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Fluoropolymer blends have been widely used as binders for exterior coatings because of their excellent resistance to ultra-violet (UV) radiation as well as to many corrosive chemical agents.It is known that the fluorinated component usually has a lower glass transition temperature and easily crystallizes in the final structure depending upon the blend composition and sample annealing condition.We investigated the effect of blend composition and annealing process (slow and fast cooling) on the surface mor... 相似文献
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P. W. J. G. Wijnen T. P. M. Beelen K. P. J. Rummens H. C. P. L. Saeijs R. A. Van Santen 《Journal of Applied Crystallography》1991,24(5):759-764
The influence of pH, cations, F− and temperature on the aggregation, gelation and ageing of aqueous silica gels has been investigated using small-angle X-ray scattering (SAXS). At both low and intermediate pH (pH = 4 and 6–8, respectively), fractal aggregates are formed during gelation from silicic monomers or small oligomers. The fractal dimensionality is 2.20 (5), to be expected for reaction-limited cluster–cluster aggregates. During ageing silicic ions dissolve from the more soluble peripheral primary particles and condense in the crevices of the core of the aggregates. This process results in an increase in the size of the scattering primary particles and in a decrease in the fractal dimensionality of the silica aggregates from D = 2.25 to D = 2.0 or D = 1.85. The ageing processes are strongly enhanced by a slight increase in the pH of the solution, addition of fluorine anions and a rise in temperature. The aged silica gel is constructed of an ensemble of densified aggregates of which the density variation within the aggregates is larger than that of the freshly prepared aggregates. The primary particles of which the aggregates are built grow during ageing from molecular size to dense particles in the nanometer range, the ultimate radius depending on the type and duration of the ageing process. 相似文献
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Jeukens CR Jonkheijm P Wijnen FJ Gielen JC Christianen PC Schenning AP Meijer EW Maan JC 《Journal of the American Chemical Society》2005,127(23):8280-8281
We have prepared 5 nm diameter, micrometer long tetra(p-phenylenevinylene) (OPV)-based nanofibers on a graphite surface. The fluorescence emission of an individual fiber shows a profound polarization over its entire length that directly corresponds to its orientation on the substrate. Quantitative analysis of the fluorescence polarization, including the depolarizing effect of the underlying graphite, evidences the high degree of organization within chiral fibers with the OPV molecules perpendicular to the fiber axis. The control of the internal order within self-assembled fibers, and the ability to measure it, is a crucial step to obtain uniform organic fibers that can be applied in nanosized electronics at room temperature. 相似文献
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In order to study the influence of hydrogen-bond interactions on the accelerations of Diels-Alder reactions in water and highly aqueous mixed solvent systems, second-order rate constants for the Diels-Alder reaction of acridizinium bromide (1a) with cyclopentadiene (CP) have been measured in aqueous media and organic solvents. Only modest rate accelerations were found in water-rich media. This is attributed to the absence of hydrogen-bonding groups in the reactants. Comparison with cycloadditions of CP with 9-carbomethoxyacridizinium bromide (1b), acrylonitrile (3), and methyl vinyl ketone (4), which do contain hydrogen-bond acceptors, reveals substantially larger aqueous accelerations. These results demonstrate that hydrogen bonding is a major factor in aqueous accelerations. Also rate constants for the cycloaddition of CP to 1a in surfactant solutions were determined. Micellar catalysis is observed in SDS solutions, due to binding of both the diene and the dienophile to the anionic micellar surface. 相似文献
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