Molecular Simulation Of Small Molecule Gases Diffusion In Common Packaging Polymers

In this work, molecular dynamics simulations and Grand canonical Monte Carlo were used to study the diffusions of oxygen and water in several common packaging polymer membranes by Materials Studio software.COMPASS force field was used in the whole simulation procedure. The initial model was constructed with three-dimensional periodic boundaries, and just the periodic boundary which had the lowest energies was selected. Then the model need to carry on energy optimization and simulated annealing procedure until the polymer model's minimum total energy retained constant. To simulate the diffusion coefficient of gas in polymer membranes, the polymer unit cells needed to mix into a certain amount of gas moleculars. Thus Grand canonical Monte Carlo (GCMC) method was used to study the number of oxygen adsorbed in the polymer unit cell by the means of the Sorption module in Materials Studio software. After absorbed, the mixed system was equilibrated again by repeated NVT and NPT molecular dynamics simulations. Finally, NVE molecular dynamics simulation was used to get the diffusion procedure and trajectory of gas in polymer membranes. Diffusion coefficient of gas was calculated from the mean-square displacement (MSD) of gas molecular by means of the Einstein equation. The results are as follows:(1) It is workable that calculating the diffusion coefficient of small molecules of gas in the packaging polymer membranes. Diffusion coefficients of oxygen and water in PE, PP, PS and PVC increases as the temperature increases and decreases as the degree of polymerization increases.(2) Through the study of the influences of pressure on diffusion coefficient of gas in polymers, it shows that the influences of pressure are different in different polymer membranes.(3) Oxygen diffusion coefficient in polyethylene is significantly affected by humidity. The diffusion coefficient of oxygen when relative humidity was equal to 100% is much greater than the one when relative humidity was equal to 0%.(4) During the research of the diffusion of the mixed gases of oxygen and nitrogen in polyethylene, it is found that nitrogen makes the diffusion coefficient of oxygen in PE increasing greatly.(5) Compared diffusion coefficients in four polymers, it is found PVC has the best barrier property to oxygen and water, PS followed, and PE has the worst barrier property to oxygen and water.In general,calculating the diffusion coefficient of small molecules of gases in the packaging polymer membranes is workable, and this paper provides some guidance and basis for developments in packaging materials.