Ab initio study of the binding strength of POSS-cation complexes

Polyhedral oligomeric silsesquioxane (POSS) is a cage-shaped molecule comprised of alternating silicon and oxygen atoms that have found increasing use as filler in polymer nanocomposites. The presence of four oxygen atoms on each face of the POSS cage suggests that the molecule forms a stable complex with a cationic atom or molecule. Although experimental evidence exists for gas-phase binding, the lack of an estimate of this binding strength makes it difficult to predict if complex formation is feasible in condensed systems. This paper presents an ab initio DFT study of the binding strength of POSS-cation complexes based on cation type, POSS cage functionalization, and the presence of a counterion. The binding strength of an unfunctionalized (i.e. with H-terminated corners) POSS molecule with a Li+ ion is found to be similar to80 kT at room temperature in vacuo. In a poly(styrene) solvent, the polarity of the medium is estimated to affect little this binding energy. Cyclohexyl ligand substitution is found to increase the binding strength by an additional 30 kT. In the presence of a tightly bound counterion (Cl-), the binding energy drops to the still substantial value of similar to20 kT. POSS is found also to bind cationic surfactants (through study of the model +NH3CH3) at similar to25 kT, suggesting a possible new route to POSS modification to hinder agglomeration. This study supports the conclusion that POSS-cation complex formation is feasible in the presence of polymer. (C) 2004 Published by Elsevier Ltd.