Liquid crystalline material

A belief that solid interfaces are easier to understand than liquid ones shifted emphasis to the former but the subjects are not really separable, and the advances in the one are giving impetus to the other. There is increasing interest in films of biological and of liquid crystalline materials because of the importance of thin films in microcircuitry (computer chips ), there has been in recent years a surge of activity in the study of deposited mono- and multilayers. These Langmuir-Blodgett films are discussed in Section XV-7. 

Liquid Crystallinity. The Hquid crystalline state is characterized by orientationaHy ordered molecules. The molecules are characteristically rod-or lathe-shaped and can exist in three principal stmctural arrangements nematic, cholesteric, and smectic (see Liquid crystalline materials). 

More recently, Raman spectroscopy has been used to investigate the vibrational spectroscopy of polymer Hquid crystals (46) (see Liquid crystalline materials), the kinetics of polymerization (47) (see Kinetic measurements), synthetic polymers and mbbers (48), and stress and strain in fibers and composites (49) (see Composite materials). The relationship between Raman spectra and the stmcture of conjugated and conducting polymers has been reviewed (50,51). In addition, a general review of ft-Raman studies of polymers has been pubUshed (52). 

The three phenylenediamines are all white soHds when pure, but darken after standing in air. They are all very soluble in hot water, for example, about 700 g of 0- or -phenylenediamine are soluble in 100 mL of water at 100°C whereas at room temperature only 4 g dissolve. The physical properties of the phenylenediamines and some of their more important derivatives are given in Table 1. Certain imines of -phenylenediamine are Hquid crystals (1). For example, N,N -bis[ -(octadecyloxy)ben2yhdene]-/)-phenylenediamine [24679-0508] i2is amp of 129, 147, and 183°C (see Liquid crystalline materials). 

Cjg H37 (CH3 )2CH2CH2CH2Si(OCH3 )3C1 , tend to orient Hquid crystals perpendicular to the surface (see Liquid crystalline materials) parallel orientation is obtained on surfaces treated with /V-methy1aminopropy1trimethoxysi1ane [3069-25-8] CH2NHCH2CH2CH2Si(OCH2)3 (25). 

However, momomers based on these units alone have extremely high melting points and are intractable. This has led to the use of a variety of techniques to produce more tractable materials which may be processed without degradation whilst retaining many of the features of liquid crystalline materials. These techniques include ... 

A relatively new class of high-performance carbon fibers is melt-spun from mesophase pitch, a discotic nematic liquid crystalline material. This variety of carbon fibers is unique in that it can develop extended graphitic crystallinity during carbonization, in contrast to current carbon fibers produced from PAN. 

X-ray diffraction has been used for the study both of simple molten salts and of binary mixtures thereof, as well as for liquid crystalline materials. The scattering process is similar to that described above for neutron diffraction, with the exception that the scattering of the photons arises from the electron density and not the nuclei. The X-ray scattering factor therefore increases with atomic number and the scattering pattern is dominated by the heavy atoms in the sample. Unlike in neutron diffraction, hydrogen (for example) scatters very wealdy and its position cannot be determined with any great accuracy. 

Other liquid-crystalline materials that have been investigated by X-ray scattering include single- and double-chained pyridinium [33] and N-substituted 4-(5-alkyl-l,3-dioxan-2-yl)pyridinium salts [34]. In the former case, diffraction analysis allowed an explanation for the differences in mono- and di-substituted salts to be proposed. 

In general, X-ray data are used in conjunction with other techniques to obtain as full a picture as possible. For liquid-crystalline materials, differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) are conventionally used. 

Even within a particular class of polymers made by step-growth polymerization, monomer composition can be varied to produce a wide range of polymer properties. For example, polyesters and polyamides can be low-Tg, amorphous materials or high-Tg, liquid crystalline materials depending on the monomer composition. 

One type of material that has transformed electronic displays is neither a solid nor a liquid, but something intermediate between the two. Liquid crystals are substances that flow like viscous liquids, but their molecules lie in a moderately orderly array, like those in a crystal. They are examples of a mesophase, an intermediate state of matter with the fluidity of a liquid and some of the molecular order of a solid. Liquid crystalline materials are finding many applications in the electronics industry because they are responsive to changes in temperature and electric fields. 

In most devices the liquid crystal molecules are confined between two thin walls which act as capacitor plates. This allows the determination of the dielectric properties of the liquid crystalline material through the simple relations... 

---