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Interferometric Studies of Optical Anisotropy and Radial Differences in Orientation in Polymer Fibers

Mathematical formulas are derived for the shape of two-beam interference fringes crossing a cylindrical multilayer fiber, as well as an equation to determine the mean refractive index at any point along the diameter of the fiber. Radial birefringence of a cylindrical fiber is also reported. The derived mathematical formulas are applied to determine the variation of the refractive indices and birefringence at any point along the diameter of two polypropylene samples with draw ratios of 4 and 5.2. Refractive index values, when light vibrates parallel and perpendicular to the fiber axis, and birefringence are larger at the edges than at the bulk of the fiber. The Lorentz-Lorenz equation is applied to calculate the polarizability of the polymer material per unit volume at different points along the diameter of the fiber. The results are illustrated by microinterferograms.