Phase anisotropic

Many ceUulosic derivatives form anisotropic, ie, Hquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety of solvents (56,57). The nature of the polymer—solvent interaction determines the concentration at which hquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birefringent polymer solution. Strong organic acids, eg, trifluoroacetic acid are most effective and can produce an anisotropic phase with concentrations as low as 28% (58). Trifluoroacetic acid has been studied with cellulose triacetate alone or in combination with other solvents (59—64) concentrations of 30—42% (wt vol) triacetate were common. 

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

Lyotropic liquid crystals are those which occur on the addition of a solvent to a substance, or on increasing the substance concentration in the solvent. There are examples of cellulose derivatives that are both thennotropic and lyotropic. However, cellulose and most cellulose derivatives form lyotropic mesophases. They usually have a characteristic "critical concentration" or "A point" where the molecules first begin to orient into the anisotropic phase which coexists with the isotropic phase. The anisotropic or ordered phase increases relative to the isotropic phase as the solution concentration is increased in a concentration range termed the "biphasic region." At the "B point" concentration the solution is wholly anisotropic. These A and B points are usually determined optically. 

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. 

It bears repeating that the values are effective diffusivities and that, in fact, diffusivity is a function of surfactant concentration, as shown by interferometry for the Li phase of Ci2(EO)5 [9]. For the anisotropic phases diffusivity is also a function of orientation, and Dej depends on the number and orientation of domains of the phase as formed during dissolution. Thus, the value shown in Table 1 for DgHi of Ci2(EO)6 is intermediate between the diffusivities parallel and perpendicular to the rodlike micelles measured in fully oriented samples of the hexagonal phase Hi for this surfactant [26]. 

Fig. 9. Phase diagrams of quasi-ternary systems containing two different molecular weight samples a PHIC-toluene with (Ni, N2) = (4.46,0.38) [73] b schizophyllan-water system with (Nt, N2) = (0.930, 0.0765) [75,76]. (O, A) experimental coexisting isotropic phase ( , , ) experimental coexisting anisotropic phase dashed segments, experimental tie lines the shadowed triangular region, the IAA triphasic region thick full curves, theoretical binodals thin full segments, theoretical tie lines...

Reaction Cavities of Alkanophenones in Neat Solid and Liquid-Crystalline Phases. As mentioned previously, solid-state studies on the Norrish II processes of alkyl aryl ketones are unambiguous with respect to the triplet multiplicity of the reactive excited state. On the other hand, a bulky aryl auxochrome can create complications during the transformation of the excited triplet states to photoproducts in neat anisotropic phases. 

For the investigation of the polymerization in l.c. s two aspects are of interest. For polymerizable m-l.c. s, which are in the well defined anisotropic phase, model systems... 

In most of these unit cell analyses the assumptions are made that a) strains are small, so that infinitesimal elasticity theory applies, b) the phases are of uniform density and elastic properties and c) the phases are isotropic. Some extensions to the cases of anisotropic phases have been made54 . 

These results of Walpole61 include as special cases those of Hill47 and of Hashin and Shtrikman48. For anisotropic phases Walpole58 gives bounds on the five elastic moduli of an aligned array of transversely isotropic elements and for randomly oriented fibrous inclusions in an isotropic matrix. For the former case (alignment) the bounds are expressed in terms of phase concentration q and the quantities k, 1, m, n, p defined as follows k - 1/2 (Cjj + C, m — 1/2 (Cn — C22), = C13, n = C33, p = C44 = C55. 

Circular anisotropic phase group of binder-phase anisotropic carbon textures that are distinguished by approximately circular domains (ASTM D-5061). 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

The crystal structures of transition metal compounds and minerals have either cubic or lower symmetries. The cations may occur in regular octahedral (or tetrahedral) sites or be present in distorted coordination polyhedra in the crystal structures. When cations are located in low-symmetry coordination environments in non-cubic minerals, different absorption spectrum profiles may result when linearly polarized light is transmitted through single crystals of the anisotropic phases. Such polarization dependence of absorption bands is illustrated by the spectra ofFe2+ in gillespite (fig. 3.3) and of Fe3+in yellow sapphire (fig. 3.16). 

When the blend is heated up melting of the PET-crystallites turns out to be the key process. This process is simultaneously the onset process for liquid liquid phase separation which elapses rapidly. The liquid-liquid phase separation in turn permits the formation of the anisotropic phase. When the PET content of the blends exceeds 60% the formation of the anisotropic phase is hampered and finally impossible. 

If monomeric liquid crystals are polymerised, the polymers also show in many cases the liquid-crystalline effects. In addition, their viscosity during flow is unexpectedly low in the anisotropic phase state. 

Eq. (6.36) is, as a matter of fact, no guarantee that an anisotropic phase will really be observable. In the present state of the art, it is impossible to predict the nature and even the possible existence of a mesophase from the structural formula of a polymer. 

The general description of Eq. (16.60) is in agreement with experiment for a variety of lyotropic LCPs. A consequence of the theory is that for a polydisperse system of rods fractionation will occur longer rods will become part of the anisotropic phase and the smaller ones of the isotropic phase. The major drawback to his lattice theory is that it is athermal, just like the Onsager model and therefore also fails to predict the occurrence of the clearing temperature. 

The absorbances at both 500 and 610nm, which appeared first, became larger with increasing number of carbons, reached a maximum of Cj n, and then became smaller with further increases in the carbon number. Presumably this is due to crystallization (orientation) of the long alkyl chains. As shown in Figure 9.6, the dication and counteranion moieties in an anisotropic phase are situated next to each other at the site, and the distance between both ions is large compared with that in an isotropic (disordered) phase. Thus the electron transfer from counteranion to dication hardly takes place via the exciplex mechanism. 

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