Modern Polymer Spect..

tiate highly overlapped IR bands, since asynchronous cross peaks should develop
among these bands.
From the sign of asynchronous cross peaks, one can obtain temporal information
about the perturbation-induced reorientation processes of transition dipoles and
their corresponding chemical groups. A positive peak in an asynchronous spectrum
indicates the transition dipole with the vibrational frequency 1’1 reorients hc;fbr.r that
for 1’2. If the sign is negative, reorients ufi~r
1’2. However. this temporal relationship
is reversed for perpendicularly reorienting pairs of transition dipoles, i.e., the
synchronous correlation intensity at the same spectral coordinate becomes negative.
In Figure 1-1 1, the reorientational motions of functional groups contributing to the
molecular vibrations with frequencies B and D occur before those for A and C. More
detailed discussions on the properties of 2D 1R spectra are found elsewhere [3).
1.4 Instrumentation
Dynamic infrared spectra suitable for 2D correlation analysis can, in principle, be
measured using any conventional IR Spectrometer [ 71. The spectrometer, however,
must be equipped with the ability to stimulate samples by some physical means and
measure the resulting time-dependent fluctuations of the IR signals. Both dispersive
monochromators (231 and Fourier transform infrared (FT-IR) instrumentation 151
have been successfully used to measure dynamic 2D 1R spectra. Although welldesigned
dispersive spectrometers can often achieve better signal-to-noise ratios
(S/N) over small spectral regions, FT-IR measurements cover much broader spectral
regions in less time and are, more importantly, readily available commercially.
Conventional rapid-scan FT-IR spectrometers are not well suited for these types
of measurements when the dynamic strain frequencies of interest are in the range
between 0.1 Hz and 10 kHz. Using a step-scanning interferometer substantially
simplifies dynamic 2D FT-IR measurements. Figure 1-12 shows an example of a
n
Rolahng-Blade
chopper. wc
nred
P*er
Figure 1-12. A schematic view
of a DIRLD spectrometer based
on a small-amplitude mechanical
deformation 1231.