## 3 Results and Discussion
### Preparation and Properties of the Silica-Filled (PAA-co-PAM)-DPNR Composites  
#### 3.3.1 Cure Characteristics  
The natural rubber compounds mixed with different concentrations of silica were prepared by using a two-roll mill. The optimal temperature and time for vulcanization were determined by a moving die rheometer (MDR). The cure characteristics of all the compounds obtained from the MDR at a vulcanizing temperature of 150 \({}^{\circ}\)C are shown in Table 4. For the compounds without silica, the (PAA-_co_-PAM)-DPNR compounds exhibited a shorter scorch time and longer cure time when compared to the DPNR compound. In this case, the DPNR was functionalized with a copolymer consisting of carboxylic acid and amide groups. These additional groups may affect the vulcanization process. The vulcanization reaction was accelerated by the presence of amine-containing substances, resulting in a decrease in scorch time [60]. On the other hand, vulcanization retardation occurred, which might have increased the vulcanization time, as is seen in the case of existing methacrylic acid in rubber composites [61]. After that, the silica-filled rubber composites were prepared by mixing them with different concentrations of silica at 10 and 20 phr. Generally, silica contributes to the vulcanization system by delaying the rate of vulcanization [62]. Considering the chemical structure of silica, it possesses silanol groups that can adsorb the accelerators in the system, resulting in higher scorch times and cure times [63]. In the case of silica-filled DPNR, it was clearly observed that the scorch time and cure time of DPNR-10 Si and DPNR-20 Si were higher than that of the DPNR compound without silica, and, consequently, the cure rate index decreased. However, the (PAA-_co_-PAM)-DPNR with 10 and 20 phr of silica showed an increase in the cure rate, with shorter scorch and cure times when compared to silica-filled DPNR using the same amount of silica. It could be that the polar functional groups contained in the modified DPNR play an important role by interacting with silica in the mixing step. Therefore, the adsorption of the accelerators was reduced, resulting in an increase in the cure rate. The presence of (PAA-_co_-PAM) and silica in the rubber compounds revealed important features. At 10 phr of silica loaded into the (PAA-_co_-PAM)-DPNR compounds, a longer scorch time and shorter cure time were observed when compared to the (PAA-_co_-PAM)-DPNR without silica. For  
Figure 8: TGA and DTG thermograms of DPNR, P10-DPNR, and P30-DPNR. the processing, the longer scorch time enhances processing safety, while a shorter cure time is preferred in terms of better productivity [60]. When the silica content was increased to 20 phr, the scorch time and cure time increased for the modified rubber compounds using 10 phr of silica. Although some silica interacted with the modified DPNR, more of the free silica from the higher loadings had the ability to adsorb the accelerators, leading to the retardation of the vulcanization process. In addition, the minimum torque (M\({}_{\rm L}\)) of the (PAA-_co_-PAM)-DPNR was higher than that of the DPNR. It was also found that the M\({}_{\rm L}\) increased with an increase in silica content in the compounds. The M\({}_{\rm L}\) is related to the viscosity of the rubber compounds. The increase in M\({}_{\rm L}\) was probably due to the PAA-_co_-PAM chains and the rigid silica particles restricting the motion of the rubber chains [64]. Moreover, the delta torque (M\({}_{\rm H}\)-M\({}_{\rm L}\)) of the (PAA-_co_-PAM)-DPNR was higher than that of the DPNR, and the addition of silica presented an increase in the delta torque. The delta torque was related to the crosslinking density of the vulcanizates. These results corresponded with the swelling tests of the rubber composites in toluene, as shown in Figure 9. The high crosslinking density of the rubber vulcanizates leads to a reduction in the swelling degree [65]. It was observed that the DPNR had a swelling ratio of 4.12.