Molecular Dynamics Simulation On Diffusion Properties Of Polymer-inorganic Hybrid Membranes

Understanding the effects of incorporated inorganic particles on diffusion properties of polymer-inorganic hybrid membranes at molecular level is crucial to design hybrid membranes with tailored structure and desired properties. In this study, molecular dynamics simulation has been employed to investigate the effects of inorganic particles on the diffusion behavior of benzene and cyclohexane in poly(vinyl alcohol)-based hybrid membranes. The detailed atomistic structures of two classes of polymer-inorganic hybrid membranes were built, which included the class I hybrid membranes such as PVA-graphite and PVA-silica hybrid membranes, and the"class II"hybrid membrane, such as PVA-silica hybrid membranes.Firstly, the polymer-inorganic interaction energies were investigated. Next, the interchains distance of the polymer was calculated by x-ray scatter, and found that the interchains distance was influenced by incorporated inorganic particles. Polymer chain mobility was analyzed by mean-square displacement (MSD) and glass transition temperature (Tg), and found that incorporation of inorganic particles into PVA destroyed the crystalline in the polymer and increased the polymer chain mobility. Free volume characteristics of membranes were investigated by Connolly surface, and found that free volumes in hybrid membranes were larger than those in PVA control membranes. The free volume increase in hybrid membranes arise from two aspects: (1) the polymer-inorganic interface where a gap may appear because of the difference properties of polymer and inorganic particles. (2) the loose packing of the polymer chains in the polymer matrix because of incorporated inorganic particles. The simulation results of FFV showed the same tendency with the PALS measurement. Finally, the diffusion coefficients of benzene and cyclohexane in the membranes were calculated by Einstein relation, and found that the diffusion coefficients in hybrid membranes were larger than those in the control membrane.The differences of diffusion properties between class I and class II hybrid membranes were also investigated by molecular dynamics simulation. The interaction between organic phase and inorganic phase in class II hybrid membranes was stronger than that in class I hybrid membranes because of the covalent bonds between organic and inorganic particles. The interachains distance, chains mobility and free volume in class II hybrid membranes were both enhanced much higher than those of class I hybrid membranes. Accordingly, the diffusion coefficients of benzene and cyclohexane in class II hybrid membranes were increased more remarkably.Furthermore, it was found that the diffusion coefficients of benzene and cyclohexane in the membranes exhibited exponential relation with the inverse...