Process unzipping

Various studies have been made on the effects of radiation on lactide/glycolide polymers (24,38,58). Gilding and Reed (24) reported the effect of y rays on Dexon sutures. Those results confirmed that deterioration of the sutures occurs but that random chain scission is not the primary mechanism. Number average-molecular weight Mn showed a dramatic decrease at doses above 1.0 Mrad. Thus, unzipping of the polymer chain appeared to be the more dominant process, at least in the case of polyglycolide. 

Pyrolysis GC/MS is limited in application to those studies in which the compounds formed are capable of being analysed by GC, that is it is only reasonably suitable for low molecular weight products. Many synthetic polymers degrade (pyrolyse) by processes that may simply be described as either random scission (e.g., polyolefins), unzipping to produce mostly monomers (e.g., PMMA)... 

Emersion of an electrode from electrolyte with its double layer intact is now a widely accepted phenomenon and technique. Not only is it a phenomenon which deserves careful consideration and study, but also a process which opens up a new set of experimental methods to the study of the electrochemical double layer. Electrode emersion involves the careful removal of an electrode from electrolyte under potentiostatic control, usually hydrophobically 11-5). When fairly concentrated electrolyte parts ("unzips") from the electrode surface during hydrophobic emersion, the double layer remains essentially intact on the electrode surface and no electrolyte outside the double layer remains. This phenctnenon is not due to the presence of organics or other impurities as seme have suggested. The emersion process works well with rigorously clean electrode surfaces (5). 

Thirdly, experiments concerned with the whole process of conformational change and the unzipping process (e.g., [49]) studied the mechanical stability of individual double-stranded DNA molecules. It was foimd that the B-S transition of A-DNA occurred at 65 pN, followed by a second conformational transition during which the DNA double helix melted into two... 

When heated rapidly in small portions the product undergoes a mild explosion and gives high yields (80% to 95%) of methanal and benzenecarbalde-hyde. The mechanism may be a kind of unzipping process, starting from a break in the chain and spreading toward each end ... 

Because the rates of this reaction are very rapid, normal sampling techniques were not satisfactory and an infrared technique was used. This esterification reaction was shown to be about 100 times faster than the disproportionation reaction and inter-intra-molecular assistance was also found to be important. This assistance seems to be a common pattern in acid-catalysed processes of oligosiloxanols in inert solvents. In dioxane solvent the redistribution kinetics can be interpreted in terms of an unzipping mechanism. The ratedetermining step is terminal silanol cleavage by water forming dimethylsilanediol which rapidly reacts with other substrate silanols (Scheme 4). 

Should the process take place with some number of intermediate steps, such as the unfolding of only part of the protein chain or an incomplete unzipping of the duplex, then the fractional change in the absorbance will not be linearly related to the composition of the mixture A + B. Under these circumstances, the melting curve is often broader and spread out over a larger temperature interval than would occur for the simple two-state process. The broadening leads to a... 

Depolymerization is a special case of thermal degradation. It can be observed particularly in polymers based on a, a -disubstituted monomers. In these, degradation is a reversal of the synthesis process. It is a chain reaction during which the monomers are regenerated by an unzipping mechanism. This is due to the low polymerization enthalpy of these polymers. For the thermal fission of polymers with secondary and tertiary C-atoms, higher energies are required. In these cases elimination reactions occur. This can be seen very clearly in PVC and PVAC. 

The normal unzipping process of PMMA decomposition was obstructed in the case of the acrylate copolymers, thus reducing the evolution of the flammable methylmethacrylate (MMA) monomer. 

An important step in the thermal degradation of many polymers such as PMMA involves depropagation. Here the polymer molecule or radical fragment rapidly unzips in such a way that produces mainly a monomer. This process may be modeled simply by a rapid reaction where an z-mer is converted to an (i - l)-mer plus a monomer P —> P x + Px. In reality, this is a gross oversimplification of the process and a more sophisticated model will be discussed subsequently and also later in the chapter in relation to the thermal degradation of PMMA. 

Effect of Fiber Degradation on the Corrosion Solution. Hydrolysis and oxidation of protein and cellulose have been described in the literature primarily with the focus on degradation in industrial processing conditions. In alkaline conditions, amino acids are released from silk in a chain unzipping mechanism in acidic conditions, the scissions are random (8,9). As the polymer deteriorates, free carboxyl and amine end groups are formed. Tyrosine oxidizes to a quinone this reaction gives aged silk its yellow coloration. Amorphous areas of the fiber are attacked first. 

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