Nematic structures

Finally, whenever in matter there is no long range positional but still long-range orientational order, we have ordered liquids, instead of solids. The X-ray spectral features of an ordered liquid with a smectic structure is the occurrence of one or a few meridional sharp reflections in the fiber spectrum, plus polarized halos for an ordered liquid with nematic structure the occurrence of polarized haloes, only. 

Several structure sizes caused by microphase separation occurring in the induction period as well as by crystallization were determined as a function of annealing time in order to determine how crystallization proceeds [9,18]. The characteristic wavelength A = 27r/Qm was obtained from the peak positions Qm of SAXS while the average size of the dense domains, probably having a liquid crystalline nematic structure as will be explained later, was estimated as follows. The dense domain size >i for the early stage of SD was calculated from the spatial density correlation function y(r) defined by Debye and Buche[50]... 

A physicist would say the liquid crystal adopted a twisted nematic structure. 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

Note 5 From a crystallographic point of view, the uniaxial nematic structure is characterised by the symbol Dooh in the Schoenflies notation (Wmm in the International System). 

Fig. 8a,b. Nematic structure induced by moderate external shear in lower M samples a DP 15, specimen quenched from 300 °C b DP 39, specimen quenched from 330 °C... 

In many cases the disorder introduced into rigid-rod structures in order to depress the crystalline melting point to a temperature below the degradation temperature is such that the polymer can crystallise only to a small extent or not at all. In such cases, the polymer exhibits a frozen nematic structure at temperatures below the glass transition, with a typical micaceous sheen. 

Table 9. LC polymers with supposed nematic structure... 

The first success was achieved when optically active (chiral) monomeric units were combined with a nematic LC polymer 105,123,143,144). The approach was based on the idea that a cholesteric mesophase may actually be realized as a helical nematic structure. Then by chemical binding of chiral and mesogenic units into a chain, accomplished by copolymerization or copolycondensation (in case of linear polymers) of nematogenic and optically active compounds, it was found feasible to twist a nematic mesophase and obtain copolymers of cholesteric type (Table 13). 

The following three possible models, as illustrated in Fig. 19, are suggested. We know P8-0-PIMB molecules form helical nanofilaments. If 5CB molecules align parallel to the groves of the filaments, we can expect helical structure of 5CB shown in Fig. 19a, b. If we assume that nanofilaments align rather parallel to each other (Fig. 19a), 5CB molecules form a twisted nematic structure. But this is not always... 

Fig. 19 Possible model to show enhanced optical activity by nonchiral 5CB molecules, (a) 5CB molecules form twisted nematic structure between helical nanofilaments, (b) 5CB molecules form superhelical structure around a helical nanofilament [9]...

The super birefringent effect (SBE-LCD) reported by Scheffer and Nehring from Brown Boveri in Baden, Switzerland, uses the optical interference of two normal, elliptically polarised modes of transmitted light generated by a high-tilt, highly-twisted nematic structure viewed between two polarisers set in an unusual way, i.e. the input polarisation direction is not parallel to the nematic director at either substrate surface and the polarisers are not crossed at 90°, see Figure 3.10. 

The static homeotropically aligned guest-host nematic mixture (plus guest dye and chiral dopant) is optically transparent and, therefore, the display appears colourless in the non-activated state. The lower limit of the pitch for a given cell gap, before a twisted nematic structure becomes energetically more favoured than the homeotropic nematic structure, is determined by the djp ratio ... 

Twisting a nematic structure around an axis perpendicular to the average orientation of the preferred molecular axes, one arrives at the molecular arrangement commonly called cholesteric (Kelker and Hatz, 1980). The twisted nematic phase is optically uniaxial, however with the axis perpendicular to the (rotating) director. Such a mesophase combines the basic properties of nematics with the implications of chirality The structure itself is chiral and as a consequence, a non-identical mirror image exists as it is shown schematically in Fig. 4.6-7. Besides the order parameters mentioned before, the essential characteristics of a cholesteric mesophase are the pitch, i.e., the period of the helical structure as measured along the twist axis, and its handedness, i.e., whether the phase is twisted clockwise or anticlockwise. 

By polarizing light microscopy we saw the typical focal conic smectic texture of the BDVHD hcmopolyester mesophase but observed no distinctive smectic or nematic structure of the BDA/50 EG/50 HD copolyester cooled from the isotropic melt and held in the mesophase many hours. Also on an x-ray diffractometer trace of the copolyester we did not see the characteristic peak due to the spacing between the layers of the smectic mesophase but did see... 

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