Degradation mechanisms chain

Because of the complex hydrodynamics associated with GPC systems, it is difficult to arrive at a simple correlation between GPC operational parameters and chain scission kinetics. At least five degradation mechanisms (given below with the associated flow field) may be operative in the different parts of the column, during a standard GPC analysis ... 

The thermal degradation of alkylbenzene sulfonates in alkaline media is important because of the application at elevated temperatures. The half-lives, with respect to thermal degradation, of several commercially available sulfonates were estimated at hundreds to thousands of years at 204° C. The degradation mechanism was predominately a clipping of the alkyl chain to yield an alkylbenzene sulfonate with the phenyl group attached to the a-carbon however, desulfonation also occurred [1624]. 

In random degradation molecular mass decreases early, while in chain degradation the molecular mass of the polymer remains almost constant. Characterisation methods for molecular mass are thus very sensitive methods to follow random degradation. In contrast, as monomer is produced in chain depolymerisation, weight loss measurement techniques are the best methods to follow this kind of degradation. (Chapters 10-12, in Section IV, of this book focus on the methods used in the molecular characterisation and analysis of polymer degradation and polymer degradation mechanisms.)... 

The most important degradation mechanism of asparagine and glutamine residues is formation of an intermediate succinimidyl peptide (6.63) without direct backbone cleavage (Fig. 6.29, Pathway e). The reaction, which occurs only in neutral and alkaline media, begins with a nucleophilic attack of the C-neighboring N-atom at the carbonyl C-atom of the Asn side chain (slow step). The succinimide ring epimerizes easily and opens by hydrolysis (fast step), as shown in Fig. 6.27, to yield the iso-aspartyl peptide (6.64) and the aspartyl peptide (6.65) in a ratio of 3 1. 

Three different degradation mechanisms were proposed. In the first mechanism, the hydroxyl radical attacks atrazine by hydrogen abstraction from the secondary carbon of the ethylamino side chain, producing a free radical as shown in Equation (6.135). 

Early research of ionomer membrane degradation was conducted in the context of PEM electrolyzers. The detection of fluoride and other chain fragments in the condensed effluent water indicates the decomposition of PFSA ionomer and has long been noticed. Baldwin15 reported the detection of fluoride in the effluent of PEM electrolyzer and believed that it is the result of membrane mechanical failure. Extensive research has been conducted to elucidate the reaction pathways for membrane decomposition. Many controversial results and mechanisms have been reported in the literature, demonstrating the complex nature and the current inadequate understanding of the membrane degradation mechanisms. 

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