Birefringence molecular orientation

Optical Properties. When light falls on an object, it is either partially absorbed, reflected, or transmitted. The behavior of the object as it relates to each of these three possibiUties determines visual appearance. Optical properties of fibers give useful information about the fiber stmcture refractive indexes correlate well with fiber crystalline and molecular orientation and birefringence gives a measure of the degree of anisotropy of the fiber. 

Many papers deal with the crystallization of polymer melts and solutions under the conditions of molecular orientation achieved by the methods described above. Various physical methods have been used in these investigations electron microscopy, X-ray diffraction, birefringence, differential scanning calorimetry, etc. As a result, the properties of these systems have been described in detail and definite conclusions concerning their structure have been drawn (e.g.4 13 19,39,52)). 

Birefringence is one of the simplest methods for the characterization of molecular orientation in polymers. The polarizability of a structural unit is usually not equivalent in all directions, leading to three independent refractive indices along its principal axes. In an isotropic sample, a single averaged macroscopic refractive index is observed whereas birefringence or trirefringence is observed... 

The pyrazine LB films was expected to show uniaxial birefringence because the LB film possess uniaxial molecular orientation. Accordingly, the refractive index in the film plane no and film thickness W was determined by the analysis of TE modes. Then, the refractive index perpendicular to the film plane ne was obtained form the analysis of the TM modes. 

Aromatic polyimides have many anisotropic imide rings and benzene rings, and they are easy to orient by a film-forming process. Molecular orientation in polyimide films causes in-plane/out-of-plane birefringence (AnJ. Russell et al. have reported the A/ / of conventional PMDA/ODA. On the other hand, spin-coated polyimide films just after preparation do not cause Ann, as noted in the... 

Polymeric filaments produced in this way have a diameter of 100 to 300 fim and show anisotropy in their properties, indicating molecular orientation. For instance the birefringence of this fiber is 0.0073. Table 1 compares the modulus measured both in axial and in lateral direction, with the modulus of the same material cured under isotropic condition at 80 C. The increase in modulus in axial direction is obvious. The decrease of the modulus at higher temperatures can be ascribed to both the glass transition and melting of crystalline areas. 

It has been demonstrated that molecular orientation can be achieved starting with a low molecular weight species which is oriented in an elongational flow and subsequently cured under UV-irradiation. The orientation of the monomer is frozen-in by the ultra-fast process of polymerization and crosslinking. Both extrusion and stretching can be carried out at relatively low temperatures and pressures. Polymer filaments produced in this way are definitely anisotropic as is evidenced by their birefringence and by a strong increase of the tensile modulus and a decrease of the thermal expansion coefficient in the axial direction. 

Infrared dichroism is one of numerous methods used to characterize molecular orientation. The degree of anisotropy of the strained pol3rmers may also be accurately characterized by other techniques such as X-ray diffraction, birefringence, sonic modulus, polarized fluorescence and polarized Raman spectroscopy [2]. These techniques directly probe the orientational behavior of macromolecular chains at a molecular level, in contrast to the macroscopic information provided by mechanical measurements. 

In regime III, the flow field is very strong and shear-induced molecular orientation becomes important. According to birefringence measurements for anisotropic HPC/H2O solutions and HPC/ m-cresol solutions, the molecular orientation is a monotoni-cally increasing function of the steady state shear rate. 

In many cases in the manufacturing of materials, the molecular orientation results because there was molecular orientation in the melt. This review will not be concerned with the measurement of orientation in the liquid or molten state. Generally the technique of flow birefringence is used for such measurements. Recent comprehensive... 

Birefringence can be defined as the difference in the refractive indices of two perpendicular directions as measured with light polarized along these respective directions. Molecular orientation will in general give rise to non-zero birefringence. This results because the polarizability... 

Morgan, H. M. Correlation of molecular orientation measurements in fibers by optical birefringence and pulse velocity methods. Textile Res. J. 32, 866... 

Ward, I. M. The correlation of molecular orientation parameters derived from optical birefringence and sonic velocity methods. Textile Res. J. 34,806 (1964). 

Orthoscopic examination with crossed polars is carried out first of all to determine the isotropism or the anisotropism of a sample. The polarization colors, the defects and variation in molecular orientation, and the orientation pattern or texture of liquid crystals are observed in this examination. With a heating stage the temperature of phase transition is also determined. In addition, with use of a compensator, the determination of vibration directions of the ordinary and extraordinary rays, the determination of relative retardation and birefringence are possible. In this section, the optical basics for orthoscopic observations are briefly outlined. The description of textures frequently observed for polymeric liquid crystals is given in Section 4.1.4. 

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