Modern Polymer Spect..

186 3 Vibrational Spectra cis a Probe of Structural Order
I I I I I I
1000 900 800 700
v cm-'
Figure 3-49. Band progression
due to CH2 rocking
modes for n-nonadecane in
the orthorhornbic phase (at
0°C) and in the a phase.
Notice at 25 "C the appearance
of a few weak bands
and the disappearing of the
crystal field splitting.
(c) At T, the mode at 1375 cni-' (U) shifts to 1378.5 cm-'.
(d) At T, a weak, clear and broad absorption appears at 766 cni-* and remains a
clear, weak and broad band in the melt (Figure 3-49).
(e) At T, additional peaks are observed in the infrared at 766, 804, 845, 866, 877,
940, and 956 cm-' (Figure 3-49). Some of these modes may be assigned to the
even j values of the P band progression.
(f) At T, in the CH2 wagging progression we clearly locate WS, W7, WS, and W11
as in the case of the solid, but the frequencies of these components are rigidly
shifted toward higher frequencies for a few wavenumbers (Figure 3-50). The
other CH2 wagging motions are overlapped by the CH7 U inode which shifts to
-1378 cm-l.
(8) At T, the CH; ,B mode shifts from 891 to 889 cin-l.
(h) At T, a weak, but unquestionable peak arises at 1342 c1n-l (Figure 3-50).
Upon melting:
(i) At T, the band progression of the CH2 wagging disappears (Figure 3-50).
(j) At T,ll the peak at 1342 c1ii-l gains considerable intensity (Figure 3-50).
(1) At T,, the two well-known defect modes absorptions near 1353 cm-' and 1370
cni-' appear together with the broad peak centered near 1306 cm-' (Figure
3-50).
In the case of the Rainan spectrum the most meaningful observations are the following:
(m) Above T, the features of the k = 0 modes of the infinite trans-planar molecule
are still the dominant features (Figure 3-51b).
(n) At T, the lattice band at 98 cm-' disappears.
(0) At T, the LAMI frequency shifts from 124.5 to 123.2 cm-' (Figure 3-52).