Modern Polymer Spect..

17E (ir 1, R): (iii) z = 4, t = 1, 18 atoms per unit cell, D: point group, 17A (R)
+17B, (ir 11, R) +17B: (ir I, Ri +17Bi (ir 1, Ri.
The knowledge of the dispersion curves, of the spectroscopic activity of the k = 0
phonons and their precise frequencies has allowed us to locate the energy gaps,
where to expect localized defect modes, and to predict where the activation of the
density of states singularities (strong peaks in g(v) with no activity in IR and/or
Ranian) due to lack of symmetry because of disorder could generate extra absorption
or scattering.
Since we take for granted that the actual samples of PVC contain geometrical as
well as configurational defects at non-negligible concentrations, calculations were
extended [90] with the introduction of: (i) conformational defects as isolated units
or as possibly interacting multiple units; and (ii) isolated or possibly interacting
configurational defects. Of necessity, when configurational defects are introduced,
local conformational distortions must be considered so as to justify the existence of
an energetically plausible defect. NET was applied for chains consisting of 200
chemical units (i.e., matrices of 3600 x 3600 were constructed), while IIM was
restricted to shorter chains made up by 50 units (900 x 900).
The distribution of defects was random and the concentrations ranged from an
isolated unit to 8'Yo defects. The construction of the large matrices was made from
smaller submatrices associated to each chemical unit in a given conformation or
configuration. The detailed algebra and the complete expressions for such submatrices
are available in [90] and can be applied to any case of polymers made up
by chiral chemical repeat units.
The attention was mainly focused on the C-C1 stretching range of the supposedly
mostly syndiotactic chain. Polymerization in urea clathrates provides the 'purest'
sample of planar syndiotactic PVC [130]. Such a chain shows a large energy gap in
the CCl stretching range (-650 to 820 cm-'). It is very likely that defect modes
from other structures may generate localized gap modes which may be used as
useful structural probes.
While we refer to [90] for a thorough analysis of the complex structural situation,
in Figures 3-33 and 3-34 we give two examples of the frequency and vibrational
displacements for an isolated GG or an isolated isotactic diad defect in a fillly
syndiotactic planar zig-zag host chain. It must be noted that gap frequencies are
different, but that the vibrational displacements associated to these gap modes involve
many chemical units (-20). The complexity of the problem is shown in Figure
3-35 where the density of vibrational states of a realistic model of configurationally
disordered PVC is compared with the experimental spectrum.
3.16 Structural Inhomogeneity and Raman Spectroscopy
of LAM Modes
The problem raised in Section 3.14.1 requires further discussion. From longitudinal
accordion motion (LAM) spectroscopy of a variety of fatty acids Vergottin et 211.