Fibers interference colors

Interference colors. The observations described up to now in this list are obtained with the source light polarized, but the analyzer left out of the optical path. Inserting the analyzer provides additional information and confirmation. If the fiber is isotropic, the field will appear black. Anisotropic materials will show interference colors, the pattern of which is used to determine thickness and birefringence. When a fiber is mormt-ed and rotated, it shows maximum contrast at certain rotation angles and it disappears into the black field of view at other rotation angles. 

The angle at which the fiber is invisible is called the extinction angle, and it repeats every 90° of rotation, as shown in Figure 14.12. At the point of maximum contrast (45°), the colors observed in the fiber are called the interference colors and should not be confused with the true color of the fiber. A colorless nylon fiber will show vivid interference colors because of the way polarized light interacts with the pseudocrystalline nature of file fiber, not because of the way the fiber absorbs or reflects visible light. 

Figure 14.13 illustrates how interference colors are formed. If polarized light enters a fiber and vibrates in a plane parallel to the light s long axis, the light is imaffected and the field of view remains dark. However, at orientations other than parallel, the polarized light is split into two... 

Figure 14.12 Anisotropic fiber under crossed polars. When the long axis of the fiber is parallel to the polarizer, the fiber disappears into the black background. Rotated 45°, the fiber shows the brightest interference colors. 

Interference colors coupled to fiber diameter can be used to obtain birefringence without resorting to the use of different mounting media. This is accomplished with a Michel-Levy chart, shown in the color insert, and microscope accessories called compensators or wave plates. Recall that a birefringent material breaks polarized light into a fast and a slow component. The thicker the sample, the greater is the offeet. Suppose a fiber is rotated such that interference colors are at their most vivid and... 

Compensators are used in this context when the difference between fast and slow components is relatively small. On the Michel-Levy chart, there is a large band to the left where the interference colors are whiti and not very informative. A compensator adds a constant offset to the small offset created by the fiber. The labels on the bottom of the chart correspond to fixed ofifeets that can be added to increase the contrast. A compensator is a crystal that is inserted in the light path after the sample, but before the polarizer. Compensators are labeled by the amount of offset they add. A full wave-plate compensator adds 550 nm, for example, corresponding to the x-axis labels on the Michel-Levy chart. A quarter wave plate would add 137.5 nm. A full wave-plate compensator is also called a first-order red compensator, because of the red color it imparts to the background under crossed polars. 

M Figure 14.14 Use of a compensator. Fibers with small retardation (<300 nm) appear white under crossed polars. Adding a compensator increases the retardation by a constant amount and makes it easier to visualize on the basis of interference colors. The notation "Rf" refers to retardation factor. 

Okuyama, M. Eyelash cosmetic composition containing light interference color fibers. Jpn. Kokai Tokkyo Koho JP 2005314395, 2005 Chem. Abstr. 2005, 143, 446245. 

Bicomponent technology has been used to introduce functional and novelty effects other than stretch to nylon fibers. For instance, antistatic yams are made by spinning a conductive carbon-black polymer dispersion as a core with a sheath of nylon (188) and as a side-by-side configuration (189). At 0.1—1.0% implants, these conductive filaments give durable static resistance to nylon carpets without interfering with dye coloration. Conductive materials such as carbon black or metals as a sheath around a core of nylon interfere with color, especially light shades. 

An industrial microscope with a long-working distance 20 X objective is used for the collection of the chromatic interference patterns. They are produced by the recombination of the light beams reflected at both the glass/chromium layer and lubricant/steel ball interfaces. The contact is illuminated through the objective using an episcopic microscope illuminator with a fiber optic light source. The secondary beam splitter inserted between the microscope illuminator and an eyepiece tube enables the simultaneous use of a color video camera and a fiber optic spectrometer. 

By inserting either an interference filter or a colored filter, it is possible to select a more or less extended region of the spectrum likewise, by adding an optical fiber it is possible to direct the beam where desired. This set-up best exploits the characteristics of these powerful lamps, and offers an excellent choice for the irradiation of small surfaces. Consequently, spectrophotometric cuvettes or cylindrical cuvettes are used for the irradiation, which involves small volumes. Such restrictions, as well as the high price and short lifetime of the lamp and its accessories, favors the use of these arcs for kinetics studies and quantum yield measurements, rather than for preparative photochemistry. 

Polyferrocenylsilanes can be fabricated into films, shapes, and fibers using conventional polymer processing techniques. The dimethyl derivative 3.22 (R=R = Me), which has been studied in the most detail, is an amber, film-forming thermoplastic (Fig. 3.7a) which shows a Tg at 33°C and melt transitions (T ) in the range 122-145 °C. The multiple melt transitions arise from the presence of crystallites of different size, which melt at slightly different temperatures [65, 100). Poly(ferrocenyldimethylsilane) 3.22 (R=R =Me) can be melt-processed above 150°C (Fig. 3.7b) and can be used to prepare crystalline, nanoscale fibers (diameter 100 nm to 1 pm) by electrospinning. In this method, an electric potential is used to produce an ejected jet from a solution of the polymer in THF, which subsequently stretches, splays, and dries. The nanofihers of different thickness show different colors due to interference effects simUar to those seen in soap bubbles... 

Contamination, which interferes in forensic fiber identification, is very frequent. Contaminants, such as blood and oil, interfere in the color evaluation of... 

---