Spinning anisotropic

Following the technological breakthroughs which led to the discovery of (1) the liquid crystalline behavior ofpara-oriented aramids26 and (2) a novel method for spinning anisotropic liquid crystalline polymer solutions,27 Kevlar aramid fiber was produced and commercialized by the DuPont company in 1972. Other fibers based on aromatic polyamide compositions, which were produced and commercialized by other companies, were Technora (Teijin, Japan), Teijinconex (Teijin, Japan), andTwaron (Akzo, The Netherlands). Additionally, SVM is a fiber produced in the Former Soviet Union and it was announced in 1990 that a new aramid fiber had been introduced by Hoechst, in Germany. 

Higher terms, biquadratic in the spin operators opS, also can occur.18 Spin-spin anisotropic interactions of the type D [S S2] are known for electrons,19 where D is a parameter vector this expression is the antisymmetric part of the most general bilinear spin-spin interaction. Observation of a field-induced magnetization in SrCu2(B03)2 via 11B NMR has revealed the presence of such an interaction therein.20... 

Electron spin-nuclear spin Anisotropic part... 

Correlation time, rotational (tr)> 427,487,489 Coupling, nuclear spin-spin anisotropic, 400 calculations of, 49-60... 

Following the technological breakthroughs which led to the discovery of ( ) the liquid crystalline behavior of para-oriented aramtds and (2) a novel method for spinning anisotropic liquid crystalline polymer solutions, " Kevlar aramid fiber was produced and commercialized... 

Woessner D E 1962 Spin relaxation processes in a two-proton system undergoing anisotropic reorientation J. Chem. Rhys. 36 1-4... 

In equation (bl. 15.24), r is the vector coimecting the electron spin with the nuclear spin, r is the length of this vector and g and are the g-factor and the Boln- magneton of the nucleus, respectively. The dipolar coupling is purely anisotropic, arising from the spin density of the impaired electron in an orbital of non-... 

M continually decreases under the influence of spin-spin relaxation which destroys the initial phase coherence of the spin motion within they z-plane. In solid-state TREPR, where large inliomogeneous EPR linewidths due to anisotropic magnetic interactions persist, the long-time behaviour of the spectrometer output, S(t), is given by... 

The electron-spm echo envelope modulation (ESEEM) phenomenon [37, 38] is of primary interest in pulsed EPR of solids, where anisotropic hyperfme and nuclear quadnipole interactions persist. The effect can be observed as modulations of the echo intensity in two-pulse and three-pulse experiments in which x or J is varied. In liquids the modulations are averaged to zero by rapid molecular tumbling. The physical origin of ESEEM can be understood in tenns of the four-level spin energy diagram for the S = I = model system... 

Because the spin-orbit interaction is anisotropic (there is a directional dependence of the view each electron has of the relevant orbitals), the intersystem crossing rates from. S to each triplet level are different. 

Technora. In 1985, Teijin Ltd. introduced Technora fiber, previously known as HM-50, into the high performance fiber market. Technora is based on the 1 1 copolyterephthalamide of 3,4 -diaminodiphenyl ether and/ -phenylenediamine (8). Technora is a whoUy aromatic copolyamide of PPT, modified with a crankshaft-shaped comonomer, which results in the formation of isotropic solutions that then become anisotropic during the shear alignment during spinning. The polymer is synthesized by the low temperature polymerization of/ -phenylenediamine, 3,4 -diaminophenyl ether, and terephthaloyl chloride in an amide solvent containing a small amount of an alkaU salt. Calcium chloride or lithium chloride is used as the alkaU salt. The solvents used are hexamethylphosphoramide (HMPA), A/-methyl-2-pyrrohdinone (NMP), and dimethyl acetamide (DMAc). The stmcture of Technora is as follows ... 

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

We finish this section by comparing our results with NMR and incoherent neutron scattering experiments on water dynamics. Self-diffusion constants on the millisecond time scale have been measured by NMR with the pulsed field gradient spin echo (PFGSE) method. Applying this technique to oriented egg phosphatidylcholine bilayers, Wassail [68] demonstrated that the water motion was highly anisotropic, with diffusion in the plane of the bilayers hundreds of times greater than out of the plane. The anisotropy of... 

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Tile ESR spectra of the radical anions, generated by one-electron reduction of the a-oxothioketone 173 and the dithiete 172, were determined, and spin densities were calculated from the coupling constants and, especially, from the anisotropic values (87CB575). 

Notice that the associated spin model has the following three properties (1) it is, in general, anisotropic (i.e. a-2 / CI3), (2) its set of coupling constants hi, hij, /1123) are interdependent (this should be obvious from equation 7.63, which provides a parameterization of each of these seven constants in terms of our original four independent conditional probabilities, aj (equation 7.58)), and (3) its partition function, Z, can be calculated exactly. 

From the selection rules of the 6j coefficients (.89), it follows that the biquadratic terms cannot mix the S = I levels with higher spin states. By contrast, the anisotropic symmetric and antisymmetric terms, whose magnitude is related to that of the isotropic component (89), can give rise to a substantial mixing. However, a detailed quantitative model is needed to verify whether the peculiar magnetic properties of [3Fe-4S] + centers can be explained by this mixing. 

The main objectives of this article are (i) to give an account of the simple theory related to spin-lattice relaxation-rates, in a language that is directed, as far as possible, to the practising chemist rather than to the theoretician (ii) to caution against uncritical use of this simple theory for systems that are strongly coupled, or undergoing anisotropic reorientation, or both (hi) to introduce the pulse n.m.r. experiments that are used to measure spin-lattice relaxation-rates, and to stress the precautions necessary for accurate... 

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