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ATKS-Chemiluminescence:

Chemiluminescence (CL), the emission of light without emission of heat as the result of a chemical reaction, has become very useful technique for studying oxidation of organic materials. Investigations are based on measuring light emission resulting from the oxidation processes of the organics due to e.g. thermal decomposition, mechanical stress or photo-oxidation. Because of the very high sensitivity of CL, the measurements can be carried out with very small samples (microgram range) and at temperatures at which detection of the oxidation by other techniques is difficult or impossible.

Goals

Organic materials reacting with environmental oxygen change their optical, mechanical and chemical properties, therefore the knowledge of their oxidation kinetics is of particular importance, especially for stability, quality, safety and guarantee purposes. A prerequisite for the correct evaluation of the kinetic parameters of the oxidation processes is the application of a sensitive method which allows the collection of the experimental data at low temperatures, closer to real application conditions.

Principe

Chemiluminescence is the generation of visible electromagnetic radiation by the release of energy from a chemical reaction. During relaxation of excited- toward ground state, photons are emitted, this especially occurs during oxidation reactions. The precise mechanism of exited states formation during oxidation is not yet entirely understood, it is believed to be based on a termination of two peroxide radicals as proposed in a Russel-mechanism.

Luminescence is a term used for various phenomena, originating from electronically excited states. Luminescence is a ‘cold light’, not an incandescent light. The emission of photons results from the relaxation of excited electrons (triplet state) into their ground-state. This is a fast process: the delay between the excitation and the light emission is in the range of 10-10 seconds. Chemiluminescence includes all luminescence phenomena resulting from chemical reactions.

The fact that organic substances undergoing oxidation emit light has been recognized already in 1669 by Robert Boyle. In the last decades the Chemiluminescence has gained wide acceptance as a sensitive method to study the oxidative degradation of organic solids and liquids.

Chemiluminescence during oxidation of the organic materials originates from radical reactions. The first degradation step is the formation of unstable alkyl radicals, which immediately reacts with environmental oxygen to form peroxy radicals. These reacts further and transform into different species in an accelerating degradation cycle (auto-oxidation, see figure below). It is normally attributed to a transition of excited triplet-carbonyl-functions (3R=O*) into their ground state. The spectral range of the light emitted varies according to the type of substances involved. In most cases the Chemiluminescence is located in the short wave region of the visible spectrum, generally between 380 and 450 nm. However, there are well-known exceptions: the relaxation of 1O2 can be detected in the infrared region at approx. 1200 nm.

The required energy to form these excited states (290-340 kJ mole-1) may be supplied by basically three different chemical mechanisms:

• The termination two peroxy radicals in a Russel mechanism [1] being strongly exothermal (460 kJ mole-1) [2]: The CL-emitter is an excited “triplet” carbonyl state (right part of above figure).

• The direct homolysis of hydroperoxides followed by a cage reaction leads to an excited carbonyl state and is combined with the evolution of 315 kJ mole-1 [3].

• The metathesis of alcoxy or peroxy radicals provides 374 kJ/mole and 323 kJ/mole, respectively [4].

It has been shown, that the CL signal intensity reveals the existence of two kinetic stages during oxidative degradation of organic materials: The first one is correlated with the concentration of peroxide groups [5], the second stage corresponds to the oxidation propagation by hydrogen abstraction responsible for carbonyl formation [6].

REFERENCES

[1] Celina, M., George, G., ‘A heterogeneous model for the thermal oxidation of solid polypropylene from chemiluminescence analysis’, Polym. Degr. Stab.,40, 1993, pp. 323-335.

[2] Vasiliev, R., Progr. Reaction. Kinetics, 4, 1967, 305.

[3] Reich, L., Stivala, S., ’ Some remarks on the origin of weak chemiluminescence during polymer oxidation’ Makromol. Chem., 103, 1967, pp. 74

[4] Quinga, E., Mendenhall, G., ‘Chemiluminescence from Hyponitrite Esters. Excitet Triplet States from Dismutation of Geminate Alkoxyl Radical Pairs’ J. Am. Chem. Soc., 105, 1983, pp. 6520.

[5] Billingham N., Then, E.‚ Chemiluminescence from peroxides in polypropylene. Part I: Relation of luminescence to peroxide content’ Polym. Degrad. Stab., 34, 1991, pp. 263.

[6] Audouin L. Bellenger, V., Tcharkhtchi, A., Verdu, J.‚ ‘Oxyluminescence of Cross-Linked Amine Epoxies: Diglycidylether of Bisphenol A-Diaminodiphenyl Sulfone System’ Polymer Durability. Clough R., Billingham N., Gillen K. (Eds). (1996), American Chemical Society: Washington DC, pp. 223-234.