Molecular weight distribution, silicate

Analytical Procedure. The structures of silicate anions in the solutions and solids have been examined with the trimethylsilylation technique combined with gas-liquid chromatography and "Si NMR. The molecular weight distribution was measured by applying gel permeation chromatography to the trimethylsilylated derivatives. 

In any case, dispersion obtained via in-situ polymerization may improve the barrier and thermal properties based on a tortuosity argument alone but may or may not improve the mechanical performance, depending on the level of polymer/silicate interactions. Without thermodynamic compatibility, the silicate layers may even collapse to form multilayer stacks if the nanocomposite is heated. Finally, all of these techniques become somewhat complicated by any changes the silicate layers cause in terms of the polymer s molecular weight distribution as compared to silicate-free controls. 

Many zeolites were crystallized from strongly basic solution containing silicate and aluminate. In strongly basic solution, silicate exists as polymerized-state polysilicates with a variable distribution of molecular weight. Aluminate reacted with polysilicate to form various zeolite structures via condensation reaction under hydrothermal or solvothermal conditions. Therefore, in the study of the synthetic chemistry of zeolites, one of the key issues is to understand the existing state and reactivity of the polymerized silicate. 

The results in Table 5.1 indicated that silicate ions with a low degree of polymerization existed in sodium silicate solution with the possible structures shown in the second column of Table 5.1. Besides there low-polymerization silicate ions, highly polymerized silicate ions with more complex structures and higher molecular weights (10 000-50 000) existed as well, and could be measured by the TMS-GPC method. Because there are still some technical problems in using GPC for the separation of silicate ions, it is very difficult to determine the distribution of highly polymerized silicate ions. 

By summarizing the main important evidences, we report in Table 11.1 a few examples of functional olefin material-based oligomers and polymers, used as matrix and/or compatibilizer in PO/clay nanocomposites they are reported on the basis of the type of functionality and its distribution along the backbone, molecular weight, and kind of interactions (as suggested by the authors) with layered silicate platelets. This is not exhaustive... 

It will be noted that in these experiments the solutions were made from TMA hydroxide and colloidal silica. It is possible that if TMA had been added to a dilute solution of polysilicic acid of low molecular weight at a ratio of 3.3 and then vacuum evaporated to 10% silica, solubility equilibrium would become established and the distribution of silica species would be like that in sodium silicate solutions. 

From the above results, it could be concluded that the addition of excess amount of sodium ions into the crystallization system has apparent effect on the particulate properties of the product. At low batch alkalinity, the additional sodium ions causes de-aggregation of the final products, rendering the particles with more uniform size distributions. At high batch alkalinity, the excess amount of sodium ions triggers the surface condensation reactions on the crystalline end products. However, the crystallization rate is not enhanced by the increase of batch sodium ion content, indicating that the determining factor of crystallization of ZSM-5 zeolites in SDA-free system is concentration of low molecular weight silicate species, determined by batch alkalinity. 

The thermodynamic properties of silicate melts however, are complicated by the presence of a large number of different silicate anions. While silicate minerals are usually monor disperse, containing only a single type of anion (e.g. SiojJ in olivine), molten silicates contain a distribution of different polymeric silicate anions of different molecular weights and are thus polydisperse systems. The presence of a distribution of silicate anions in the melt can be inferred from the mixing properties of silicate melts (Richardson, 1956 Masson, I965). However, it has recently become possible to separate some of the... 

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