Modern Polymer Spect..

182 3 T7ibmtional Spectra CIS a Probe of Structurd Order
I- P
u+w
I I I I I 1
IW 1400 1200 loo0 000 600
v cm-'
Figure 3-46. Survey infrared spectrum of n-nonadecane in the orthorhonibic phase. Band progressions
are indicated. For the labeling of the modes see text. R = skeletal stretching, p = CHj
deformation.
The infrared spectrum of C19 in the solid orthorhonibic phase is shown in Figure
3-46; we use this spectrum as reference in the present analysis. The Raman spectrum
of the same material is given in Figure 3-47.
The identification of the large number of normal inodes in the infrared and
Raman spectra of these molecules becomes indeed unfeasible. Life becomes easier
if we consider C19 as a segment of polyethylene made up by 17 CH2 units and try
to located the 17 normal modes lying in each frequency branch of the dispersion
curves of infinite polyethylene (Figure 3-1). Each mode is then characterized by a
phase shift q, between adjacent CH2 units, qj = 2nj/17, (here j = 0,. . . , 17 - 1).
The possibility of observing all noniial modes depends on their symmetry and on
their intrinsic intensity in infrared or Raman, as well as on the dispersion of each
phonon branch. As discussed in Section 3.9.2, we expect to observe in the infrared
spectrum band progressions with decreasing intensity for branches with reasonable
dispersion, while for flat branches band progression will be strongly compressed
with strong overlapping.
The experimental observation predicted by theory has been discussed previously
in detail by Snyder and Schachtaschneider [9, 341. From the dispersion curves
of Figure 3-1 it follows that 110 clear band progressions are expected for CH2
stretching (d, overlapping) and CH2 bending (6, overlapping), while progressions
are expected for CH2 wagging (w, very weak in IR); in the CH? wagging progression
we clearly locate Ws, W7, WS, and W11; the other CH2 wagging motions are