Characterization of Fibrous Materials by Two and Multiple-Beam Interferometry

Abstract

In this work two-beam, multiple-beam and manual variable wavelength (VAWI) interferometric techniques are applied to study the dispersion properties and structure deformation of polymer. fibres. These techniques are used to measure the refractive indices, refractive index profiles and birefringence for deformed and un­ deformed fibres. The fibre drawing devices attached with interference microscopes and systems are used to investigate and detect various types of deformations associated with stretched polymer fibres. Chapter (1) includes a general introduction and previous work for characterization polymer fibres by different interferometric techniques. In chapter (2), the description of experimental set-up and general theoretical considerations of the interferometric techniques (two-beam, VAWI and multiple-beam • interference systems) are presented. Also, this chapter includes the basic concepts of automatic fringe analysis of multiple-beam microinterferograms. In chapter (3) the polymer fibres are divided into three groups according to their birefringence. These groups suggested to be low, medium and high birefringence. The low birefringence fibres produce small optical path difference o < 3A., where A. is the wavelength of monochromatic light used. The high birefringence fibres produce large optical path length difference o » 3A..The medium birefringence fibres are between low and high birefringence fibres. These fibres produce relatively significant optical path difference o>3A.. In part-! of this chapter the conventional two-beam, multiple-beam and conventional variable wavelength interferometry (VA WI) techniques are used as suitable techniques for measuring the spectral dispersion curves of these medium birefringence fibres. These fibres are polyester sample 1.5 denier type 3 ST, polyester sample 1.5 denier type 360, polyester sample 4 denier type LM-SL, polyester fibres produced from Egyptian manufacturer and staple sample from ICI. Using the results of refractive indices n! and n' at different monochromatic wavelengths, the dispersion and oscillation energies of these fibres are calculated. A comparative study is carried out between results using various interferometric techniques. It is found that multiple­ beam interferometric technique is more accurate and sensitive technique for measuring the dispersion properties of fibres. In part-2 of this chapter, low birefringence fibres are characterized. In this case the conventional VAWI technique is not suitable for measuring the spectral dispersion of birefringence of these fibres. A method is suggested to overcome the difficulty of measuring the spectral dispersion curves of low birefringence fibres. This method depends on variable wavelength interferometry (VA WI). The suggested method is used to measure the spectral dispersion of birefringence of polypropylene fibres with a draw ratio of 4 (low birefringence fibre). This method is also used to measure the spectral dispersion of birefringence of a polyester staple from ICI. Also, the conventional VAWI method is used to measure the spectral dispersion of this fibre to confirm the results obtained by the suggested method. In prt-3 of this chapter, the highly oriented fibres • are studied. In case of interferometric techniques, these highly oriented fibres give large optical path length differences 0>>3/.. and the connection between the fringes crossing the fibre and the immersion liquid medium is not clear.