Studies On Diffusion Coefficients Of Migrants In Plastic Packaging By Molecular Dynamics

Migration from plastic food packaging materials into food initiates widespread concern of the consumers on food safety. Migration process can be divided to three different but interrelated stages:diffuse through polymers, dissolve at the interface between polymer and food, and disperse into food bulk, respectively corresponding to "diffusion, dissolution and dispersion" Given some reasonable assumptions on such above three stages, the migration mathematics models based on Fick's second law can be obtained. There has one key model parameter for most of migration mathematics models, diffusion coefficient, which decides the migration dynamic process. The most important question for the migration mathematics models is to obtain the diffusion coefficients of migrant in polymer during the migration process.The paper first analyzes in detail the influence factors of diffusion coefficients, such as the properties of polymer, the properties of migrants, temperature, interaction between polymer and food, et al. Then, the paper take the polyolefins as examples to analyze the difference of diffusion coefficients between experimental values and the values predicted by four kinds of diffusion estimation models, and take non-polyolefins as examples to discuss the applicability of Brandsch model which is one of diffusion estimation models. Besides, the paper reviews the migration test method of diffusion coefficients, such as contact with solvent and without solvent. The principle design of method without solvent is consistent with the idea of the method to obtain diffusion coefficients by molecular dynamics simulations.It is very difficult for experimental test methods to obtain diffusion coefficients while it is hard to determine the model parameters for diffusion models. Not only the experimental test methods but also the diffusion estimation models are all lack of the intrinsic acknowledge about the microcosmic structures and diffusion mechanisms. In recent years molecular dynamics simulation based on Newton's classical mechanics have been the tools to study the polymer material structures as well as its diffusion mechanisms. The paper reviews the basic theories of molecular dynamics methods, such as the development history, principle, technical details and molecular force-fileds. The application of molecular dynamics simulations on small molecules diffusion in polymer are summarized from three respects-diffusion mechanisms, influences factors and quantitative description.Molecular dynamics simulations are used to study diffusion of oligmers in amorphous poly(methyl methacrylate). A number of amorphous models with periodic boundary conditions are generated by changing the chains length of PMMA, polymerization degree and contents of oligomers. The results of molecular dynamics simulations show that the simulations time to reach dynamics equilibrium increases exponentially when the number of molecules in the systems increases. The diffusion coefficients reduce with the increase of polymer chains. The contents of oligomer do not significantly affect the diffusion coefficients and the weak discrepancy may be result from the system errors. The diffusion coefficients reduce rapidly with the increase of polymerization degree. Taking limonene as the model migrant, the diffusion properties of three types of polypropylene materials are studied by using molecular dynamics method. The diffusion coefficients of limonene through PP-H, PP-B and PP-R are evaluated from the limiting slope of the mean square displacements as a function of time. The diffusion coefficient of Limonene in PP-H at 313K is 3.39×10-9cm2/s, which is consist with the values of the literature reports 2.1×10-9cm2/s. The free volume in polymer matrix plays vital roles in diffusion of low molecular-weight through polymer materials. A Connolly surface is calculated to define the free volume. The results suggest that the accessible free volume reduces with the probe radius increase. Another phenomenon can be observed that not only the amount of accessible free volume in PP-R cell model is larger than that of PP-H and PP-B, but also the free volume combines a piece of area. That can also explain why the diffusion coefficients of limonene in PP-R are higher than that of PP-H and PP-B. The movement of limonene molecules through PP-H, PP-B and PP-R cell model at different simulation time suggest that hopping diffusion mechanism is not suitable to explain the diffusion of limonene in polypropylene. For a long time limonene molecules slowly squirm in the polymer simulation cell rather than jump. The movement trajectories of limonene molecules in PP-R have very wide range and reveal that the movements of limonene in PP-R are more active. That is consistent with the calculated diffusion coefficients.The diffusion coefficients of 13 kinds of small molecules with molecular weights ranging from 32 to 339 g/mol in amorphous PET are calculated based on molecular dynamics simulation. By comparison of diffusion coefficients simulated by MD simulation techniques, predicted by Brandsch model and by experiments, the accuracy of Brandsch model and MD simulation techniques for estimation of diffusion coefficients of migrants in PET is evaluated. The activation energy is calculated by Arrhenius equation which characterizes the relationship between diffusion coefficient and temperature. It is shown that the MD simulation yields acceptable activation energy. Connolly surface method is used to calculate the amount, fraction and morphology of free volume. The results show that the fraction of free volume reduces rapidly with the increase of the probe radius. The temperature leads to the increase of mobility of polymer chains, furthermore induces the fast change of channel between free volume cavities in polymer matrix. Some small free volume cavities conjoin together and form the larger cavities which exactly accommodate migrant molecules. Thus, it facilitates the diffusion of migrant molecules in polymer matrix. The diffusion trajectories suggest that the molecules in first class move actively, but the molecules in class third class move limitedly and the movement mobility of molecules in second class is between that of first class and third class.The study suggests that MD simulation may be a useful approach to calculate the diffusion coefficients and understand the polymer structure and diffusion mechanism.