Molecular Simulation Of Diffusion And Solution Of Organic Molecules In Polyethylene Membranes

With the rapid development of theory disciplines, such as quantum mechanicsand molecular mechanics, the molecular simulation technique has been widely used inmany fields of scientific research. Individuals take more and more attention tomolecular simulation which plays an increasingly important role. Polyethylene (PE) ispolymers which formed by the polymerization of vinyl monomers. It is the mostwidely used species in synthetic resin, and can be widely used in food packaging,industrial packaging, chemical packaging, agricultural film, construction film and soon. The process of diffusion and solution of organic molecules in polyethylenemembranes is studied by molecular simulation，thus the molecular structure andbehavior can be simulated at the molecular level， and the mechanism ofsolution-diffusion can be understood from a microscopic point.In this paper, the processes of diffusion and solution of benzene, toluene andethylbenzene in polyethylene membranes are simulated by Materials Studio softwarethrough molecular simulation method，that can obtain the diffusion coefficient andsolubility coefficient of the organic molecules in the polyethylene membrane，thechanges are studied about the diffusion coefficient and solubility coefficient withtemperature and the size of the organic molecular weight，which can get thesolution-diffusion mechanism.1. The study of simulation method(1) Use molecular dynamics simulation to study the diffusion coefficients oforganic molecules and cluster analysis method to analyze the curve of the meansquare displacement. Use Monte Carlo simulation to simulate the solubilitycoefficients of organic molecules and the limit method to analyze adsorptionisotherm.(2) Analyze the degree of polymerization and the number of molecules how toimpact the diffusion coefficients. Comprehensive considered the changes of diffusioncoefficients, software and computer, ultimately determines the degree ofpolymerization of polyethylene is360and the number of organic molecules is10.2. The diffusion process simulation of organic molecules in the polyethylene filmThe diffusion coefficients of benzene, toluene and ethylbenzene in polyethylenefilm are calculated and the diffusion processes of the organic molecules in the polyethylene film are described by molecular dynamics simulation. The results showthat the diffusion coefficients of the organic molecules in polyethylene film decreasewith increasing molecular weight at the same temperature, while they graduallyincrease with temperature for the same kinds of organic molecule. The displacementsof organic molecules and the paths of molecular motion in the membrane are obtained,and the track results show that organic molecules in the polyethylene membrane arenot a continuous diffusion but the jump diffusion.3. The solution process simulation of organic molecules in the polyethylene filmThe solubility coefficients of benzene, toluene and ethylbenzene in polyethylenefilm are calculated by Monte Carlo simulation, thus a reasonable model is established,and the solution processes of organic molecules in the polyethylene membrane aredescribed.(1) Obtain the curve of van der Waals interaction energy, the distribution curve ofnon-bond interaction energy, the map of molecular density distribution and the map ofenergy distribution. From the curve of van der Waals interaction energy, it can judgethe simulation system has reached equilibrium within the set number of steps, so it isreliable for the simulation results. From the distribution curve of non-bond interactionenergy, it can be seen that the energy states are more or less with benzene, toluene,and ethylbenzene in the polyethylene film. From the map of density distribution, itcan be seen directly the concentrate adsorbed regions of the three kinds of moleculesin polyethylene membrane. From the map of energy distribution, it can be seendirectly the easiest adsorbed place of the three kinds of molecules in polyethylenemembrane. These micro-level understanding above is complement with physicsexperiments.(2) The simulation results show that the solubility coefficients decrease with theincreasing molecular weight at the same temperature and they also decrease withincreasing temperature for the same kinds of organic molecule.(3) Calculate the diffusion coefficient and solubility coefficient of benzene,toluene and ethylbenzene to get the permeability coefficient, and the results show that:the permeability coefficients decrease with the increasing molecular weight at thesame temperature and they also decrease with increasing temperature for the samekinds of organic molecule. It is consistent with the trends of the solubility coefficient,which indicate that for the same process of membrane penetration, the solutionprocess is at a decisive place, and the permeability coefficient depends on the solubility coefficient of organic molecules.