Types of birefringence

When the literature is surveyed a number of different terms arise to describe the type of birefringence observed. We will try to establish at the onset what some of these categories are and to comment on their origin and relative importance in polymeric materials. 

This arises when there is a physical ordering of optically anisotropic elements e.g. chemical bonds) along some preferential direction— see Fig. 3. This can occur in polymers by aligning amorphous or crystalline chains as by an extension or drawing deformation. Orientation birefringence is the quantity that is most generally measured (or desired) 

Intrinsic birefringence, An°, is a special case of orientation birefringence and is, by definition, the maximum in orientation birefringence. It follows that the intrinsic birefringence will depend on whether one has an amorphous chain or a crystal. In fact, for a crystal, three intrinsic birefringence values exist due to the three dimensionality of a unit cell. However, only two of these are independent. 

Until recently, this origin of apparent optical anisotropy had been generally neglected for birefringence measurements on polymeric solids. The phenomenon arises when the medium contains at least two phases each having a different refractive index and at least one dimension 

Illustration showing lunv deformation birefringence may arise as a result of a clumge in packing caused by an external deformation. 

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All other types of birefringent polarizers listed above exploit the same physical principle and differ only in their construction. Immersion media are more often used instead of the air gap in order to achieve a rigid device. Birefringent polarizers are mainly used in Raman spectroscopy for polarization of the radiation of a laser light source. 

That response theory encompasses a great number of of properties of various kind, was nicely illustrated in [145] and [146], which presented and analyzed results from the theory of this review applied to five different types of birefringences. These were the Kerr, Cotton-Mouton, Buckingham, Jones and Magneto-electric birefringences... 

Thus, there are two types of birefringence, one with respect to the light rays and the other with respect to the wave normals. The relative importance of the two types depends on the laws of refraction. 

The adsorption double refraction results from orientated adsorption of solvent molecules on the polymer frame. There are indications that this effect plays a part in the double refraction of certain systems However, a theoretical approach to this type of birefringence does not exist. Notwithstanding these difficulties, Treloar ... 

Optical microscopy in polarized fight is done for samples of thin film (maximum thickness of the film should be less than a few micrometers in order to get a sharp pictiue of the structure). This optical transmission method allows the detection of the type of birefringence (negative or positive) but is insufficient to assign the lamellar structure. Satisfactory results are achieved for structure emits with dimensions of some micrometers. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

Birefringence (or double refraction) is the decomposition of a light ray into two rays when it passes through certain types of crystalline material. This occurs only when the material is anisotropic, that is, the material has different characteristics in different directions. Amylose and amylopectin polymers are organized into a radially anisotropic, semicrystalline unit in the starch granule. This radial anisotropy is responsible for the distinctive... 

However, if an LC substance is heated, it will show more than one melting point. Thus, liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and a solid crystal. For instance, an LC may flow like a liquid but have the molecules in the liquid arranged and/or oriented in a crystal-like way. There are many different types of LC phases that can be distinguished based on their different optical properties (such as birefringence). When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have a distinct texture. Each patch in the texture corresponds to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered. Liquid crystal materials may not always be in an LC phase (just as water is not always in the liquid phase it may also be found in the solid or gas phase). 

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. 

Three common types of electrooptic effects are illustrated in Figure 8 i.e, quadratic and linear birefringence and memory scattering. Also included in the figure is a typical setup required for generating each effect along with the observed behavior shown in terms of light intensity output (I) as a function of electric field (E). 

A second type of behavior existing in the PLZT s is the linear (Pockels) effect which is generally found in high coercive field, tetragonal materials (composition 3), This effect is so named because of the linear relationship between An and electric field. The truly linear, nonhysteretic character of this effect has been found to be intrinsic to the material and not due to domain reorientation processes which occur in the quadratic and memory materials. The linear materials possess permanent remanent polarization however, in this case the material is switched to its saturation remanence, and it remains in that state. Optical information is extracted from the ceramic by the action of an electric field which causes linear changes in the birefringence, but in no case is there polarization reversal in the material. 

During the studies of phase behaviour two types of liquid crystalline phases were identified. LC material was viscous and exhibited intense "white" birefingence. material was apparently homogeneous but of low viscosity and exhibited "multi-coloured" birefringence. The liquid crystalline phases observed in the equilibrium studies of surfactant concentrations up to 25 are unlikely to take part in the self-emulsification process due to the presence of two-phase regions between L2 and liquid crystalline phases however, LC material may account for the improved stability of emulsions formed by 25 surfactant systems (Table II). Figure 4c indicates that by increasing the surfactant concentration to 30 the... 

For polymer melts another type of apparatus has been designed in order to measure flow birefringence in the same plane (17). This apparatus is of the cone-and-plate type. In this apparatus the light beam is directed in a radial direction. The principles of other arrangements, which were designed for the measurement of flow birefringence in a plane perpendicular to the plane of flow, will receive special attention in Section 1.5. 

At this point it should be noted that the conclusion drawn from flow birefringence measurements, viz. that p22 — p33 of polymer systems is very small compared with pn — pn is not always supported by other types of measurement. With the aid of pressure measurements in the walls of various rheometers (e.g. cone-and-plate apparatus) results have been obtained by a number of authors (refs. 26, 43, 44), showing that p23 — p33 should be positive and can have values up to 20 per cent of Pn Pta- 1-7 suggests for the investigated polyisobutylene solution... 

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