Page 510 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Testing and Spectrometric Characterization of Polymers 473

refractive index n is equal to the ratio of the sine of the angles of incidence, i, and refraction, r, of
light passing into the polymer.

n = sin i/sin r (13.18)

The magnitude of n is related to the density of the substance and varies from 1.000 and 1.3333
for air (actually in vacuum) and water, to about 1.5 for many polymers and 2.5 for white pigment
titanium (IV) oxide (titanium dioxide). The value of n is often high for crystals and is dependent on
the wavelength of the incident light and on the temperature. It is usually reported for the wavelength
of the transparent sodium D line at 298 K. Typical refractive indices for polymers range from 1.35

for polytetrafluoroethylene to 1.67 for polyarylsulfone.

13.10.2 OPTICAL CLARITY
Optical clarity or the fraction of illumination transmitted through a material is related by the Beer-
Lambert relationship:

o
log I/I = −AL and I/I = e −AL (13.19)
o
where the fraction of illumination transmitted through a polymer (I/I ) is dependent on the path
o
length of the light (L) and the absorptivity of the polymer at that wavelength of light (A).
Clarity is typical for light passing through a homogeneous material, such as a crystalline ordered
polymer or completely amorphous polymer. Interference occurs when the light beam passes through
a heterogeneous material in which the polarizability of the individual units varies causing interfer-
ence that disrupts optical clarity.

13.10.3 ABSORPTION AND REFLECTANCE

Colorless materials range from being almost totally transparent to opaque. The opacity is related to
the light-scattering process occurring within the material. Incident radiation passes through nonab-
sorbing, isotropic, optically homogeneous samples with essentially little loss in radiation intensity.
Actually, all materials scatter some light. The angular distribution of the scattered light is complex
because of the scattering due to micromolecular differences.
Transparency is defined as the state permitting perception of objects through a sample.

Transmission is the light transmitted. In more specific terms, transparency is the amount of undevi-
ated light, that is, the original intensity minus all light absorbed, scattered, or lost through other
means. The ratio of reflected light intensity to the incident light intensity is called the absorption

coeffi cient.
13.11 WEATHERABILITY

Polymers are used in almost every conceivable environment. They are tested for their ability to inter-
act with radiation, weather, and microorganisms. Weathering includes the ability to resist attacks by
freezing and heating cycles, resistance to frictional damage caused by rain and air, and infl uence of
low and high temperatures as the polymeric materials are used. Many of the results are measured
using chemical testing as described in the following section or “real time/condition” testing.
Moisture is an important factor for some polymers, especially those with noncarbon backbones,
where hydrolysis, and subsequent degradation, can bring about drastic changes in chain length and
consequently, polymer properties. Such attack can occur on the surface and is indicated, as can
many chemical attacks, by a discoloration generally followed by crazing and cracking. Other attack
can occur within the matrix with the polymer absorbing moisture.

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