Although discotic liquid crystals are attractive functional materials, their use in electronic devices is often restricted by high melting and clearing points. Among the promising candidates for applications are [15]crown‐5 ether‐based liquid crystals with peripheral n‐alkoxy side chains, which, however, still have melting points above room temperature. To overcome this problem, a series of o‐terphenyl and triphenylene [15]crown‐5 ether derivatives was prepared in which δ‐methyl‐branched alkoxy side chains of varying lengths substitute the peripheral linear alkoxy chains. The mesomorphic properties of the novel crown ethers were studied by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction. δ‐Methyl branching indeed lowers melting points resulting in room‐temperature hexagonal columnar mesophases. The mesophase widths, which ranged from 87 to 30 K for o‐terphenyls, significantly increased to 106–147 K for the triphenylenes depending on the chain lengths, revealing the beneficial effect of a flat mesogen, due to improved π–π interactions. 相似文献
In this study, chitosan‐zinc oxide nanoparticles were used as a sorbent of miniaturized matrix solid‐phase dispersion combined with flotation‐assisted dispersive liquid–liquid microextraction for the simultaneous determination of 13 n‐alkanes such as C8H18 and C20H42 in soil samples. The solid samples were directly blended with the chitosan nanoparticles in the solid‐phase dispersion method. The eluent of solid‐phase dispersion was applied as the dispersive solvent for the following flotation‐assisted dispersive liquid–liquid microextraction for further purification and enrichment of the target compounds prior to gas chromatography with flame ionization detection. Under the optimum conditions, good linearity with correlation coefficients in the range 0.9991 < r2 < 0.9995 and low detection limits between 0.08 to 2.5 ng/g were achieved. The presented procedure combined the advantages of chitosan‐zinc oxide nanoparticles, solid‐phase dispersion and flotation‐assisted dispersive liquid–liquid microextraction, and could be applied for the determination of n‐alkanes in complicated soil samples with acceptable recoveries. 相似文献
Liquid crystal trimers based on the hydrogen bonding dimerization of 4‐{n‐[4‐(4‐m‐alkoxy‐phenylazo)phenoxy]alkoxy}benzoic acid (BAm‐n) have been synthesized and characterized. Temperature‐dependent FTIR spectroscopic studies showed that the carboxylic acid groups in BAm‐n are associated to form H‐bonded cyclic dimers both in their crystalline and liquid crystalline phases. The trimers exhibited enantiotropic liquid crystalline behaviour except for BA1‐3 which showed monotropic behaviour, and the mesophases changed from nematic to smectic phase, with the increase of length of the spacer and the terminal substituents. Pronounced odd–even effects in the melting temperatures, clearing temperatures and nematic–isotropic enthalpy changes were observed. 相似文献
We study the important n‐cyanobiphenyl (with n= 4–8) series of mesogens, using modelling and molecular dynamics simulations. We are able to obtain spontaneously ordered nematics upon cooling isotropic samples of 250 molecules. By using the united‐atom force field developed herein, we show that the experimental isotropic–nematic transition temperatures are reproduced within 4 K, allowing a molecular‐level interpretation of the odd–even effect along the series. Other properties, like densities, orientational order parameters and NMR residual dipolar couplings are also reproduced well, demonstrating the feasibility of predictive in silico modelling of nematics from the molecular structure. 相似文献
A general model for all kinds of size‐dependent melting points, free of any adjustable parameter, is extended to illustrate the size‐dependence of the melting temperature of polyethylene (PE). The model prediction for the depression of the melting temperature of PE is consistent with the calorimetric experimental results as shown in the Figure where the melting temperature of lamellae PE crystals as a function of the thickness of the crystals is presented.
The Tm(D) function of PE shown as a solid line in terms of Equation ( 4 ). The necessary parameters for use of Equation ( 4 ) are shown in Table 1 . The symbols ○ and • denote the experimental evidence of Tm(D) of linear alkanes 14 and cyclic alkanes, 14 respectively. The values of D are determined by D = 0.1273n 6 with 0.1273 nm being the orthorhombic C C lattice distance in the c‐direction. 5 The symbol □ denotes the experimental evidence of chain‐extended PE. 6 相似文献
Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C1–n‐C4 alkanes on Zn‐modified high‐silica zeolites ZSM‐5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C? H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n‐butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C1–n‐C4 alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites. 相似文献
Crystal structures and thermal properties of cobaltocenium salts with bis(perfluoroalkylsulfonyl)amide (CnF2n+1SO2)2N anions [n=0 ( 1 ), 1 ( 1 a ), 2 ( 1 b ), 3 ( 1 c ), and 4 ( 1 d )] and the 1,1,2,2,3,3‐hexafluoropropane‐1,3‐disulfonylamide anion ( 2 ) were investigated. In these solids, the cations are surrounded by four anions around their C5 axis, and stacking of these local structures forms two kinds of assembled structures. In the salts with even n ( 1 , 1 b , and 1 d ), the cation and anion are arranged alternately to form mixed‐stack columns in the crystal. In contrast, in the salts with odd n ( 1 a and 1 c ), the cations and anions independently form segregated‐stack columns. An odd–even effect was also observed in the sum of the phase‐change entropies from crystal to melt. All of the salts exhibited phase transitions in the solid state. The phase transitions to the lowest‐temperature phase in 1 , 1 a , and 2 are accompanied by order–disorder of the anions and symmetry lowering of the space group, which results in the formation of an ion pair. Solid‐state 13C NMR measurements on 1 a and 1 b revealed enhanced molecular motions of the cation in the higher‐temperature phases. 相似文献
The available data on the melting and boiling points in homologous series of normal 1-fluoro-, 1-chloro-, 1-bromo-, and 1-iodoalkanes were critically analyzed. A correlation was observed between the melting and boiling points of halogenated even and odd n-alkanes. This allowed us to obtain an interpolation equation for predicting compound melting points. 相似文献