Birefringence biaxial orientation

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. 

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. 

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. 

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 usual approach used to characterise the molecular orientation in biaxially oriented samples by IR spectroscopy is used to measure spectra with polarisation in all three directions - machine, transverse and normal (or thickness). However, the latter measurement is rather difficult to make experimentally. A new approach to characterising the molecular orientation in both uniaxially and biaxially oriented samples of PETP is employed. It makes use of the ratio of the absorption bands near 1250 and 1725 cm-1, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus it is possible to determine individually the orientation functions with respect to the machine and transverse directions. The validity of functions determined in this way is confirmed by comparison with birefringence results. 9 refs. 

Polymers may exhibit a biaxial orientation. The segmental orientation function is in this case a function of two angular variables, i.e. /(, v), as shown in Figure 10.7. The in-plane orientation is different in the zx and zy planes (Figure 10.7). There are several methods commonly used to determine chain orientation in-plane birefringence, wide angle X-ray diffraction, small angle X-ray diffraction, infrared spectroscopy and sonic modulus measurements. In the case of uniaxial orientation there is only... 

Biaxial Birefringence Birefringence resulting from biaxial orientation. 

Biaxially oriented film n. Polymeric film (i.e., polyethylene) which as been strained or stretched in one direction which produces optical and physical birefringence or most importantly, high strength one the axial or long direction. 

The theoretical descriptions of biaxial orientation and the relationships to physical properties are not as well developed as those of uniaxial orientation. In amorphous polymers, birefringence can measure total molecular orientation and can be used to predict properties. For semicrystalline polymers, a more... 

A major topic of interest relating to film structure is the effect of crystallinity on the deformation mechanism. The optical properties of biaxially oriented films were studied in 1957 by Stein [109] who determined the full set ofbirefrin-gences, by measuring the optical retardation as a function of the tilt of a PS film. Samuels [110] used complementary techniques of x-ray scattering, TEM of surface replicas and birefringence measurement in a study of the microstructure... 

The biaxial orientation can be assessed by optical methods measuring the in-plane birefringence in the three orthogonal directions. The three Ah values may be derived by tensor summation of eq. (9.16) ... 

Ac° and Aam° are the intrinsic birefringences of crystalline and amorphous regions and A/iform is form birefringence. For biaxially oriented systems (79)... 

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... 

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