Molecular order

Because possesses a spin with / = 1, the deuteron resonance for a single molecular orientation exhibits two lines, which correspond to the two transitions with Am = 1 in the energy level diagram (Fig. 3.2.1). The lines appear symmetric with respect to the Larmor frequency Ul, 

The wideline spectram, consequently, consists of two overlapping powder spectra, which possess mirror symmetry with respect to cul. This is the Pake doublet. In addition to the two observable single-quantum transitions ( Am = 1), a double-quantum transition ( /iml = 2) exists which can be detected indirectly. Its transition frequency is the sum of both single quantum transitions. 

With the help of (3.2.1), it is readily seen that co2q does not depend on the orientation angles a and of the principal axes system. Therefore, the transition corresponds to a narrow resonance which can be favourably exploited for space encoding in imaging. 

Molecular order is determined conventionally by X-ray diffiaction and in some other cases by neutron scattering. Highly ordered stmctures lead to sharp reflexes, and weakly 

Molecular order is descnhedhy the orientational distribution function P(0) [Mcbl]. This is the probability density of finding a preferential direction n in the sample under an angle 0 in a molecule-fixed coordinate frame (Fig. 3.2.2(a)). For simplicity, macroscopically uniaxial samples with cylindrically symmetric molecules are considered. Then, one angle is sufficient to characterize the orientational distribution function. In practice, not the angle 0 itself but its cosine is used as the variable and for weak order the distribution function is expanded into Legendre polynomials P/(cos 0), 

Orientational order of LCPs is conveniently described in terms of two differoit order parameters, characterizing the average orientation of the mesogenic units within a molecular domain (microorder) and the macroscopic alignment of the domains (see Fig. 3) [10,35]. The measurement of these parameters is important for relating macroscopic physical properties of LCPs to their molecular structure. Various methods have been used to measure order parameters in these systems, including the use of X-ray diffraction [128-130], birefringence [103, 104], and linear dichroism [104,131], but these methods can not essentially separate the two types of orientational order. 

Dynamic magnetic resonance techniques do not suffer from this deficiency. In particular, the use of CSL probes [8, 101, 102] offers the chance of determining the micro- and macroorder unequivocall This is demonstrated in Fig. 16. The spectra refer to the side chain polymers 2 (M = 14000) and two different temperatures (T = 382 K, left column T = 263 K, right column). Drastic spectral changes are observed when the sample is rotated (top row, q = 0 central row, q = 50°, bottom row, Q = 90°). Comparison of the angular variation in the nematic (left column spectra) and supercooled smectic phase (right column spectra) reveals the crucial effect of the microorder on the spectral feature. 

In Fig. 17 the microorder parameters S z [34] of the CSL probes in the various systems are plotted as function of the reduced temperature T = T/T j [35,99,100]. They refer to LCP 1 (M = 14000, full circles), LCP 2 (M = 14000, open circles) and the low molecular weight analogue 5 (open triangles). One sees that in the iso- 

Lowering the temperature increases Szz for all systems to the same limiting value of Szz 0.65 in the nematic phase [35, 99, 100]. Note, that the order parameter curve of polymer 1 exhibits a horizontal slope at the glass transition. A further jump of Szz. observed for the polymers 2, indicates an additional smectic A phase with a limiting value of Szz 0.9. No such discontinuity is detected for the monomeric liquid crystal 5, exhibiting a smectic A phase, likewise. Comparison of the order parameters of Fig. 17 with those of NMR [108-110] and birefringence studies 

A recent detailed x-ray diffraction (XRD) study of PPy/PF6 during electrochemical cycling revealed new insights into the microstructure.153 In the fully oxidized state, the XRD patterns could be interpreted as dopant ions homogeneously dis- 

Although most of the available literature supports the planar trans arrangement for electrochemically prepared PPy, one study suggests an alternative conformation. Davidson and coworkers154 found clear evidence of a helical structure produced from an all cis coupling of pyrrole rings. These workers used dodecyl sulfate as dopant, but also argue that all published literature on XRD of PPy is also consistent with the helical structure. Furthermore, the helical structure is identical to that proposed for poly(3-alkylthiophenes) as described further in Chapter 6. 

Although molecular anisotropy has been noted in several PPy films, the overall degree of crystallinity is very low in these materials. The work by Davidson and coworkers154 on thin PPy films (prepared at very short polymerization times) provides evidence of crystal formation, but the crystal growth is not maintained as the film thickens. Thus, the polymer first formed on the electrode surface may have a high degree of order but does not extend into the bulk structure of thick films. 

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The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Finally, engineered surfaces may contribute to the understanding of adhesion (172). Control of adhesion is essential to a large number of industrial processes and is often associated with various problems, but currendy (ca 1997) there is Htde if any understanding of how specific molecular ordering and interactions at the surface may affect adhesion. 

In most carbon and graphite processes, the initial polymerization reactions occur in the Hquid state. The subsequent stages of crystal growth, heteroatom elimination, and molecular ordering occur in the soHd phase. The result is the development of a three-dimensional graphite stmcture. 

The liquid crystal polymers consist of rod-like molecules which, during shear, tend to orient in the direction of shear. Because of the molecular order the molecules flow past each other with comparative ease and the melts have a low viscosity. When the melt is cooled the molecules retain their orientation, giving self-reinforcing materials that are extremely strong in the direction of orientation. 

Polyamides such as nylon 6, nylon 66, nylon 610, nylon 11 and nylon 12 exhibit properties which are largely due to their high molecular order and the high degree of interchain attraction which is a result of their ability to undergo hydrogen bonding. 

Fig. 15. A schematic model illustrating the concepts of basic structural unit, BSU, and local molecular ordering, LMO [c.g., 116].

Because biomolecules normally exist in liquid water, this article will be largely concerned with their ordered structures in aqueous media and therefore with hydration effects. In order to understand better the influence of solute-solvent interactions on molecular order, also solvation in organic liquids will be considered to some extent. 

Melt structure High shear at a temperature not far above the melting point may cause a melt to take on too much molecular order. In turn, distortion could result. 

The representation of hard-block domain structure shown in Scheme 4.8, implying rigid, crystallike molecular order, can be misleading because hard blocks are, at best, microcrystalline (as are soft blocks). Although microcrystallinity can be readily obtained, it requires careful selection of raw materials,... 

One type of material that has transformed electronic displays is neither a solid nor a liquid, but something intermediate between the two. Liquid crystals are substances that flow like viscous liquids, but their molecules lie in a moderately orderly array, like those in a crystal. They are examples of a mesophase, an intermediate state of matter with the fluidity of a liquid and some of the molecular order of a solid. Liquid crystalline materials are finding many applications in the electronics industry because they are responsive to changes in temperature and electric fields. 

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