obtained from the power law fitting has enabled us to locate the tricritical point to be very close to x=0·04, which is in agreement with the results obtained previously by high resolution calorimetric [1] and X-ray scattering studies [2].     

Phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D2O system as determined by 2H and 14N NMR

The high resolution phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D₂O system has been mapped out using ²H and ¹⁴NNMR spectroscopy. The ¹⁴N quadruple splittings are more than an order of magnitude larger than corresponding ²H splittings, while the line widths are only two to three times larger. Thus, ¹⁴NNMR offers an order of magnitude improvement over ²H NMR in the resolution of the spectra from coexisting phases. The ²H spectra of samples in biphasic regions are often complicated by chemical exchange of D₂O molecules between coexisting phases, particularly at low TMAHFN concentrations. Analysis of the ²H line shapes of a TMAHFN/D₂O sample with a weight fraction of TMAHFN of 0.230 obtained at various times following cooling of the sample into the isotropic/nematic biphasic region shows that the mean diameter for the dispersed nematic droplet grows from about 7 to about 26 μm over a period of 2 h. At a mean droplet size of 7 μm the exchange of TMA⁺ ions between the coexisting phases is slow on the NMR time-scale and exchange effects are not observed in ¹⁴N spectra. The TMAHFN/D₂O phase diagram exhibits the generic form of those of the CsPFO/water and APFO/D₂O systems, which are the only other systems composed of stable discotic micelles for which high resolution phase diagrams are currently available, but the nematic phase is displaced to smaller TMAHFN concentrations. Specifically, a discotic nematic phase N_D⁺, intermediate between an isotropic micellar phase I and a lamellar phase L, exists for weight fractions of TMAHFN between 0.149 (φ = 0.105) and 0.420 (φa = 0.325) and temperatures between 277.3 and 327.6 K.     

X-ray diffraction, Raman study and electrical properties of the new mixed compound [Rb0.44(NH4)0.56]2HgCl4 · H2O

Differential scanning calorimetry of [Rb_0.44(NH₄)_0.56]₂HgCl₄ · H₂O material showed three anomalies at 340, 355 and 424 K, respectively. The room temperature phase has space group Pcma (a=8.433(1) Å, b=9.1817(9) Å and c=11.954(1)). Phase II (T=350 K) is disordered and exhibits orthorhombic symmetry (a=8.456(13), b=9.202(9) and c=12.011(10) Å). Hydrogen bonding, the nature and the degree of structure (dis)order and the mechanisms of the transitions are discussed. The dielectric constant ^′ at different frequencies and temperature revealed a phase transition at T=340 K related to NH₄⁺ reorientation and H⁺ diffusion, and a characteristic increase above 355 K, which might be due to loss of water of crystallization. Transport properties in this compound appear to be due to an Rb⁺/NH₄⁺ and H⁺ ions hopping mechanism.     

X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines

We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4'-methoxyazoxybenzene two types of layering were found. One corresponds to the fluctuations of the smectic density wave with a monolayer wavevector q₁, and the other is due to the partial bilayer fluctuations with the incommensurate wavevector q₂ ≠ q₁/2. The temperature dependences of the X-ray scattering intensity and the longitudinal correlation length for both types of layering in the nematic phase are presented. The critical behaviour in the vicinity of the smectic A-nematic phase transition occurs for a fluctuation mode, either q₁ or q₂, depending on the position on the liquid crystal phase diagram. The influence of the molecular structure of cyano-substituted pyridines on the formation of layered structures of different types is also discussed.     

Raman spectroscopy of liquid crystalline N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]-3-aminopyridines

New homologous series of N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]-3-aminopyridines were synthesized, they exhibited a nematic phase. The temperature dependence of their Raman spectra has been observed in the spectral range 900-1800 cm^-1. Some Raman bands show a marked change in their intensity and frequency through the phase transition from crystalline solid to nematic. These bands are ascribed to the vibrational modes related to the core part of the molecule. Such behaviour can be explained by the change of molecular conformation related to the core. Some members of these series exhibited photochromism in the solid state.     

Thermal Characteristics of Room Temperature Inorganic Phase Change System Containing Calcium Chloride

A novel inorganic calcium-based phase change material(PCM-Ca) consisted of 47.1%(mass fraction) water, 47.7% calcium chloride, 2% potassium nitrate, 2% potassium bromide and 1.2% strontium chloride with a solid-liquid phase change temperature of 21.4 ℃ was investigated systematically. Among the components of PCM-Ca, calcium chloride and water act as the latent heat storage materials, and potassium nitrate, potassium bromide and strontium chloride work as the modifier, thickener and nucleating agent, respectively. Thermochemical properties including melting point, latent heat, density and thermal conductivity of the PCM-Ca were measured experimentally. The experimental results indicate that the melting latent heat, thermal conductivity at the melting point and density at room temperature for the PCM-Ca are 203.3 kJ/kg, 1.3637 W·m^-1·K^-1 and 1.55×103 kg/m³, respectively. Moreover, a thirty-run-cycling test showed that the PCM-Ca has a good thermal characteristic with no phase segregation or supercooling, and the maximum deviations of latent heat and phase change temperature are only 0.2% and 1.6%, respectively.     

