Molecular orientational order

The anisotropy of the liquid crystal phases also means that the orientational distribution function for the intermolecular vector is of value in characterising the structure of the phase [22]. The distribution is clearly a function of both the angle, made by the intermolecular vector with the director and the separation, r, between the two molecules [23]. However, a simpler way in which to investigate the distribution of the intermolecular vector is via the distance dependent order parameters Pl+(J") defined as the averages of the even Legendre polynomials, PL(cosj r)- As with the molecular orientational order parameters those of low rank namely Pj(r) and P (r), prove to be the most useful for investigating the phase structure [22]. 

DelRio FW, Steffens KL, Jaye C, Fischer DA, Cook RF (2010) Elastic, adhesive, and charge transport properties of a metal-molecule-metal junction the role of molecular orientation, order, and coverage. Langmuir 26 1688-1699... 

Note 2 Diselinations are defects in molecular orientational order in contrast to dislocations that are defects in molecular positional order. 

Molecular orientational order in adsorbed monolayers can be inferred indirectly from elastic neutron diffraction experiments if it results in a structural phase transition which alters the translational symmetry of the 2D lattice. In such cases, Bragg reflections appear which are not present in the orientationally disordered state. Experiments of this type have inferred orientational order in monolayers of oxygen (41) and nitrogen (42) adsorbed on graphite. However, these experiments have not observed a sufficient number of Bragg reflections to determine the molecular orientation by comparing relative Bragg peak intensities with a model structure factor. 

The simplest phase, which contains only molecular-orientational ordering, is the nematic. The term "nematic" means thread in Greek. All known nematics have one symmetry axis, called the director, n, and are optically uniaxial with a strong birefringence. The continuous rotational symmetry of the isotropic liquid phase is broken when the molecules choose a particular direction to orient along in the nematic phase. Since the nematics scatter light intensively, the nematic phase appears turbid. 

Samulski, E. T., and Dong, R. Y, Chain ordering and molecular orientational order in liquid crystals, J. Chem. Phys., 77, 5090-5096 (1982). 

The sensitivity of deuteron NMR to the molecular orientational order and to director field configurations turned out to be extremely useful in studies of liquid crystals confined into snbmicrometer pores. Moreover, the large surface-to-volume ratio of these composite systems render the interfacial and surface phenomena, induced by the liquid crystal-surface interactions, accessible even to an essentially integrative technique like NMR. Since the discovery of polymer dispersed liquid crystals (PDLCs) in 1986 [4], NMR of selectively deuterated liquid crystals was used to discriminate unambiguously among various director structures in cavities, resulting from an interplay between elastic forces, morphology and size of the cavity, and surface interactions. These structures include the escaped-radial, planar axial, planar-polar, and... 

The LC state is a unique condition in which long-range molecular orientational order persists in the absence of various types of short-range translational order. 

Since the first discovery of the liquid crystalline phase over one hundred years ago, the classification of the distinct liquid crystalline phases in small-molecule liquid crystals has been well established (7,2). As shown in Figure 1, the least ordered liquid crystalline phase is the nematic phase that only possesses molecular orientational order due to the anisotropy of the molecular geometric shape. The next ordering level introduced is the layer structure in addition to the molecular orientation to lorm a smectic A (S/J or a smectic C (Sc) phase. Following the phase the hexatic B (Ho), smectic crystm B (So) and smectic crystal E (S ) phases are observed. In this series the long axis of the molecules is oriented perpendicular to the layer surface while order is increasingly developed from positional order normal to the layer in bond... 

A. Sanehez Castillo, Polarized micro-Raman spectroscopy. A modem technique to study the molecular orientational order of nematic and smectic liquid crystals. Doctoral thesis, Verlag Dr. Hut, Miinchen, 2013... 

The molecular anisotropy also manifests itself in a variety of spectroscopic techniques. Dichroism, the difference in absorption coefficients of linearly polarized light measured in orthogonal directions, is another phenomenon wherein one can extract average molecular orientational order present in the nematic phase. The dichroic ratio D = A Ax (see Chapter 7), the ratio of the intensities of the absorption band of a characteristic transition measured with the polarized incident radiation parallel to n, A, relative to that with it perpendicular to n. Ax, is given by... 

Studying molecular orientational order in soft materials, providing information on the ordering of specific bonds. The intensity of the Raman scattered radiation depends on the orientation of the chemical bond with respect to the polarization direction of the radiation. Raman microscopy is used to obtain information on the orientation of molecules at a surface, by scanning a Raman microprobe across it. Photon correlation spectroscopy is usually understood to refer to visible radiation, but the same method can be used to analyse the fluctuations of Raman (and IR) radiation. This provides information on the dynamics of molecular motions, since molecular rotation will modify the vibrational frequencies. This can be used to determine rotational diffusion coefficients. 

The relationship between macroscopic properties and molecular properties is a major area of interest, since it is through manipulation of the molecular structure of me-sogens, that the macroscopic liquid crystal properties can be adjusted towards paricu-lar values which optimize performance in applications. The theoretical connection between the tensor properties of molecules and the macroscopic tensor properties of liquid crystal phases provides a considerable challenge to statistical mechanics. A key factor is of course the molecular orientational order, but interactions between molecules are also important especially for elastic and viscoelastic properties. It is possible to divide properties into two categories, those for which molecular contributions are approximately additive (i.e. they are proportional to the number density), and those properties such as elasticity, viscosity, thermal conductivity etc. for which intermolecular forces are responsible, and so have a much more complex dependence on number density. For the former it is possible to develop a fairly simple theory using single particle orientational order parameters. 

For the simplest kind of SmA phase, denoted SmAi, there is a layer structure with a periodicity d slightly less than the molecular length Iq. This effect is due to a combination of imperfect molecular orientational order Pj) < 1) and conformational disorder in the molecules. This leads to a number of equally-spaced low-angle (00/) pseudo-... 

Sg, see Fig. 1, all have more order than S, and S. In order to discuss the different smectic phases it is necessary to distinguish molecular orientational order and bond orientational order. The latter may occur also in a substance composed of isotropic molecules, and is defined as... 

On the other hand, discotic liquid crystal material exhibits a colunm phase regardless of the difference in molecular orientation order. The intermolecular distance in the colunms is generally about 0.35 nm, and the actual differences in observed mobility of over three orders of magnitude (ranging from 10 to 10 cm A s) cannot be explained by differences in the intermolecular distance in the colunmar phase [10], There is not enough empirical evidence about this problem, but perhaps orientational fluctuations of the molecules in the colunm could be the dominant factor. 

Figure 6.18. Possible two types of crystal phases with different molecular orientational order in fee (fiw3and fm3m). Another important crystal phase of primitive cubic (Pa3) with rotational-ordering appears at low temperatures for Na-containing fullerides.

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