Atomistic Simulation of Poly(dimethylsiloxane): Force Field Development, Structure, and Thermodynamic Properties of Polymer Melt and Solubility of n-Alkanes, n-Perfluoroalkanes, and Noble and Light Gases

Abstract

Poly(dimethylsiloxane) (PDMS) is a widely used polymer for a number of industrial applications. In order that PDMS is selected for a specific application, accurate knowledge of its physical properties is necessary. Physical properties can be either measured or calculated based on reliable suitable methods. Molecular simulation using realistic models is a powerful tool for the elucidation of microscopic structure of polymers and the subsequent estimation of macroscopic physical properties. In this work, a force field is developed for the prediction of thermodynamic and structure properties of PDMS melts. Force field development was based on existing force fields for PDMS together with fitting to experimental thermodynamic data at ambient pressure. Extensive NPT molecular dynamics (MD) simulations were performed at different temperature and pressure values. In all cases, good agreement was obtained between literature experimental data and model predictions for the melt density. Calculations are reported also for the solubility parameter of the polymer melt at different temperatures. Furthermore, radial distribution functions for the intra- and intermolecular interactions are presented and shown to be in good agreement with previous literature work. The new force field is used subsequently for the calculation of solubility of 17 different compounds in PDMS using the Widom test particle insertion method. The solubility of n-alkanes from methane to n-hexane at 300 and 450 K and different pressures was calculated. In addition, solubility calculations for n-perfluoroalkanes at 300 and 450 K and for noble and light gases at 300, 375, and 450 K and ambient pressure were performed. Model predictions are in very good agreement with experimental data, in all cases. The infinite dilution solubility coefficient is shown to increase with temperature for very light gases and decrease for the heavier ones.