Molecular dynamics simulation of water diffusion in atactic and amorphous isotactic polypropylene

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Abstract
Molecular dynamics (MD) simulation of the diffusion of water in both atactic polypropylene (aPP) and amorphous isotactic polypropylene (iPP) models has been performed. Constant particle number, constant pressure, and constant temperature (NPT) MD was employed for both systems consisting of three polypropylene chains of 500 monomer units with ten water molecules. The density observed for the iPP model after full equilibration was slightly greater than that for the aPP model, but predicted x-ray scattering curves indicated that both systems were completely amorphous structures. In the aPP model, time duration up to 11 ns showed that the water molecules dynamically came together forming a water cluster with various sizes and again departed to a smaller size cluster or a single water sometimes. In contrast, the clustering was remarkable in the iPP model and maintained its aggregated structure over several nanoseconds. The self-diffusion coefficient, Dself, estimated from the mean-square displacement curve for the aPP model and the iPP one up to 11 ns duration showed 2.2×10−6 cm2/s and 3.1×10−7 cm2/s, respectively. The latter value corresponded well to the range of experimental diffusion coefficients.