Abstract
The changes in refractive indices and the birefringence of polyester (Egyptian PET) fibres with different annealing conditions were studied by interferometry. The polarizing interference microscope was used to determine the mean refractive indices and mean birefringence of these fibres. An acoustic method has been used to measure densities. The results are used to calculate structural parameters such as the form birefringence, virtual refractive index, harmonic mean polarizability of the dielectric, the number of molecules per unit volume, and the specific refractivity of the isotropic dielectric. Also, the number of monomer units per unit volume and the refractive indices of the fully oriented fibre, when using monochromatic light vibrating parallel and perpendicular to the fibre axis, were calculated and found to be 1.803 and 1.562. The effect of the annealing conditions on the refractive index was studied. The generalized Lorentz-Lorenz equation given by de Vries was used to determine PET fibre structural parameters. Comparisons between the Hermans optical orientation function formula and the corrected formula by de Vries are given.
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