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

Molecular models of three amino-functionalized cubic polyhedral oligomeric silsesquioxanes (POSS) have been studied at 293 K 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.

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► We modelled three amino-functionalized polyhedral oligomeric silsesquioxanes (POSS).
► Their bulk amorphous structure was studied using molecular dynamics (MD) simulations.
► The organic pendant arms are intertwined, the inorganic siloxane cages are dispersed.
► An hydroxyl group on a phenyl ring increases cohesion through additional H-bonds.
► Replacing an aromatic by a cyclohexyl ring leads to molecular disorder.