Modern Polymer Spect..

3.1 7 Femi Resorinnces 165
2
L 1
2900 2800
frcgurlcr I cm-' 1
I
[ , , I , !
3000 2900 28oC
Figure 3-37. Examples of the changes with physical states of the Raman spectrum of polyniethylene
systems: (a) Raman spectrum of crystalline orthorhombic polyethylene in the CH stretching region;
(b) Raman spectrum of polyethylene in the molten state in the same frequency range.
from the mixing, because of anharmonic terms in the potential, of two (or more)
eigenstates (one fundamental and one (or more) overtone( s) or combination( s))
which belong to the same symmetry species. As a result, generally the two (many)
interacting levels repel each other and the generally weak overtone(s) (or combination(s))
borrows (borrow) intensity from the much stronger fundamental [2, 31.
Fermi resonance has for a long time been a well-known and explicable fact in the
case of small molecules [135]; the case of hydrocarbon chains was simply and
clearly presented by Lavalley and Sheppard [ I361 for the molecule of hexadeuteropropane
(CD3-CH?-CD3), which consists of an isolated CH? group between two
CD3 groups with which little or no mechanical coupling is likely to occur, in a first
approximation. The molecule belongs to the symmetry. Figure 3-38a gives the
survey Raman spectrum of this molecule, while Figure 3-38b and 3-38c show an
expanded section of the Raman spectrum polarized ( 1 1 ) and depolarized (I)[137].
Normally, in the CH? stretching range we should reasonably expect two lines to
be associated with the antisymmetric (d-) and symmetric (d') CH? stretching of
species BZ (1) and A1 ( 1 1 ) respectively. Instead, we find two strong lines certainly
with A, symmetry (polarized) and one certainly with B7 symmetry (depolarized).
The dynamical situation which justifies the observed Raman spectrum is the fol-