*3.6. Morphology of the Fracture Surface of PP Composites*

PP composites were further analyzed with SEM-EDS to observe their cross-sectional fracture surface. First, each sample was submerged in liquid nitrogen for 5 min to ensure it was completely frozen. It was then removed from the liquid nitrogen and immediately snapped in half. The sample was dried to remove excess water. After freeze-fracture, the cross-section of the sample was carbon-coated and observed. Figure 9 depicts the SEM-EDS micrograph and quantitative analysis of elements present at the white spot. The elements are oxygen (O) and silica (Si). The SEM image also shows the typical fracture surfaces of neat PP and PP composite samples. This characterization technique can be used to identify the hybrid filler distribution in the polymer composite as well.

**Figure 9.** SEM images mapped with EDS analysis for distribution of oxygen (O) element represents CNF and silica (Si) element represent SiP: (**a**) PP polymer, (**b**) PP-1.25 CNF, (**c**) PP SS/CNF and (**d**) PP SP/CNF.

In Figure 9a, the rough surface observed on neat PP indicated the typical characteristic of the elastic behavior of PP [34] and the element mapping of Si and O was almost completely black in colour. In addition, it can be observed that CNFs were agglomerated in the polymer composite (white circle) in the EDS analysis and also SEM morphology, which is supported by the study of Yasim-Anuar et. al. [35]. According to the respective literature, the distribution of oxygen (O) represents CNFs (Figure 9b).

Moreover, the EDS images showed the samples PP SS/CNF and PP SP/CNF in Figure 9c,d to consist of uniform white spots of O and Si, which further confirm the good dispersion of the hybrid filler in the polymer composite. The oxygen element of PP nanocomposites increase compared with PP/CNF is due to Si-O-Si bonding formed after blending. These results are in line with the FT-IR assignment in the previous section.

Further, the SEM images present a uniform distribution of the hybrid fillers in a PP composite surface. Sample PP SS/CNF showed that the particulate fillers with a spherical and fibrous shape (red circle) were found in the composite, suggesting the applied stress effectively transferred to the filler from the polymer matrix by scattering the energy during crack propagation [36]. While on the sample PP SP/CNF, interconnected nanofibers formed with rough surfaces and irregular shapes were observed. SiP SP exhibited better interfacial bonding strength and filler dispersion in the PP composite. These results indicate robust interfacial adhesion between the PP matrix and filler due to their hydrophobicity compatibility.
