Fibers birefringence

Fibers with E modulus ranging from 30 to 50 GPa could very easily be produced. Break tenacities very generally around 400 MPa, but could be increased to approx. 600 MPa by heat treatment. The draw ratio had no significant effect on the fiber-birefringency but remained constant at 0.32, within experimental error. 

The study of fiber birefringence is not however a purely academic exercise but has definite practical applications, some of which are... 

In general, these methods have changed very little since the nineteenth century, so that the older references still have value. Wiley and Hobson s review [9] covers refractometer methods and immersion methods, whereas Brown, McCrone et al. [10, II] deal with dispersion staining. Billmeyer [12], Ellis [13], and Schael [14] deal specifically with polymer films, where it must be borne in mind that, as in the case of fibers, birefringence may result from the processing conditions. 

An analysis of the theoretical methods of calculation of the ideal fiber birefringence has been presented in the literature [308], Using a modified Lorentz-Lorenz equation, theoretical birefringence was calculated by considering intermolecular interactions. The calculations showed considerable discrepancies between the theoretical and the experimental values. 

The orientation of the crystalline region is measured by x-ray diffraction techniques. The orientation in the less-ordered amorphous region is determined by x-ray, infrared measurements, sonic modulus technique [183], or by separating the crystalline and amorphous contributions to the fiber birefringence [79]. The average orientation of the crystalline regions is specified by the Hermans Stein orientation functions ... 

Forer, A. 1965. Local reduction of spindle fiber birefringence in living Nephrotoma suturalis (Loew) spermatocytes induced by ultraviolet microbeam irradiation. J. Cell Biol., 25 (Mitosis Suppl.) 95-117. 

Figure 7a. Anatomy of a real fiber, b. Intrinsic and extrinsic mechanisms of fiber birefringence. From C. P. Poole and S. Nagel, [10], Polarization effects in lightwave systems, in Optical Fiber Telecommunications, illA, pp. 115-161, I. P. Kaminow and T. L. Koch, Eds., Academic Press, San Diego, CA (1997).

Fig. 15.2 POM photographs of isotropic (b) and anisotropic (d) all-cellulosic based composites with 4% w/w HPC of Avicel fibers [birefringent rods in images (b) and (d), respectively]. Images (a) and (c) are the POM photographs of the isotropic (a) and anisotropic (c) HPC matrix. White arrows in images (c) and (d) indicate the shear direction. All images were taken under crossed polars. Images were obtained from references [12] (a and b) and [14] (c and d)...

With pntrpese to specify the occurred in the samples structure developments caused by the SHMM at temperature 85 C has been measured and the fibers birefringence. The obtained results pertaining to the birefringence values depending of the ap>plied load show a wide... 

Measurement of Other 0[4ical Properties. Measurement of other optical properties such as strain-, electro-, magneto-, and acoustooptical effects involve specialized techniques beyond the scope of this chapter. The same is true for measurements of transmittance, refractive index profile, dispersion, and other important properties of optical communications fiber. Birefringence and strain-optical measurements will, however, be discussed in Chap. 6, in relationship to annealing and strengthening of glass. 

Noncrystalline domains in fibers are not stmctureless, but the stmctural organization of the polymer chains or chain segments is difficult to evaluate, just as it is difficult to evaluate the stmcture of Hquids. No direct methods are available, but various combinations of physicochemical methods such as x-ray diffraction, birefringence, density, mechanical response, and thermal behavior, have been used to deduce physical quantities that can be used to describe the stmcture of the noncrystalline domains. Among these quantities are the amorphous orientation function and the amorphous density, which can be related to some of the important physical properties of fibers. 

Refractive Index. The refractive index parallel to the fiber axis (s) is 1.478 for acetate and 1.472 for triacetate. The index perpendicular to the axis (co) is 1.473 for acetate and 1.471 for triacetate. The birefringence, ie, the difference between S and CO, is very low for acetate fiber and practically undetectable for triacetate. 

A common measurement usehil in predicting threadline behavior is fiber tension, frequentiy misnamed spinline stress. It is normally measured after the crystallization point in the threadline when the steady state is reached and the threadline is no longer deformed. Fiber tension increases as take-up velocity increases (38) and molecular weight increases. Tension decreases as temperature increases (41). Crystallinity increases slightiy as fiber tension is increased (38). At low tension, the birefringence increases as tension is increased, leveling off at a spinline tension of 10 MPa (1450 psi) (38). 

Fig. 13. Elongation to break as a function of birefringence for undrawn, hot-drawn, and cold-drawn annealed fibers (6) , undrawn , cold-drawn,...

Optical properties also provide useful stmcture information about the fiber. The orientation of the molecular chains of a fiber can be estimated from differences in the refractive indexes measured with the optical microscope, using light polarized in the parallel and perpendicular directions relative to the fiber axis (46,47). The difference of the principal refractive indexes is called the birefringence, which is illustrated with typical fiber examples as foUows. Birefringence is used to monitor the orientation of nylon filament in melt spinning (48). 

Optical Properties. When light falls on an object, it is either partially absorbed, reflected, or transmitted. The behavior of the object as it relates to each of these three possibiUties determines visual appearance. Optical properties of fibers give useful information about the fiber stmcture refractive indexes correlate well with fiber crystalline and molecular orientation and birefringence gives a measure of the degree of anisotropy of the fiber. 

Optical properties of fibers are measured by light microscopy methods. ASTM D276 describes the procedure for fiber identification using refractive indexes and birefringence. Other methods for determining fiber optical properties have been discussed (3,38—44). However, different methods of determining optical properties may give different results (42). 

The quantitative assessment of the overall orientation of PET fibers is generally made on the basis of fiber optical anisotropy measurements, i.e., measurements of the optical birefringence of the fiber. The determination of the value of optical birefringence makes it possible to determine the value of Hermans function of orientation based on the equation ... 

Figure 10 Electrical resistivity of PET fiber versus birefringence of the fiber.

Table 14 Directional Refractive Indices (nn, nj.) and Birefringence (An) of Differently Drawn PET Fibers...

Linearly polarized, near-diffraction-hmited, mode-locked 1319 and 1064 nm pulse trains are generated in separate dual-head, diode-pumped resonators. Each 2-rod resonator incorporates fiber-coupled diode lasers to end-pump the rods, and features intracavity birefringence compensation. The pulses are stabilized to a 1 GHz bandwidth. Timing jitter is actively controlled to < 150 ps. Models indicate that for the mode-locked pulses, relative timing jitter of 200 ps between the lasers causes <5% reduction in SFG conversion efficiency. 

Lagorceix, H., Reynaud, R, 1995, Squeezing highly Birefringent Fiber for an Accurate Nondestructive Determination of Principal-Axis Orientation along the Waveguide application to Fiber Babinet Compensator Implementations Optical Fiber Technology 1, 171... 

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