Interface between end-functionalized PEO oligomers and a silica nanoparticle studied by molecular dynamics simulations

Fully atomistic molecular dynamics (MD) simulations have been carried out on a series of bulk melt pure PEO oligomers and PEO oligomer-silica systems, which differed by their macromolecular end groups. A realistic hybrid model of a silica nanoparticle combining an ionic core as well as the fine-tuning of the surface thickness and number of OH groups per unit surface area was used. The PEO oligomers were decorrelated in all systems under study in order to prevent any artifacts related to the preparation procedure. Significant changes were found to occur in the immediate vicinity of the interface with flattened PEO backbones arranged in densily packed shells and stabilized by the added PEO-silica interactions. Their conformations were also more coiled in order to better adapt to the surface structure. While methyl end groups did not show special characteristics other than their steric effect, hydroxyl end groups had a much higher affinity for the surface and tended to position themselves perpendicular to the surface, thus forming dynamic hydrogen-bonding complexes between the hydroxyl oxygens and the silanol hydrogens. The range of influence of the nanoparticle was evident for structural properties only up to two or three molecular layers, 10-15 Angstrom, but was approximately twice that for dynamical ones.