Modern Polymer Spect..

3.1 7 Fermi Resonancrs 163
stants. This model aimed at reproducing the class of dimeric fatty acids or fatty
alcohols. This is the most important case both for its dynamical implications
and for the consequences on structural determination as indicated at the beginning
of this section. The modeling [133] considers two chains with one
heavier mass at one end joined head-to-head by a spring with variable spring
constants. Such a spring mimics the hydrogen bonds which link the two carboxyl
groups.
The results of the calculation indicate that LAMl extends over the whole dimer,
with the node at the center of the dimer between the two carboxyl groups. The frequency
of LAMl has to be searched at much lower frequencies and the frequencies
of the band progression of higher-order LAM modes are totally different from those
identified in the spectra of these materials by the French group. The new band
progression is easily found in the low frequency Raman spectra (Figure 3-36), thus
giving further experimental support to what was shown by defect-mode spectroscopy
(see Section 3.14.1). namely that fatty acids in the crystal do not contain conformational
kinks and are fully trans-planar.
3.17 Fermi Resonances
The next step in OUT analysis is to focus on additional spectroscopic manifestations
of molecular order and disorder which are not only intellectually interesting as
physical phenomena, but also become of extreme importances as markers for the
experimental routine characterization of the very many systems containing segments
of polyinethylene chains.
We are faced with the following experimental facts which need to be understood
and possibly used for analytical and structural determinations. The Raman spectrum
of polyethylene (or practically of any polymethylene chain) in the solid alltrans
structure is very similar (or identical) to that given in Figure 3-37a; upon
melting, the overall appearance of the Raman spectrum is that given in Figure
3-37b.
3.17.1 Principles
The experimental fact is that whenever a molecule contains a few CH? groups the
Ranian spectrum shows a very characteristic pattern of lines in the CH2-stretching
frequency range. Since they are strong they can be observed in a variety of materials,
even under the most awkward experimental conditions. Indeed, they can be
observed for monolayers of organic or biological inaterials spread on surfaces, in
aqueous solutions, in microscopic crystals, etc. Moreover, the spectrum shows
drastic changes in frequencies and intensities when the molecular structure or the
environment changes. For this reason the Raman spectrum is at present a very