Depolymerization with aging

The hydrolysis of Pu(IV) can result in the formation of polymers which are rather intractable to reversal to simpler species. This has led often to incorrect conclusions about the nature of the plutonium species present and the validity of their equilibrium constants. The kinetics of depolymerization take a different course as the polymer ages such that while freshly prepared hydroxides are easily decomposed, aged polymers require quite rigorous conditions. A reasonable model for this process involves initial formation of aggregates with OH bridging which dehydrate with aging (Choppin 1983). 

Plutonium(IV) polymer is a product of Pu(IV) hydrolysis and is formed in aqueous solutions at low acid concentrations. Depolymerization generally is accomplished by acid reaction to form ionic Pu(IV), but acid degradation of polymer is strongly dependent on the age of the polymer and the conditions under which the polymer was formed (12). Photoenhancement of Pu(IV) depolymerization was first observed with a freshly prepared polymer material in 0.5 HClOh, Fig. 3 (3 ). Depolymerization proceeded in dark conditions until after 140 h, 18% of the polymer remained. Four rather mild 1-h illuminations of identical samples at 5, 25, 52, and 76 h enhanced the depolymerization rates so that only 1% polymer remained after the fourth light exposure (Fig. 3). 

Enhancement of depolymerization rates for aged polymer (polymer with high degree of PuC>2 character (13) were observed... 

These polycations can be further hydrolyzed or react with each other to give a large number of possible polycations. The olatlon reaction occurs without a change In solution acidity but does change the concentration of hydrolyzed species In solution. Dilution causes the reversal of reaction 2 (I.e., depolymerization) and the Introduction of new hydrolyzed species Into solution. The new species formed can then re-equilibrate through the hydrolysis reaction. The kinetics of aging depend on the relative rates of the olatlon and hydrolysis reactions. 

In Figure 1, the pH of the batch diluted solutions Is plotted against the dilution ratio, C/Co, where C Is the molar concentration of Al In the dilute solution and Co Is the molar concentration of Al In Chlorhydrol (6.43 M). The pH of the most concentrated solution was still Increasing after ten days aging. (The higher viscosity of the concentrated solutions may contribute to the slower equillbriatlon). At higher dilution (lower C/Co values), new equilibria can be Introduced due to depolymerization reactions. For the zero day curve, pH continuously Increases with... 

The oxidation of the film follows first-order kinetics with rates close to those found for decreases in tear strength and depolymerization of cotton and rayon fabrics. The reaction mechanisms appear not to be affected by temperature. The infrared spectra of the film and of the water extract of the aged film are essentially the same as those found for naturally aged linen. 

The mechanism of formation of zeolites is very complex, stemming from the diversity of chemical reactions, including various polymerization and depolymerization equilibria, nucleation and crystal growth processes. The physical and chemical nature of the reactants, which typically involve a source of aluminum and silicon along with hydroxides and salts determine the formation of zeolites. Physical effects such as aging, stirring, and temperature also play an important role. These effects lead to the complexity of zeolite formation, but are also responsible for the large number of frameworks that can be synthesized and the rich chemistry associated with this area. Cl. 21... 

Figure 9. Representation of microsyneresis (24). Immersion of a water-aged sample in ethanol is accompanied by partial esterification of the gel surface, network depolymerization, and solvation smaller, more uniform pores result. The reverse process, immersion of an ethanol-aged sample in water, causes almost complete hydrolysis, network condensation, and apparent phase separation into water-rich and polymer-rich regions larger pores with broader size distributions result. (Reproduced with permission from reference 67.)...

The pulp is first steeped in an aqueous solution of sodium hydroxide (17 18%), which causes the fibers to swell and converts the cellulose to sodium cellulosate, commonly called alkali cellulose or white crumb. After steeping, the swollen mass is pressed to obtain a precise ratio of alkali to cellulose and then shredded to provide adequate surface area for uniform reaction in subsequent process steps. The alkali cellulose is aged under controlled conditions of time and temperature to depolymerize the cellulose by oxidation to the desired DP prior to reacting with carbon disulfide to form sodium cellulose xanthate. The xanthate, which is a yellow to orange crumb, is dissolved in dilute sodium hydroxide to yield a viscous orange-colored solution called viscose. The solution is filtered, deaerated, and ripened to the desired coagulation point (called salt index) appropriate for spinning. 

Another way to achieve rapid depolymerization is to irradiate the cellulose with a beam of accelerated electrons [146-148]. In this case, the time required for the depolymerization is of the order of seconds. This technology has been investigated as a means to eliminate the need to age alkali cellulose in the viscose process, and rayon has been successfully made from irradiated pulp [149,150]. The process, however, has not been adopted by the industry because, like double steeping, material handling poses special requirements. 

Shredders are usually jacketed to permit cooling or heating as required. Mechanical action and the heat involved will cause depolymerization, which affects subsequent aging requirements. It is important to avoid excessive drying, carbonation, or localized wetting from condensation of the crumb within the shredder. Carbonation, due to reaction of CO2 from air with the alkali on the crumb, forms sodium carbonate, which blocks access of CS2 to the... 

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