IR spectroscopy differences

Absorbance subtraction can be considered as a spectroscopic separation technique for some problems in polymers. An interesting application in FT-IR difference spectroscopy is the spectral separation of a composite spectrum of a heterophase system. One such example is a semicrystalline polymer which may be viewed as a composite system containing an amorphous and crystalline phase53). In general, the infrared spectrum of each of these phases will be different because in the crystalline phase one particular rotational conformation will predominate whereas in the disordered amorphous regions a different rotamer will dominate. Since the infrared spectrum is sensitive to conformations of the backbone, the spectral contributions will be different if they can be isolated. The total absorbance A, at a frequency v of a semicrystalline polymer may be decomposed into the following contributions... 

Figure 5.1-11 IR difference spectroscopy for quality control of impact resistant PVC (see text).

Figure 5.1-12 IR difference spectroscopy for the determination of the microstructure of polybutadiene in the presence of polystyrene.

Photobiological systems (chromoproteins) can conveniently be studied by FT-IR difference spectroscopy. Light is an ideal trigger. It is physiologi-... 

Successful concepts for "reaction-modulated" IR difference spectroscopy use the multiplex advantage of FTIR spectroscopy or the availability of high-intensity laser IR sources. A kinetic photometer using tunable IR diode lasers as sources for the mid-infrared has been developed in our laboratory and will be described elsewhere [6]. It covers the time-domain from approx. 500 nsec to some seconds. A second approach is time-resolved FTIR spectroscopy using a rapid-scanning interferometer, several scans can be recorded per second and the time-domain of slow reactions thus be covered [7]. The following schemes illustrate both concepts ... 

It is this aspect that makes the combination of infrared (IR) spectroscopy with electrochemical reactions so attractive. The infrared spectrum in the wavelength range from 4 to 20 pm reflects the structural properties of a polypeptide molecule, both for backbone and the side chain conformations. However, the potential of IR spectroscopy is better exploited with reaction-induced IR difference spectroscopy. 

Pyromellitic-dianhydride based polyamide is the most photostable of the above group (4.69). The photolytic decomposition in air of polyimide films based on a dianhydride and a diarylamine with hexafluoroisopropylidene 6F bridging groups 4.71 and 4.72) is extremely rapid, leading to efficient chain cleavage and subsequent photo-oxidative decomposition. On the basis on the photo-oxidation of several model compounds and IR difference spectroscopy, it has been shown that the photodecomposition process occurs via decomposition of the arylimide linkage [996, 998] ... 

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