Negative birefringence

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. 

Unstretched PPX films exhibit an inherent negative birefringence, the optical axis of which is perpendicular to the plane of the film. The refractive index along the optical axis is lower than the refractive index observed in the plane of the film, the difference being = —0.075 0.001 (42). Where... 

Copolymers and blends of BPA-PC and Modified PS. Theoretically, a blend or copolymer of 60 parts BPA-PC (positively birefringent) and 40 parts PS (negatively birefringent) should yield a product free of birefringence (Fig. 25) (207). In spite of modifications to PC to improve the compatibiUty, no blend could be produced which would be optically isotropic and thus suitable as a substrate material. The same holds tme for PC/PS-copolymers (208) in which... 

Reduction or even complete compensation of birefringence by mixing polymers with positive birefringence (PC, PVC, PETP, PPE, PVDF, etc) with polymers with negative birefringence (PMMA, PS, PAN, etc) has been the consistent strategy. 

In thin sections natural graphite is translucent, strongly pleochroic, and uniaxial. It has a negative sign of birefringence and two extinctions per revolution under crossed Nicol prisms. The atomic number of carbon accounts for its low absorption coefficient for x-rays and electrons. 

Figure 17 Isothermal melting of Ziegler-Natta isotactic poly(propylene). (a) Spherulites with mixed birefringence at Tc = 148°C. The top middle figure displays the melting for the same thermal history, (b) Subsequent to crystallization, the temperature was raised to 171°C spherulites acquire negative birefringence, (c), (d) and (e) Isothermal melting at 171°C for 80, 200 and 300 min, respectively. Reproduced with permission from W.T. Huang, Dissertation, Florida State University, 2005. (See Color Plate Section at the end of this book.)...

Dynamic behaviour. Time dependent curves of the birefringence An(t) induced by applying successively a positive and a negative electric... 

Tumbling regime At very low shear rates, the birefringence axis (or the director) of the nematic solution tumbles continuously up to a reduced shear rate T < 9.5. While the time for complete rotation stays approximately equal to that calculated from Eq. (85), the scalar order parameter S,dy) oscillates around its equilibrium value S. Maximum positive departures of S(dy) from S occur at 0 n/4 and — 3n/4, and maximum negative departures at 0 x — k/4 and — 5it/4, while the amplitude of oscillation increases with increasing T. 

Atactic PVAc prepared in a free-radical polymerization is crosslinked by means of benzoyl peroxide. The resulting elastomertic networks are studied in elongation, both unswollen and swollen with triethylbenzene, over the range 273 - 363 K. The most important experimental results obtained are values of the network birefringence, which is negative. Calculations carried out to interpret the birefringence are based on Monte-Carlo simulations of the atactic structure, and on the RIS theory. 

Potassium chlorate is monoclinic it has strong negative birefringence, and the vibration direction for the lowest refractive index lies in the ac plane, making an angle of 56° with the c axis. The conclusion that the low pyramidal (that is, more or less flat) chlorate ions have their oxygen triangles normal to this direction of least refractive index is correct within 1° (Zachariasen, 1929). 

Gout can be diagnosed by the presence of negatively birefringent monosodium urate crystals in aspirated synovial fluid examined by polarized-light microscopy. Here, crystals are within polymorphonuclear leukocytes. 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

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