Modern Polymer Spect..

1.7 Coizclirsions 29
derived in a straightforward manner. Using this generalized method, the 2D correlation
analysis of any time-dependent spectral signals can be carried out. Highly
nonlinear time-dependent reactions of crosslinking polymer systems, for example,
are followed by the time-resolved IR measurement [65, 661. The resulting sets of
time-dependent IR spectra can be transformed into 2D correlation spectra to yield
valuable information to probe details of curing reactions of polymers.
One of the very promising new developments in the recent 2D correlation analysis
of polymers is the application of this versatile technique to the study of spectral
changes recorded as functions of a general physical variable, which is no longer
limited to time. For example, it is possible to apply the 2D correlation analysis
to the near-IR spectral changes corresponding to the development of disorder in
a hydrogen-bonded polyamide system induced by the rising temperature [67]. Of
course, spectral changes of polymers induced by other variables, such as pressure,
age, composition, and the like, may be analyzed in the same way.
1.7 Conclusions
2D IR spectroscopy based on the correlation analysis of the individual timedependent
behavior of localized characteristic reorientational motions of various
submolecular moieties comprising a system has been shown to be very useful for a
broad range of applications in the study of complex polymeric materials. In 2D IR,
the spectral resolution is substantially enhanced by spreading the overlapped IR
bands along the second dimension. The presence or lack of chemical interactions or
connectivity among functional groups located in various parts of the polymer system
are detected. The relative reorientation directions and the order of realignment
sequence of submolecular units are also provided.
Notable discoveries made through the use of 2D IR spectroscopy include the fact
that local dynamics of side groups can proceed independent of the polymer main
chain, and there can be abrupt changes in the side group realignment mechanism
above and below the glass transition temperature. It is possible to probe the niicroscopic
spatial distribution of submolecular components of polymers, especially in
phase-separated systems such as semicrystalline polymers and block copolymers,
and the degree of interactions between various components can be estimated.
Specific interactions between Components in polymer mixtures are quite effectively
identified. The application of 2D IR spectroscopy certainly is not limited to the
characterization of traditional synthetic polymers. Complex macromolecules of
biological origin can also be readily studied by this technique. The recent extension
of the 2D correlation concept to (1) spectroscopic study of polymers using probes
other than IR, (2) nonsinusoidally varying spectral changes, and 13) dependence
on physical variables other than time has greatly expanded the scope of possible
spectroscopic applications of this technique to the study of polymeric materials.