Birefringence biaxial

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Biaxial Orientation. Many polymer films require orientation to achieve commercially acceptable performance (10). Orientation may be uniaxial (generally in the machine direction [MD]) or biaxial where the web is stretched or oriented in the two perpendicular planar axes. The biaxial orientation may be balanced or unbalanced depending on use, but most preferably is balanced. Further, this balance of properties may relate particularly to tensile properties, tear properties, optical birefringence, thermal shrinkage, or a combination of properties. A balanced film should be anisotropic, although this is difficult to achieve across the web of a flat oriented film. 

Only two birefringence indices are independent and necessary to describe the anisotropy of a biaxial system, while a single birefringence measurement is required for uniaxial samples because nx — ny. 

Opt. Colorless in transmitted light. Often exhibits optical anomalies due to mechanical deformation or aggregation of crystals at times biaxial with small 2F due to the anomalies. Abnormal interference colors, due to marked change of birefringence with wavelength, are often observed. 

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. 

Two types of phase-matching exist for SHG Type I, where the two fundamental photons are of the same polarization, and Type II, where they are orthogonally polarized. The phase- matching direction and one of the principal vibration directions of the crystal may be coincident in biaxial and uniaxial (birefringent) crystals. This situation, called noncritical phase-matching, is quite... 

Figure 1. The biaxial indicatrix, whose principal axes correspond to the refractive indices na < n(i < ny. The optic axes are indicated by dashed lines and the optic angle is denoted 2V. N is the principal birefringence.

J. A. van Aken and H. Janeschitz-Kriegl, Rheol. Acta, Simultaneous measurement of transient stress and flow birefringence in one sided compression (biaxial extension) of a polymer melt, Rheol. Acta, 20,419 (1981). 

Biaxially oriented films such as PET and PEN are birefringent. For LC displays which depend on light of known polarization this means that birefringent films, which would change the polarization state, are unlikely to be used as substrates. Films based on amorphous polymer are not birefringent and are more suitable for LC displays. Birefringence is not an issue with OLED, electrophoretic displays, or, indeed, some LC displays. 

Biaxial orientation of crystalline plastics generally improves clarity of films. This occurs because stretching breaks up large crystalline structures into smaller than the wavelength of visible light. With uniaxial orientation, the result is an anisotropic refractive index and thus birefringence, especially in crystalline plastics. 

The continuous-phase morphology is firmly established for copolymers of HBA and ET, mentioned above, by the recent work of Windle and collaborators They observed birefringent domains in sections of oriented pellets obtained by extrusion of these copolymers. The combined techniques of X-ray diffraction and optical birefringence revealed biaxial order within the domains, instead of the uniaxial order... 

Birefringence The difference between the refractive index for the extraordinary and ordinary rays (he — no) in a uniaxial or biaxial crystal. If a crystal, such as quartz, is positively birefringent, no is less than ns, and the velocity of the ordinary-ray is greater than that of the extraordinary ray. The reverse is true for a negatively birefringent crystal. 

Optic axis A direction in a birefringent crystal along which the ordinary and extraordinary rays travel with the same velocity so that only one image is seen. Uniaxial crystals have one such axis biaxial crystals have two. 

Before proceeding with discussion of the measurement of birefringence of a fiber it should be pointed out that if biaxial orientation exists in a film all three birefringence values must be determined to characterize the optical anisotropy. 

The measurement of optical activity in single crystals has been studied most extensively by transmission techniques, although recent development of emission methods has also been reported (2,1). Transmission methods are limited to those compounds which can be obtained in suitably large and perfect single crystals. In addition, only in non-biaxial crystals can the complications of linear birefringence be avoided easily. (However, a two-angle method for cancelling these effects has... 

---