Linearly polarized light, birefringence

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. 

An interstellar dust cloud containing aligned particles may be looked upon as a linearly birefringent (and possibly linearly dichroic) medium (van de Hulst, 1957, p. 58) the cloud acts like a retarder. We showed at the end of Section 2.11 that linearly polarized light becomes circularly polarized upon transmission by a retarder. The first clear evidence for this kind of polarization mechanism was reported by Martin (1972), where light from the Crab Nebula was the source of linearly polarized incident light. 

The most commonly encountered manifestations of chiroptical phenomena are circular birefringence (also known as optical rotation), optical rotatory dispersion (ORD), and circular dichroism (CD). An explanation as to the nature of circularly and linearly polarized light is provided, and the origins of the various chiroptical effects are discussed. In each instance, a concise summary of the calculations used by workers in the field to report the results of their investigations is provided. 

Figure 9.2. Conversion of linearly polarized light to elliptically polarized light passed through a birefringent crystal.

After passage through a birefringent material of thickness e, the two components of linearly polarized light show a phase difference 6 equal to ... 

FIG. 13.10 Birefringence cycles on freshly prepared p4MAN films. For film a, the linearly polarized light is turned on at t = 0 for 500 s after a 500 s relaxation period, the sample is in situ heated at a rate of 5°C/min to I75°C and then cooled to room temperature. For film b, the sample is cooled rapidly when the maximum birefringence is reached at approximately t = 2200 s as the temperature reached 120 C. Reprinted with permission from reference 41. Copyright 2000 American Chemical Society. 

A comparison has been made between the reorientational behavior of a series of copolymers of DR with MMA (13, Figure 4) and that of series of blends of a DR homopolymer (12) with PMMA to elucidate the sequential effect of the DR monomer units on the orientability caused by intramolecular interactions between dye chromophore groups in polymer solids.79 The maximum birefringence induced by linearly polarized light for the blends80 increased linearly with an increase in the DR fraction. In contrast, the copolymers showed a nonlinear... 

Jones birefringence (JB) [140-143] this birefringence is observed when linearly polarized light traverses a sample in the presence of both an electric and a magnetic field perpendicular to the direction of propagation but (unlike in the case of MEB) parallel to each other. The theoretical expression for JB is identical to that for MEB, but it should be noted that the two effects correspond to different experiments with the optical axes in the JB directed at 45° with respect to those of the other three birefringences (KE, CME and MEB). 

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