Modern Polymer Spect..

98 3 I i’brational Spectra as a Probe of Structural Ordrv
4. The situation represented by Eq. (3-36) is at present of particular interest when
polyconjugated molecules (polyenes and polyaromatic systems) are considered.
The role played by vibrational infrared and Ranian spectroscopy in this new
field of material science has been essential for the understanding of the new
physical and chemical phenomena associated with the existence of extended n
electron delocalization. The reader is referred to specialized references for a
thorough discussion on the spectroscopy of these systems [4143]. For these
systems recent studies have pointed out the occurrence of characteristic normal
modes which show ‘frequency dispersion’ with conjugation length, i.e., the
characteristic fi-equencies lower by adding repeating units which tale part in the
conjugation [41-431. Dealing with a chain molecule, the elements (Lo),m, and
entering Eqs. (3-33) and (3-34) are necessarily large. The physically
relevant problem is to discover whether, to what extent, and at which distance
the elements of AF change by adding conjugated units. The solution of this
problem would shine some light on the distance and the extent of electronic
interactions in polyconjugated systems 1441. This is a challenge to spectroscopists,
theoretical quantum chemists, and computational chemists.
5. It is obvious that the addition of strong electron donors or acceptors at a site
away from the functional group of interest will modify the electronic environment
and possibly the geometry at the site of the substitution. Certainly for
particular internal coordinates, large AF and/or AG must occur, but their contribution
to the frequency shift of the group frequency of interest is reduced to
zero since the corresponding elements of L or L-‘ which relate the motion of
the functional group and the motions at the site of the substituent are all zero
(Eq. (3-33)).
3.5 Towards Larger Molecules: From Oligomers to
Polymers
Molecular spectroscopy and lattice dynamics of oligonieric and polymeric materials
have been treated thoroughly in several textbooks or articles [16-181. We wish to
point out here a few fundamental concepts which form the basis of the understanding
of the spectra of disordered materials which will follow in this discussion.
Let LIS first consider the vibrations of polymers considered as one-dimensional
perfect and infinite lattices. The usual basic assumptions are the following:
1. The polymer is obtained by forming a one-dimensional chain of chemical units
linked with a pre-assigned cherwicnl seqzirnce. For sake of example, we consider
propylene (CH~-CH=CHZ) as the prototype of a monomer unit capable of
producing a polypropylene chain. The first step is to make a chain of polypropylene
in which all monomers are chemically linked head-to-tail (cheiizicul