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21.
S. G. McNamee T. J. Bunning S. S. Patnaik C. M. McHugh C. K. Ober W. W. Adams 《Liquid crystals》2013,40(5):787-794
Abstract The presence of periodic diffuse meridional reflections in the X-ray diffraction pattern of the nematic phase of a pentamethylcyclosiloxane compound with pendant mesogenic groups is examined as a function of temperature and frequency of an applied electric field. These reflections are thought to be caused by a tendency of the mesogens to pack in columns of molecules that have no side-to-side registry (uncorrelated strings). Column lengths obtained from the intercepts of plots of δs 2 versus s 2, where s is the scattering vector and δs is the width of the reflection, indicate association of 3–6 mesogens depending on temperature and frequency. The length over which the strings are correlated are compared to values obtained from a magnetically aligned sample quenched into a glassy nematic phase. The development of a pseudo-layered packing arrangement with decreasing temperature is also discussed. The appearance of a symmetric 4-spot pattern at temperatures slightly above the crystallization temperature suggests the formation of layered domains with local ordering similar to smectic C packing. Layer correlation lengths are calculated and compared for the electrically (110 Å) and magnetically (180 Å) aligned samples. 相似文献
22.
A new model for non-leptonic weak interactions has been recently proposed which is based on the introduction of a right handed charm and strangeness changing weak current. We show that this model can be made to fit the data on B→B′π and B→B′γ decays only if several ad hoc assumptions of large SU3 breaking are made. We also review the success of the conventional Cabibbo current-current model in fitting all of the non-leptonic data. 相似文献
23.
H. Watanabe Kunihiro Osaki Mutsuo Matsumoto Dobrin P. Bossev Cathy E. McNamee Masaru Nakahara Ming-Long Yao 《Rheologica Acta》1998,37(5):470-485
Nonlinear rheological features were investigated for an aqueous solution of tetraethylammonium perfluorooctyl sulfonate (C8F17SO3
–N+(C2H5)4; abbreviated as FOSTEA). In the solution (c=0.045 mol/l; 28.3 g/l), spherical micelles of FOSTEA were connected with each other to form threads of pearl-necklace shape.
These threads were further organized into a transient network to exhibit linear relaxation characteristic of living polymers,
single-mode terminal relaxation widely separated from faster relaxation processes. Nonlinear relaxation experiments against
large step-strains γ(≤8) revealed that the terminal relaxation mode had a γ-insensitive relaxation time but its relaxation
intensity exhibited significant damping (much stronger than that for entangled polymers). In contrast, the relaxation time
and intensity for the fast relaxation modes first increased and then decreased with increasing γ. Under shear flow, the FOSTEA
threads exhibited strong thinning of the viscosity. These nonlinear features of the FOSTEA threads were compared with those
of other threadlike micelles, analyzed on the basis of an empirically introduced constitutive equation, and discussed in relation
to strain/low-induced scission of the living threads.
Received: 20 February 1998 Accepted: 30 July 1998 相似文献
24.
PC. M. Gubbens G. J. Boender A. M. van der Kraan K. H. J. Buschow 《Hyperfine Interactions》1992,68(1-4):329-332
DyFe12?xMx (M=Si,Ti,Cr) were studied with57Fe and161Dy Mössbauer spectroscopy. The deduced coupling constants JRFe of these compounds are about equal. It is found that Si is substituting one particular Fe-site, while Cr is substituting Fe randomly. In DyFe10Si2 two Fe-sites favour a c-axis anisotropy, while the third one favours the basal plane. 相似文献
25.
The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperature range. At temperatures near clearing, the net degree of orientation diminished. Cooling through the mesophase with an applied electric field resulted in much larger orientation parameters than could be obtained by aligning at a fixed temperature in the mesophase. 相似文献
26.
The paper develops a construction for finding fully symmetric integration formulas of arbitrary degree 2k+1 inn-space such that the number of evaluation points isO((2n)k)/k!),n . Formulas of degrees 3, 5, 7, 9, are relatively simple and are presented in detail. The method has been tested by obtaining some special formulas of degrees 7, 9 and 11 but these are not presented here. 相似文献
27.
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29.
Two efficient approaches for the synthesis of isoindolin-1-one derivatives in phosphonium salt ionic liquids are described. The palladium-catalyzed carbonylation-hydroamination reaction of 1-halo-2-alkynylbenzene with amines afforded the substituted 3-methyleneisoindolin-1-ones in good yields and high selectivities in favor of the Z-isomers. The target molecules could also be synthesized by the Sonogashira coupling-carbonylation-hydroamination one-pot reaction of dihalides, alkynes, and amines. Both processes can be conducted under mild conditions and tolerate a wide array of functionalized substrates. 相似文献
30.
McNamee CE Tsujii Y Matsumoto M 《Langmuir : the ACS journal of surfaces and colloids》2005,21(24):11283-11288
Atomic force microscopy was used to characterize an anatase TiO2 surface, prepared by the helical vapor preparation method. The forces between two bare TiO2 surfaces were measured in the presence of water at various pH values. This TiO2 isoelectric point (iep) was characterized by the presence of only a van der Waals attraction and was measured at pH 5.8; this value is similar to that for a rutile TiO2 surface. The adsorption mechanism of a nonionic surfactant molecule to this anatase TiO2 surface was investigated by measuring the forces between two such TiO2 surfaces at their iep pH in the presence of linear dodecanol tetraethoxylate (C12E4), a poly(ethoxylene oxide) n-alkyl ether. C12E4 was seen by the presence of steric forces to adsorb to the uncharged TiO2 surface. For low surfactant concentrations, C12E4 adsorbed with its hydrophobic tail facing the TiO2 substrate, to reduce its entropically unfavorable contacts with water. Additional surfactant adsorption occurred at higher surfactant concentrations by the hydrophobic and hydrophilic interactions between the surfactant tails and heads, respectively, and gave sub-bilayers. A two-step adsorption isotherm was subsequently proposed with four regions: (1) submonolayer, (2) complete monolayer, (3) sub-bilayer, and (4) bilayer. The absence of a long-range repulsive force between the two TiO2 surfaces in the presence of the C12E4 surface aggregates indicated that a C12E4 nonionic surfactant aggregate did not possess charge. 相似文献