Hydrogen permeation mechanism in functionalized graphene oxide/polyamide 6 nanocomposites

Understanding hydrogen transport in polymer nanocomposites requires correlating permeation parameters with structural factors. Here, functionalized graphene oxide (fGO) was incorporated into polyamide 6 (PA6) through in situ polymerization, and an integrated analysis of permeation behavior and structural factors was used to identify the dominant barrier mechanism in fGO/PA6 nanocomposites. Scanning electron microscopy and Raman mapping revealed relatively uniform fGO dispersion at low loadings, whereas aggregation occurred at 1.0 wt%. The hydrogen permeability coefficient decreased from 3.55 × 10 −14  cm 3  cm cm −2 ·s −1  Pa −1 for neat PA6 to 1.89 × 10 −14  cm 3  cm cm −2 ·s −1  Pa −1 at 0.5 wt% fGO, corresponding to a reduction of 46.8%. Analysis of diffusivity and apparent solubility showed that the barrier enhancement was diffusion dominated, despite increased apparent solubility. Temperature-dependent permeation measurements from 23 to 85 °C showed that hydrogen permeability increased with increasing temperature, while 0.5 wt% fGO/PA6 nanocomposites retained lower permeability than neat PA6 throughout the tested temperature range. Nielsen model fitting, dynamic mechanical analysis, and positron annihilation lifetime spectroscopy indicated that diffusion suppression arose from the combined effects of a tortuous diffusion pathway, restricted segmental motion, and reduced free volume fraction, as reflected by weakened β relaxation, an α relaxation shift from 33.4 to 36.3 °C, and a free volume fraction decrease from 1.23% to 1.13%. Differential scanning calorimetry and X-ray diffraction revealed slight crystallinity changes and essentially unchanged crystal structure, indicating that crystalline variations were not the primary origin of barrier enhancement. These results provide a comparative structure-transport analysis for diffusion-dominated hydrogen transport in fGO/PA6 nanocomposites.