External impurity

MAGISTERIUM — is a Chemical State which follows the process of extraction, and in which a matter is developed and exalted by the separation of its external impurities. In this manner are all the parts of natural and homogenous concretion preserved. But they are so exalted that they almost attain the nobility of essences. Whence also there is left almost the same mass or quantity which nature gave. It is, however, impossible to prevent at all times the loss of a certain proportion of the essential matter along with the foreign matter which is removed, and a small percentage of such loss may in certain cases be overlooked. From which you may understand that we find what the chemists call alteration is here of great use. 

Sasaki K, Yoshizumi T, Haga K, YosMtomi H, Hosoi T, Shiratori Y, Taniguchi S (2013) Chemical degradation of SOFCs external impurity poisoning and internal (fiffusion-related phenomena. ECS Trans 57 315... 

The irrtrinsic materials aging mechanisrrts can generate products that contaminate the catalyst or PEM. However, the problem of the imderstanding of the materials aging phenomena becomes more complex if we consider that a realistic PEMFC would also operate with H (in the case of hydrogen-fed PEMFCs) and O2 contaminated with external impurities (Fig. 11.4). 

External impurities Such impurities are contained in the sample but not incorporated in the polymer structure. 

During synthesis, processing and storage, the polymer can be contaminated or blended with a variety of external chemical spedes which may contain chromo-phore and photoreactive groups. Some typical external impurities found in commercial plastics are ... 

Energy transfer from the internal and/or external impurities (donors) to the chromophoric groups (acceptors) in a polymer. 

The state of the amorphous phase, the structure of the crystalline phase, defects in the crystals and external impurities all influence the formation and behaviour of radicals in solid polymers. Mechano-radicals are formed in mechanically stressed samples. One of the characteristics of mechano-radicals is high reactivity at low temperatures compared with the corresponding radicals formed by photo-irradiation [136, 2012, 2013]. 

Low molecular free radicals formed from the photocleavage of external impurities (RH) may abstract hydrogen from the polymer molecule (PH) giving polymer alkyl radicals (P ) ... 

External impurities, which may contain chromophoric or photoactive groups, including ... 

Polyethylene should, in theory, be photo-oxidatively stable on the basis of its formal -(-CHj—CH2 structure however commercial polyethylenes are not photostable. This can be due to the presence of internal and external impurities (photoabsorbing chromophores) which are formed during synthesis and processing (cf. section 3.1.6). The UV irradiation of polyethylene in air causes an increase in its absorption spectrum (Fig. 3.5). 

A further restriction on possible reactions is imposed by the elementary photochemical principle that light must be absorbed by the polymer if a reaction is to occur. However, most pure polymers do not absorb at wavelengths longer than 300 nm, and hence should not be affected by solar radiation. However, most polymers degrade when subjected to solar radiation, and this has been attributed to the presence of small amounts of internal and/or external impurities which absorb light and initiate oxidative chain reactions within the polymer. A detailed study of such reactions is difficult because of the frequently unknown nature of the trace impurities which initiate the reactions and of the multiplicity of products formed in the photo-oxidative degradation (cf. section 2.21). 

The experimental evidence demonstrates that the formation of the first C-C bond should be mechanistically separated from the fiorther pathways, which contribute to the formation of olefins. Primary traces of any external impurities play an important role only at the initial step as soon as the first hydrocarbon species are formed, a jump in activity is observed. Whatever is the mechanism behind the formation of the first C-C bond, the main practical conclusion of the investigations is a parallel formation of ethylene and propylene as secondary products at high conversion level of methanol. 

Initiation stabe where free radicals R are formed from the photolysis of internal and external impurities (some of them... 

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