A Quantum Chemistry Based Force Field for Poly(dimethylsiloxane)

A classical force field for poly(dimethylsiloxane) (PDMS) and its oligomers has been derived on the basis of intermolecular binding energies, molecular geometries, molecular electrostatic potentials, and conformational energies obtained from quantum chemistry calculations on model compounds. The force field accurately reproduces the molecular properties of the model compounds obtained from quantum chemistry, including the Si−O−Si bond linearization energy. Molecular dynamics simulations performed on PDMS of various molecular weights between 310 and 1571 using the quantum chemistry based force field yielded good agreement with experiment for their densities, enthalpies of vaporization, and X-ray structure factors. The characteristic ratio of PDMS was also found to be in good agreement with experimental values. Molecular dynamics as represented by the melt self-diffusion coefficient and viscosity at 298 K as a function of molecular weight were also found to be in reasonable agreement with experimental values. Finally, the intermediate incoherent structure factor of PDMS from MD simulations was in excellent agreement with the quasi-elastic neutron scattering results for large Q values and in satisfactory agreement for small Q values.