Electron Spin Resonance Imaging (ESRI) of Degradation and Stabilization Processes in Polymers:

Abstract

Polymeric materials, exposed to heat, mechanical stress and ionizing or UV irradiation, undergo degradation in the presence of oxygen due to the formation of reactive intermediates such as free radicals R., RO. and ROO., and hydroperoxides ROOH.1, 2, 3, 4, 5, 6 The degradation process can be accelerated by chromophores, free radicals and metallic residues from the polymerization reactions. Often the deleterious effects are not immediately detected, but develop over longer periods. The gradual changes in the polymer properties observed in many systems, including polyolefins, and the ultimately grave results are due to trapped radicals that react slowly, to peroxy radicals that decompose in time with formation of reactive radicals and gas molecules, and to trapped gases that lead to local stresses and to cracking.2,5 While the timescale of these changes may vary, the final results are dramatic: degradation of the structure and collapse of the mechanical properties. The accelerated rate of ozone depletion in the stratosphere due to environmental factors is expected to raise the level of UV-B radiation (≈290–320 nm), thus adding severity to the problem of degradation and urgency to the need for solutions.