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Atomistic Simulation of Poly(dimethylsiloxane): Structure, Thermodynamic and Diffusion Properties to Gases and N-Alkanes

Ioannis G. Economou and Zoi A. Makrodimitri. Institute of Physical Chemistry, National Center for Scientific Research “Demokritos”, GR-15310, Aghia Paraskevi, Greece

Poly(dimethyl siloxane) (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 conditions. 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. The new force field is used subsequently for the calculation of solubility of seventeen (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 K and 450 K and for noble and light gases at 300 K, 375 K 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.

Finally, the atomistic force field is used to calculate the permeability properties of the polymer to light gases and to n-alkanes. The torsional potential barrier is re-estimated compared to the original force field. Chain dynamics and chain sizes are affected by this change while thermodynamic properties of the melt and of PDMS mixtures remain unaffected. The diffusion coefficients of penetrants to PDMS are calculated at different temperatures based on long molecular dynamics simulations. Subsequently, the permeability coefficients are estimated and ideal mixture selectivities are evaluated for binary hydrocarbon mixtures. In all cases, agreement with literature experimental data is ranged from good to excellent. Calculations for mixed penentrant permeation reveal that, in the presence of a second penetrant species, solubility and diffusion coefficients increase, in agreement with recent experimental evidence.