## 2 Materials and Methods
### Experimental Techniques
#### 2.3.5 Mechanical Properties  
Quasi-static tensile tests were carried out at room temperature using an Instron(r) 5969 tensile testing machine (ITW Test & Measurement and Equipment, Lake Bluff, IL, USA) equipped with a 1 kN load cell. The tests were performed on ISO 527 1 BA specimens at a crosshead speed of 10 mm/min, and at least ten specimens were tested for each composition. In this way, the determination of the maximum stress (\(\sigma_{max}\)) and of the strain at break (\(\epsilon_{b}\)) was performed. The elastic modulus (E) was determined through tensile tests by using the same machine equipped with an Instron(r) 2620-601 extensometer, having a gauge length of 12.5 mm, at a crosshead speed of 0.25 mm/min. It was calculated as the secant modulus considering the stress levels associated to the strain values of 0.05% and 0.25%, as reported in the ISO-527 standard. Regarding the modeling of the mechanical properties of the prepared blends, three different models were selected: series model (Equation (2)), parallel model (Equation (3)), and Maxwell-Eucken (ME) model (Equation (4)). The series and parallel models represent the upper and lower bounds of the mechanical properties, respectively. On the other hand, the Maxwell-Eucken (ME) model is able to predict the mechanical properties of a blend characterized by a droplet-matrix morphology [35]. \[P_{b}\;=\;fP_{PA6}+(1-f)P_{PCL} \tag{2}\]\[P_{b}\ =\ \left(\frac{f}{P_{PA6}}+\frac{1-f}{P_{PCL}}\right) \tag{3}\]  
\[P_{b}\ =\ \frac{P_{PA6}f+P_{PCL}(1-f)\frac{3P_{PA6}}{2P_{PA6}+P_{PCL}}}{f+(1-f) \frac{3P_{PA6}}{2P_{PA6}+P_{PCL}}+P_{PCL}} \tag{4}\]  
where \(P_{b}\) is the property of the blend,\(f\) is the volume fraction of PA6 in the sample, \(P_{PA6}\) is the property of neat PA6, and \(P_{PCL}\) is the property of neat PCL. The fracture toughness of the blends was evaluated according to the procedure described in the ASTM D5045 standard. The tests were carried out on Single Edge Notched Bending (SENB) specimens, having dimensions of 44 \(\times\) 10 \(\times\) 5 mm\({}^{3}\), an initial notch length of 5 mm, and a span length of 40 mm. At least ten specimens were tested for each composition. Tests in quasi-static mode were performed in three-point bending configuration by using an Instron(r) 5969 electromechanical testing machine, at a crosshead speed of 10 mm/min. Tests in impact mode were performed using a Charpy impact machine provided by CEAST, following the ISO 17281 standard, and the load-displacement curves were recorded using a top extensometer in the hammer. A mass of the hammer equal to 0.5 kg, a starting angle of 60\({}^{\circ}\), and an impact speed of 1.5 m/s were utilized. From the load-displacement curves, the maximum load sustained by the samples (\(P_{max}\)) was determined, and it was thus possible to obtain the critical stress intensity factor (\(K_{IC}\)), both in quasi-static and impact conditions, according to the expressions reported in Equations (5) and (6):  
\[K_{IC}=\left(\frac{P_{max}}{BW^{1/2}}\right)\cdot f(x) \tag{5}\]  
\[f(x)=6x^{1/2}\ \frac{1.99-x(1-x)\big{(}2.15-3.93x+2.7x^{2}\big{)}}{(1+2x)(1-x)^{3/2}} \tag{6}\]  
where \(B\) is the thickness of the specimen, \(W\) is the width of the specimen,\(f(x)\) is a calibration factor, where \(x=a/W\), and \(a\) is the notch length.