In the field of organic/inorganic hybrid polymers with the application as thermal insulating materials, ethylene-bridged polysilsesquioxane films are promising materials with enhanced thermal insulating properties due to the presence of intramolecular void spaces in their frameworks. With the aim of increasing the content in intramolecular void spaces to further enhance the thermal property of the resulting material, this paper describes the hydrosilylation reaction of hydrodimethyl-silylated oligomethylsilsesquioxane (MSQ-SiH) and octavinyl polyhedral oligomeric silsesquioxane (Vinyl-POSS) to produce a hybrid film with intramolecular void spaces, which was colorless and transparent due to the formation of covalent SiCCSi bonds. As a control experiment, a polymethylsilsesquixane (MSQ) film and an MSQ film containing 5 wt % of Vinyl-POSS (composite film-5) were prepared. Hybrid film-5 (prepared from MSQ-SiH and 5 wt % of Vinyl-POSS) exhibited lower density and thermal diffusivity (1.19 g/cm3 and 1.21 × 10-7 m2/s) than the MSQ film (1.34 g/cm3 and 1.36 × 10-7 m2/s) and composite film-5 (1.37 g/cm3 and 1.58 × 10-7 m2/s). Hybrid film-5 showed a 5% weight loss temperature (Td5) and a 10% weight loss temperature (Td10) of 382 and 453 °C, respectively, which were lower than those of the MSQ film (Td5 of 436 °C and Td10 of 481 °C) and higher than those of composite film-5 (Td5 of 325 °C and Td10 of 453 °C). Positron annihilation lifetime technique revealed that the hole volumes estimated from the value of the positronium lifetime component decreased in the order hybrid film-5 > MSQ film > composite film-5. Taken together, the results demonstrate that the thermal insulating property of the polysilsesquioxane film was improved by the formation of intramolecular void spaces around the POSS molecules.
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