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Operating the Optional Equipment
.
103
Infrared Polarizer Option
An absorption band in the infrared range occurs when a vibration is accompanied
by a change in dipole moment. The electric vector of the incident radiation must
have a component in the direction of the dipole moment change.
In polarization spectroscopy, the absorption bands of greatest interest are those in
which the direction of dipole moment change is related to a bond direction, for
example, the nitrile stretching vibration. If, in a particular sample, all the bonds of a
particular type are aligned in a specific direction, the strength of the absorption
depends on the polarization of the incident radiation, that is, whether the electric
vector is parallel to or perpendicular to the bond direction.
For example, stretching an acrylic fiber aligns the molecules with the general
direction of the polymer chains parallel to the fiber axis, and the nitrile groups tend
to be oriented perpendicular to the axis. If the spectrum is collected with the
infrared radiation polarized perpendicular to the axis, the nitrile absorption peak is
much stronger than if the spectrum is collected with radiation polarized parallel to
the axis. The ratio of the two intensities (called the
dichroic ratio
) is a measure of
the extent of alignment of the nitrile groups and thus of the polymer chains.
By the use of the microscope, polarization spectra can be collected on very small
samples. These include:
•
Single filaments (typically 14
x
70 μm);
• Films;
•
Single crystals;
•
Liquid crystal polymers.