Molecular dynamics simulation of diffusion of simple gas molecules in a short chain polymer

Abstract

Diffusion of simple gas molecules in polymers has been studied using molecular dynamics (MD) computer simulations. Two model systems were used, each composed of short chain molecules having twenty segments and small molecules interacting with the segments. One of the systems is a mimic of polymethylene with oxygen as a solute. For the other system, potential parameters are identical except that the torsional potential of the chains is set to zero to alter the chain flexibility. Self-diffusion coefficients of the small molecules and relaxation times of internal rotations of the chains were calculated at around the density of amorphous polyethylene and at temperatures of 250 to 300 K above glass transitions of the systems. Behavior of the models obtained within the time scale of usual MD simulations agrees well with experimental observations for amorphous polymers above glass transition temperatures. The free volume theory of simple liquids is found to be applicable to the diffusion as well as the relaxation. The diffusion strongly depends on amount of free volume so that thermal expansion of the system considerably affects the diffusion. Both dynamic properties show Arrhenius-type temperature dependence regardless of the chain flexibility. Although the amount of free volume is a dominant factor for the diffusion, the distribution and dynamics of free volume, which depend on the chain flexibility, clearly affect the diffusion for a fixed amount of free volume; the high chain flexibility causes large diffusion coefficients and a low apparent activation energy of diffusion.