Infrared spectrum without interference

Fig. 14. Infrared spectra of carbon fiber (A) transmission spectrum without water interferenr (B) transmission spectrum with water interferenr (C) transmission spectrum with water interference in the tockground KBr pellet (D) PAS -FTIR sttectrnm (E) DRIFT spectrum [2441. Reprinted with permission from C.Q. Yang and J. R. Simms, Fuel, 74 (1995) 543. CoRTight (1995) American Chemical Society...

This method of classification is useful for pure polymer samples without significant modification or in the absence of additives. In the presence of a polymer modification or blended additives, a misinterpretation may result because of interference from other components. Usually the amounts of additives used in a formulated product are relatively low, and their presence is seldom a major interference. An exception is experienced with certain plasticizers, in which the concentration is often high. A common example is plasticized poly(vinyl chloride), which is a mixture of poly(vinyl chloride), a stabilizer, and a plasticizer such as dioctyl phthalate (often diisooctyl isomer). In this example, features associated with the plasticizer dominate the infrared spectrum. Certain additives, such as fillers (calcium carbonate, for example) may also be misleading, and can confuse the spectral interpretation. For example, products fabricated from poly(vinyl chloride) are used for construction and piping, and these are typically formed from a blend of poly(vinyl chloride) and calcium carbonate. The two examples provided are the common cases where the additives dominate the infrared spectra, and these are sufficiently popular combinations that the spectra are easily recognized. 

The same principle is used in dispersive spectroscopy to measure the thickness of an infrared sample cell or a film sample. As is illustrated in Fig. 7, some of the radiation striking the sample is transmitted without any reflection, while some of the radiation is internally reflected at the interfaces before passing through the sample. The transmitted radiation and the 2n-times-internally-reflected radiation interfere, and interference patterns appear in the spectra. The patterns are spectral artifacts, not vibrational or rotational bands, and are entirely a function of the thickness of the sample. They are very apparent in almost every polymer spectrum. 

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