**2. Materials and Methods**

#### *2.1. Materials*

Poly (propylene glycol) bis(2-aminopropyl ether) − Mn ≈ 2000 Da (PPG2000, Sigma Aldrich, St. Louis, MO, USA), 4,4 -diaminodiphenylmethane (DADPHM, Sigma Aldrich, St. Louis, MO, USA), and diphenylmethane-4,4 -diisocyanate (MDI, technical product Desmodur® 44V20L, Covestro, Leverkusen, Germany) were pre-dispersed in multiwalled carbon nanotubes in a polyester polyol based resin (MWCNTs, Graphistrength® CPU2-30, Arkema, Colombes, France; Graphistrength® CPU2-30 is a MultiWall Carbon Nanotubes (MWCNT), at a concentrate that is used as an additive for polyurethane-based materials, coatings or adhesives. It contained 30 wt% of MWCNT, perfectly dispersed in a polyester polyol. Typical final MWCNT loadings in the final compounds were in the range 0.1 to 2 wt%, depending on the host matrix characteristics, targeted performances, processing methods, and conditions. Acetone (Sigma Aldrich, St. Louis, MO, USA), was used as received.

#### *2.2. Methods*

#### 2.2.1. Preparation of Polyurea-Polyurethane Nanocomposite Films

Five distinct types of polyurea-polyurethane nanocomposites were synthesized (Table 1) to obtain polymeric films with different mechanical properties.


**Table 1.** The composition of polyurea nanocomposite films.

For the synthesis of the blank sample, the diamines (PPG2000 and DADPHM) and the isocyanate (MDI) were solubilized separately in acetone, obtaining two solutions: Solution **A** (containing the two components with amino functional groups, PPG2000: DADPHM molar ratio 1:2) and Solution **B** (containing the component with isocyanate functional groups). The molar ratio between the isocyanate and the amino (primary amine) groups was maintained for all reactions at 1:1. Solutions **A** and **B** were vigorously stirred for 10 to 15 s at room temperature, and then the mixture was transferred on a Petri dish placed on a perfectly horizontal surface. The reaction mixture was maintained at 20 ◦C and relative humidity of between 50% and 55%, until the reaction was complete. After 24 h, a polyurea film with a thickness of about 0.4 mm was obtained.

Polyurea-polyurethane-based composites were obtained using the same procedure, according to the compositions described in Table 1. The only difference was the introduction of carbon nanotubes in solution **A**, thus obtaining Solution **C**. To obtain the best possible dispersion of MWCNT in the polyurea matrix, solution **C** was subjected to the action of ultrasound for one hour, before being mixed with solution **B**. For a better understanding of the behavior of the synthesized polymeric films on impact with a projectile, samples consisting of aluminum plates coated with polyurea-polyurethane nanocomposites (described in Table 1) were prepared by a casting technique (Scheme 1).

**Scheme 1.** Representation of the fabrication steps for the polyurea-polyurethane nanocomposite films.

The nanocomposites, obtained as described above, contain a hybrid polymeric matrix consisting of both polyurea and polyurethane zones. The polyurethane regions surround the MWCNTs, since these carbon nanotubes were pre-dispersed in a polyester polyol matrix before being part of the polyurea matrix. Even though these polyurethane areas are present in the nanocomposite, their concentration is neglectable in comparison with the polyurea matrix.

