Modern Polymer Spect..

3.7 The Case of Isotactic Polypropylene - A Tesbook Cme 113
The fact which needs to be justified is that in some analytical determinations of
the concentration of crystalline material in a polymer bulk, correlations are found
between the intensity of the regularity bands and the data from X-ray diffraction
experiments (which measures scattering from 3D periodicity). The measure of the
intensity in infrared or Raman provides the amount of material organized as 1 D
straight chains. If the ‘rIinylzrtti’ model is accepted, the measure of the intensity
of the regultrr-ity bands [65, 661 gives an estimate of the concentration of straight
spaghetti in an otherwise disordered environment of ‘boiled’ spaghetti (conformationally
disordered chains). Whether the chains are packed in a crystal (3D order)
or exist in a sort of ‘liquid crystalline’ arrangement (1D order) cannot be revealed
from k = 0 regularity bands.
True crystallinity bands have been observed for only a few polymers and their
origin experimentally verified by isotopic dilution studies [70]. We may quote the
classical prototype case of orthorhombic polyethylene [68, 691, orthorhombic polyoxymethylene
[7 I], aiid-with caution-possibly a few others [72, 731.
3.7 The Case of Isotactic Polypropylene -
A Texbook Case
It is beneficial here to discuss the case of isotactic polypropylene (IPP) as a worked
example of the analysis of the vibrational spectrum of a polymer molecule. Let us
assume that polymerization of propene with Ziegler-Natta catalysts has produced a
fully head-to-tail isotactic polymer.
Let us first consider the vibrational spectra (infrared) of IPP in the melt (Figure
3-9a) [74]. In this physical state, polymer chains possess a conformationally irregular
structure like any liquid branched n-alkane. We expect the infrared spectrum to
consist of relatively few bands easily identified as the group frequencies of CH3 and
CH2 and CH groups. Other modes may be identified with caution, but their location
is irrelevant to the present discussion. The experimental infrared spectrum of
Figure 3-9a is in full agreement with the expectations.
It must be pointed out that the infrared spectrum of liquid IPP shows a few bands
(especially the band at 973 cmr’) which are neither observed in the spectrum of
atactic PP nor in the spectrum of liquid methyl-branched hydrocarbons. Moreover,
in the Rainan spectra of molten IPP, three characteristic tacticity bands have been
located at 1002, 973 and 398 cnir’ which allow IPP to be distinguished from the
syndiotactic stereosionier (993, 966 and 310 cnr’). It follows that the ‘isotactic’
chiral units generate vibrations mostly localized on the asymmetric carbon atom,
but with some coupling with the neighboring units. This allows the chirality of the
neighbors to be probed. These bands become characteristic of the tacticity and
allow us to distinguish between isotactic and syndiotactic sequences [75, 761.
Next, let the sample of molten IPP solidify. Polymer chains are allowed through
their conformational flexibility to reach the minimum energy conformational struc-