Infrared spectroscopy carbonyl index

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

As part of a multi-technique investigation (see also discussion under mid-infrared spectroscopy later), Corrales et al. [13] plotted the carbonyl index for films prepared from three grades of polyethylenes a high-density PE (HDPE), a linear low-density PE (LLDPE) and a metallocene PE (mPE) (see Figure 5). In this study, the data trend shown in Figure 5 correlated well with activation energies derived from the thermal analysis, which showed that the thermal-oxidative stability followed the order LLDPE > mPE > HDPE, whereas the trend... 

Undoubtedly the most powerful method of study in academic laboratories has been ir spectroscopy, advanced rapidly in recent years by the development of FTIR and microspectroscopy and by rapid improvement in sampling methods. Infrared spectroscopy can easily detect the formation of hydroxyl and carbonyl species, and a common technique is to monitor the time dependence of the so-called carbonyl index, the ratio of the intensity of the carbonyl absorption envelope to that of a chosen reference band. 

Adamus et al. [26] studied copolymers with units of beta-butyrolactone (HB) and2-hydroxyhexanoic acid (2HHA) with ESI. The mixture of HB and 2HHA was stirred and heated at 70°C. The heating was continued until the racemic beta-butyrolactone was consumed, as determined by infrared (IR) spectroscopy (disappearance of beta-lactone carbonyl absorption band at 1815/cm).The copolymer yield was higher than 80%. The subsequent SEC analysis allowed to estabhsh that the polydispersity index is reasonably large ( I =1.6). 

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