Temperature effects on the composition and microstructure of spray-dried nanocomposite powders.

Porous composite powders, prepared by spray drying of silica and polybromostyrene nanoparticles, were calcined at various temperatures up to 750 degrees C. The structure in these powders are quantitatively investigated by ultra small-angle X-ray scattering, thermogravimetric analysis, and nuclear magnetic resonance measurements. It has been found that the polybromostyrene latex is efficient in templating mesopores. However, polybromostyrene remains almost completely in the interstitial micropores in the grain after the spray-drying process. A post thermal treatment of the powders has been applied from 250 up to 750 degrees C. We found that the hydrocarbon part of the polybromostyrene is decomposed and leaves the micropores at around 350 degrees C. However, it is demonstrated that a significant amount of bromine remains in the interstitial micropores between the silica particles. At around 600 degrees C, the silica nanoparticles start to fuse with each other and a coalescence of the micropores takes place. At still higher temperature, around 750 degrees C, the micropore network totally disappears, and the growth in pore size occurs due to the coalescence of the mesopores with a significant decrease of the total porosity. During this process, the silica network densification is accompanied by a lowering of the specific surface area.