## 3 Results and Discussion
### Preparation and Properties of the Silica-Filled (PAA-co-PAM)-DPNR Composites
#### 3.3.3 Mechanical Properties of Composites  
Figure 14 displays the stress-strain curve obtained from the tensile testing of the DPNR and (PAA-_co_-PAM)-DPNR mixed with different silica contents. It was observed that the stress gradually increased after deformation. After that, the stress sharply increased. This is because of the occurrence of the strain-induced crystallization of the natural rubber chains during deformation. The sample was tested under tension mode until it broke. From the results, comparisons between the tensile strength, elongation at break, and modulus at 100, 200, and 300% strain are shown in Figure 15a-c, respectively. The P10-DPNR and P30-DPNR exhibited good mechanical properties by showing an enhanced tensile strength and modulus compared to the DPNR. The tensile strength of P10-DPNR and P30-DPNR were 15.21 and 14.33 MPa, respectively. The DPNR had a tensile strength of 10.02 MPa. At the same time, the modulus values, under a 300% strain, for P10-DPNR and P30-DPNR were found to be 2.01 and 5.26 MPa, respectively, which were higher than that of the DPNR (1.58 MPa). The elongation at break of P10-DPNR (750.75%) was not much different from that of the DPNR (743.58%). When the grafting percentage was increased, as was the case for P30-DPNR, its elongation at break was reduced to 563.91%. This was probably because the polyacrylic acid (the Tg of PAA was 100-125 \({}^{\circ}\)C) [70; 71] and polyacrylamide (Tg of PAM was 190 \({}^{\circ}\)C) [72] became more rigid and restricted the movement of the chains in the natural rubber during deformation. For the silica-filled composites, an increase in tensile strength and modulus was obtained. This indicates that silica acts as a reinforcing filler in the rubber matrix and can improve the mechanical properties of the composites. For example, the tensile strength of P10-DPNR-10 Si increased by 1.55 and 2.35 times when compared to P10-DPNR and DPNR, respectively. Moreover, the tensile strength of P10-DPNR-10 Si increased by 1.55 times when compared to DPNR-10 Si, suggesting strong interactions between the silica and natural rubber after modification. It was noticed that the tensile strength of those composites with 10 phr of silica significantly increased when compared to those samples without silica. However, the tensile strength of P10-DPNR-20 Si saw no difference when compared to P10-DPNR-10 Si. The addition of 10 phr of silica did not affect the elongation at break of the composite. In addition, at 20 phr of silica, some properties were reduced due to the aggregation of the silica particles at high content loading. Therefore, the addition of 10 phr silica would be suitable for the reinforced composites. Figure 13: 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**) P30-DPNR; (**e–h**) P30-DPNR-10 Si; (**i–l**) P30-DPNR-20 Si. Figure 16a displays the tear strength of those composites prepared from DPNR and (PAA-_co_-PAM)-DPNR, with the addition of various contents of silica. It was observed that the tear strength of the silica-filled DPNR composites seemed to decrease with an increase in silica content. Natural rubber molecules are composed of isoprene units, which are  
Figure 14: Stress-strain curve of the (a) DPNR; (b) DPNR-10 Si; (c) DPNR-20 Si; (d) P10-DPNR; (e) P10-DPNR-10 Si; (f) P10-DPNR-20 Si; (g) P30-DPNR; (h) P30-DPNR-10 Si; (i) P30-DPNR-20 Si composites. Figure 15: (a) Tensile strength; (b) elongation at break; and (c) modulus at 100, 200, and 300% strain of the silica-filled natural rubber composites. nonpolar. Therefore, a weaker natural rubber/silica interaction was obtained [73]. However, the P10-DPNR and P30-DPNR showed an increase in tear strength when compared to the DPNR. Their tear strength values increased when the silica was added. This was suggested by the strong networks being formed in the composites and the strong interactions between the modified natural rubber and silica.