Polyimide-silica hybrid membranes were characterized by dynamic mechanical thermal analysis, differential scanning calorimetry, positron annihilation lifetime spectroscopy, and gas permeability measurements in order to investigate the role of inorganic-organic interactions on chain mobility, free volume, and transport properties. Current composite theories neglect inorganic-organic interactions (or interfacial phenomena); however, the present work illustrates that this interaction is critical to the transport properties of these hybrid materials. Transmission electron microscopy of samples with low levels of alkoxysilane shows well-dispersed inorganic domains of sizes ranging from 25 nm to 100 nm. These nanodomains are shown to increase the glass transition temperature of the polymer, the free volume, and the permeability.
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