Liquid crystalline state mechanical behavior

The effect of the chemical ermstitution of the crosslinker oti the local topology of the network is the second new feature to be considered. If the crosslinker molecule is flexible it can behave like an isotropic solvent. In that case, essentially only the phase transition and phase transformatiOTi temperatures of the LC phase are affected [90]. If, however, the chemical constitution resembles that of a mesogen of the constituent polymer backbone, the history of the crosslinking process becomes important. Under these conditions the crossUnker adopts the state of order in which the final crosslink process of the network occurs and thus determines the local topology of the crosslink [120,121]. The mechanical properties and the reorientational behavior are considerably modified for networks with the same chemical constitution but crosslinked either in the isotropic or in the liquid crystalline state [122-124]. Other important aspects of the local topology at the crosslink concern the phase transformation behavior [125] as well as the positional ordering in smectic systems [126]. 

It would thus be fcwmally possible to assign the described system, which contains a liquid-crystalline phase, to lyotropic liquid crystals. Actually, this system differs fiom true lyotropic liquid-crystalline systems with respect to the mechanism of the onset of the ordoed state since only a decrease in the temperature of the C-LC transition is involved in the rigid-chain polymer-solvent system due to addition of the solvent, and in this sense, the system does not differ from thomotropic liquid-crystalline systems formed in the pure polym with an increase in the temperature. In addition, let us examine the behavior of a system with a fixed concentration of solvent cwresponding to a composition of the system V2 (ho% and below, subscripts 1 and 2 in the volume and weight compositimis of the system refer to the solvent and polymer, respectively). An increase in the temperature to Tj results in the complete transition of the system into the liquid-crystalline state, which corresponds to the usual thermotropic transition, and the solvent does not play any specific role here except for decreasing the melting point of the crystalline phase. With a further increase in the tempaature, the same ttansitions LC -> LC +1 (at and LC +1 -> I (at as in ordinary thermotropic liquid-crystalline systems take place. 

The thermal properties of polymers include their behavior during heating from the solid amorphous (glassy) or crystalline to the liquid (molten) state, but also their chemical and mechanical stability in the entire range of application. 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

From practical considerations, two properties are of prime interest The effect of liquid crystalline behavior on viscosity and the ability of the polymer to retain the ordered arrangement in the solid state. Liquid crystalline behavior during the melt results in lower viscosity, because the rigid polymeric mesophases align themselves in the direction of the flow. As a result, the polymer is easier to process. Also, retention of the arrangement upon cooling yields a material with greatly improved mechanical properties. Several thermotropic liquid crystalline copolyesters are now available commercially. 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

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