The mechanical behaviors of epoxy-terminated hyperbranched polyester (E-HBP) as toughener in different epoxy resins

Using epoxy-terminated hyperbranched polymer (E-HBP) to modify epoxy resin (EP) is an effective way to improve the toughness of EP. In the present study, two different epoxy resin systems with E-HBP are researched: a commercial diglycidyl ether bisphenol A (DGEBA) resin with anhydride as curing agent and a tetraglycidyl diaminodiphenyl methane (TGDDM) resin with diamine as curing agent. Characterization results show that the addition of E-HBP could improve the mechanical properties of the two epoxy resin systems, such as tensile strength, elongation, and modulus of elasticity. Meanwhile, the glass transition temperature (T-g) of the two systems does not decrease. However, the morphology of the tensile fracture surfaces of the two modified systems shows different behaviors. Significant plastic deformation could be observed in the fracture surfaces of the modified DGEBA/anhydride system, and particle cavitations are clearly shown in the fracture surfaces of the modified TGDDM/diamine systems. The analysis of the tensile fracture surfaces suggests that firstly E-HBP participates in the curing process of the modified resin systems, followed by the chemical-induced phase separation; finally, a gradient transition interface layer (GTIL) is formed. Apart from these, during the external loading process, the mechanical behaviors (deformation or cavitation) of the separated E-HBP particles in the modified epoxy resins are affected by the properties of the epoxy matrix itself.