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

Abstract - 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 (H 2 O), hydrogen peroxide (H 2 O 2 ) and oxygen (O 2 ) in isotactic polypropylene (iPP) and hydrogen (H 2 ) in iPP and atactic polypropylene (aPP) for time intervals up to 11 ns and in the case of H 2 O 2 up to 22 ns. We found robust cluster formation in the case of H 2 O and H 2 O 2 . 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 H 2 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 H 2 O, O 2 and H 2 in iPP, where we find good agreement for H 2 O with experimental results. By means of MD simulation the glass transition temperature (T g ) of iPP was estimated to 273.66 ± 4.21 K which is consistent with previously published experimental results.