Interfacial Structure and Dynamics of Siloxane Systems: PDMS−Vapor and PDMS−Water

Using a fully atomistic force field for polydimethylsiloxane developed by Smith et al. [J. Phys. Chem. B 2004, 108, 20340], we study the interfacial properties of polydimethylsiloxane (PDMS) as well as its interactions with water. We determine the surface tension of methyl- and hydroxyl-terminated PDMS chains with lengths between 20 and 100 repeat units and find good agreement between simulation results and experimental observations. The width of the polymer liquid−vapor interface is shown to depend on both molecular weight and temperature. The surface tension and contact angle are determined for the PDMS−water binary system using several different geometries and calculation methods. At 300 K, the surface tension of roughly 41 mN/m and contact angle of ≈108° for chains with 100 repeat units are in excellent agreement with experimental data. The width of the interface in both the PDMS and water layers increases with temperature, although the computed widths are significantly smaller than the liquid−vapor widths of the individual liquids. The diffusion constant measured for low concentrations of water molecules permeating through PDMS shows a wide degree of variation as a result of "caging" effects caused by local density inhomogeneities. At larger concentrations, aggregation of the water molecules leads to phase separation. Finally, the degrees of alignment of the methyl groups and siloxane backbones at the interface are found to decrease with temperature but are augmented in the presence of an interface with water.