## 3 Results and Discussion
### Morphological Studies  
Figure 4a,b represent FE-SEM micrographs of the nanocapsules at two different magnifications. Spherical nanocapsules are clearly seen, which guarantee easy dispersion into the resins before applying coating [15]. In addition, the rough outer surface of the capsules provides enhanced adhesion to the resin and metallic substrate. The rough surface of capsules can be attributed to the phase change of CAR to solid during the encapsulation process. To investigate the mean diameter and size distribution of synthesized nanocapsules, ImageJ software was used and the average of 50 particles' diameter was determined. Nanocapsules with a mean diameter of \(708\pm 252\) nm were found. Figure 4c shows the size distribution of the prepared nanocapsules. It can be seen that the coaxial electrospray method led to the formation of nanocapsules with a wide range of size distribution. The capsules with a wide range in the size distribution showed great potential in self-healing performance due to covering and healing all sizes of cracks, from nano to microscale [39].  
Figure 4: FE-SEM micrographs of the prepared nanocapsules at: (**a**) 5000\(\times\) and (**b**) 20,000\(\times\) magnification; (**c**) the size distribution of nanocapsules; (**d**) FE-SEM image of the fractured capsules in the epoxy composites; and (**e.f**) TEM images of the prepared nanocapsules.  
The fractured surface of the epoxy composite is shown in Figure 4d. According to Figure 4d, the core-sheath structure, along with a smooth inner layer of the nanocapsules, was formed, which confirms the FTIR results regarding the presence of CAR in nanocapsules without any reaction with SAN shell materials. Moreover, Figure 4d reveals an increased bonding of nanocapsules and the epoxy matrix, which is attributed to the high roughness of nanocapsules and the chemical reaction of unencapsulated CAR on the surfaces of the nanocapsules and epoxy matrix.  
Figure 4e shows the TEM images of nanocapsules, where the dark and bright fields are observed in these images. It proves the formation of the core-shell structure of nanocapsules while the single-core structure of nanocapsules can be observed. According to the TEM and FE-SEM images of the fractured capsules, the thickness of shells is in the range of 35-95 nanometers, and, consequently, a scratch longer than 95 nm has the ability to break the capsules' shell [40]. From these measurements, the volume fraction of the core was calculated to be in the range of 27-33%. Knowing the density of CAR and SAN solution (1.027 g/mL and 1.08 g/mL, respectively) led to the weight percentage of the core content in the range of 27% to 32% [40]. These values are in good agreement with the measured core content from the extraction method (32 wt.%).  
Thermodynamic of the Encapsulation Process  
In the coaxial electrospray method, the surface tensions of the components are important factors for the successful encapsulation process [30]. The surface tensions of the core and shell materials were measured and listed in Table 1. According to Table 1 and Equations (3) and (4), the spreading coefficient was calculated to be 3.02 mN/m. The positive spreading coefficient proves the spreading of the shell solution on CAR and confirms the successful encapsulation of CAR within the SAN shell [30].