Processes in polymer degradation

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free radical and ionic species [413]. When this application of shear is done in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the visco-elastic state. Both homolytic and heterolytic scissions are possible. The first one yields free radical and the second one ionic species. Heterolytic scissions require more energy, but should not be written off as completely unlikely [413]. Early work was done with natural mbber [413]. It swells when exposed to many monomers and forms a visco-elastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers [413]. Later the technique was applied to many different polymers with many different monomers [414]. 

Molecular weight loss occurs through the breaking of primary valence bonds. Such chain scissions may occur at random points along the polymer backbone or they may take place at the terminal ends of the polymer where monomer units are released successively. This last effect can be compared to unzipping. The response of any particular polymeric material to specific causes of degradation depends upon the chemical structure. 

This chapter presents the degradation processes in some typical commercial polymers. The materials selected were those that received major attention in the literature. 

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As is evident in the many examples selected in this chapter to illustrate the molecular characterisation and analysis of polymer degradation and oxidation processes, almost, without exception, each study or investigation cited has involved use of more than one analytical method or characterisation technique, often many more than two. This clearly emphasises the importance of using a multi-technique approach to provide supportive, complementary and confirmatory information, since each technique or method yields a different aspect of the degradation/oxidation process in polymers. 

Care must be taken when choosing an oil for rubber processing to ensure that its content of polar fractions is low. These polar fractions contain some of the most chemically active compounds in the oil. Polar materials are thought to interfere with cure characteristics and play a part in polymer degradation. These effects will also be accompanied by a drastic reduction in weather resistance, although usually heat resistance is not affected. 

M. Schulze, N. Wagner, T. Kaz, and K. A. Friedrich. Gombmed electrochemical and surface analysis investigation of degradation processes in polymer electrolyte membrane fuel cells. Electrochimica Acta 52 (2007) 2328-2336. 

To remove the polymeric binder, the green body is heated often in an oxidizing atmosphere. During this process the polymer degrades along many possible degradation pathways, which include (1) scission of the main chain, (2) reaction with side chains and substituents, and... 

The process of polymer degradation is always accompanied by a parallel competitive process of crosslinking which results in the formation of a microgel. The crosslinking depends on numerous factors such as the structure of the macro-radicals, the cage effect, the presence of various free radicals and oxygen, and the concentration of the polymers in solution. 

Summing up the results obtained in the investigations of the catalytic activity of metals on the oxidation process in polymers it may be said that this activity plays a highly important role in the degradation and ageing of polymers. 

Reich, L. and Stivala, S. S. Elements of Polymer Degradation. McGraw-Hill, New York 1971 Tkac, A. Radical processes in polymer burning and its retardation. I. ESR methods for studying the thermal decomposition of polymers in the preflame and flame zones. J. Polym. Sci. Polym. Chem. Ed., 19, 1475 (1981)... 

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