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排序方式: 共有215条查询结果,搜索用时 15 毫秒
1.
Holger Poths Erik Wischerhoff Rudolf Zentel Axel Sch nfeld Guido Henn Friedrich Kremer 《Liquid crystals》1995,18(5):811-818
Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarization Ps. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local β-relaxation a similar temperature dependence of the relaxation times τ for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer. 相似文献
2.
Patrick Mather Nino Grizzuti Glenn Heffner Mathias Ricker Willie E. Rochefort Markus Seitz Hans-Werner Schmidt Dale S. Pearson 《Liquid crystals》1994,17(6):811-826
In this paper, we report on the synthesis and detailed characterization of a new semiflexible nematic liquid crystalline polyester which could serve as a 'model' polyester for a variety of physical and physico-chemical investigations. The polymer is a nematic liquid over a wide temperature range-from the glass transition temperature at ∼95°C to the isotropic transition at ∼240°C. We expect this polyester to be particularly useful for studying the effect of flow on the orientation of liquid crystalline polymers, as well as the production and removal of disclinations. 相似文献
3.
Leusser D Henn J Kocher N Engels B Stalke D 《Journal of the American Chemical Society》2004,126(6):1781-1793
To elucidate the bonding situation in the widely discussed hypervalent sulfur nitrogen species, the charge density distributions rho(r) and related properties of four representative compounds, methyl(diimido)sulfinic acid H(NtBu)(2)SMe (1), methylene-bis(triimido)sulfonic acid H(2)C[S(NtBu)(2) (NHtBu)](2) (2), sulfurdiimide S(NtBu)(2) (3), and sulfurtriimide S(NtBu)(3) (4), were determined experimentally by high-resolution low-temperature X-ray diffraction experiments (T = 100 K). This set of molecules represents an ideal frame of reference for the comparison of SN bonding modes, because they contain short formal S=N double bonds as well as long S-N single bonds, some of them influenced by inter- or intramolecular hydrogen bonds. For comparison, the gas-phase ab initio calculations of the four model compounds, H(NMe)(2)SMe, H(2)C[S(NMe)(2)(NHMe)](2), S(NMe)(2), and S(NMe)(3), were performed. The topological features were found to be not particularly sensitive with respect to different substituents R (R = H, Me, tBu). In this paper, it is documented that theory and experiment differ in the eigenvalues of the Hessian matrix because of systematically differing positions of the bond critical points but agree very well concerning the spatial Laplacian distribution and the distinct polarization of all investigated sulfur-nitrogen bonds. Both recommend the S(+)-N(-) formulation of sulfur nitrogen bonds in 1 and 2 since all nitrogen atoms are found to be sp(3) hybridized. The planar SNx (x = 2, 3) units in the diimide 3 and the triimide 4 reveal characteristics of m-center-n-electron systems. For none of the investigated S-N bonds, a classical double bond formulation can be supported. This is further substantiated by the NBO/NRT approach. Valence expansion to more than eight electrons at the sulfur atom can definitely be excluded to explain the bonding. 相似文献
4.
Gerald Koitz Walter Fabian Hans-Werner Schmidt Hans Junek 《Monatshefte für Chemie / Chemical Monthly》1981,112(8-9):973-985
The synthesis of several 2-aminopyridines is described. The reaction of tetracyanoethylene with 3-imino-propane-carbonitrile gives 2-amino-3,4,5-tricyano-pyridines with alkyl and aryl-substituents, resp., in position 6. Nucleophilic substitution of 2-amino-6-chloro-3,5-dicyanopyridines and 2-amino-6-chloro-3,4,5-tricyano-pyridines with phenolates leads to a variety of pyridines. Spectroscopical data of absorption and fluorescence are presented and the influence of the cyano groups is discussed. The results are in good agreement with quantum chemical calculations (PPP).
Herrn Univ-Prof. Dr.A. Holasek zum 60. Geburtstag gewidmet. 相似文献
5.
