Nematic conformation

Ratios such as AvilAvg and AvtlAvj i j) are free from the orientational order of the molecular axis Szz and solely depend on the spacer conformation [Eqs. (7.3), (7.6) and (7.7)]. From an analysis of the spectra, these ratios were found to remain nearly invariant with temperature. These observations support the idea that the nematic state is populated with an ensemble of nematic conformers. Since the flexible spacer maintains liquidlike characteristics in this state, the equilibration among the conformers should be affected by the temperature, leading to a small variation in these ratios when the data are collected over a wider temperature range. 

MOLECULAR CONFIGURATION OF MAIN-CHAIN LCs 301 Nematic Conformation Random-Coil... 

FIGURE 7.12 Schematic representation of the nematic arrangement of main-chain LCs. While the orientational fluctuation of the entire molecule varies as a function of temperature, the nematic conformation of the spacer remains quite stable over the entire LC region defined by the two phase houndaries (7cn and Tni). 

Before closing Section 7.3 we would like to mention briefly the optical anisotropy of the nematic conformation of the spacer. The parameter A, representing the strength of the anisotropic interaction in theMaier-Saupe theory [Eq. (7.1)], should be related... 

TABLE 7.1 Optical Anisotropies for the Isotropic and Nematic Conformation of CBA-9 and CBA-10 Calculated Using the Group Anisotropies Determined in the Isotropic Solution ... 

A satisfactory account of the steric interaction characteristic of the dense nematic phase such as those treated here is not yet available. The treatment is largely hampered by the occurrence of the nematic conformation of the flexible spacer in the ordered state [Flory, 1989 Yoon andHory, 1989 Abe and Ballauff, 1991]. 

PVT analysis presented in this chapter has revealed that main-chain LCs resemble monomer LCs in many thermodynamic characteristics. In addition to the conventional anisotropic dispersion interactions and steric repulsions, however, special attention is required for the effect of the nematic conformation of the spacer on the molecular... 

Nematic Conformation of Flexible Spacers Involved in the Mesophase. .. 132... 

Keywords Mainchain liquid crystals Nematic conformation Crystallization Liquid-crystallization Transition entropies... 

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

Rg.5 Schematic representation of the crystal-nematic (CN) and nematic-isotropic (NI) transitions, illustrated for a trimer liquid-crystal system. The conformational distribution of the spacer remains quite stable in the nematic phase, and is termed nematic conformation... 

In the nematic state, the molecular orientations are governed by a uniaxial potential field, and thus the spacers are required to recognize a certain conformational correlation along the chain (nematic conformation), but the overall properties are still very much liquidlike. The liquid-liquid first-order transition between the isotropic and nematic states presents an interesting example to investigate the validity of the entropy separation hypothesis (see Sect. 2.2). 

On going from the isotropic to the anisotropic LC state, the orientation-dependent attractive interactions come into play [125,126] while the steric interactions (the excluded-volume effect) between the mesogenic rods are relaxed [127]. In the LC state, all molecules are required to take an asymmetric shape. Accordingly, chain segments adopt a unique conformer distribution called a nematic conformation [26,93-95,102,103,105]. The Y-Vsp relation determined in the aforementioned studies may be used to examine the mean-field potentials effective in nematic as well as isotropic hquids. After Frank [128] and HUdebrand and Scott [2,129], the intermolecular interaction potentials such as... 

Fig. 3 Side-chain LC polymer with (a) prolate nematic conformation and (b) oblate smectic conformation, (c) Representation of two hairpin defects confined to a long, thin cylinder (aftm-...

Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase... 

Keywords Conformation of the spacer Nematic conformation Odd-even effect <... 

Elucidation of the Nematic Conformation and Its Contribution to the NI Transition Entropy... 

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