Epoxy resin possesses excellent adhesion, outstan‎ding chemical an‎d thermal resistance, an‎d minimal shrinkage upon curing, which together provide significant advantages over other thermosetting polymers. Nevertheless, its highly crosslinked network structure results in intrinsic brittleness an‎d limited resistance to crack propagation. Consequently, enhancing the toughness an‎d overall mechanical performance of epoxy resins remains a fundamental an‎d ongoing research challenge. In this study, liquid rubber in the form of a carboxyl-terminated butadiene–acrylonitrile copolymer (CTBN) an‎d carbon nitride nanosheets (g-C₃N₄) were incorporated into an epoxy resin matrix to systematically investigate the resulting physical, mechanical, an‎d thermal properties. The prepared composites were characterized using tensile an‎d flexural tests, FTIR, DSC, TGA, an‎d SEM. Two distinct experimental approaches were employed to eva‎luate the effects of these modifiers on the epoxy resin system. In the first approach, liquid rubber CTBN was incorporated into the epoxy matrix at concentrations ranging from 5 to 20 phr. The results showed that the sample with 15 phr CTBN exhibited the best performance in terms of toughness, with a 19.8% increase in toughness an‎d a 24.6% improvement in elongation at break compared to neat epoxy. However, tensile strength an‎d modulus decreased by 15.33% an‎d 15.2%, respectively, while flexural strength an‎d modulus declined by 13.3% an‎d 20.3%. SEM micrographs confirmed void formation an‎d phase separation, providing direct evidence of microstructural changes caused by rubber modification. DSC an‎d TGA analyses further revealed a 5.3% reduction in glass transition temperature along with decreased thermal stability, indicating the negative impact of CTBN addition on the thermal behavior of epoxy. In the second approach, the optimum CTBN content was fixed, an‎d hybrid nanocomposites were fabricated by incorporating carbon nitride nanosheets at concentrations between 0.25 an‎d 1 wt%. This strategy allowed the eva‎luation of the synergistic effects of nanosheets on the modified epoxy. Mechanical an‎d thermal characterizations showed that the sample with 0.75 wt% nanosheets offered the most balanced performance, with improvements of 17.5% in tensile strength, 14% in tensile modulus, 6.2% in toughness, an‎d 2.4% in elongation at break compared to the CTBN-only system. SEM analysis also revealed progressive changes in fracture surface morphology with increasing nanosheet content. Moreover, DSC results indicated a slight increase in glass transition temperature, with the 0.5 wt% sample showing a 2.7% enhancement, suggesting a modest yet positive effect of nanosheet incorporation. TGA data demonstrated that carbon nitride addition had no significant influence on thermal stability. Overall, the study demonstrated that CTBN incorporation greatly improves the toughness of epoxy but compromises tensile strength, stiffness, an‎d thermal stability, reflecting a trade-off between fracture resistance an‎d other properties. In contrast, the inclusion of carbon nitride nanosheets partially offset these drawbacks, leading to improved mechanical performance while largely maintaining thermal stability. These findings highlight the synergistic effect of nanosheet incorporation an‎d suggest a promising route for developing high-performance epoxy-based hybrid composites suitable for advanced structural applications.

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افسانه فخار , محمود معصومي