Liquid crystalline polymers semi-rigid

First, we will consider the design of polyphosphazenes as side-chain liquid-crystalline materials.241 248 Side-chain liquid-crystalline polymers are a subclass of species described earlier as structure 3.72. Liquid crystallinity occurs when the rigid side groups become organized, usually in the semi-liquid state. The organization may be nematic (oriented but unlayered) or smectic (layered) as illustrated in Figure 3.25. 

In the present paper, we first review briefly the rigid rod models for liquid crystalline phase transitions. In these models, emphasis is placed on the anisotropic form and on the orientation dependent intermolecular interactions between rigid particles. Conformational studies on isolated chains have shown that liquid crystalline polymers are rather semi-rigid in character although only a narrow range of deformations is possible due to intrachain interactions. The effect of chain flexibility on the formation of liquid crystalline phases has been pointed out both experimentally and the-oretically J. 

Doi and Edwards developed the most extensive theory of dynamics of rod-like macromolecules in concentrated solutions [2,12], As discussed in the previous section, the majority of commercial TLCPs are not perfect rigid rods but are semi-rigid rods having some degree of flexibility. Although several experimental results support the validity of the rigid rod approximation for both lyotropic [13-15] and thermotropic liquid crystalline polymers [16-18], there is no complete theory on the dynamics of TLCPs. To better understand the dynamics of TLCPs in the melt, the Doi-Edwards theory on the dynamics of rod-like polymers in solution is summarized here. Readers may find further details of the theory in the original reference [2,12]. 

Liquid crystalline polymers (LCPs) exhibit ordered molecular arrangements in the melt state. The polymer molecules of LCPs are generally semi-rigid or contain rigid units. The rigid units are either along the backbone or in the side branches. LCPs are viscoelastic and anisotropic. The interested reader can learn details about LCPs from Feng and Leal (1997), Larson (1999), Fan et al. (2003), and Klein et al. (2008). 

Fortunately such systems exhibit the unusual property to be able to form liquid-crystalline liquids under certain conditions (of molecular mass, temperature, nature of solvent, concentration, etc. the liquid-crystalline liquid obtained is mostly of the nematic type. As already described in Chap. 2, two types of semi-rigid polymers can be distinguished ... 

The other and most popular way is to copolymerize the flexible segment into the polymer main chains. De Gennes (1975) predicted that incorporation of both a rigid and a flexible segment in the repeating unit should afford semi-flexible polymers exhibiting thermotropic liquid crystallinity,... 

In this section, we consider the case of solutions of rigid or semi-flexible polymers which display one or several liquid crystalline phases in a given range of concentration. The main control parameter is not flie temperature as is the case for thermotropic LCPs but rather the concentration of polymer in the solvent. There are many different kinds of lyotropic LCPs. Some are synthetic like Kevlar which has become a very important structural material with mechanical properties comparable to those of steel. Some are natural like the Tobacco Mosaic Virus and like DNA which shows a nematic and a hexagonal phase. Some are mineral like the vanadium pentoxide ribbons. In the next section, we shall first describe the lyotropic system which is probably best known, namely the tobacco mosaic virus. 

Poly(l,4-benzamide), abbreviated here as PpBA, was the first nonpeptide, synthetic condensation polymer reported to form liquid crystalline phases [11, 39,42,48-50]. Technologically important is poly(l,4-phenylene terephthalamide), abbreviated here as PpPTA. In both polymers the non-flexibility or semi-rigid chain structure is caused by the paralinked benzene rings and the partial double bond character of the carbon-nitrogen bond in the predominantly trans amide linkages. 

The procedures which employ the intrachain scattering to evaluate the molecular orientation parameters have been applied successfully to main-chain and side-chain liquid crystal polymers and to non-crystalline polymers in general. - They have recently been reviewed by Windle. We shall explore their use through the example of a semi-rigid thermotropic co-polyester The X-ray... 

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