We have employed molecular simulation to study the permeation of two different gases (CH4 and CO2) in polyethylene. The simulations have been performed at temperatures below the polymer melting point. Although under such conditions, polyethylene is in a semicrystalline state, we have used simulation boxes containing only a purely amorphous material. We showed in previous works [Memari P., Lachet V., Rousseau B. (2010) Polymer 51, 4978] that the effects of the complex morphology of semicrystalline materials on solubility can be implicitly taken into account by an ad-hoc constraint exerted on the amorphous phase. Here, it has been shown that our method can be applied not only for the calculation of equilibrium properties but also for transport properties like diffusion coefficients. In addition, the ad-hoc constraint has been theoretically related to the fraction of elastically effective chains in the material by making use of Michaels and Hausslein elastic model [Michaels A.S., Hausslein R.W. (1965) J. Polymer Sci.: Part C 10, 61]. We observe that the transport properties in amorphous regions are strongly governed by this fraction of elastically effective chains
