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8.2 DYNAMIC 2D IR DICHROISM SPECTRA OF POLYMERS
Some pertinent examples for 2D IR dichroism spectra, especially those of polymeric materials, are provided here to show how certain useful information can be effectively extracted for different types of applications of the dynamic 2D IR correlation technique. At first, a binary blend mixture system consisting of essentially noninteracting polymer components is studied. Despite the apparent simplicity of this model system, the exercise actually highlights several pertinent features of 2D IR correlation analysis applied to dynamic dichroism experiment. Next examined in detail are single-phase amorphous glassy polymers: one of the simplest form of solid-state systems. This study demonstrates the unique capability of dynamic 2D IR analysis in differentiating between highly overlapped IR bands. The improved spectral resolution arises primarily from the fortuitous submolecular level selectivity of IR dichroism responses to a dynamic mechanical perturbation. It is truly remarkable to be able obtain high-resolution mid-IR spectra for condensed-state systems by coupling a mechanical perturbation applied to a sample with a simple correlation method. The comparison between results obtained from 2D IR dichroism studies and independent IR absorbance measurements of a
Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
selectively deuterium-substituted specimen provides strong experimental support for the physical validity of high-resolution 2D IR dichroism spectra.
The apparent high-resolution capability of 2D IR dichroism spectra may be combined effectively with IR spectral band correlations among common molecular structures to probe some of the most exciting but also very complex systems, such as proteins and other biomolecules. Here, the gross overlap of IR spectral lines of amide vibration bands of protein-rich specimens can be experimentally deconvolved into individual spectral components, and the characteristic molecular vibrations of various conformational features of proteins may be studied.
8.2.1 POLYSTYRENE/POLYETHYLENE BLEND
We start with a very simple system, a binary mixture of atactic polystyrene (PS) and low-density polyethylene (PE),1 to illustrate the unique features of dynamic 2D IR spectra. It is well known that a blend mixture of polymers is often immiscible and spontaneously separates into phases because of the much reduced entropy contribution to the free energy of mixing. Such an immiscible mixture serves as an excellent model system, where molecular level interactions between components are believed to be very small. Each component of an immiscible polymer blend spontaneously segregates into relatively coarse (>10 ^m) dispersed phase domains. The volume fraction of the interfacial region is often negligibly small. Therefore, little true interaction at the molecular level will be expected between PS and PE components.
A conventional IR spectrum for this blend film is shown in Figure 8.4. Absorption peaks associated with the semicircle-stretching modes of the PS phenyl ring and CH2 deformations of the PE and PS backbones are observed. From this spectrum alone, however, it is rather difficult to determine the state of molecular level interactions between PS and PE components. The corresponding 2D IR dichroism correlation spectra (Figures 8.5-8.7) were derived from the strain-induced dynamic IR spectra measured with a simple dispersive time-resolved IR spectrometer.6 The dynamic IR measurement was carried out by mechanically perturbing the PS/PE blend sample at the room temperature with a 23 Hz small-amplitude oscillatory tensile strain (about 0.1 % amplitude) and recording the time-dependent fluctuations of IR absorbance induced by the perturbation at a spectral resolution of 4 cm-1. For this particular study, dynamic absorbance signals were used, instead of dichroism signals, for experimental simplicity.
Figure 8.5 shows a pseudo-three-dimensional fishnet representation of the synchronous 2D IR spectrum. The spectral resolution is clearly enhanced by spreading the peaks over the second dimension. While the relative magnitude of correlation intensity may be best represented by a 3D fishnet plot, it is usually more convenient to use contour map representation (Figures 8.6 and 8.7) to determine the location of peaks in 2D IR spectra. Autopeaks observed on the diagonal positions near 1454 and 1495 cm-1 in the synchronous 2D spectrum (Figure 8.6)
Dynamic 2D IR Dichroism Spectra of Polymers
Figure 8.4 Conventional (1D) IR absorbance spectrum of a mixture of atactic polystyrene and low-density polyethylene obtained at 4 cm-1 resolution. (Reproduced with permission from Ref. No. 1. Copyright (1989) American Chemical Society.)
Figure 8.5 Fishnet representation of the synchronous 2D IR correlation spectrum of a mixture of polystyrene and polyethylene. (Reproduced with permission from Ref. No. 1. Copyright (1989) American Chemical Society.)