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Organosilicon compounds with "hairy" alkyl substituents may be suitable for use in high-temperature lubrication applications. AUTHORS used Friedel-Crafts chemistry to synthesize a series of alkyl-substituted octaphenylsilsesquioxanes (example shown). The compounds, powders at room temperature, melt on heating to form viscous clear liquids. The aim of the work is to produce low-melting-temperature materials consisting of hard spherical cores and soft flexible shells that act as nanometer-size liquid ball-bearings. "We tried to reduce the crystallinity of the compounds by adding alkyl groups so that they become like hydrocarbon materials with low melting points," Laine says. "At lower degrees of substitution, the melting point decreases with increasing alkyl chain length, as expected. But surprisingly, compounds with longer chains suddenly undergo a dramatic increase in melting point as the substitution level increases." He suggests that the melting-point increase results from the increasingly hairy nature of the longer chain molecules. "The alkyl chains interdigitate like nanosized gears," he explains. "As the number and length of the hairs increase, it takes more energy to separate them."
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