Molecular Dynamics Simulation On Diffusion Properties Of Chitosan And Chitosan Composite Membranes

The study of small molecules diffusion in polymers and polymer composite materials arethe hotspot of research. Understanding the effects of polymer structures and incorporatedmediums on diffusion properties of polymer membranes at molecular lever, is crucial todesign membrane materials with specific function and desired properties. In this study,molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations wereadopted to study the diffusion of small molecules in chitosan membranes and chitosancomposite membranes, and analyzed the effects of deacetylation degree, polymerizationdegree and water content on diffusion properties of chitosan membranes. The effects of silicaand gelatin on oxygen diffusion in chitosan composite membranes were also analyzed.Firstly, the influences of deacetylation degree and polymerization degree on diffusionproperties of oxygen in chitosan mermbranes were investigated. The diffusion coefficients ofoxygen in chitosan membranes were calculated by Einstein relation. It was found that thediffusion coefficients decrease with the increase degree of polymerization (DP), and decreasefirst and then increase with the increase degree of deacetylation (DD). The results showed thesame tendency with fractional free volumes (FFV). The simulation results of oxygenpermeability showed the same tendency with experiment. Then the diffusivity of oxygen inchitosan membranes with different water contents were studied, and found that the chainmobility, FFV and diffusion coefficients were increase with the increase of water content.Finally, the diffusion coefficients of oxygen and carbon dioxide in chitosan membrane werestudied, and found that diffusion coefficients degrease with the increase molecule size andsolubility coefficient. The diffusion coefficient of oxygen was larger than carbon dioxide.Chitosan membrane has selective permeability between oxygen and carbon dioxide, and thecalculated separating factor was similar with experiment value.The effects of silica and gelatin on oxygen diffusion in chitosan composite membraneswere also studied. It was found that the interaction energy between chitosan and silicaincrease with the decrease of the nanoparticle size. The diffusion coefficients in hybridmembrane were smaller than that in the control chitosan membrane, and it decrease with thedecrease of the nanoparticle size. The interaction energy between chitosan and gelatin increase with the increase of the gelatin content. The chain mobility of chitosan, the FFV ofmembranes and the diffusion coefficients of oxygen were also increase with the increase ofthe gelatin content.Furthermore, the relationship between diffusion properties and structural parameters wasinvestigated. It was found that the diffusion coefficients of oxygen in membranes exhibitedexponential relation with the inverse of FFV, it agreed well with the free volume theory. Thediffusion properties can be predicted by the free volume properties. The change disciplines ofsimulation results in this study were similar with the experiments. Therefore, molecularsimulation can be used as a director on the selection of membrane materials.