Modeling polyethylene with the bond fluctuation model

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Abstract
This work presents an application of recently developed ideas about how to map real polymer systems onto abstract models. In our case the abstract model is the bond fluctuation model with a Monte Carlo dynamics. We study the temperature dependence of chain dimensions and of the self-diffusion behavior in the melt from high temperatures down to 200 K. The chain conformations are equilibrated over the whole temperature range, which is possible for the abstract type of model we use. The size of the chains as measured by the characteristic ratio is within 25% of experimental data. The simulated values of the chain self-diffusion coefficient have to be matched to experimental information at one temperature to obtain a scaling for the Monte Carlo time step. The melt viscosity from the simulations as determined by applying the Rouse model is then in good qualitative agreement with experimental data over the experimentally available temperature range. The activation energy as extracted from an Arrhenius fit is different because the simulations are done at constant volume. Both experimental data and the simulation, which covers a far greater temperature range, show Arrhenius behavior for the viscosity and no indication of a finite nonzero Vogel–Fulcher temperature. For one temperature (T=509 K) various time-dependent mean-square displacements are available from atomistic molecular-dynamics simulations, and are shown to be in excellent agreement with the results from the coarse-grained model.