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• W2159741569 abstract "Real time X-ray diffraction is used to examine kinetic barriers and metastability in periodic silica/surfactant nanophase composites. These materials, which are the precursors to ordered mesoporous silicas, are synthesized in a P6mm hexagonal structure using a range of base concentrations. When the composites are heated under hydrothermal conditions, they are observed first to anneal into a more ordered hexagonal structure and then to undergo a hexagonal-to-lamellar phase transition. The transition and annealing are followed in situ through low-angle X-ray diffraction as the materials are heated under a linear temperature ramp. From the variation in diffraction peak intensities with time as a function of ramp rate, it is possible to determine activation energies for the phase transition and for annealing using the Ozawa method. Activation energies both for annealing and for the hexagonal-to-lamellar phase transformation are found to correlate with the base concentration used to synthesize the composites. Materials made at the lowest pH show an activation energy of 163 ± 3 kJ/mol for the hexagonal-to-lamellar phase transition, whereas materials made at the highest pH show an activation energy of only 106 ± 3 kJ/mol. This result can be explained by a more condensed framework in materials synthesized at lower pH and, thus, the need to hydrolyze more silioxane linkages in order for the material to rearrange. The annealing process shows the opposite dependence on pH, with the highest activation energies observed for those materials synthesized at the highest pH (Ea = 58 ± 12 kJ/mol for materials synthesized at the lowest pH and 80 ± 16 kJ/mol for materials synthesized at the highest pH). This result can be explained by postulating that the activation energy for annealing is related to silica condensation, rather than silica hydrolysis. Higher-level kinetic analyses allow us to extract an activation energy from each temperature ramp and, thus, to examine the chemical changes that occur during the heating process. Composites made at the highest synthesis pHs have activation energies that increase with slower ramp rates, suggesting that silica condensation occurs during heating, which, in turn, increases the activation energies. Composites made at the lowest synthesis pHs, in contrast, have activation energies that decrease with slower ramp rates, suggesting that silica hydrolysis occurs during heating, which, in turn, lowers the activation energies. These ideas are corroborated by ex situ 29Si MAS NMR experiments, which show that materials made at the lowest pH start out with the most condensed silica framework. During annealing, the degree of condensation increases, but at the start of the phase transformation, the degree of framework condensation again decreases. This work provides a basis for understanding the relationship between synthetic parameters, silica chemistry, and the stability of silica/surfactant nanostructured composites." @default.
• W2159741569 created "2016-06-24" @default.
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• W2159741569 date "2000-05-19" @default.
• W2159741569 modified "2023-10-10" @default.
• W2159741569 title "Effect of Framework Polymerization on the Phase Stability of Periodic Silica/Surfactant Nanostructured Composites" @default.
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• W2159741569 doi "https://doi.org/10.1021/jp9944379" @default.
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