2.3. Sample Characterization

Mechanical properties were evaluated by means of tensile test measurements which were performed according to standard test methods ISO 527 [30]. The tests were performed in a universal test machine Ibertest ELIB-50-W (Madrid, Spain) with a load cell of 5 kN and a crosshead rate of 10 mm/min. Charpy’s impact resistance was also assessed in a Metrotec impact equipment (San Sebastian, Spain), using a 1 J pendulum and A-type notched specimens under the ISO 179 [31]. For each test five specimens were characterized, and the mean and standard deviation of the values were reported. The significance in the mechanical data differences was statistically analyzed using the one-way analysis of variance (ANOVA), by means of OriginPro 8 software, at 95% confidence level according to Tukey’s test for the significant differences among formulations.

Dynamic thermo-mechanical analysis (DTMA) of the samples was performed in an oscillatory rheometer AR G2 from TA Instruments (New Castle, England), in torsion mode. The rheometer was equipped with a special clamp system for solid samples. The test samples were rectangular samples sized 40 × 10 × 4 mm³ and were fabricated by injection-molding from the previously obtained pellets of each formulation. The temperature range of the test was set from −50 to 110 °C and the heating rate was set on 2 °C/min. The frequency was 1 Hz and the maximum deformation 0.1%. A liquid nitrogen atmosphere was employed during the tests. The glass transition temperatures (T_g) were determined at the maximum peaks of the tangent of the loss factor (tan δ).

The microstructure of the different formulations was observed by field emission scanning electron microscopy (FE-SEM), using a Zeiss Ultra 55 microscope at 1 kV. Samples of 1 µm thick were obtained by the environmental ultramicrotomy of the central part of the previously injected specimens. Through this technique, it was possible to obtain sufficiently smooth and flat surfaces for observation by FE-SEM and atomic force microscopy (AFM), while it was possible to appropriately reveal the microstructure of the material.

The atomic force microscopy with nanomechanical assessment (AFM-QNM) was performed in an AFM model Nanoscope II from Veeco National Instrument (Santa Barbara, CA, USA) working in peak force tapping mode by the quantitative nanomechanical measurement. QNM method based on literature [32,33] has been used. With the AFM-QNM analysis of the topography map, deformation, stiffness, and adhesion of each area was obtained [34]. An antimony (n) doped Si cantilever of 5 N/m and with a Poisson coefficient of 0.48 was used. The radius of the tip was determined at 20 nm by a calibration carried out in a standard sample of polystyrene at 5 nm, the same depth that the contact with the sample was made during the test. The calibration of the area function was done before and after the test, to certify that there was no wear during the entire test procedure. Derjagin, Muller, Toropov model (DMT) was employed to calculate the elastic modulus. This model takes into account the contact adhesive component [32].

The oxygen transmission rate (OTR) was measured with a Systech Instruments 8500 oxygen permeation analyzer (Metrotec S.A, San Sebastián, Spain) operating at room temperature and 2.5 atm. To prepare the appropriate samples for OTR measurements, masterbatch pellets were processed into 14 cm diameter film discs by using a hot press (Mini C 3850, Caver, Inc., Wabash, IN, USA) with the following pressure cycle: At atmospheric pressure for 5 min, 3 MPa for 1 min, 5 MPa for 1 min, and 10 MPa for 3 min. The films were then quenched to room temperature at atmospheric pressure. Their average thickness was around 450 μm. The film disks films were compressed between the upper and lower diffusion chamber. Pure oxygen (99.9% purity) was introduced into the upper half of the sample chamber while nitrogen was injected into the lower half. The oxygen volumetric flow rate per unit area of the film and per time (OTR, cm³ × mm m⁻² × day⁻¹) was continuously monitored until a steady state was reached. Measurements were expressed as oxygen transmission rate per film thickness (OTR·e).

Surface wettability was studied through static water contact angle (WCA) measurements by using a standard goniometer (EasyDrop-FM140, KRÜSS GmbH, Hamburg, Germany) equipped with a camera and Drop Shape Analysis SW21; DSA1 software. Ten contact angle measurements were taken in random positions, putting drops of ~2 μL distiller water onto the surface of the samples with the aid of a syringe. The average values were calculated and reported.