I n dependence on sample position between crossed polarizer and analyzer two effects can be distinguished in CdS crystals: the conventional birefringence and the mode coupling between ordinary and extraordinary waves. Both effects are clearly identified by comparing the ellipticity parameters. The mode coupling shows up as a spectral structure consisting of two peaks separated by a pronounced minimum at the isotropic point for as-grown CdS platelets. The ellipticity parameters in this case exhibit principally the same features as the optical activity. The occurrence of this effect is due to non-diagonal components of the dielectric susceptibilit,y tensor. These components can arise as a result of higher-order terms of the dielectric susceptibility tensor and/or internal stress. Expressions are derived which very well describe the spectral shape as well as the influence of an external magnetic field on the spectral characteristics due to mode coupling.
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Birefringence in heat-mechanical modified freshly moulded polyester fibers Abstract. The article submits new experimental data concerning to the role of combined thermo-mechanical treatments on the structural development of freshly moulded uncrystallized but crystallizable poly (ethylene terephthalate) (PET) fibers. The object of the present work is PET as a thermoplastic polymer with a large practical application. The report is devoted to the influence of the heat-mechanical modification temperature on the structure rearrangement in uniaxially orientated amorphous PET. The heat-mechanical modification of the investigated yarns and the optical measurements were realized by specialized gears constructed and built in the author's laboratories. The fibers heat-mechanical modification includes samples annealing at constant temperature above their glass transition temperature (T g ) without strain stress. The yarn annealing has been followed from well defined uniaxially strain-loading with values from 0 MPa up to 30 MPa during two minutes. The optical measurements were carried out by an optical system using a polarization microscope and a CCD camera. The obtained experimental data has been analyzed by Mocha-1.2 (Jandel Scientific) software. There are established dependences between the heat-mechanical modification mode and the structural rearrangements running in the studied PET samples.
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