Liquid Crystal States

We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

The homologous polybibenzoate with a central oxygen atom in the spacer, PDTMB, exhibits a liquid-crystal state at room temperature, owing to its low crystallization rate [15]. This fact is one of the main differences between P7MB and PDTMB, and is reflected on the mechanical properties of both polybibenzoates. The initial... 

The liquid crystal state represents the fourth state of matter and exists between the solid and liquid states, which form its boundaries. The liquid crystal state is reached from the solid state either by the action of temperature (thermotropic liquid crystals) or of solvent (lyotropic liquid crystals) and it is the former that will be the subject of this chapter. 

Being bordered by the solid and liquid states, the liquid crystal state has some of the order of a solid, combined with the fluidity of a liquid. As such, it is an anisotropic fluid and it is this anisotropy that has led to the widespread application of liquid crystals. 

Cholesterol s presence in liposome membranes has the effect of decreasing or even abolishing (at high cholesterol concentrations) the phase transition from the gel state to the fluid or liquid crystal state that occurs with increasing temperature. It also can modulate the permeability and fluidity of the associated membrane—increasing both parameters at temperatures below the phase transition point and decreasing both above the phase transition temperature. Most liposomal recipes include cholesterol as an integral component in membrane construction. 

The synthesis of the N03 and PFg salts of the iron(III) complex of the terdentate phenolate anion of the liquid crystal Schiff base (7) provides the first example of the coexistence of spin transition and liquid crystal properties in a single compound. The ligand now needs to be tuned to increase the spin cross-over transition temperature and decrease the transition temperature to the liquid crystal state. ... 

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

This article reviews the following solution properties of liquid-crystalline stiff-chain polymers (1) osmotic pressure and osmotic compressibility, (2) phase behavior involving liquid crystal phasefs), (3) orientational order parameter, (4) translational and rotational diffusion coefficients, (5) zero-shear viscosity, and (6) rheological behavior in the liquid crystal state. Among the related theories, the scaled particle theory is chosen to compare with experimental results for properties (1H3), the fuzzy cylinder model theory for properties (4) and (5), and Doi s theory for property (6). In most cases the agreement between experiment and theory is satisfactory, enabling one to predict solution properties from basic molecular parameters. Procedures for data analysis are described in detail. 

Liquid-crystalline polymers with stiff backbones have many static and dynamic solution properties markedly distinct from usual flexible polymers. For example, their solutions are transformed from isotropic to liquid crystal state with increasing concentration. While very high in the concentrated isotropic state, their viscosity decreases drastically as the concentration crosses the phase boundary toward the liquid crystal state. The unique rheological properties they exhibit in the liquid crystal state are also remarkable. 

The orientation dependent parameter p defined by Eq. (11) becomes unity in the isotropic state, and decreases as the polymers are uniaxially oriented. Therefore, it follows from Eqs. (9) and (10) that the wormlike hard spherocylinder system has a smaller translational entropy loss from the ideal solution in the liquid crystal state than in the isotropic state. This difference drives the system to form a liquid crystal phase. However, in order to determine the equilibrium orientation of the system, the orientation dependence of Sor has to be formulated, and this is done in Sect. 2.3. 

FIGURE 7. Optical microscope picture with crossed polarizers of the microdroplets showing a liquid-crystal state filling the pores of a GDLC thin film. The orientations in microdroplets can be observed as the result of the specific sol-gel processing (parallel polarizers gave Maltese crosses of LC microdroplets and a black image of the silica-gel substrate)... 

The rate and character of the molecular motions of both the molecules embedded in the lipid bilayer and lipid molecules themselves are strongly dependent on the temperature [19, 203], At a certain temperature tm, the gel-liquid crystal phase transition is known to occur for the membrane made of a synthetic lipid. For example, tm = 41.5 °C for the membranes from DPL. In the vesicles formed by a mixture of lipids, e.g. egg lecithin, the phase transition occurs smoothly rather than jumpwise and starts below 0 °C. Note that the permeability of lipid membranes increases notably upon transition from the liquid crystal state to the gel state [204]. 

If p-methacrylylhydroxybenzoic acid is mixed with p-cetylhydroxy-benzoic acid, a smectic form of liquid crystals results, but if it is mixed with p-nonylhydroxybenzoic acid, the resulting form is nematic above 104°C. and is smectic below this. This makes it possible to compare the polymerization behavior of the same monomer in solution, where the mutual ordering of its molecules is minimal, in a liquid crystal state with only orientation order (nematic form), and in the liquid-crystal state involving both orientation and coordination order (smectic form). 

Polymerization in the liquid crystal state involves a major increase in the molecular weight of the polymer, reaching the hundreds of thousands. The thermodynamic equilibrium between the monomer in smectic liquid crystals and the polymer is shifted completely toward formation of the polymer up to the melting point of the crystal (138°C.). The polymerization rate does not change with the degree of conversion... 

Polyhedral niosomes were found to be thermoresponsive Fig. 7 (a). Above 35 °C, there was an increase in the release of CF from these niosomes even though the polyhedral shape was preserved until these vesicles were heated to 50 °C. Solulan C24-free polyhedral niosomes do not exhibit this thermoresponsive behavior [160] due to a decrease in the interaction of the polyoxyethylene compound solulan C24 with water at this temperature (due to decreased hydrogen bonding) as identified by viscometry [161]. This observed thermoresponsive behavior was used to design a reversible thermoresponsive controlled release system Fig. 7 (b). Thermoresponsive liposomal systems which rely on the changing membrane permeability, when the system transfers from the gel state (La) to the liquid crystal state (L 3) [162], are not reversible. This is not unex-... 

According to Coulomb, there occurs a kind of one-dimensional melting with the formation of a 2-D liquid crystal state. 

The liquid crystal state represents a fourth state of matter. Located between the solid and liquid states, it is boimded by these states. 

Fig. 10a and b. X-ray Diffraction Patterns of Thermotropic Polymers Polymer 15 Obtained at Room Temperature, a and in the Liquid Crystal State, b... 

POLYMERS IN THE LIQUID CRYSTAL STATE 2.7.1 Liquid Crystal State... 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

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