Modern Polymer Spect..

Figure 1-19. An asynchronous 2D IR
spectrum of a blend of polystyrene and
1510 1470 1430 low-density polyethylene at room tern-
Wavenurnber, v(
perature [Z].
may become substantially different from those of the other. Therefore, 2D IR correlation
spectroscopy can easily differentiate individual spectral contributions from
each of the immiscible components.
Another notable feature in Figure 1-19 is the development of asynchronous
cross peaks correlating the 1459-cm-’ band attributed to the CHz-deformation
of the polystyrene backbone to the 1454-cm-’ and 1495-cm-’ bands arising from
semicircle-stretching vibrations of the phenyl side groups. The asyncronicity among
IR signals from these polystyrene bands again reflects the difference in mobilities of
backbone and side-group functionalities, as already discussed in Section 1.5.1. The
asynchronicity expected between the crystalline and amorphous bands of the polyethylene
component are not apparent due to the intense cross peaks associated with
polystyrene.
The assignments of the 1454-cm-’ and 1459-cm-’ bands can be independently
verified by selectively substituting the hydrogen atoms of polystyrene backbone
with deuterium atoms. The elimination of spectral contributions from the backbone
of polystyrene leaves only the pure phenyl vibration spectrum in this region, as
shown in Figure 1-20. The asynchronous 2D IR spectrum (Figure 1-19) differentiated
the two IR bands even without deuterium substitution. This result clearly
demonstrates the truly unique feature of 2D IR spectroscopy to enhance substantially
the resolution of highly overlapped spectral regions by effectively spreading
IR bands along the second dimension according to the characteristic reorientational
response times of individual functional groups.
1.5.4 Miscible Polymer Blends
Certain classes of polymer blends are known to be truly miscible and form homogeneous,
one-phase mixtures. A specific interaction at the submolecular level. e.g..