Hydrogen transport in semicrystalline polymers: experimental characterization and modelling

Semicrystalline polymers are nowadays substituting steels as liner materials for compressed gas pipelines and storage vessels, due to low gas permeability, low weight, high durability and good mechanical properties. This work is aimed at providing new experimental data on hydrogen permeation, diffusion, and sorption in polyethylenes and polyamides pointing out, where present, the disparity of the results coming from different test methods and different samples of the same material. All the results obtained are elaborated in order to give a comprehensive description of hydrogen permeation in semicrystalline polymers, highlighting the difference among the different polymers tested, as well as the effect of crystallinity, pressure and temperature. Moreover, the results are analyzed pointing out the differences on the transport coefficients measured, related to different experimental setups and different samples of the same material. Considering the variation of gas transport coefficient with crystallinity, this effect is investigated more in detail, through a modeling analysis based on the tortuosity. In this work, the microstructure of semicrystalline polymers is reproduced through ordered and disordered arrays made by impermeable crystallites, with different shape, disposition, aspect ratio and orientation. The preliminary analysis, focused on systems made by dispersed crystallites with low aspect ratio and crystallinity, is then extended to highly crystalline materials. The results obtained are then compared with the outcomes of existing analytical models mainly derived from effective medium approaches, to study their applicability also to these systems and possibly suggesting modifications to improve their description of tortuosity in semicrystalline materials.