Thermotropic polymer shape

The study of the molecular organization of lyotropic and some thermotropic comb-shaped polymers was of primary importance in understanding the mechanism of formation of intramolecular orientational order [9] these concepts subsequently served as the basis for notions on ordering of liquid-crystal polymers on the molecular level. 

The possibility of synthesizing a large number of comb-shaped thermotropic polymers with a different structure of the main chain [31-35], different mesogenic fragments [36-41], and a different type of connection to the main chain provides broad opportunities for their investigation [1, 2, 19, 31, 43-50]. The presence of chemical groups which cause the formation of the liquid-crystalline phase in their side chains, while the main chain of the molecule can be flexible, is a basic structural feature of these comb-sh )ed mesogenic macromolecules. 

It follows from the data in Table 3.2 and Fig. 3.3 that the rigidity of the main chain increases with an increase in the length of the side chains for both the previously studied comb-shaped macromolecules and for the macromolecules of thermotropic polymers with mesogenic fragments in the side chains. The ob-... 

The second group comprises thermotropic polymers. The phase transitions of thermotropic liquid crystals are achieved when a determined temperature range is reached. The fundamental unit that induces structural order in this kind of polymer presents with high rigidity and anisotropic shape [26,27]. Two major subclasses can be distinguished according to this shape discotic (disc-Uke molecules) and... 

The exploitation of lyotropic liquid crystalline order by Kwolek" to process Kevlar fibres established a principle—but one which was retained in the hands of specialized fibre producers. The extension from lyotropic to thermotropic polymers by Jackson and Kuhfuss opened the field to conventional thermoplastics processing. The variability of thermoplastics processing technology takes one into products of complex shapes and more complex flow histories and so an even greater interdependence between flow history, morphology and properties. 

Academic and industrial interest in liquid-crystalline polymers of the main-chain type has been stimulated by certain special properties shared by lyotropic and thermotropic systems that exhibit a nematic phase. Although these special properties affect both the processing into fibres and other shaped articles and the physical behaviour of the products, the product behaviour is at least partly attributable to the novel processing behaviour. 

On the other hand, the interest towards this field is accounted for by the possibility to create polymeric systems, combining the unique properties of low-molecular liquid crystals and high molecular compounds, making it feasible to produce films, fibers and coatings with extraordinary features. It is well-known that the utilization of low-molecular thermotropic liquid crystals requirs special hermetic protective shells (electrooptical cells, microcapsules etc.), which maintain their shape and protect LC compounds from external influences. In the case of thermotropic LC polymers there is no need for such sandwich-like constructions, because the properties of low-molecular liquid crystals and of polymeric body are combined in a single individual material. This reveals essentially new perspectives for their application. 

These are macromolecules that can align into crystalline arrays while they are in solution lyotropic) or while in a molten state thermotropic). Such liquids exhibit anisotropic behavior [51,52]. The regions of orderliness in such liquids are called mesophases. Molecular rigidity found in rigid rod-shaped polymers, for instance, is the chief cause of their liquid crystalline behavior. It excludes more than one molecule occupying a specific volume and it is not a result of intermolecular attractive forces. Some aromatic polyesters or polyamides are good examples, like polyphenylene terephthalate ... 

A disadvantage of Kevlar is that the polymer Is a "lyotropic" liquid crystal, that Is, one that becomes liquid crystalline when dissolved In an appropriate solvent. The need to remove solvent during the fabrication process limits lyotropic LCP s to formation of thin fibers or films. In order to Form thick articles or parts with more complicated shapes requires a "thermotropic" LCP, that Is, one that becomes liquid crystalline upon heating.[4]... 

It has been well recognized that melt blending of a thermotropic liquid crystalline polymer (LCP) and an isotropic polymer produces a composite in which fibrous LCP domains dispersed within the blend act as a reinforcement il ). The so-called insitu composite possesses several advantages in comparison with the inorganic reinforced thermoplastic composites. Firstly, LCP lowers the blend viscosity in the actual fabrication temperature range (3-5), Hence, the enhanced processability endows moldability for fine and complex shaped products. 

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