Molecular dynamics simulation of polypropylene: diffusion and sorption of H2O, H2O2, H2, O2 and determination of the glass transition temperature

Molecular dynamics (MD) simulations in the canonical (NVT) and the isothermal-isobaric (NPT) ensemble using COMPASS III molecular force fields were performed to study the penetrant diffusion of water (H2O), hydrogen peroxide (H2O2) and oxygen (O2) in isotactic polypropylene (iPP) and hydrogen (H2) in iPP and atactic polypropylene (aPP) for time intervals up to 11 ns and in the case of H2O2 up to 22 ns. We found robust cluster formation in the case of H2O and H2O2. Further, the diffusion coefficients for all these systems were estimated by mean-square displacement analysis. Our results are consistent with previously published experimental and computational data except for the diffusion of H2 in polypropylene where our results are one and two orders of magnitude higher, respectively. Grand Canonical Monte Carlo (GCMC) simulations were used to determine the sorption loading and saturation concentration of H2O, O2 and H2 in iPP, where we find good agreement for H2O with experimental results. By means of MD simulation the glass transition temperature (Tg) of iPP was estimated to 273.66 ± 4.21 K which is consistent with previously published experimental results.