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Molecular simulation and theoretical modeling of polyhedral oligomeric silsesquioxanes

Polyhedral oligomeric silsesquioxane (POSS) molecules are unique nanometre-size inorganic/organic hybrid structures based on a (SiO1.5)8 core. Depending on the functionalization of the POSS cages, the resulting systems can be solid or liquid, or, upon crosslinking, turned into a network. While much is known experimentally about the chemical synthesis of POSS systems, very little theoretical understanding exists at the molecular level or beyond. Of particular interest is the study of the effect of tether groups on the thermodynamic properties and phase behaviour of POSS fluids, which is addressed in this work through both molecular simulation and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). In this application of SAFT-VR to POSS molecules the hetero-SAFT-VR approach is used. The POSS cage is represented as a single sphere and the tether groups by one or more smaller spheres, depending on chain length. Since very limited experimental data is available for the thermodynamic properties of POSS systems, molecular dynamics simulations are used to obtain pseudo-experimental data with which to validate the theoretical approach. Good agreement is obtained between the simulation data and the theoretical results and predictions are made for systems yet to be studied by simulation or experiment.