Nematic liquid crystal phase molecular features

The most straightforward way to produce partially oriented solute molecules is to orient them in anisotropic solvents. Good solute orientation can be achieved in stretched polymer sheets [94, 95, 96]. Homogeneously oriented nematic liquid crystals are perfectly clear and are thus excellently suited as anisotropic solvents for optical polarization experiments. Moreover, the liquid crystal method allows the performance of polarization experiments in fluid media. This unique feature of the liquid crystal method can be exploited for polarization studies of metastable molecular species (e.g. excited complexes) formed by a diffusion-controlled process. The ordered glasses produced by rapid cooling of a uniformly aligned nematic phase can be used for phosphorescence polarization experiments. 

The majority of the existing molecular theories of nematic liquid crystals are based on simple uniaxial molecular models like sphe-rocylinders. At the same time typical mes-ogenic molecules are obviously biaxial. (For example, the biaxiality of the phenyl ring is determined by its breadth-to-thick-ness ratio which is of the order of two.) If this biaxiality is important, even a very good statistical theory may result in a poor agreement with experiment when the biaxiality is ignored. Several authors have suggested that even a small deviation from uniaxial symmetry can account for important features of the N-I transition [29, 42, 47, 48], In the uniaxial nematic phase composed of biaxial molecules the orientational distri-... 

Many liquid crystal phase transitions involve broken continuous symmetries in real space and their interactions on a molecular scale are short range [1]. As a result, fluctuations have long been known to be an important feature of liquid crystal phase transitions even weakly first order (discontinuous) ones. Compared to major advances in our understanding of fluctuation controlled second-order (continuous) phase transitions, relatively little is known about fluctuation phenomena (critical phenomena) at first-order phase transitions such as the nematic-isotropic transition. 

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... 

Understand how the molecular arrangements characteristic of nematic, smectic, and cholesteric liquid crystals differ from ordinary liquids and from each other. Be able to recognize the features of molecules that favor formation of liquid crystalline phases. (Section 11.7)... 

In the mixture of a polymer and a liquid crystal, the nematic-isotropic phase separation and the isotropic-isotropic one with an upper critical solution temperature has been often observed [3-9]. Typical examples are the mixtures of (p-ethoxybenzylidene)-p- -butylaniline (EBBA), and polystylene (PS) or polyetylen oxide(PEO) [3] and the mixtures of 4-cyano-4 - -heptylbiphenyl (7CB) with polymethyl methacrylate (PMMA) or PS [7]. These solutions consist of a low molecular weight liquid crystal and a sufficiently flexible polymer. The main feature of the... 

The first molecular field theory of biaxial nematics was presented by Preiser [14] indeed it was his prediction which stimulated the hunt for thermotropic biaxial nematics. An alternative vision of the theory was then given by Straley [17] and, although not primarily concerned with liquid crystals, Boccara et al [33] have presented a theory applicable to uniaxial and biaxial nematic phases. The key feature of these theories is the potential of mean torque which is written using a second rank interaction as... 

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