Effect of polymer molecular weight and deposition temperature on the properties of silica aerogel/hydroxy-terminated poly(dimethylsiloxane) nanocomposites prepared by reactive supercritical deposition

Abstract

Monolithic nanocomposites of silica aerogels with hydroxy-terminated poly(dimethylsiloxane) (PDMS(OH)) were prepared by reactive supercritical deposition technique. The depositions were performed by using PDMS(OH) having two different molecular weights (M-n = 2750 g/mol and 18,000 g/mol) and at three different temperatures (313.2 K, 323.2 K, 333.2 K) and the effects of deposition temperature and polymer molecular weight on the properties of nanocomposites were investigated. The polymer uptake of the nanocomposites was found to increase with increasing deposition temperature indicating faster reaction rates at higher temperatures. PDMS(OH) molecules with lower molecular weight were homogenously distributed throughout the cylindrical composites. On the other hand, the samples that were deposited with high molecular weight PDMS(OH) were not homogenous with a higher polymer concentration near the surface than at the center. The pore volumes and BET surface areas of the nanocomposites decreased upon deposition of the polymer. The reductions in pore volumes were higher by a factor of two than the volume of the deposited polymer indicative of blocking of pores. Moreover, the compressive modulus of the nanocomposite was found to be more than three times greater than the compressive modulus of the native silica aerogel.