Liquid-crystalline polymers long-range orientational order

Note 2 A liquid-crystalline polymer can exhibit one or more liquid state(s) with one- or two-dimensional, long-range orientational order over certain ranges of temperatures either in the melt (thermotropic liquid-crystalline polymer) or in solution (lyotropic liquid-crystalline polymer). 

Smectic liquid crystalline polymers have more ordered structures than nematic liquid crystalline polymers, as their molecular arrangements have not only long-range orientational order, but also positional order. The positional order refers to the layer packing structures of the polymers. The less ordered smectic liquid crystals, such as smectic A, are true one-dimensional crystals. The packing structure of the smectic A is illustrated in Figure 5.6. The smectic A phase can be considered as convolution of a layer of two-dimensional liquid, i.e., a layer of randomly packed hard rods that are uniaxially oriented in the direction of the layer normal, and a one-dimensional lattice as shown in Figure 5.7. 

The main reason for the current and future ever-growing interest in liquid crystalline polymers lies in their unusual properties [89], The effective alignment of molecular backbones in LCP is claimed to produce properties even superior to engineering thermoplastics. The long-range orientational ordering of the liquid crystalline polymers leads to anisotropic mechanical, optical, magnetic, and electrical properties. 

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. 

In conventional LC phases the motion of the molecule is restricted only by the anisotropic interactions with its neighbors. This leads to the formation of the orientational long range order and in the case of smectic phases, to an additional lamellar sti ucture. However, completely different conditions normally exist in a liquid crystalline polymer. 

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