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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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10.3 TEMPERATURE-DEPENDENT 2D NIR OF AMORPHOUS POLYAMIDE
Temperature-dependent 2D NIR spectroscopy, as in the case of small molecules discussed in Chapter 9, is a powerful tool in the investigation of hydrogen
200
Generalized 2D Correlation Studies of Polymers and Liquid Crystals
bonding in associated polymers, such as polyamides. The structures of polyamides have long been studied by using IR, Raman, and NIR spectroscopy, which provide useful information about their conformation and the nature of their hydrogen bonds.36 37 Wu and Siesler investigated the thermal behavior of a totally amorphous polyamide by means of 2D NIR correlation spectroscopy.23 In the NIR region, bands due to the stretching modes of free and hydrogen-bonded NH groups are much better resolved. Bands due to free NH groups can be observed much more clearly because of the large anharmonicity constant of the free NH group. Thus, NIR studies of polyamides should provide valuable information about the temperature dependence of hydrogen bonding.
Figure 10.16 shows NIR spectra of an amorphous polyamide in the region 7000-5400 cm-1 measured over a temperature range of 25-200 C.23 Table 10.1 summarizes assignments for NIR bands of the amorphous polyamide. Bands in the 6100-5400 cm-1 region due to the overtones and combination bands of CH and CH2 groups do not show significant temperature-dependent variations, while the intensities and shapes of bands in the 6800-6300 cm-1 region due to the amide groups vary with temperature. Of note is that the intensity of a band at 6765 cm-1, arising from the first overtone of the stretching mode of free NH group, increases markedly as a function of temperature, indicating the increase in the proportion of free NH groups. A band at about 6535 cm-1 decreases in intensity with a small upward shift, whereas the intensities of bands around 6650 cm-1 increase slightly with temperature.
Synchronous and asynchronous 2D NIR spectra in the 6900-6200 cm-1 region generated from the temperature-dependent spectral variations of the polyamide
Wavenumber/cm 1
Figure 10.16 NIR spectra in the 7000-5400cm-1 region of the amorphous polyamide measured over a temperature range of 25-200 C. (Reproduced with permission from Ref. No. 23. Copyright (2000) American Institute of Physics.)
Temperature-dependent 2D NIR of Amorphous Polyamide
201
Table 10.1 NIR band assignments of the amorphous polyamide (from Ref. No. 23)
Wavenumber (cm-1) Intensity Assignment
6765 w 2 x v (NH)f
6520 m 2 x v (NH)b
6010 m 2 x v (CH)ar
5980 m 2 x v (CH)ar
5900 m 2 x v (CH)ar
5810 m 2 x vas (CH2)
5690 m 2 x vs (CH2)
4877 m v (NH)b + AmideII
4659 m 3 x AmideII
4611 m v (NH)b + AmideIII
4350 s vas (CH2) + 5 (CH2)
(A) (B)
6900
6800
T 6700
I
? 6600
$
.o
I 6500 <6 to
? 6400 6300
6200 6300 6400 6500 6600 6700 6800 6900 6200 6300 6400 6500 6600 6700 6800 6900
Wavenumber 1/cm-' Wavenumber 1/cm-'
Figure 10.17 (A) Synchronous and (B) asynchronous 2D NIR spectra in the
6900-6200cm-1 region of the polyamide obtained from 25 to 200 C. (Reproduced with permission from Ref. No. 23. Copyright (2000) American Institute of Physics.)
are shown, respectively, in Figure 10.17(A) and (B). In the synchronous spectrum, a broad autopeak is developed at 6765 cm-1, indicating that the intensity of the band due to the first overtone of the stretching mode of free NH groups [v (NHf)] increases significantly during the course of the temperature increase. Two negative cross peaks observed at (6535, 6765) and (6250, 6765 cm-1) suggest that the intensities of the bands at 6535 and 6250 cm-1, arising, respectively, from the first overtone of the stretching mode of bound NH groups [v (NHb)] and possibly from a combination of NH (bound) and CH stretching modes, decrease with temperature.
The corresponding asynchronous spectrum (Figure 10.17(B)) develops a cross peak at (6765, 6535 cm-1). The sign of this cross peak suggests that
202
Generalized 2D Correlation Studies of Polymers and Liquid Crystals
the temperature-induced spectral change takes place earlier (i.e., at a lower temperature) at 6535 cm-1 [2 x v (NH)b] than that at 6765 cm-1 [2 x v (NH)f]. The cross peak shows obvious asymmetry, indicating the possible existence of a band near 6650 cm-1, probably arising from a first overtone of the NH stretching mode of weakly hydrogen-bonded amide groups (Figure 10.18(C)). This observation suggests that the intermolecular hydrogen bonding is gradually weakened during the course of heating. Also of interest in the asynchronous spectrum is the cross peak at (6780, 6740 cm-1). This result indicates that the band at 6765 cm-1 consists of two components. Those at 6780 and 6740 cm-1 probably originate, respectively, from the totally free NH (Figure 10.18(A)) and the free-end NH (Figure 10.3(B)) groups.
Figure 10.19(A) and (B) shows synchronous and asynchronous 2D NIR correlation spectra in the 6200-5400 cm-1 region generated from the temperature-dependent spectral variations (from 25 to 200 C) of the same amorphous polyamide.23 The first overtones of the aliphatic CH2 stretching modes and the aromatic CH stretching modes are expected to appear in the 6100-5400 cm-1 region. Three autopeaks at 6010, 5980, and 5900 cm-1 are due to the first overtones
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