Anisotropic birefringence

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. 

The clarity of a translucent crystalline polymer, such as polypropylene (PP), may be improved by biaxial orientation. Monoaxial orientation of a crystalline film produces an anisotropic birefringent film. 

Isotropic, no birefringence observed B = Bip ic, areas of bii ringence and areas of extinction A = Anisotropic, birefringence observed throughout... 

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. 

Uniaxial film is an anisotropic birefringent film with only one optical axis. For simplicity, let us limit our discussions to non-absorption uniaxial films. From the viewpoint of optical axis orientation, uniaxial films can be classified into a film and c film. An a film s optical axis is parallel to the film surface, while a c film s optical axis is perpendicular to the film surface. 

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... 

Polarizing light microscopy employs crossed polarizers to view the sample. With isotropic specimens, the field of view is dark, while anisotropic, birefringent samples or areas of a sample will appear bright. Polarizing microscopy is employed to view spherulitic structure [66-68] and deformation morphologies (crazes, shear banding) [69] in polymer blends. Samples for... 

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). 

For perspective, we first consider a z-directed plane wave propagating in an unbounded, slightly anisotropic (birefringent) medium characterized by refractive index n - n + dn, when the electric field is x-directed, and n = h+dny, when the electric field is y-directed, where n is the refractive index of the isotropic medium, and dn, Pny are constants. In an isotropic medium the plane wave can be polarized in any direction, but in the anisotropic medium there are only two polarizations, characterized... 

Figure 8.4. Interaction of a linearly polarized light with an anisotropic (birefringent) molecule. We therefore have...

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