Aging and Degradation of Polymer Blends
The selection of polymers and polymer blends for use as specific materials requires the consideration of how these will withstand the environmental conditions to which these will be subjected. The long term stability of a polymer will depend on its aging
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AGING AND DEGRADATION OF POLYMER BLENDS
J. M. G. Cowie1, I. J. McEwen1 and R. McIntyre2
1
Heriot-Watt University, Edinburgh EH14 4AS, Scotland
2
Millenium Inorganic Chemicals, Linconshire England
14.1
Abstract
The selection of polymers and polymer blends for use as specific materials requires the consideration of how these will withstand the environmental conditions to which these will be subjected. The long term stability of a polymer will depend on its aging characteristics both physical and chemical. Physical aging is important mainly when the materials have a substantial amorphous content as it is manifest in volume contraction of the solid caused by small scale relaxation processes in the glassy state. This can be followed by enthalpy and volume relaxation methods or mechanical measurements such as creep, stress-relaxation and dynamic mechanical methods. The systems have been modeled in a number of ways and those of Moynihan, Hodge, KAHR, Petrie-Marshall and Cowie-Ferguson are described. In blends, the general observation is that aging rates differ from those of the individual components. Both homogeneous and heterogeneous blend (and composites) behavior is described. The external effects of the environment on polymer blends are chemical in nature, and normally lead to degradation of the polymers. Chain scission, depolymerization and reactions on the side-chain substituents all contribute to overall deterioration of blend properties. These are described for blends containing polyvinyl chloride, polystyrene, acrylics and polyolefins mixed with a variety of other polymers. The general features of radiation damage and the detrimental effects of processing are reviewed. These degradation reactions contribute to the ‘weathering’ of blends. This is best studied by long term exposure to appropriate environmental conditions and the subsequent methods of assessing the results using ESR, FTIR, XPS and chemical methods are covered. Because many of the degradation reactions involve radicals, ESR is a particularly powerful approach and as surfaces are the first areas of attack techniques such as FTIR and XPS are being used extensively in this work. Examples of the application of these techniques to coatings are detailed.
L.A. Utracki (Ed.), Polymer Blends Handbook, 977-1021. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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14.2
J. M. G. Cowie, I. J. McEwen and R. McIntyre
Introduction
The long-term stability of polymeric materials is a matter of considerable importance, both to materials scientists and to engineers. Two types of aging occur that lead to changes in the properties of polymers; chemical and physical. Chemical aging normally leads to modification of the polymer chain and may involve chain scission, oxidation, dehalogenation, loss of pendant groups, hydrolysis, and crosslinking, all of which are chemical reactions. Physical aging is a manifestation of small scale relaxation processes that take place in the amorphous regions of a glassy polymer, causing volume contraction and densific
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