Multistage degradation

Regnier, N. and Guibe, C. (1997) Methodology for multistage degradation of polyimide polymer. Polym. Degrad. Stab., 55, 165-172. 

N. Regnier and C. Guibe, Methodology fo" multistage degradation of polyimide polymer. Polymer Degradation arul Stability, 55, 165 (1997). 

The main limitation of the technique is the problem of achieving radiochemical, as distinct from chemical, purification of the final labeled product. Purification of the irradiated sample is necessary to remove radiation-induced degradation products and labile tritium. Many of the degradation products are not only chemically similar to the parent compound, but also possess much higher specific activities. For complete purification it is necessary to use multistage processes, such as gas and column chromatography, countercurrent distribution, and fractional distillation. Distribution of isotope within a molecule is generally random and nonuniform however, in some circumstances useful specificity can be achieved.17... 

To minimize degradation of impurities, low-speed (1800 rpm) pumps with open impellers should be used. Where solids are large and floc-like, it may be advisable to use a recessed impeller centrifugal pump or a multistage piston pump with suitable aircushioning chambers. Reciprocating pumps should have ball valves. Gear or rotary pumps may be used where solids are not abrasive and filter aid is not present. 

Research into the biological activity of phytochemicals has a long history. Initially, the focus was on natural product chemistry, with degradation studies providing a basis for structure determination that was confirmed by elegant multistage synthesis. Many key reactions of organic chemistry were first developed out of the specific synthetic requirements of this area. 

Using laser microprobe techniques and mass spectrometry, Lum [66] investigated PBT degradation under vacuum. A complex, multistage, decomposition was observed, which included two major reaction pathways. Initial degradation was said to occur via the familiar cyclic transition state to provide acid chain ends and unsaturated ends. The subsequent reactions of the unsaturated chain ends were said to proceed in two ways. Initially, it was proposed that degradation occurred via an ionic process to produce THF ... 

The thermal degradation of poly(butylene terephthalate) was examined with the aid of a laser microprobe and mass spectrometry [506]. A complex multistage decomposition mechanism was observed that involves two reaction paths. The initial degradation takes place by an ionic mechanism. This results in an evolution of tetrahydrofuran. This is followed by concerted ester pyrolyses reactions that involve intermediate cyclic transition states and result in formation of 1,3-butadiene. Simultaneous decarboxylations occur in both decomposition paths. The latter stages of decomposition are... 

Electric current leads to degradation of a LCM and reduces the lifetime of the display. Impurities influence the stability of the material and accelerate electrodegradation. Therefore a multistage purification, consisting for example of recrystallization, and column chromatography, to remove conducting impurities (intermediate products, water, and CO2), is necessary. Usually the specific conductivity of a LCM is lower than 10 -10 Cm/cm and corresponds to the intrinsic conductivity. 

Steroidal saponins are isolated from dried plant material including aerial parts, roots, and bark and are usually partitioned into a polar organic solvent before purification via reverse phase (RP) HPLC. The planar structure and stereochemistry of the isolated steroidal saponins is then typically determined using a combination of multistage mass spectrometry (MS"), ID and 2D NMR experiments, chemical degradation and derivatization, and enantioselective gas chromatography (GC). 

The degradation of polyethylene and the product yield in fluidized bed reactors " and multistage distillation columns" have also been studied. 

Practically all slurry pumps are single stage. Multistage pumps are limited to mine dewatering applications. Slurry pumps are rubber lined whenever they are designed to handle particles finer than 6 mm or Because rubber is susceptible to thermal degradation when the tip speed of the impeller exceeds 28 m/s or 5500 ft/mrn, ruhher-Uned pumps are typically reserved for a maximum head of 30 m (98.5 ft) per stage. 

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