Multi-scale modeling of gas transport properties in semi-crystalline polymers

The research presented here aimed at modeling gas transport properties in semi-crystalline polymers with particular attention to hydrogen gas. The first part of the work was dedicated to developing a multi-scale modeling approach for evaluating gas sorption in semi-crystalline High-Density Polyethylene which couples Lattice-Fluid theory with Molecular Dynamics simulations. The model belongs to the so-called 'constraint pressure models' which implicitly describe the perturbation induced by the impermeable crystalline domains on the amorphous matrix using a fictitious constraint pressure parameter. The second and main part of the work focused on the molecular reproduction and simulation of the explicit semi-crystalline material accounting for different intercalations between the two phases (surface fraction of tie-chains). Quantitative information on hydrogen sorption and diffusion coefficients were derived using Widom test particle insertion method and the Mean Square Displacement analysis coupled with a macroscopic model for the estimation of the crystallites' induced tortuosity, respectively. The permeability coefficient was finally estimated according to the solution-diffusion model. The results were in agreement with the experimental results obtained in an experimental campaign that was conducted in parallel and other literature references.