Modern Polymer Spect..

3.8 Density of Vibrational States and Neutuopi Scattering 117
1360
I/‘\
i r
/ A
1040
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81 0
I
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I
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Figure 3-11. Raman spectrum of isotactic polypropylene from ‘smectic’ (- - -) to crystalline (-)
(tridimensional order) obtained after long annealing processes at low temperatures. Raman regularity
bands split into doublets (crystallinity bands) due to phonons at r (k, = k, = k, = 0) (from
[781).
The case of isotactic polypropylene becomes an interesting texbook case which
shows clearly and separately the many steps in the formation of a polymeric
material.
3.8 Density of Vibrational States and Neutron Scattering
The next step in the development of vibrational spectroscopy of large organic
molecules is to realize that other physical measurements, such as neutron-scattering
spectroscopy, can provide essential information on the normal vibrations of organic
molecules. This information is complementary to that provided by infrared and
Raman spectroscopy which is intrinsically limited by the optical selection rules.
Indeed, the limitations to the spectroscopic activity imposed by symmetry selection
rules are very strict and select only a few phonons. Moreover, when some sort of
molecular disordering is introduced into the system (and this is always the case),
symmetry is removed, selection rules are relaxed, and the spectra show sharp
absorptions (or Raman scattering) originating only from characteristic localized
vibrations floating on top of a broad background absorption whose origin needs to
be explored and understood.