Degradation mechanisms chemistry

Yu, J., Yi, B., Xing, D., Liu, R, Shao, Z. and Fu, Y. 2003. Degradation mechanism of polystyrene sulfonic acid membrane and application of its composite membranes in fuel cells. Physical Chemistry Chemical Physics 5 611-615. 

A second tier of tests should be designed for compounds already detected as sensitive. The goals of the second tier are the isolation of impurities and the investigation of the degradation mechanism. It is not possible to propose general protocols, as in this case the chemistry of the substance must be fully considered. In this phase we can study, by comparison with... 

Photosynthesis in all photosynthetic organisms is blocked by triazines, as well as by other PS II herbicides, when isolated thylakoid systems are tested. However, in intact plants, they express either different inhibitory potency or no inhibition. This shows that the specificity of these photosynthesis herbicides to certain weeds is not related to a difference in the chemistry of their primary target, but rather is attributed to degradative mechanisms, translocation, and translocation mechanisms. 

Mao, Y. Schoeneich, C. Asmus, K. D. Radical mediated degradation mechanisms of halogenated organic compounds as studied by photocatalysis at Ti02 and by radiation chemistry, in Photocatalytic Purification and Treatment of Water and Air D. F. Ollis H. Al-Ekabi, eds., Elsevier Science Publishers B.V. Amsterdam, 1993, p. 49. 

In this chapter we will examine the atmospheric degradation mechanisms of the following important classes of anthropogenic molecules alkanes, alkenes, aromatics, nitrogen oxides, S()2, CFCs and Halons, and finally HFCs and HCFCs. Our intent is not to give an exhaustive account of the photochemical oxidation of every man-made chemical species but rather to present examples of the degradation mechanisms of a few representative members of each class of pollutant. First, we need to consider the general features of atmospheric chemistry. 

The complexity associated with unraveling the precise degradation mechanism of any given alkane can be appreciated by considering the case of n-hexane. Initial OH radical attack leads to the formation of three different alkoxy radicals, each of which can either react with O2, decompose, isomerize (via several possible pathways), or undergo a combination of these possible loss processes. Our understanding of the atmospheric chemistry of alkoxy radicals is rather crude at present and this is an area of active research [49]. 

In addition to OH radicals, unsaturated bonds are reactive towards O3 and NO3 radicals and reaction with these species is an important atmospheric degradation mechanism for unsaturated compounds. Table 4 lists rate constants for the reactions of 03 and NO3 radicals with selected alkenes and acetylene. To place such rate constants into perspective we need to consider the typical ambient atmospheric concentrations of O3 and NO3 radicals. Typical ozone concentrations in pristine environments are 20-40 ppb while concentrations in the range 100-200 ppb are experienced in polluted air. The ambient concentration of NO3 is limited by the availability of NO sources. In remote marine environments the NO levels are extremely low (a few ppt) and NO3 radicals do not play an important role in atmospheric chemistry. In continental and urban areas the NO levels are much higher (up to several hundred ppb in polluted urban areas) and NO3 radicals can build up to 5-100 ppt at night (N03 radicals are photolyzed rapidly and are not present in appreciable amounts during the day). For the purposes of the present discussion we have calculated the atmospheric lifetimes of selected unsaturated compounds in Table 4 in the presence of 100 ppb (2.5 x 1012 cm 3) of O3 and 10 ppt (2.5 x 108 cnr3) of NO3. Lifetimes in other environments can be evaluated by appropriate scaling of the data in Table 4. As seen from Table 4, the more reactive unsaturated compounds have lifetimes with respect to reaction with O3 and NO3 radicals of only a few minutes ... 

The environment is an important factor affecting the rate and degree of biodegradation of polymer substrates. The other key aspects determining biodegradability are related to the chemical composition of the polymer. The polymer chemistry governs the chemical and physical properties of the material and its interaction with the physical environment, which in turn affects the material s compostability with particular degradation mechanisms. 

Summary Summary of drug chemistry of substance Discussion of potential impurities and degradation products Degradation mechanisms and relevance Preformulation data summary... 

Furthermore, chemical models to be developed can be applied in the EUROCHAMP network for the solution of specific problems in atmospheric chemistry, e.g. development and validation of degradation mechanisms of organic pollutants that are of paramount importance, the investigation of atmospheric reactivity as an overall property under various conditions or the influence of alternative fuels or solvents as well as bio fuels on tropospheric chemistry (3 objective). 

Hydrolysis is the principal degradation mechanism for the condensation polymers. From the point of view of chemistry, the equilibrium molecular weight of these polymers is determined by the H O concentration at given temperature, T. However, owing to the moisture absorption from the air, the reaction equilibrium is shifted toward depolymerization. The rate of hydrolytic depolymerization depends on the moisture content, T and the presence of catalyst. Since these polymers are also subject to free-radical and oxidative processes (that lead to formation of unsaturations, hence the... 

Reviews the chemistry behind pyrolysis mechanisms, degradation mechanisms, and essential instrumentation considerations... 

The fundamental degradation chemistry of PET and PEN should be very similar. Actually, thermal degradation studies show strong similarities in the degradation behavior of these related polymers. Identical primary and secondary thermal degradation mechanisms have been proposed [63]. 

This chapter reviews PCL- and PLC-based materials, their chemistry, properties, degradation mechanisms and processing techniques. In the last part of the present... 

Photodegradation chemistry has evolved into a highly practical state over the last few years to where commercial products are available and others are being evaluated. The degradation mechanisms are understood to the point of property loss for the plastics. The gap on environmental acceptability still needs attention, it is not sufficient to expect low molecular weight fragments to be biodegradable, this must be demonstrated. 

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