Effective Interactions between Polyhedral Oligomeric Sislesquioxanes Dissolved in Normal Hexadecane from Molecular Simulation

Polyhedral oligomeric silsesquioxanes show promising applications as organic-inorganic nanocomposite building blocks that can be used, for example, to enhance the properties of polymeric materials. In this work radial distribution functions, potentials of mean force, and self-diffusion coefficients are obtained from molecular dynamics simulations for polyhedral oligomeric silsesquioxane (POSS) monomers dissolved in normal hexadecane in the temperature range 400-1000 K. The calculated potentials of mean force show a marked dependence on temperature and on the organic substituents tethered to the silsesquioxane cages. Selected simulation snapshots are used to elucidate the mechanism by which two POSS monomers approach each other. The results obtained are compared to calculations for the potentials of mean force between POSS monomers dissolved in poly(dimethylsiloxane) reported in earlier work [Striolo, A.; McCabe, C.; Cummings, P. T. J. Phys. Chem. B 2005, 109, 14300]. Additionally, from the simulated potentials of mean force we calculate osmotic second virial coefficients as a function of temperature. Our results indicate that in both hexadecane and poly(dimethylsiloxane) the osmotic second virial coefficient is negative for Si8O12H8 monomers at all temperatures considered, while that for Si8O12(CH3)8 monomers is negative at low temperatures and becomes positive as the temperature increases. The theta temperature (i.e., the temperature at which the osmotic second virial coefficient equals zero) is approximately 550 and 700 K for Si8O12(CH3)8 in poly(dimethylsiloxane) and in normal hexadecane, respectively.