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Two dimensional correlation spectroscopy applications in vibratioal and optical spectroscopy - Isao N.

Isao N. Two dimensional correlation spectroscopy applications in vibratioal and optical spectroscopy - Wiley publishing , 2004. - 312 p.
ISBN 0-471-62391-1
Download (direct link): twodimensionalcorrela2004.pdf
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8.2.6 TOLUENE AND DIOCTYLPHTHALATE IN A POLYSTYRENE MATRIX
The understanding of the origin and specific nature of miscibility in multicomponent mixtures of different chemical compounds is of great scientific interest, from both fundamental research and practical technological points of view. The macroscopic thermodynamic description of solutions cannot provide adequate molecular level insight into the underlying mechanisms of solubilization and miscibility, necessary for proper design of various mixtures. 2D IR dichroism spectroscopy, which is sensitive to specific submolecular scale interactions of chemical groups, may provide useful background information for the study of miscible multicomponent systems.
The first example of the miscible blend study is a mixture system consisting of a small amount (<10wt%) of toluene added to a matrix of atactic polystyrene. The polystyrene used for this experiment is perdeuterated to minimize the overlap
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Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
of the spectral features of toluene and polystyrene. Toluene is a good solvent for polystyrene, and at a low level of addition, it serves as an effective plasticizer for polystyrene to reduce the glass transition temperature. The sample was prepared from a dilute solution of polystyrene in toluene. The solvent was evaporated gradually for a given amount of time in open air to form a uniform film. The film was then placed in a container saturated with toluene vapor until later use. The polystyrene sample film containing a small amount of toluene was analyzed with a dynamic spectrometer at -30 C under a dynamic deformation applied at a frequency of 23 Hz and a strain amplitude of 0.1 %.
The synchronous 2D IR spectrum (Figure 8.18) of the toluene/polystyrene mixture system shows that the strain-induced molecular reorientation of toluene is fully synchronized with the motion of polystyrene. The entire spectral plane is filled with strong synchronous correlation peaks. On the other hand, the corresponding asynchronous 2D IR spectrum (not shown) has few correlation intensities beyond the noise level of the instrumental detection limit. At this temperature, the asynchronicity among motions of constituent groups of polystyrene, such as side-chain phenyl and backbone methylene, is rather weak. The whole system, including toluene molecules, is reorienting under the applied strain as a coordinated unit with few local independent motions. While this finding by itself
Synchrnous 2D IR Dichroism Spectrum
3160 2980 2800
Wavenumber, V)
Figure 8.18 Synchronous 2D IR spectrum of PS/toluene blend. (Reproduced from I. Noda et al., Appl. Spectrosc., 54, 236A (2000). Copyright (2000) Society for Applied Spectroscopy.)
Dynamic 2D IR Dichroism Spectra of Polymers
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is not very exciting, it is the first demonstration that strain-induced reorientational motions of small molecules can be observed by dynamic IR measurement and analyzed with 2D correlation analysis.
The second mixture system studied here provides a more interesting result.17 This time, a small amount of dioctylphthalate (DOP) is mixed with deuterium-substituted atactic polystyrene. The new sample was made by adding a small amount of DOP to a toluene solution of polystyrene. The toluene was then fully removed by a prolonged evaporation to produce a thin uniform film with residual DOP. The DOP-containing sample film was analyzed with the dynamic spectrometer under conditions similar to the case for the toluene/PS mixture. DOP is a commonly used plasticizer for various polymers due to its affinity to a wide variety of polymeric materials. The chemical structure of DOP has a slight overlap with toluene; DOP also has an aromatic ring. However, instead of a single methyl group as in toluene, DOP has two long fatty octyl ester chains attached to the ring. The presence of ester chains makes this compound much less volatile, which is an important requirement for a good plasticizer.
Unlike the case for the toluene/polystyrene mixture, the asynchronous 2D IR spectrum of the DOP/polystyrene mixture (Figure 8.19) shows the development
Asynchrnous 2D IR Dichroism Spectrum
-----1---1---1---1----1--1---1---1---1----r ^
3160 2980 2800
Wavenumber, vj
Figure 8.19 Asynchronous 2D IR spectrum of PS/DOP blend. (Reproduced from I. Noda et al., Appl. Spectrosc., 54, 236A (2000). Copyright (2000) Society for Applied Spectroscopy.)
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Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
of numerous asynchronous cross peaks with significant correlation intensities between bands assignable to the ester side chains of DOP and those for backbone groups of polystyrene. According to this result, DOP molecules are moving more or less independently of the reorientational motions of polystyrene under the applied dynamic deformation. The result is somewhat surprising since such strong asynchronicity among different compounds is more commonly observed among the components of phase-separated systems.
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