Nematic mesophase, diffusion

Cholesteric mesophases in general resemble the nematic mesophase when observed by both small-angle X-ray diffraction, SAXD, and WAXD. The smectic mesophase, in contrast, produces both diffuse rings at 4--5 A and sharp rings at a distance generally, but not necessarily, equal to the repeat length of the monomer unit, between 15 and 50 A. 

The new FC technique made possible extensive measurements of the self-diffusion constans Dg and Dj of liquid crystals, parallel ( ) and perpendicular (-L) to the director axis, with v ues as low as 10 m s . The most challenging requirements exist for nematic mesophases because their low viscosity facilitates flow in the nonequilibrium state and so necessitates the fast FC procedure to establish the magic angle rotation of the director... 

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

Through the study of the different topics considered in this article, it was shown how X-ray scattering is a useful tool to characterize the most salient features of the mesophases of LCPs. For instance, a simple procedure can be used to measure the nematic order parameter and it is so far valid for all kinds of LCPs based on rod-like moieties. In the case of main-chain polymers, useful information about the conformation of the repeat unit can also be deduced from the diffuse scattering. In the case of side-chain polymers, not only the smectic period but also the amplitude and shape of the smectic modulation can be derived from the measurement of the smectic reflection intensities. Moreover, fluctuations and localized defects may be detected through their contribution to the diffuse scattering. The average distance between lyotropic LCPs can be measured as a function of concentration which tells us the kind of local packing of the particles. 

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... 

Liquid-crystalline stationary phases are particularly useful to separate isomers. Isomers that are more elongated in shape (e.g.,p-xylene) are retained in a column filled with LCSPs longer than isomers that are less elongated in shape (e.g., m-xylene). The retention of isomers of chromatographed substances considerably depends on the type of the mesophase of the liquid-crystalline stationary phase. This results from the fact that the type of mesophase affects the dissolution and diffusion of isomers of the chromatographed substance to a different degree. On the whole, nematic liquid crystals show the best separation properties in relation to isomers but, in some cases, very good separation can be obtained on smectic and cholesteric stationary phases. 

Diffusivities of binary, ternary and multi-component liquid crystalline mixtures, e.g. of soap (potassium laurate (PL), water [25, 58], and lipid (dipalmitoylphosphatidylcho-line (DPPC) [25, 59] systems in lamellar, hexagonal, cubic, nematic and micellar mesophases [25,60,61] have been studied extensively by pulsed-field-gradient NMR [25] and optical techniques [62], partly because of their intimate relation to the structure and dynamical performance of biological membranes [18]. The main distinction from thermotropic phases is that for layered structures a noticeable diffusion occurs only within the layers (i.e. lateral, frequently written as Dl, but in our notation DjJ, whereas it is negligibly small and difficult to detect across the layers [60-62] (transverse migration, for bilayers denoted by flip-flop ) so the mobility is essentially two dimensional, and the anisotropy ratio is so great that it is seldom specified explicit-... 

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