Covalently bonded graphene oxide–carbon nanotube hybrid nanofillers for achieving high-performance polyamide 6 composites with superior mechanical properties and thermal conductivity

The rise of hydrogen energy places stringent demands on the mechanical strength and thermal conductivity of polyamide 6 (PA6) liners used in hydrogen storage tanks. However, the presence of weak interfaces in PA6 composites significantly hinders the efficient transfer of the intrinsic mechanical and thermal conductivity properties of the reinforcing phases. In this work, hydroxyl-functionalized carbon nanotubes (CNT-OH) were covalently grafted onto graphene oxide (GO) to construct a hybrid nanofiller, which was subsequently surface-functionalized with hexamethylene diisocyanate (HDI), yielding a reactive nanofiller (fGO + CNT-OH + HDI) rich in isocyanate groups. This functionalized nanofiller served as an activator during the in situ anionic ring-opening polymerization of ε-caprolactam, enabling the fabrication of PA6 composites with strengthened nanofiller/nanofiller and nanofiller/matrix interfacial interactions. Owing to the synergistic reinforcement from the hybrid nanofiller and its uniform dispersion driven by in situ polymerization, the resulting composite containing only 0.2 wt% fGO + CNT-OH + HDI exhibited outstanding mechanical performance, with a 36.3% increase in tensile strength and an exceptional elongation at break of 130.86%. In addition, the thermal conductivity was improved by 46.3%. This interfacial engineering strategy provides a promising pathway toward the development of high-performance liner materials for reliable and safe hydrogen energy applications.