Nematic order microscopic

Figure 2 (4,5) summarizes some key points on what was the first reported and wel 1-characterized thermotropic polymer. The Eastman workers melt copolymerized p-acetoxybenzoic acid with poly(ethylene terephthalate) (PET) to form a series of thermotropic aliphatic-aromatic polyesters (X7G polymers). Some of these compositions showed the characteristic melt optical anisotropy typical of nematic small molecules, that is, the melt between crossed polaroids on a hot stage microscope transmits light. This nematic order was persistent. Of course, shear birefringence is also shown by isotropic polymer melts but light transmission decay is quite rapid. 

In the study of dielectric relaxation, temperature is an important variable, and it is observed that relaxation times decrease as the temperature increases. In Debye s model for the rotational diffusion of dipoles, the temperature dependence of the relaxation is determined by the diffusion constant or microscopic viscosity. For liquid crystals the nematic ordering potential contributes to rotational relaxation, and the temperature dependence of the order parameter influences the retardation factors. If rotational diffusion is an activated process, then it is appropriate to use an Arrhenius equation for the relaxation times ... 

The helical arrangement of the molecules in a cholesteric phase has been described in an earlier chapter. On a sufficiently microscopic scale one cannot distinguish between cholesteric and nematic ordering. However, as we consider larger and larger volumes of the two types of material, a difference in the molecular ordering begins to be-... 

Figure 7.1 Illustration of different aggregation states obtained (from left to right) by increasing temperature crystal (K), smectic C (SmC), nematic (N) and isotropic (I). Row a shows macroscopic appearance of samples in row b, short-range microscopic ordering is represented (each bar represents a molecule) thermotropic phase diagram of row c illustrates relevant transition temperatures (Tm melting temperature Tsmc-N transition temperature between SmC and N Tc clearing temperature) row d shows different texture of different states as seen through polarizing microscope (with crossed polars, isotropic phase appears black).

Some liquid crystals form one or more smectic phases. These display a variety of microscopic structures that are indicated by the letters A, B, C, and so forth. Figure 23.9c shows one of them, the smectic A structure the molecules continue to display net orientational ordering, but now, unlike in the nematic phase, the centers of the molecules also tend to lie in layers. Within each layer, however, these centers are distributed at random as in an ordinary liquid. TBBA enters the smectic A phase at 200°C, before undergoing transitions to two other more ordered smectic phases at lower temperatures. 

Based on microscopic structural details liquid crystals form three separate ordered phases nematic, smectic, and cholesteric. 

In general, chiral nematic polymer liquid crystals (LCP) cannot form monodomains in which the rodlike polymers have a spatially uniform orientation within the sample. Typically, because of the high density of orientational defects, the LCPs are textured, with a distribution of polymer orientation. Microscopically, the polymer chains have a preferred orientation with a relatively narrow distribution around the average orientation. Macroscopically, the variation in space of the orientation results in a domain structure. Defects and orientational variations give rise to the polydomain texture and the overall LCP sample may be randomly ordered (Fig. 3). 

The constituent molecules of a cholesteric phase roust contain at least one center of chirality and frequently include a steroidal ring system. They are arranged with their long axes (directors) parallel, but without longitudinal order. A convenient description of a cholesteric arrangement (which is microscopically incorrect(3a)) places the molecules in nematic like "layers" which are very slightly twisted with respect to those above and below (Fig. 1). The angle and direction of... 

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