## 3 Results and Discussion
### Characterization of (PAA-_co_-PAM)-Modified Deproteinized Natural Rubber
#### 3.2.3 Particle Size and Surface Properties  
The particle size distribution and size of the prepared samples were examined using a dynamic light-scattering (DLS) technique, the results of which are displayed in Figure 6a and Table 3, respectively. The DPNR particles had a mean size of 0.610 \(\pm\) 0.078 \(\upmu\)m. After modification by graft copolymerization, the particle size increased. When the monomer was added to 10 phr, the mean particle size was 2.533 \(\pm\) 0.080 \(\upmu\)m. The particle size decreased to 0.874 \(\pm\) 0.005 \(\upmu\)m when a 30 phr monomer was used. The results were consistent with the transmission electron microscopy. A reduction in particle size was found with an increase in monomer content. The increase in monomer content resulted in an increase in the resulting (PAA-_co_-PAM) covering the rubber particles, causing the rubber particles to be separated from each other, as shown in Figure 6b. Moreover, the PAA-_co_-PAM that covered the DPNR particles had the ability to stabilize the rubber particles through electrosteric repulsion [53]. As can be seen from the zeta potential measurements from Table 3, the DPNR exhibited a negative zeta potential (\(-\)20 mV). This slightly negative value may be attributed to the remaining proteins in the system. The surface charges of P10-DPNR and P30-DPNR were observed to be \(-\)33.2 and \(-\)64.5 mV, respectively. The highly negative values might have resulted from the carboxylate groups containing the PAA-_co_-PAM adsorbed onto DPNR particles after modification. They can provide electrosteric repulsion to prevent aggregation and sufficiently stabilize the rubber particles, which are dispersed in the water phase [54].  
Figure 5: TEM images of (**a,b**) DPNR; (**c,d**) P10-DPNR; (**e,f**) P30-DPNR.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No**} & \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**Monomer Content**} & \multicolumn{2}{c}{**Particle Size (\(\upmu\)m)**} & \multirow{2}{*}{**Zeta Potential**} & \multirow{2}{*}{**Tg 2** **} \\ \cline{3-3} \cline{5-6}  & & **(phr)** & & **SEM 1** ** & **DLS** & & \\ \hline
1 & DPNR & 0 & 0.384 \(\pm\) 0.089 & 0.610 \(\pm\) 0.078 & \(-\)20.1 & \(-\)65.0 \\
2 & P10-DPNR & 10 & 2.086 \(\pm\) 0.506 & 2.533 \(\pm\) 0.080 & \(-\)33.2 & \(-\)64.0 \\
3 & P30-DPNR & 30 & 0.805 \(\pm\) 0.222 & 0.874 \(\pm\) 0.005 & \(-\)64.5 & \(-\)62.9 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Particle size, zeta potential, and Tg of DPNR and (PAA-_co_-PAM)-DPNR with different monomer contents.  
Figure 7 shows the contact angle of the DPNR and the (PAA-co-PAM)-modified DPNR films. The contact angle was also used to determine the surface properties of the prepared samples. The DPNR film had a contact angle of 95.5\({}^{\circ}\). The contact angle tended to decrease when the grafting percentage of the modified DPNR increased. The P10-DPNR and P30-DPNR had a contact angle of 61.2\({}^{\circ}\) and 35.2\({}^{\circ}\), respectively, which decreased by 0.36-0.63 times when compared to the DPNR. The decrease in the contact angle of the modified DPNR indicated that the modified DPNR films were more hydrophilic than the DPNR version [55]. Therefore, grafts using PAA-co-PAM can enhance the polarity of natural rubber. This might be useful for preparing composites by mixing them with polar reinforcing agents, such as silica, to improve the mechanical properties of the composites.