The structure of amino-functionalized polyhedral oligomeric silsesquioxanes (POSS) studied by molecular dynamics simulations

Abstract

Molecular models of three amino-functionalized cubic polyhedral oligomeric silsesquioxanes (POSS) have been studied at 293K using classical molecular dynamics (MD) simulations. These hybrid organic/inorganic systems were all based on the T8R8 form, but they differed by the end-groups of their organic pendant chains, i.e. either R=–(CH2)3–NH–CO–C6H5OH, R=–(CH2)3–NH–CO–C6H6 or R=–(CH2)3–NH–CO–C6H11. Model densities, volumes, cohesive energies, void spaces, X-ray diffractograms and intermolecular as well as intramolecular interactions were analysed. The differences in bulk and structural properties between both aromatic-based organic chains are fairly small, although the added hydroxyl groups bring additional cohesion by modifying the amide⋯amide H-bonding network. On the other hand, replacing an aromatic by a cyclohexyl ring leads to molecular disorder, larger POSS⋯POSS separations and a lower density. The pendant arms of the three POSS under study were found to be intertwined with a preference for intermolecular interactions because of steric considerations. Such an interdigitation also allowed for the parallel stacking of the ring end-groups at short distances, thus bringing extra cohesion to the systems. However, there was no evidence of long-range crystalline order.