## 3 Results and Discussion
### Preparation and Properties of the Silica-Filled (PAA-co-PAM)-DPNR Composites
#### 3.3.4 Dynamic Mechanical Analysis  
The thermomechanical behavior of the silica-filled natural rubber composites was studied by temperature sweep testing. Figure 17a,b display the storage modulus (E\({}^{\prime}\)) and loss tangent (tan 5) of the silica-filled natural rubber composites as a function of temperature from \(-80\) to \(100\)\({}^{\circ}\)C, respectively. From Figure 15a, all samples had a high E\({}^{\prime}\) value in the glassy state at a low temperature. The E\({}^{\prime}\) decreased when a higher temperature was applied due to the increase of mobility of the chain segments. The E\({}^{\prime}\) dramatically decreased when the temperature was in the range of \(-50\) to \(-10\)\({}^{\circ}\)C, which corresponds to the transition region. Then, when the temperature increased from \(-10\) to \(100\)\({}^{\circ}\)C, the E\({}^{\prime}\) did not change much within this range. From the results, the 10-DPNR and P30-DPNR vulcanizates exhibited a higher E\({}^{\prime}\) compared to the DPNR for all temperatures used. The increase in E\({}^{\prime}\) was related to the amount of grafting percentage. For example, at a temperature of \(30\)\({}^{\circ}\)C, the E\({}^{\prime}\) of the DPNR vulcanizate was 1.36 MPa, while the E\({}^{\prime}\) values of the P10-DPNR and P30-DPNR vulcanizates increased to 1.42 and 10.81 MPa, respectively. This was attributed to their higher stiffness after modification. When studying the effect of silica in the composites, the P10-DPNR with various contents of silica (10 and 20 phr) was compared to P10-DPNR without the addition of silica, as shown in Figure 15a (inset). The E\({}^{\prime}\) of the silica-filled P10-DPNR was higher than that of the P10-DPNR and tended to increase with increasing silica content due to the stronger modified rubber-filler interaction [75], suggesting that  
Figure 16: (**a**) Tear strength and (**b**) hardness of the silica-filled natural rubber composites.  
more energy was required for its deformation. In addition, the Tg values can be estimated from the tan \(\delta\) at the transition region, as shown in Figure 15b and Table 5. It was found that the Tg values for P10-DPNR and P30-DPNR shifted to a higher temperature when compared to the DPNR. It was also observed that the shift in Tg for the silica-loaded P10-DPNR was higher (temperature) than for the unloaded silica sample. The Tg of DPNR-10 Si shifted to a lower temperature. This may be due to the weak interaction between the DPNR and silica [76]. The Tan \(\delta\) is a crucial parameter to determine the viscoelastic properties of the materials, which is the ratio of the loss modulus to the storage modulus. From Table 5, the highest tan \(\delta\) peak was found for DPNR, which exhibited a more viscous response due to having the highest chain mobility. A reduction in the tan \(\delta\) peak height was observed after grafting with (PAA-co-PAM) and/or the addition of the silica particles. For the same amount of silica, the tan \(\delta\) peak height for P10-DPNR-10 Si was lower than that of DPNR-10 Si. This could indicate that the restriction of rubber chain movement (via the grafting with PAA-co-PAM) and the strong interaction between the modified rubber and silica demonstrated a change in the Tg and tan \(\delta\) peak [77]. This resulted in an enhanced elastic behavior in the composites. Therefore, the (PAA-co-PAM)-modified DPNR and the incorporation of silica exhibited a higher storage modulus and lower tan \(\delta\) peak, indicating higher stability to the deformation when compared to the DPNR.