In this work, we have used an oligomer model compound approach for the understanding of the vibrational
spectra of the principal forms of polyaniline. Selected model compounds have been studied by Raman and
infrared (IR) spectroscopies. The observed vibrational bands are assigned to the expected modes, by revealing
the relationships between molecules. In a more theoretical part, dynamical calculations on the different model
compounds and their related polymers are presented. The use of model compounds allows us to reach better
sets of force constants, as soon as we assume that the parameters can be transferred to the polymeric forms.
The obtained values of these force constants are discussed in regards to the oxidation state of polyaniline and
its corresponding geometrical structure.
The effect of polyaniline deuteration on the Raman spectra of this
polymer has been studied in detail. Four
types of samples have been prepared: nondeuterated polyaniline,
polyaniline deuterated selectively on the
ring, polyaniline deuterated selectively on nitrogen, and polyaniline
deuterated both on the ring and on the
nitrogen. Selective deuteration allowed for experimental
verification of the Raman modes assignment proposed
for different types of polyaniline. In particular, the
contribution of C−H bending to C−C stretchings in
benzoid and quinoid rings of the polyaniline chain has been confirmed.
In addition, selective deuteration
enabled us to identify, for the first time in polyaniline, the band
associated with N−H bending deformations.
Raman spectroelectrochemical investigations of the four
above-mentioned types of polyaniline samples gave
evidence that in protonated polyemeraldine two types of structures
coexist, namely, semiquinone radical cation
(polaronic lattice) and quinoid dication (bipolaronic). The
vibrations of the former are in resonance with the
energy of the blue (457.9 nm) excitation line, whereas the vibrations
of the latter undergo resonance
enhancement if the red (676.4 nm) line is used. The calculated and
experimentally observed Raman modes
for both polaronic and bipolaron structures are in very good agreement
in all four cases of differently deuterated
polyaniline.
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