A new powerful class of low-molecular-weight amphiphilic compounds has been synthesized and their structure-property relationships with respect to their gelation ability of organic solvents have been investigated. These compounds are able to gel organic solvents over a broad range of polarity. Especially polar solvents such as valeronitrile and gamma-butyrolactone can be gelled even at concentrations far below 1 wt %. It was found that the gelation ability of these asymmetrically substituted p-phenylendiamines depends on a well-balanced relation of the terminal head group, the units involved in hydrogen bonding (amide or urea groups), and on the length of the alkyl chain. With this class of new gelators it is possible to tailor thermal and mechanical properties in different organic solvents and open various application possibilities. 相似文献
6.
F. E. G. Henn R. M. Buchanan N. Jiang D. A. Stevenson 《Applied Physics A: Materials Science & Processing》1995,60(5):515-519
Alternating current (ac) conductivity and permittivity measurements were carried out on yttriastabilized cubic zirconia, with yttria varying from 9.6 to 25.5 mol%. In this composition range, the dielectric constant measured at 103 Hz decreased from 29.8 to 23.2 as the yttria concentration increased. A plot of the ac conductivity versus frequency exhibited a plateau conductivity (dc) and a frequency () dependent term '(). An analysis of '() with a suitable theoretical approach based on a pairs-approximation model, resulted in a calculated binding energy for the yttrium-associated oxygen vacancy of 0.29±0.05 eV, which compares favorably with values obtained by different means in previous studies. The experimental determination of the frequency dependence of the conductivity and its analysis provides an effective basis for understanding the ionic conductivity mechanism. 相似文献
7.
Hans-Werner Schmidt 《Monatshefte für Chemie / Chemical Monthly》1987,118(2):217-228
The synthesis of several new derivatives of 2-amino-4-hydroxybenzonitrile2 and their herbicidal screening data are described. Thus, halogenation and furtherSandmeyer-Reaction of2 leads to the 2,3,5-trihalogenated derivatives5 a–e. Reaction of ethyl chloroformate with the amino group of2 gives6 a, acylation7 a, b and8. The 3,5-dihalogenated compounds10 a–e are obtained from6 a and7 a, b by bromination or iodation. The hydroxyl group of6 a, 7 a, b and10 a, c, e reacts with N,N-disubstituted carbamyl chlorides to give6 c, d, 9 a, b, 11, 12 and13 a, b. 相似文献
8.
Different methods for the separation and identification of eighteen non-steroidal antirheumatics (non-steroidal, anti-inflammatory drugs) and the application of these methods for systematic toxicological analysis are reviewed. A method for separating these compounds by high-performance liquid chromatography using an RP-18 column and mixtures of acetonitrile and acetate buffers as eluents is presented and the detectability of these substances with respect to their different UV maxima is discussed. 相似文献
9.
Zusammenfassung Es werden die Eigenschaften und Reaktionen von 1,3-Dichloruretidindion, einem neuen Dimeren des Chlorisocyanats, beschrieben. Durch thermische Isomerisierung entsteht der bereits bekannte N,N-Dichlor-N-carbonylharnstoff. Die IR-Spektren beider Verbindungen werden mitgeteilt.
Mit 2 Abbildungen 相似文献
Chemistry of chloro-isocyanate, III: 1,3-Dichlorouretidinedione, a new chloroisocyanate dimer
The properties and reactions of 1,3-dichloro-uretidinedione, a new dimer of chloro-isocyanate, are described. Its thermic isomerization gives the known N,N-dichloro-N-carbonyl-urea. The IR-spectra of both compounds are published.
Mit 2 Abbildungen 相似文献
10.
The order in thin films of a combined liquid crystalline polymer is studied by X-ray reflection. Films of thicknesses of less than 200 nm on float glass are investigated as a function of temperature. The polymer with mesogenic groups in the main and side-chains exhibits smectic and cholesteric mesophases. Measurements in the smectic phases show a Bragg peak and smectic layers are oriented parallel to the substrate. The sample is thus macroscopically ordered by the influence of substrate and free surface. The film surface is very smooth after spincoating; surface roughness is typically 0.8 nm. First annealing of samples leads to a significant roughening of the free surface; roughness increases to 2.1 nm. Order as a function of film thickness depends on the interaction of the polymer with the substrate and free surface. These interactions give rise to a typical correlation length of perturbations in smectic ordering. 相似文献