## 3 Results and Discussions
### Mechanical Properties
The specimens containing PCL did not completely fulfill the elasticity requirements reported in the ASTM D5045 standard, owing to the excessive plasticization of the PCL phase within the blends, and thus the tentative value of the fracture toughness (KQ) was considered in this plot. The obtained KIC value for neat PA6 is 2.8 MPa\(\cdot\)m\({}^{1/2}\), and, interestingly, for low amounts of PCL, both KIC and GIC increase due to toughening mechanisms played by the presence of PCL [65]. The increase of both KIC and GIC at limited PCL concentrations could be attributed to the toughening mechanism known as particle debonding [66; 67]. This mechanism is responsible for an alteration of the stress state in the surrounding matrix enabling the matrix to yield at low-stress levels, and thus initiating the plastic deformation inside the matrix, with a consequent increase in the absorbed fracture energy [68; 69]. This hypothesis will be confirmed by the results of the microstructural analysis reported hereafter. On the other hand, at higher percentages of PCL, the fracture toughness slightly decreases due to the lower fracture toughness of PCL. However, the observed drop in fracture toughness at elevated PCL amounts is not dramatic. Overall, the fracture toughness under quasi-static conditions of the PA6 is not negatively affected by the introduction of PCL, even if the two polymers are immiscible. Concerning the results obtained in impact mode, it can be observed that for low concentrations of PCL, the KIC of the blends is comparable with that of neat PA6, and the slight decrease observed for elevated PCL content can be probably attributed to the limited adhesion between the PCL and the PA6 matrix. Moreover, the KIC values calculated in impact mode are significantly higher than those reported in quasi-static conditions, and a possible reason can be related to the higher deformation speed, which may lead to the activation of different fracture mechanisms. Figure 9: Results of quasi-static tensile tests on the prepared PA6/PCL blends. (**a**) Elastic modulus, (**b**) ultimate tensile strength, (**c**) elongation at break. Figure 10 reports the SEM micrographs of the tested SENB specimens in both quasi-static and impact mode. The surface of the tested specimens in quasi-static conditions is noticeably different with respect to that observed in impact mode. In quasi-static mode, it is possible to see that the fracture surface of the PA6_20 PCL and PA6_30 PCL blends is quite irregular, the PA6 matrix is plastically deformed, and the voids formed during the particle debonding process are clearly visible. Particle debonding and PA6 matrix deformation dissipates a large amount of energy; in these conditions crack propagation is hindered [70]. The observed morphology clearly explains the improvement in the fracture toughness of the prepared blends observed in quasi-static conditions. On the contrary, the surfaces of the specimens tested in impact mode are much smoother, with no plastic deformation phenomena. Indeed, the particle debonding toughening mechanism does occur, since the voids left by PCL domains can be clearly detected, but this effect is counterbalanced by the lack of plastic deformation of the PA6 matrix [71].