## 3 Results and Discussion
### Preparation and Properties of the Silica-Filled (PAA-co-PAM)-DPNR Composites
#### 3.3.2 Morphology of Silica-Filled Natural Rubber Composites  
The morphology of the silica-filled DPNR, silica-filled P10-DPNR, and silica-filled P30-DPNR composites, with varying silica content, was examined by scanning electron microscope, as demonstrated in Figures 11-13, respectively. The cross-section surface of all vulcanizates (by freeze fracturing with nitrogen liquid) showed a smooth surface with some compounding ingredients appearing on their surfaces. When the silica was added into the system, good distribution of the silica particles in the natural rubber matrix was obtained for the composites with 10 phr of silica, as observed from the SEM/EDS mapping. However, some aggregation of the silica particles was observed under high-loading content (20 phr). The samples were then subjected to tensile tests, and the fracture surface was examined by SEM. The tensile fracture surface of the DPNR was smooth, as shown in Figure 11c,d. We observed holes and small cracks throughout its surface. This is because the compounding ingredients are pulled out from the tensile force, which was attributed to poor adhesion with the natural rubber matrix [67]. For the P10-DPNR in Figure 12c,d and \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**No** & **Samples** & **Natural Rubber** & **Silica** & **T\({}_{\rm 82}\)** & **T\({}_{\rm 60}\)** & **ML** & **MH** & **MH-ML** & **CRI** \\  & **(phr)** & **(phr)** & **(min)** & **(min)** & **(dNm)** & **(dNm)** & **(dNm)** & **(dNm)** & **(min\({}^{-1}\))** \\ \hline
1 & DPNR & 100 & 0 & 1.55 & 3.71 & 0.20 & 8.71 & 8.51 & 46.29 \\
2 & DPNR-10 Si & 100 & 10 & 2.50 & 6.22 & 0.30 & 8.82 & 8.52 & 26.88 \\
3 & DPNR-20 Si & 100 & 20 & 3.24 & 6.45 & 0.36 & 9.57 & 9.21 & 31.15 \\
4 & P10-DPNR & 100 & 0 & 0.91 & 5.35 & 0.50 & 9.99 & 9.49 & 22.52 \\
5 & P10-DPNR-10 Si & 100 & 10 & 1.61 & 3.62 & 0.50 & 11.05 & 10.55 & 49.75 \\
6 & P10-DPNR-20 Si & 100 & 20 & 2.02 & 3.94 & 0.60 & 12.78 & 12.18 & 52.08 \\
7 & P30-DPNR & 100 & 0 & 1.16 & 4.05 & 0.50 & 10.95 & 10.45 & 34.60 \\
8 & P30-DPNR-10 Si & 100 & 10 & 1.33 & 3.26 & 0.55 & 12.51 & 11.96 & 51.81 \\
9 & P30-DPNR-20 Si & 100 & 20 & 1.35 & 4.67 & 1.37 & 19.16 & 17.79 & 30.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Cure parameters obtained from MDR at a vulcanizing temperature of 150 \({}^{\circ}\)C.  
the P30-DPNR in Figure 13c,d, the embedment of the compounding ingredients into the surface was observed. The morphology clearly changed after tensile measuring, leading to a rough surface. The modification of natural rubber by PAA-_co_-PAM grafting strongly affects its fracture morphology, indicating the rigid structure of the modified rubber [68]. For the DPNR with the addition of silica at 10 phr in Figure 11g,h and 20 phr in Figure 11k,l, cracks in the rubber matrix at the region between the natural rubber and the silica cluster were found due to the weak interaction and incompatibility of both of the components. In the case of the silica-loaded (PAA-_co_-PAM)-DPNR samples, uneven microscopic layers and rough surfaces were observed. The SEM images in Figures 12 and 13g,h,k,l clearly revealed the extended region of the NR matrix-silica clusters after deformation [69]. Therefore, the grafting of DPNR with polar components might improve the interaction between natural rubber and fillers. This overall remarkable morphology change was due to the strong interfacial adhesion between the modified DPNR and the silica. Figure 10: Schematic representation of (PAA-co-PAM)-DPNR/silica interactions. Figure 9: Swelling ratio of the silica-filled natural rubber composites. Figure 11: SEM images of freeze fracturing, corresponding EDS (the red spots indicate silica particles), and tensile fracture surface at \(\times\)1000 and \(\times\)10,000 magnifications of (**a–d**) DPNR; (**e–h**) DPNR-10 Si; (**i–l**) DPNR-20 Si.