Optical biaxiality

In the apatites from Japan, Akizuki et al. (1994) found biaxial optical character with domains of different optical orientation and 2V (Fig. 27). Although the optical properties indicate a monoclinic or triclinic structure, electron and X-ray diffraction data showed no reflections inconsistent with / 63/m symmetry or that indicate a superstructure. They postulated that the dissymmetrization is due to ordering of OH and F (Hughes and Rakovan, this volume) along growth steps during incorporation. 

The symmetry of liquid crystalline phases can be categorized in terms of their orientational and translational degrees of freedom. Thus nematic, smectic and columnar phase types have respectively three, two, and one degrees of translational freedom, and within each type there can be different phases depending on the orientational or point group symmetry. Their optics (uniaxial, biaxial, optically active) are determined by the point group symmetries, which are listed in Table 1 for common liquid crystal phases the optical symmetries of variants of these phases can usually be established directly from their structures. 

An increasing phase biaxiality obviously disturbed the smooth optical periodicity of the cholesteric phase structure. Whether these defects were due to a modification of the continuous twist of the main director or to the biaxial optics... 

This is sufficient for samples possessing uniaxial optical symmetry, ie, fibers and uniaxially stretched films. However, for oriented samples possessing biaxial optical symmetry (ie, orthorhombic), all three principal refractive indexes are not equal, and A/ii3 and A 23, the out-of-plane birefringences, need to be measiu-ed (eqs. 20 and 21). An optical bench is employed (92,93), equipped with a simple goniometer, which enables the rotation of the sample about one of its main axes lying in the plane of the sample (Fig. 4). 

During the optical coat work stress examination method the upper plate of the head of some of the bolts was covered with an optical coat work (Fig. 4). On the head of some other bolts strain gauges were stuck which measured the plain biaxial stress state in the middle of the top surface of the head of the bolt (3.5 x 3 mm). The magnetic probe detected average stresses up to 0.1 mm depth in an area of 14 mm diameter in the middle of the head of the bolt. 

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. 

Interior acute angle between optic axes of biaxial mineral. 

Biaxial crystals offer the possibiUty of coincidence of the phase matching direction with one of the optic axes. This highly desirable situation, called non-critical phase matching, is quite tolerant of divergence of the incident beam from the most efficient phase matching direction. 

The Bertrand lens, an auxiliary lens that is focused on the objective back focal plane, is inserted with the sample between fully crossed polarizers, and the sample is oriented to show the lowest retardation colors. This will yield interference figures, which immediately reveal whether the sample is uniaxial (hexagonal or tetragonal) or biaxial (orthorhombic, monoclinic, or triclinic). Addition of the compensator and proper orientation of the rotating stage will further reveal whether the sample is optically positive or negative. 

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

Kim, J. C Cakmak, M. and Zhou, X Effect of composition on crystalline orientation and optical properties of biaxially stretched PEN/PEI films, in Proceedings of the 55th SPE ANTEC 97 Conference, May 5-8, 1997, Toronto, ON, Canada, Society of Plastics Engineers, Brookfield, CT, 1997, Vol. 2, pp. 1588-1592. 

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. 

Note 3 The E, J and K phases have herringbone organizations of the molecular short axes and so the mesophases are optically biaxial. 

Fig. 52. Convergent light figures, a. Uniaxial crystal with optic axis parallel (left) and slightly inclined (right) to line of vision. 6. Biaxial crystal with acute bisectrix parallel (left) and inclined (right) to fine of vision.

Biaxial crystals under similar optical conditions produce convergent light figures like that shown in Fig. 52 6, when the acute bisectrix of the optic axes lies along the line of vision and the vibration directions... 

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