Melts anisotropic

A hot-stage-equipped polarizing microscope was used for measurement of these parameters. The anisotropic melting temperature (Tn) was determined as the onset temperature of stir-opalescence observed on the hot-stage. The liquid crystalline-isotropic transition temperature (71) was also determined by the use of the hot-stage-equipped microscope. 

Polymers that consist entirely of rigid-rod units form anisotropic melts at a very early stage of melt polymerisation if they form them at all. This observation suggests that... 

Brown, G. Wolken, J. (1979) Liquid crystals and biological structures. NY Academic Press 103Bemal, J. (1933) Liquid crystals and anisotropic melts. Trans. Faraday Soc. vol. 29, pg. 1082... 

FIG. 15.43 Rheological behaviour of isotropic vs. anisotropic melts. All graphs are double logarithmic. 

Wissbrun earlier observed a very long relaxation time and high elasticity for anisotropic melts of aromatic polyesters, as well as several other types of flow anomalies. Unfortunately, in most of these earlier studies, the rheological behavior of liquid crystal melts of polymers could not be directly compared with that of the isotropic phase of the same polymers because of their high clearing temperatures. 

It is during the plastic (fluid) stages of carbonization that the most important features of coke structure are formed, particularly porosity (6) and pore wall structure. During the plastic stage, the optically isotropic parent material undergoes a phase transition to an optically anisotropic melt which finally... 

The typical LCP character, resulting from the combination of LC and polymeric features, is primarily reflected in the phase behaviour. In analogy to LMLCs, LCPs exhibit various mesophases (see Fig. 2), often broadened in comparison to those of the monomeric systems. Likewise, the polymeric nature also dominates the behaviour in the solid phase below the anisotropic melt. Here, semicrystalline or glassy states are reported, which are very common for conventional non-mesomorphic polymers. 

The substituted polyamides with long alkyl- and alkoxychains [33] are highly soluble and form anisotropic melts above their melting temperature. These polyamides are typical examples of sanidic liquid crystalline polymers. Generally, no lyotropic behavior is observed. The temperature stability is obviously substantially lowered due to the substitution with alkyl chains. 

M. S. Connolly. Optically anisotropic melt forming aromatic copolyesters based on t-butylhydroquinone. US Patent 4664972, assigned to E. I. Du Pont de Nemours and Company (Wilmington, DE), May 12, 1987. 

A distinctive feature of semirigid polymers such as p-aramids is that their solutions develop molecular orientation under shear or extension with great ease. This results in a unique difference in properties in the direction of shear or extension vs. those perpendicular to the shear direction. There are two classes of materials that have this characteristic lyotropic, which form anisotropic solutions and thermotropic, which form anisotropic melts. As aramids do not melt we will focus here on lyotropic systems. Anisotropic solutions differ from isotropic solutions in many physical characteristics including light depolarization, rheological properties, phase behavior, and molecular orientation. 

In the past few years a considerable number of papers were published which were concerned with liquid crystalline structures in polymeric systems. Different routes were employed to obtain polymers with liquid crystalline structures or even thermodynamically stable liquid crystalline phases 1,, In general monomers containing mesogenic groups - groups which are known to have a tendency towards the formation of liquid crystalline structures, or rigid groups were used. Cases are known where the monomers exhibit liquid crystalline phases Z) In that case the polymerization can be performed in anisotropic melts frozen-in liquid crystalline structures and textures can be obtained in many instances i . In other cases the monomers do not display liquid crystalline phases. The formation of liquid crystalline polymer structures may nevertheless be possible due to the restriction of the motions of the individual repeat units -3),... 

Main chain polymers. Due to the rigidity of the monomer units stiff polymer chains are obtained which according to the theory should have a tendency towards the formation of anisotropic melts The properties... 

Process for Producing Amorphous Anisotropic Melt-Forming Polymers Having a High Degree of Stretchability and Polymers Produced by Same US Patent 6207790. International Patent Catalogue C 08 G 63/00. 2001. 

Like their low-molecular-weight analogs, LCPs display characteristic colored textures in polarized light. Thus, the opalescence of lyotropic polyaramides in solution can be viewed with the naked eye. For thermotropic polymers the birefringence of the anisotropic melt is often easily detectable in a polarizing microscope. 

Of special importance is in this case that the backbone of the copolymer can change chemically by trans-esterification (see Fig. 3.47). In fact, heating poly(ethylene terephthalate) and acetoxybenzoic acid in the melt is a common synthetic route to PETcoOB. The inclusion of oxybenzoate groups makes poly(ethylene terephthalate) increasingly stiffer (see Fig. 1.50). In POBcoON, oxynaphthoate adds an additional off-set of the large zig-zag of the benzoate. As a result of this chain stiffening, the PETcoOB with more than 30 mol-% oxybenzoate shows an anisotropic melt, as for a nematic main-chain liquid crystal (see Fig. 5.136) and the system POBcoON has a mesophase character over the whole concentration range. 

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