Enhanced thermal and mechanical properties of epoxy composites at ultra-low loading of functionalized MoS2 nanosheets

Abstract. In this study, single-step branched polyethyleneimine (PEI)-assisted exfoliation of molybdenum sulfide nanosheets (MoS2-PEI) was carried out. These functionalized MoS2-PEI nanosheets were employed as toughening agents for epoxy composites. The loadings of nanosheets were kept lower than 1 wt.%. The mechanical and thermal properties, and interfacial interactions of epoxy composites were investigated. The epoxy composites have shown ∼67% and ∼101% enhancements in fracture toughness (KIC) in fracture energy (GIC), respectively, at nanosheets loadings as small as 0.09 wt.% (EP/MoS2-PEI-0.09), KIC has shown a direct linear relationship with the surface free energy and is highest at 52 mJ.m-2 for the EP/MoS2-PEI-0.09 composite. However, the surface free energy values of EP/MoS2-PEI-0.16 and EP/MoS2-PEI-1 composites decreased to 48 mJ.m-2 and 45 mJ.m-2. The overall flexural modulus (E) and strength (σ) were not highly responsive to the addition of the MoS2-PEI nanosheets. Furthermore, the thermal stability and thermomechanical properties of the epoxy composites improved significantly. The optimum MoS2-PEI nanosheet loading was observed to be 0.09 wt.%, beyond this a gradual decrease in thermal stability and mechanical properties was observed. The significant improvement in thermal and mechanical properties of the epoxy composites could be accredited to the good interfacial interaction between the MoS2-PEI nanosheets and epoxy matrix at the interface and the inherent strength, high aspect ratio, and excellent barrier effect of PEI molecules.