By Jan F. Rabek
In this ebook on actual features and useful points of polymer photodegradation Rabek emphasizes the experimental paintings at the topic. crucial characteristic of the ebook is the actual interpretation of polymer degradation, e.g. mechanism of UV/light absorption, formation of excited states, strength move mechanism, kinetics, dependence on actual homes of macromolecules and polymer matrices, formation of mechanical defects, practics in the course of environmental growing older. He contains additionally a few facets of polymer photodegradation in environmental and house condition.
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Extra resources for Photodegradation of Polymers: Physical Characteristics and Applications
First, both degradation and crosslinking take place predominantly in the amorphous part of the material. All radiation-induced processes start at specifically radiation sensitive sites (chromophoric groups) in the polymeric material. Therefore both chain scission and crosslinking are not introduced randomly. It is generally accepted that the photo-oxidation processes are initiated at the surface, which gives rise to gradient of deteriorated material across the specimen thickness (cf. Sect. 12).
These are conditions for which the Stern-Volmer kinetic law may be applied. (i) The energy migration rate (kM) between polymer-bound chromophores depends on : (i) polymer-bound chromophore separation; (ii) average mutual chromophore separations; (iii) distribution of chromophore separations - a break in a sequence of chromophore can impose a barrier to energy migration; (iv) spatial relationships that exist between adjacent or non-neighbouring chromophores. The experimental results show that migration of energy prior to the transfer occurs in poly(vinylbenzophenone) and poly(phenyl-vinylketone) but not in poly(methylvinylketone), since the critical transfer distance determined in this latter case has the expected value for transfer by electron exchange interaction.
For non-viscous fluids, the diffusion coefficient (D) for a small molecule is usually of the order 10- 5 cm 2 S-I , while in a viscous or solid medium, D can be of the order of 10- 10 cm 2 S-I. 10 Solvent Effects The solvent may influence the rate or efficiency of an energy transfer in the following ways. (i) Viscosity effect (cf. Sect. 11). (ii) Solvent effects on the energy levels of the donor and acceptor (absorption spectra and the overlap integral). g. reversible photochemical reaction between excited donor and solvent, such as a proton transfer, which leads to quenching of the donor).