Page 463 - Carrahers_Polymer_Chemistry,_Eighth_Edition

426 Carraher’s Polymer Chemistry

containment is accomplished by using cladding and coating materials differing in refractive indexes
from the core material. In the step-index approach, the index of refraction of the cladding is slightly
less than that of the core. Here the output pulse is a little broader than the input because the light
travels on slightly different paths as they travel through the fiber. This is overcome through the use of

graded- index materials where impurities such as boron oxide or germanium dioxide are added to the
silica glass so that the index of refraction varies parabolically across the cross section of the optical
fiber core. Thus, the velocity of light varies according to where it is within the core, being greater at

the periphery and less at the center. Thus, light that must travel a longer pathway through the outer
periphery travel faster than those close to the center somewhat balancing themselves to minimize
distortion.
Organic core optical materials are also in use. Such materials, in comparison to the silica cored
materials, are lighter in weight, offer better ductility, have larger core diameters, and are less sen-
sitive to vibrational stresses. They are often considered for shorter distances. For the step-index
approach, where the refractive index of the cladding material should be a little less than that of the
core material, a number of materials are possible. Two often used combinations are a polystyrene
core with poly(methyl methacrylate) cladding and a poly(methyl methacrylate) core with fl uorinated

polymers as the cladding. Since the fiber core is to be optically clear, amorphous organic polymers
are used. Also, to avoid unwanted scattering, the polymer must be of high purity since impurities
will cause sites of differing refractive indexes and the associated difference in the speed of propa-
gation of the signal. The organic fibers are generally made by melt spinning with the core-cladding

structure formed by extrusion.

The types of optical loss for both silica-glass cored fibers and organic cored fibers are similar,

but do differ in relative importance. In the visible wavelength region, there are both absorption and
scattering losses. Absorption losses include higher harmonic molecular vibration modes in the infra-
red region and electronic transitional absorption in the ultraviolet region. Scattering losses include
Rayleigh scattering and loss due to imperfections in the waveguide structure, mismatching of the

core-cladding boundary interface, and birefringence due to fiber drawing as the fiber was formed.
12.14 ASBESTOS
Asbestos has been known and used for more than 2,000 years. Egyptians used asbestos cloth to
prepare bodies for burial. The Romans called it aminatus and used it as a cremation cloth and for
lamp wicks. Marco Polo described its use in the preparation of fire-resistant textiles in the thirteenth

century. Asbestos is not a single mineral but rather a grouping of materials that give soft, thread-
like fibers. These materials will serve as an example of two-dimensional sheet polymers containing

−4
two-dimensional silicate (Si O ) anions bound on either or both sides by a layer of aluminum
4 10
hydroxide (Al(OH) ; gibbsite) or magnesium hydroxide (Mg(OH) ; brucite). Aluminum and mag-
3 2
nesium are present as positively charged ions. These cations can also contain varying amounts of
water molecules associated with them as hydrates. The spacing between silicate layers varies with
the nature of the cation and amount of its hydration.
Due to its fibrous nature and ability to resist elevated temperatures (compared with most organic-

based fabrics) it is used to make fabrics for the production of fire-resistant fabric, including laboratory

glove-wear. Shorter fibers were used in electrical insulation, building insulation, and in automotive
brake linings. Though asbestos has been known for thousands of years, it has only recently become
known that asbestos can be dangerous. For instance, asbestos miners and manufacturing personnel
who worked with it for 20 years or longer are 10 times more likely to contract asbestosis. Families
of these workers and those living near the mines also have a greater than average chance of getting
asbestosis. Asbestosis is a disease that blocks the lungs with thick fi brous tissue, causing shortness
of breath and swollen fingers and toes. Bronchogenic cancer (cancer of the bronchial tubes) is prev-

alent among asbestos worker who also smoke. Asbestos also causes mesothelioma, a fatal cancer of
the lining of the abdomen or chest. These diseases may lay dormant for many years after exposure.

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