Polyhedral Oligomeric Silsesquioxanes Surfactants

Abstract : There has been recent interest in using polyhedral oligomeric silsesquioxanes (POSS) as the smallest particles of silica for reinforcement of polymers. These materials are monodisperse, have tailored functionality (including a hydrophobic coating) and should thus act as model nanoparticulate filler. Feher et al have shown that sol-gel condensation into cage compounds can yield two thermodynamically stable main isolates: a fully condensed R8T8 cube and an incompletely condensed R7T4D3(OH)3 trisilanol. (T refers to a SiO3/2 and D is a SiO2/2 moiety in a silicate framework) There are large differences in the solubility of POSS in common organic solvents based on the substitution of the cages. For example, a fully condensed iBu8T8 cube is more soluble than a similar cyclohexyl Cy8T8 cube which in turn is more soluble than a cyclopentyl Cp8T8 substituted cube. Furthermore the breaking of symmetry in the incompletely condensed cubes could also be expected to enhance solubility/compatibility. The dispersion of POSS will likely determine how well the material can act as a "nanofiller." Farmer et. al. have recently performed molecular dynamics simulations which suggest that POSS molecules have no tendency for aggregation in a polymeric matrix if they are originally well dispersed.