(675c) Optimization of a Reactive Force Field for Modeling the Polycondensation of Siloxanes

We investigate the structures of silicates obtained by successive hydrolysis and condensation reactions of siloxanes using Molecular Dynamics simulations. To characterize the system accurately, the ReaxFF force field previously used to describe the thermal decomposition of polysiloxanes was extended to model the bulk condensation of siloxanes. The parameter set was refined by using multiple algorithms. Both ab-initio calculations and bulk structure information are used to develop the model. Density functional theory calculations compute the energies of several locations along the reaction coordinate for hydrolysis and condensation reactions conditions. Geometric data such as bond lengths and angles, as well as the energies of their distortion are used. In order to accurately model the electrostatics of the system, point charges are derived in DFT using an electrostatic potential method (CHELPG). We also use experimental data for bulk phases, such as the enthalpy of vaporization, liquid density, and crystal lattice parameters, to constrain the long range forces during the optimization. A reasonable fit is found to available data, and a useful tool for modeling the gelation of silanes has been developed.