| Abstract: | SUMMARY. Benefiting from the above mentioned advantages offered by the in situ experimental setup we have proposed methods of paper spectra standardisation: 1. Spectra normalisation using an internal standard defined as an integral absorbance of the CH 1900 cm-1 band in the range 3000-2800 cm-1, and denoted as standard absorbance; 2. Removal of water molecular vibrations from carbonyl vibration region by preliminary desorption of H2O from samples and by recording the spectra at elevated temperatures; 3. Introduction of the oxidation index, defined as a ratio of integrals of bands at 1730 cm-1 to that at 1620 cm-1. Complementary, to remove water vibrations from the CO spectrum region, isotope exchange was applied and optimised. The applicability of this method for ageing experiments is, however, limited, because the properties of D-cxchanged samples arc different from original prolonated samples. The boundary conditions of experiments selected by us proved to be able to discriminate between hydrolysis of glycosidic bond and oxidation of carbon atoms in glycopyranose anomcrs. Accordingly, under the conditions of various gas mixtures (dry air, water vapour, humid air) two types ol experiments were performed: (i) short ageing tests at three selected temperatures (100. 150, 250 °C) and (ii) longer isothermal tests. Following the changes in spectra in the carbonyl range as they increased with oxidation time (an example in Figure I), various intermediates of cellulose oxidation were distinguished starting from the least oxidised monocarbonyl groups, through dikctoncs to aldehydes and carboxyles. The band assignment was supported by the spectra fitting by mixed Lorentzian-Gaussian function optimization and literature data. The changes in band intensities during paper oxidation were found to correspond to a parallel-consecutive mechanism. In the first approach a simple model taking into account both band positions, and presumed reaction mechanism, is proposed and is schematically presented in Figure 2. The assumptions were verified by independent experiments. The position of-COOH stretching mode at 1730 cm-1 was confirmed by the experiment utilizing the acidic groups neutralization with a dilute KOH solution (Figure 3). As a result of the reaction with KOH, the intensity of vibrations at 1730 cm-1 weakened (curve b). At the same time the sub-bands coming from other intermediate producls ol cellulose oxidation revealed. |
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