Hi all,

I want to calculate the diffusion coefficient of gas into polymer. I followed the tutorial procedure for calculating the diffusivity of a gas in a polymer.

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Equilibration Phase

At first, I constructed an amorphous cell containing 3 chains of polymers (each chains consists of 30 repeat unit) and 3 molecules of methane, then I minimized the energy of system. Next, Annealing was performed temperature ranges from 300 K to 500 K with time step 20 K, total simulation time was set to 100ps. After that, for getting the experimental density, 100ps NPT was performed ( Ewald and atom based summation methods were used for electrostatic and van der Waals interaction, respectively). Density approached the experimental one.

Production Phase

500ps NVT was performed on equilibrated cell. I got the graphs of MSD vs. time for each molecules, which are attached to post.

The graphs of MSD, which I see in papers, are straight line in middle with a little fluctuation at first and end, but I can’t get this trend in my graphs. I don’t know what is the reason of this type of  trend in my MSD graphs, whether my procedure isn't correct or time of production phase is short.

Any help will be appreciated.

Regard,

nima