A molecular dynamics simulation and quantum chemistry study of poly(dimethylsiloxane)-silica nanoparticle interactions

We have conducted atomistic molecular dynamics (MD) simulations of poly(dimethylsiloxane) (PDMS) melts in contact with bare silica and modified (with hydroxyl and trimethylsiloxy groups) silica surfaces using a quantum chemistry-based force field. MD simulations accurately predicted changes in PDMS dynamics with addition of SiO ₂ observed in quasielastic neutron scattering experiments. In MD simulations, the density of PDMS near the bare silica surface was found to be much greater and the dynamics of interfacial PDMS much slower than that observed for unperturbed PDMS melts due to strong Van der Waals dispersion forces between PDMS and the silica surface. The presence of hydroxyl and trimethylsiloxy groups on the silica surface resulted in a decrease in the density of interfacial PDMS and a speedup in polymer dynamics relative to those observed for PDMS near the bare silica surface due to increased separation between PDMS and the silica surface and a corresponding decrease in the strength of the dispersion attraction between the polymer and the surface. Despite the presence of strong hydrogen bonding interactions between small molecule models for hydroxylated silica and PDMS observed in our quantum chemistry studies, no significant hydrogen bonding was observed in the MD simulations of PDMS in contact with hydroxylated silica surfaces. Nevertheless, interactions between hydroxyl groups and PDMS oxygen atoms were found to play a role in determining structure and dynamics of interfacial PDMS for surfaces with low to moderate degrees of hydroxylation.