Modern Polymer Spect..

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1-14. A synchronous 2D IR
spectrum of a thin film of atactic
polystyrene in the CH-stretching vibration
region at room temperature.
Regular one-dimensional spectra of
the same system are provided at the
top and left of the 2D spectrum for
, I I l I I I I I 315" reference. Shaded areas represent
I50 2950 2750
Wavenumber. VI
negative correlation intensity.
1.5.1 Amorphous Polymers
A homogeneous one-phase system consisting of a single. noncrystallizing homopolymer,
such as atactic polystyrene [3, 38-41 ] or poly(methy1 methacrylate) [42-
441, provides one of the least complicated testing grounds for the applicability of
the 2D IR technique for the study of local dynamics of polymers. By using 2D IR,
surprisingly rich information can be extracted even from a simple polymeric system
[25, 39-41 1. Figure 1-14 shows a contour-map representation of the synchronous
2D IR correlation spectrum of an atactic polystyrene film in the CH-stretching
region. A three-dimensional fish-net plot of the same 2D IR spectrum corresponding
to Figure 1-14 has already been shown in Figure 1-1.
DIRLD data used for the 2D correlation analysis was obtained by applying a
23-Hz dynamic strain with an amplitude of 0.1% to a thin solution-cast film of atactic
polystyrene (MI, 150,000) at room temperature 1391. The resulting dynamic dichroism
spectra have already been shown in Figures 1-5 and 1-6. The time-dependent
reorientations of transition dipoles associated with the molecular vibrations of
backbone and side groups are all observable in the original dynamic dichroism
spectra. However, much more detailed features are recognized in the 2D correlation
spectrum.
Autopeaks at the diagonal positions of Figure 1-14 represent transition dipoles
that are susceptible to reorientational motions in the polystyrene sample. Peaks at
2855 cm-I and 2925 cm-l are, respectively, assignable to the symmetric and antisymmetric
CH2-stretching vibrations of the methylene groups in the backbone of
the polystyrene chain; those peaks above 3000 cm-' arise from the phenyl ring CHstretching
vibrations and indicate the reorientation of the side groups. The appearance
of positive synchronous cross peaks at spectral coordinates corresponding to
the two methylene bands indicates the transition dipoles associated with these two