Flow-induced Structural Evolution in Graphene Oxide-grafted Carbon Nanotube/Polyamide 6 Composites with Enhanced Barrier Performance, Mechanical Properties, and Thermal Conductivity

Hot pressing is widely used to tailor the structures and properties of polymer composites. However, how it drives structural evolution and thereby affects barrier, mechanical, and thermal properties remains insufficiently understood. In this work, graphene oxide-grafted carbon nanotube/polyamide 6 (GO- g -CNT/PA6) composites were subjected to hot pressing at different radial expansion ratios ( ε =0, 2, and 4) to investigate the flow-induced structural evolution and the resulting property changes. Scanning electron microscopy and transmission electron microscopy observations revealed that hot pressing promoted the rearrangement and preferential alignment of the hybrid nanofillers, while two-dimensional wide-angle X-ray diffraction confirmed the enhanced crystalline orientation after hot pressing. Differential scanning calorimetry showed increased crystallinity in both the neat PA6 and GO- g -CNT/PA6 composites. Representative positron annihilation lifetime spectroscopy analysis further indicated a reduction in the fractional free volume, whereas the characteristic size of individual free-volume holes remained nearly unchanged. The results indicate that the optimal hot-pressing condition is property-dependent. A moderate deformation at ε =2 is more favorable for improving the hydrogen barrier and tensile properties, whereas a stronger deformation at ε =4 is more effective in promoting in-plane heat transport. These findings establish a correlation between the processing, structural evolution, and multifunctional properties of GO- g -CNT/PA6 composites, providing guidance for the design of PA6-based liner materials for Type IV hydrogen storage vessels.