Anisotropic birefringent phase

Solutions of rigid macromolecules exhibit more complex viscosity behavior than solutions of flexible chains. They exhibit a maximum in viscosity as a function of concentration [37,38, 71,72] (see Fig. 3.10). This viscosity behavior is associated with the phase separation which occurs in these systems. The anisotropic birefringent phase has the lower viscosity and the viscosity decreases significantly when it becomes the continuous phase (see Section 3.5). 

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. 

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

The birefringence of the phase, if it is anisotropic. Birefringence is the difference in the approximate reciprocals of the light velocities of the fast and slow rays in other words, the difference in indices of refraction of the fast and slow rays. 

The polarised tight is now affected by the anisotropic, birefringent nature of the phase stmcture and a bright texture appears that is representative of the defects within the stmcture. Ideally, both the focal conic texture and the homeotropic texture should be... 

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. 

Comparison of these two polarizations shows that P2 Pi- Hence, in an isotropic medium such as a gas or a liquid x " = 0 and second order phenomena are not observable. Thus, only anisotropic media such as certain crystals are suitable for three-wave mixing processes. A consequence of a crystal being anisotropic is that it exhibits birefringence. However, the crystal birefringence enables phase matching to be achieved resulting in efficient generation of the new wave. 

The birefringence An of a nematic phase depends on the anisotropic polarizabilities a, a L and the degree of order S (Eq. (3), p. 114). The polarizabilities ae and of a nematic phase parallel and perpendicular to the director respectively obey the following relations 53)... 

Circular birefringence will induce a differential retardation in the phase of the orthogonal states of circularly polarized light. Circular dichroism, on the other hand, results in anisotropic attenuation of left- and right-circularly polarized light. The Jones matrix of circularly dichroic materials is normally written as ... 

Ellipsometry can also be applied to transmission measurements linear birefringence and dichroism of an anisotropic sample cause differences in amplitudes and phase shifts for waves of different polarization azimuths. Such experiments seem to be of considerable interest for partially ordered systems such as liquid crystals (cf. Sec. 4.6), here the degrees of polarization P and Ppf, reveal information related to order and reorientation processes (Korte et al., 1993 Reins et al., 1993). 

Cellulose Mesophases. An anistopropic phase is formed in a 6% (w/w) solution of cellulose in TFA-CH2Q2 (60 40 sN) and remains anisotropic for at least 16 days (Table I). In TFA-CH2CI2 (70 30 l ) solutions birefringence was observed as... 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

The subsequently so-called mesophase combines its fluidity with some typical anisotropic properties of the solid, crystalline state, for example birefringence. In the years which followed this phenomenon became the subject of intensive study [3] and it was found that many compounds with rod-like molecular geometry formed liquid crystalline phases [4]. 

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