Oxidative degradation techniques

As it has been shown earlier, the quantification of LCC linkages with traditional wet chemistry methods is limited mostly to relative quantification of carbohydrate sites linked to lignin. In contrast, our quantitative 2D NMR approach [21] allows for quantification of lignin sites involved in LCC linkages of different types. However, it does not provide information on the specific carbohydrate linkage sites yet. Therefore, a combination of quantitative NMR analysis and appropriate wet chemistry methods, such as a routine carbohydrate analysis, along with the methylation and DDQ oxidation degradation techniques, could be the best approach for comprehensive LCC analysis. 

Schole and co-workers [8] have reported on the characterisation of polymers using an oxidative degradation technique. In this method the oxidation products of the polymers are produced in a short pre-column maintained at 100-600 °C just ahead of the separation column in a gas chromatograph. The oxidation products are swept on to the separation column and detected in the normal manner. 

Schole and co-workers [31] have applied an oxidative degradation technique to the study of PS. In this technique the PS sample is mixed with a support in a pre-column which is mounted at the inlet to a GC column. 

Comparison with Other Oxidative Degradation Techniques [120]... 

TABLE 3. - Present level of technical development of oxidative degradation techniques based on hydroxyl radical generation [120]. 

Analysis of Trace or Minor Components. Minor or trace components may have a significant impact on quaHty of fats and oils (94). Metals, for example, can cataly2e the oxidative degradation of unsaturated oils which results in off-flavors, odors, and polymeri2ation. A large number of techniques such as wet chemical analysis, atomic absorption, atomic emission, and polarography are available for analysis of metals. Heavy metals, iron, copper, nickel, and chromium are elements that have received the most attention. Phosphoms may also be detectable and is a measure of phosphoHpids and phosphoms-containing acids or salts. 

Polyether-based thermoplastic copolyesters show a tendency toward oxidative degradation and hydrolysis at elevated temperature, which makes the use of stabilizer necessary. The problem could be overcome by incorporation of polyolehnic soft segments in PBT-based copolyesters [31,32]. Schmalz et al. [33] have proposed recently a more useful technique to incorporate nonpolar segments in PBT-based copolyesters. This involves a conventional two-step melt polycondensation of hydroxyl-terminated PEO-PEB-PEO (synthesized by chain extension of hydroxyl-terminated hydrogenated polybutadienes with ethylene oxide) and PBT-based copolyesters. 

A useful concise source to a collective description of the processes of oxidative degradation of polymers and coverage of the analytical tools used to investigate and measure them is Chapter 14 in Ref. [1]. This reference also discusses the limitations of many of the techniques, and contains a more extensive list of references. 

XPS and SIMS are both surface sensitive and surface-specific techniques that may be used to probe surface oxidation/degradation of polymers. They use X-rays and ions, respectively, to characterise surface species. Both XPS and SIMS are operated under ultra-high vacuum, UHV, conditions. 

Imaging chemiluminescence technique. Chemiluminescence analysis is suitable for studying the early stages of the thermal oxidation of rubbers. A weak emission of light formed by chemical reactions appears during the oxidative degradation of hydrocarbons. This technique can be used to depth profile the oxidation of rubbers. MR... 

Cylindramide (90) is a cytotoxic compound isolated from Halichondria cylindrata (7.10 s % wet wt.), but very likely of bacterial origin [148]. The structure was deduced from spectroscopic analysis, the relative stereochemistry (only) of the bicyclo[3.3.0]octene unit by NOESY techniques, and the absolute stereochemistry of the amino acid of the tetramic acid ring (25,35) from the recovery of erythtro-L-(3-hydroxyomithine from oxidative degradation of 90. It showed cytotoxicity to B16 melanoma cells (IC50 0.8 (ig/ml). 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

This review is the result of several investigations performed by the authors and other research groups where chemiluminescence has been used to study oxidative degradation of polyolefins, either as the main technique or as a complement to other techniques. 

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