Dilatometric and high temperature X-ray diffractometric studies of La1−xMxCrO3 (M=Sr, Nd, x=0.0, 0.05, 0.10, 0.20 and 0.25) compounds

The phase transition, bulk and lattice thermal expansion behaviour of the strontium and neodymium substituted lanthanum chromites have been studied by dilatometry and high temperature X-ray powder diffractometry from room temperature to 1123 and 1073 K, respectively, in static air. The studies revealed that the temperature of the orthorhombic to rhombohedral phase transition, which occurred at 550 K in undoped LaCrO₃, decreased on substitution of Sr²⁺ ions and increased on substitution of Nd³⁺ ions, systematically. However, the coefficients of average linear and volume thermal expansion (_l and _v) of LaCrO₃ showed a marginal increase on Sr²⁺ substitution to different extent, whereas a reverse trend was observed with Nd³⁺ substitution. The phase transition temperatures and _l and _v of the compounds as determined by dilatometric and high temperature X-ray diffractometric methods are reported.     

可快速充放电聚吡咯/碳纳米管复合材料电化学聚合与表征

采用恒电流法制备了具有可快速充放电性能的对甲基苯磺酸根(TOS-)掺杂聚吡咯/功能化单壁碳纳米管(PPy-TOS/F-SWNTs)复合材料,扫描电镜(SEM)结果表明该复合材料呈纳米棒状构成的多孔结构,棒径约为70nm;比表面积(BET)测试分析表明该复合材料有着较高的比表面积(12.64m2.g-1)和大的介孔孔隙率(20-40nm).循环伏安(CV)、电化学阻抗谱(EIS)和恒电流充放电(GC)电化学分析表明该材料具有优异的快速充放电性能,在800mV的电位窗和2.5A.g-1(功率密度为2kW.kg-1)的电流密度下该材料具有211F.g-1的比容量(能量密度为18.7Wh.kg-1),而当充放电电流高达80A.g-1(功率密度为60kW.kg-1)时比容量仍可达141.8F.g-1(能量密度为12.6Wh.kg-1),同时该材料还表现出优异的稳定性,在10A.g-1大电流下经历10000圈循环后容量仍保持95.2%.     

Extraction and DFT study on the complexation of H3O with hexaethyl p-tert-butylcalix[6]arene hexaacetate

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H₃O⁺(aq) + 1· Na⁺(nb)  1·H₃O⁺(nb) + Na⁺(aq) taking place in the two-phase water–nitrobenzene system (1 = hexaethyl p-tert-butylcalix[6]arene hexaacetate; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K_ex (H₃O⁺, 1·Na⁺) = −0.6 ± 0.1. Further, the stability constant of the 1·H₃O⁺ complex in water saturated nitrobenzene was calculated for a temperature of 25 °C as log β_nb (1·H₃O⁺) = 6.8 ± 0. 2. By using quantum mechanical DFT calculations, the most probable structure of the 1·H₃O⁺ complex species was predicted. In this complex, the hydroxonium ion H₃O⁺ is bound partly to three carbonyl oxygen atoms by strong hydrogen bonds and partly to three alternate phenoxy oxygens by somewhat weaker hydrogen bonds.     

Membrane properties of mixed ganglioside GM1/phosphatidylcholine monolayers

The interaction between ganglioside G_M1 (GM1) and --dipalmitoylphosphatidylcholine (DPPC) in mixed monolayers was investigated using surface pressure measurements and atomic force microscopy (AFM), and the effects of GM1, surface pressure and temperature on the properties of the membranes were examined. Mixed GM1/DPPC monolayers were deposited on mica using the Langmuir–Blodgett (LB) technique for AFM. GM1 and DPPC were miscible below the 0.2 mole fraction of GM1 and there was attractive interaction between GM1 and DPPC. The AFM images for the GM1/DPPC monolayers (X_GM1 < 0.2) at 30 mN m⁻¹ and 25 °C indicated a percolation pattern which means a micro phase separation: namely, the mixed film composed of GM1 and DPPC phase-separated from the DPPC liquid-condensed film. The AFM images for the mixed monolayers at 33 mN m⁻¹ indicated a specific morphology when the surface pressure was varied from 30 to 40 mN m⁻¹. The percolation pattern in the AFM image at 25 °C came to be destroyed with increasing temperature and completely disappeared at 45 °C. The change in the morphology of mixed GM1/DPPC monolayers on varying the surface pressure and temperature is thought to be related to signal transduction and a preventive mechanism against viral infections in the human body.     

An X-ray scattering study of the lamellar/nematic/isotropic sequence of phases in decylammonium chloride/H2O/NH4Cl

An X-ray scattering study is presented of the lamellar/nematic/isotropic sequence in the lyotropic system DACI/H₂O/NH₄Cl. The whole reciprocal space of monocrystalline samples oriented in magnetic fields are reconstructed from their two dimensional sections on photographic films. Intense diffuse scatterings are observed in the lamellar phase, around and away from the Bragg spots. Their evolution close to the lamellar/nematic transition reveals the presence of intense structural fluctuations. They take place over temperature ranges which are significantly greater than those associated with the smectic/nematic transitions in thermotropic liquid crystals. A similar situation is observed in the isotropic phase in the vicinity of the nematic/isotropic transition.