### 2.2.2. Characterization

The morphology of the polyurea-polyurethane-MWCNTs nanocomposite films was investigated by SEM (scanning electron microscopy) using a Tescan Vega II LMU SEM instrument (TESCAN, Brno, Czechia) at 10 keV acceleration voltage. The distribution of the MWCNTs inside the polyurea-polyurethane matrix was examined via micro-CT technique. The SkyScan micro-CT attachment allowed for converting the Tescan Vega II LMU SEM to an X-ray microtomograph for non-destructive imaging and for measuring of the object's internal microstructure of specimens. Analysis parameters: Exposure time—4 s per projection at electron beam currents of 100 nA; accelerating voltage—30 KeV; step size—1◦; scanning time—24 min. Reconstruction was performed by the NRecon program which used float-point data values for internal calculations during reconstruction, and afterward allowed the operator to define the density window as a range of the reconstructed values. The full set of reconstruction results was visualized by the program DataViewer® 2D/3D Micro-CT Slice Visualization (Micro Photonics Inc., Allentown, PA, USA). A thermogravimetric analysis (TGA) of the synthesized nanocomposites was performed on a Thermal Analysis Q500 instrument (TA Instruments, New Castle, DE, USA). Samples of about 2 mg were heated under nitrogen flow, with 10 ◦C/min, from 25 ◦C to 700 ◦C. The glass transition temperatures (Tg) of the polyurea-polyurethane nanocomposites were established using differential scanning calorimetry (DSC). All the samples of around 10 mg were analyzed using a NETZSCH DSC 204 F1 Phoenix instrument (NETZSCH, Selb, Germany), under nitrogen flow, at 10 ◦C/min heating rate, in two heating/cooling cycles, between −80 and 200 ◦C. Stress–strain curves were obtained using an Instron 3382 testing machine (Instron, Norwood, MA, USA). The samples were prepared for the tensile tests by cutting the nanocomposite films in a rectangular shape, at standard dimensions for tensile specimen, with 5 mm width and 100 mm length. For each specimen, the rate of the extension was set at 500 mm/min, and the separation of the initial jaws was set at 50 mm (plain jaw faces). For each type of nanocomposite film, five tensile tests were carried out and the average of the measured values and the standard deviation for each point was registered. For a more accurate approach of the interpretation of these tests, the values of true stress (σT) and true strain (εT) were employed for determining Young's modulus values, which were calculated according to the mathematical model described in [32]. The definition of true stress (σT) states that this σ<sup>T</sup> signifies the instantaneous applied load divided by the instantaneous cross-sectional area. True stress is related to engineering stress (σ) through the following equation: σ<sup>T</sup> = σ (1 + ε). The definition of true strain (εT) states that ε<sup>T</sup> signifies the rate of instantaneous increase in the instantaneous gauge length. The true strain is related to engineering strain (ε) by ε<sup>T</sup> = ln (1 + ε). A comparative multigraph containing all the true stress/true strain values characteristic for each synthesized material was plotted to evaluate the influence of the nanofiller on their mechanical properties. This multigraph was designed to show only the curves with the closest parameters to the mean values from each set of specimens. The dynamic and mechanical behavior of the samples was evaluated in single cantilever bending mode between −80 and 200 ◦C, with a controlled heating rate of 5 ◦C/min, using a TRITEC 2000 B-Dynamic mechanical analysis (DMA, Martignat, France) instrument.

For the evaluation of the behavior of polyurea-polyurethane nanocomposite films in a dynamic regime, a series of experimental measurements were performed on aluminum metal plates coated with polyurea and polyurea-polyurethane nanocomposites. The measuring instruments used were an ultra-high-speed video camera PHOTRON (Photron, Tokyo, Japan) and a PCB force transducer (PCB Piezotronics, Depew, NY, USA).

The samples used for these tests consisted of an aluminum metal plate with a thickness of 0.5 mm and a free diameter of 100 mm, on which a layer of polyurea-polyurethane nanocomposite of approximately 1 mm was previously deposited. Each sample was fixed (Figure 1b) and orientated, with the uncoated side towards the projectile launching direction. The experiments were carried out using a Hopkinson bar air propulsion system (Figure 1c), utilizing a spherical head projectile (Figure 1a). The impact strength obtained at the impact between the projectile and the samples was measured using a piezoelectric sensor connected to the mounting bracket of the tested aluminum plate. These experiments were performed on three types of materials: neat aluminum plate, polyurea coated aluminum plate (PU), and polyurea-polyurethane-MWCNTs nanocomposite-coated aluminum plates (PU-NC2, PU-NC3, and PU-NC4).

**Figure 1.** Hopkinson bar set-up for experimental testing of the polyurea-polyurethane-MWCNTs nanocomposite-coated aluminum plates in dynamic regime: (**a**) Spherical head projectile; (**b**) sample before the experiment (coated on the backside with polyurea (PU) or polyurea-polyurethane-MWCNTs nanocomposite (PU-NC)); (**c**) image captured during the experiment, at the moment of the impact of the projectile with the sample.
