Liquid crystalline state viscosity

Fig. 10. The concentration dependence of viscosity for a PBA solution in dimethylacetamide (-(-LiCl) C is the critical concentration of the transition into the liquid crystalline state, U is the maximum viscosity at the point of the liquid crystalline transitions (according to >)...

Abrupt dropping of the viscosity by transition into the liquid-crystalline state... 

Polymer crystallization may also complicate the transition from the amorphous state to the intermediate liquid crystalhne state. Even small crystalline fractions in a polymer prevent free movement of macromolecules This is the case, for example, with cellulose. Cellulose fibres, obtained via cellulose xanthate (viscose), in the process of forming partially crystallize. Although the amorphous fraction in these fibres is large (up to 70-75 %), at a short-term heating of the fibres above the glass-transition temperature (240-260 °C) only slight self-elongation of fibres is observed, which can be attributed to the transition to the liquid crystalline state at the expense of the amorphous fraction... 

The liquid-crystalline state has been known for more than a century, as the so-called mesophase. It has a finite viscosity and at the same time anisotropic physical properties. Mesogenic systems do not lose all their long-range order suddenly above their melting points instead, as their temperature is increased, the positional long-... 

A final point of similarity between these two polymers is the occurrence of a critical concentration for a given polymer-solvent combination. Above this point the viscosity falls with further increase in concentration as illustrated in Fig. 6 for poIy(p>benzamide) in hydrofluoric acid. This is a consequence of the transition to a lyotropic liquid crystalline state. 

The phenomenological aspects of LCP rheology has been reviewed by the author [2]. More recently [14, 15], further data were obtained to compare the viscosity of the same polymer in the isotropic and liquid crystalline states. As with small molecule LC s, the viscosity of nematic LCP s was found to be lower than that of the isotropic forms. There are other aspects of the LCP rheology which require discussion, but first the evidence from theoretical considerations Is described. 

As with solutions of extended-chain polyamides (12), the critical concentration is affected by polymer molecular weight, being higher as molecular weight decreases (Fig. 7). At low inherent viscosities solubility may be insufficient for the attainment of a liquid crystalline state. On the other hand, solvent-polymer interaction may be so strong as to prevent the development of liquid crystalline order. 

The rheology of MCLCPs is complex. In general, the shear viscosity of MCLCPs is much lower than that of conventional polymers at a comparable molecular weight, and the transition from the isotropic state to the liquid-crystalline state is generally accompanied by a significant decrease in melt viscosity. At the onset of nematic behaviour, the melt viscosity of the MCLCPs is three decades less in order of magnitude than that of a similar but non-mesogenic polymer. 

Characterization439 Inherent viscosity before and after solid-sate polymerization is 0.46 and 3.20 dL/g, respectively (0.5 g/dL in pentafluorophenol at 25°C). DSC Tg = 135°C, Tm = 317°C. A copolyester of similar composition440 exhibited a liquid crystalline behavior with crystal-nematic and nematic-isotropic transition temperatures at 307 and 410°C, respectively (measured by DSC and hot-stage polarizing microscopy). The high-resolution solid-state 13C NMR study of a copolyester with a composition corresponding to z2/zi = 1-35 has been reported.441... 

If one follows the solution viscosity in concentrated sulfuric acid with increasing polymer concentration, then one observes first a rise, afterwards, however, an abrupt decrease (about 5 to 15%, depending on the type of polymers and the experimental conditions). This transition is identical with the transformation of an optical isotropic to an optical anisotropic liquid crystalline solution with nematic behavior. Such solutions in the state of rest are weakly clouded and become opalescent when they are stirred they show birefringence, i.e., they depolarize linear polarized light. The two phases, formed at the critical concentration, can be separated by centrifugation to an isotropic and an anisotropic phase. A high amount of anisotropic phase is desirable for the fiber properties. This can be obtained by variation of the molecular weight, the solvent, the temperature, and the polymer concentration. 

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