Grafting of branched polyethyleneimine onto carbon nanotubes to efficiently enhance the lap shear strength of epoxy adhesives

In this study, the lap shear strength of the epoxy-based adhesive was significantly enhanced by incorporating branched polyethyleneimine dendrimer (PEI) functionalized carbon nanotubes (CNTs) and a phase-separated triblock copolymer. The level of PEI grafting was controlled by adjusting the CNT to PEI dispersion ratio from 1:0.1 up to 1:5, which led to grafting percentages ranging from 5% to 51%. The lap shear strength was increased significantly for all PEI treatments with an optimum increase of 122% for a grafting ratio of 1:1 compared to unmodified epoxy. At lower grafting ratios, the PEI led to a narrow distribution of lower average lap shear strength. The thinner PEI coating was less efficient at dispersing the CNT and fully reacting with the epoxy matrix. The highest grafting ratios improved the dispersion of the CNT and enabled much higher loading in the epoxy. However, at these high PEI concentrations, heterogeneous curing reactions evolved which created localised stress concentrations under loading, causing a reduction in the lap shear strength. This is the first study where PEI grafting levels have both been optimized in CNT and triblock copolymer adhesives and also directly related to adhesive shear strength based on chemical and morphological changes.