Liquid crystalline mesophases director

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic... 

Not only can molecules constituting the oriented phase be thus studied. Solutes present in this mesophase will also be oriented through operation of anisotropic intermolecular forces. An example is that of deuterium-labeled D gramicidin. When dissolved in a nematic mesophase, it displays a series of deuterium doublets. Their residual splittings 6v are almost temperature-independent. This points to a rigid structure for the peptide, an helix that reorients about the director of the liquid crystalline phase (18). 

Since most liquid crystalline phases used today are lyotropic mesophases with a relatively small magnetic susceptibility anisotropy, it should be noted that the orientation of the director is also influenced by the inertial torque when... 

When drawn into fibers from the melt, these polymers are observed to have two preferred orientations, with the director of one mesophase parallel to the flow direction and that of the other mesophase perpendicular to the flow direction. Also presented are the first linear viscoelastic data of polymers which form liquid crystalline glasses. 

When the molecules that form a liquid-crystalline phase are chiral, the structure of these mesophases can have an additional property. In the chiral nematic phase (N ) the director precesses about an axis perpendicular to the director and describes in this way a helix (Figure 2.7). The pitch of a chiral nematic phase is the distance along the helix over which the director rotates over 360°. The chiral nematic phase is sometimes... 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

The measured dipolar couplings are often converted into bond order parameter CH useful for the structural and conformational studies [56-58]. The molecular order parameter S in THE6 mesophase is conveniently estimated from the heteronu-clear dipolar splitting Av for the aromatic protonated C4 carbon. Since the C-H bond vector for this site is perpendicular to the molecular rotation axis and the liquid-crystalline director is aligned perpendicular to the external magnetic field,... 

If we approximate the liquid-crystalline molecules by rod-like particles, then the basic thermotropic mesophases can be visualized schematically as in Fig. 4.5. The nematic phase is the least organized of all liquid-crystalline phases and usually appears just below the isotropic phase. In this phase the centers of mass of the elongated molecules have three translational degrees of freedom and thus are distributed randomly as in an ordinary isotropic liquid. However, the long axes of the neighbouring molecules are preferentially aligned with respect to an axis which is called the director, usually denoted by a unit vector n. If the molecules have mirror symmetry, the ordinary nematic phase is observed. It is nearly always characterized by uniaxial symmetry and equivalence of n and — n. For molecules without mirror symmetry, however, the spatial variation of n leads to an helical structure. If one considers a plane perpendicular to the helix axis, then the direction of n is the same in this plane, but it... 

In solid materials, intermolecular forces hold the constituting molecules into fixed positions giving rise to well-defined crystalline lattices. On the other hand, in the liquid state, such interactions are overcome by the vigorous vibration of molecules which force them to move randomly. The liquid crystal state, also known as mesophase, is an intermediate phase of condensed matter which combines the fluidity typical of ordinary liquids and the characteristic molecular arrangement of crystalline solids (Gray 1998). In fact, within a mesophase, all molecules tend to point to a common direction, called director, n (Fig. 18.1). This is in contrast to the isotropic liquid state where molecules show no organisation. 

Fig. 1.33 These figures trace the development of the diffraction pattern of a sample of a discotic mesogen as the crystalline solid is heated tmd passes through a succession of mesophases, ending as the isotropic liquid. Note the stepwise way in which reflections broaden and become diffuse, as various types of order in the stmctures tire lost. As in the similar scheme for smectic phases given in Fig. 1.30, the edge-on view of the molecular discs drawn on the left corresponds to the alignment of the diffraction pattern shown in the centre and again, for all mesophases, the sample as a whole is taken to have a random orientation of domtiins, giving rotational symmetry around the director...

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