Polymer Surfaces photochemical degradation

ESCA should be regarded as a powerful technique providing information complementary to that from other branches of spectroscopy, but with unique advantages which mean that for many studies of polymeric systems it may well be the most important. In particular aspects of polymer chemistry, such as dynamic studies of thermal or photochemical degradation, and in studies of polymeric films produced at surfaces by chemical reaction (e.g. fluorination, oxidation, etc.) the information derived from ESCA studies is not obtainable by other techniques. The application of ESCA to such problems is considered briefly in this article, however, more extensive discussion of particular aspects are given in the paper on surface fluorination. 

A special mention has to be made concerning the PVC-wood composites subjected to accelerated weathering they retained their initial mechanical properties in a higher proportion than the originating polymer [35, 58, 59]. The possible explanation of this behavior may be that the photo-oxidation takes place only at the surface of the composite where wood particles, considered as a chromophore material because of the presence of carbonyl groups, undergo photochemical degradation. Hence, all composite samples exhibited a more intense discoloration than the neat PVC. 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

Figure 10.9. Photochemical reactions taking place at the surface of a TiO, pigment (I) photo reduction of the titanium atom resulting in the production of hydroxyl radical (II) reduced titanium atom reacting with oxygen to form an unstable complex (III) reaction of the complex with water to produce peroxyl radicals. Both the hydroxyl and peroxyl radicals can react with the polymer matrix and initiate degradation.

Solids can be classified according to their organizational structure and, in the context of photochemical reactions, be classified as glasses, polymers, and crystals. Furthermore, a solid-state photochemical reaction only takes place in a thin layer of the solid surface, while in a dilute solution all the molecules are equally exposed to the radiation. In a tablet, the radiation has been estimated to reach a penetration depth of 0.03 cm, and the faded layer apparently does not increase upon further exposure (Carstensen, 1974). A study of nifedipine in the solid state demonstrated that the degradation rate was inversely proportional to the thickness of the powder bed (Marciniec and Rychcik, 1994). 

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