Liquid crystalline polymers molecular features

Liquid crystalline polymers exhibit anisotropy in extruded and molded articles as a result of preferential orientation of LCP domains or individual chains. Reference (21 highlights some of the molecular structural features of LCP s that account for their fundamental anisotropy. These include the large aspect ratio of the individual polymer chains and their tendency to form aligned, highly crystalline domains. 

The side chain liquid crystalline polymer as storage display has such advantages as easy erasure, long working life, high contrast and reliable storage, etc. The molecular features can be tailored. The future of its application is expected to be bright. 

Mesogenic groups can be incorporated into polymeric systems [7]. This results in materials of novel features like main chain systems of extraordinary impact strength, side-chain systems with mesogens which can be switched in their orientation by external electric fields or—if chiral groups are attached to the mesogenic units—ferroelectric liquid crystalline polymers and elastomers. The dynamics of such systems depends in detail on its molecular architecture, i.e. especially the main chain polymer and its stiffness, the spacer molecules... 

The racemic mixture of all four components LP2, LU2, DP2, and DU2 yielded long superhelices of opposite handedness that coexisted in the same sample, pointing to the occurrence of spontaneous resolution through chiral selection in molecular recognition-directed self-assembly of supramolecular liquid-crystalline polymers. Such chiral selection features of self-organized entities are of general significance in connection with the questions of spontaneous resolution and of chirality amplification. This was confirmed by subsequent studies on a variety of helical supramolecular polymers (see Chapters 2, 3, and 6). 

The molecular structure of liquid crystalline polymers is one of their characteristic features. It comprises rigid, rod-like macromodules which align in the melt to produce liquid structures. Although more expensive than some competing polymers, LCP possess better flow characteristics to fill the thin walls of modern connectors, for example. 

We are all familiar with gases, liquids and crystals. However, in the nineteenth century a new state of matter was discovered called the liquid crystal state. It can be considered as the fourth state of matter (although plasmas are also candidates for this accolade). The essential features and properties of liquid crystal phases and their relation to molecular structure are discussed in this chapter. Specifically, the focus is on thermotropic liquid crystals (defined in the next section). These are exploited in liquid crystal displays (LCDs) in digital watches and other electronic equipment. Such applications are outlined later in this chapter. Surfactants and lipids form various types of liquid crystal phase but this was discussed separately in Chapter 4. Finally, this chapter focuses on low molecular weight liquid crystals, liquid crystalline polymers being touched upon in Section 2.10. 

The parameter /r tunes the stiffness of the potential. It is chosen such that the repulsive part of the Leimard-Jones potential makes a crossing of bonds highly improbable (e.g., k= 30). This off-lattice model has a rather realistic equation of state and reproduces many experimental features of polymer solutions. Due to the attractive interactions the model exhibits a liquid-vapour coexistence, and an isolated chain undergoes a transition from a self-avoiding walk at high temperatures to a collapsed globule at low temperatures. Since all interactions are continuous, the model is tractable by Monte Carlo simulations as well as by molecular dynamics. Generalizations of the Leimard-Jones potential to anisotropic pair interactions are available e.g., the Gay-Beme potential [29]. This latter potential has been employed to study non-spherical particles that possibly fomi liquid crystalline phases. 

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. 

Polymers that exhibit liquid crystallinity, either in the melt or in their solutions, typically consist of comparatively rigid structures that confer high extension on the backbone of the macromolecule. This molecular feature is obviously conducive to the axial order that is the mark of a nematic fluid. ... 

The most direct evidence of the crystallinity in polymers is provided by x-ray diffraction studies. The x-ray patterns of many crystalline polymers show both sharp features associated with regions of three-dimensional order, and more diffuse features characteristic of molecularly disordered substances like liquids. The occurrence of both types of feature is evidence that ordered regions (called crystallites) and disordered regions coexist in most crystalline polymers. X-ray scattering and electron microscopy have shown that the crystallites are made up of lamellae which are built-up of folded polymer chains as explained below. 

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