## 4 Conclusions  
(PAA-co-PAM)-_grafted_ deproteinized natural rubber particles were successfully prepared through emulsion graft copolymerization in the latex stage. The optimum conditions were a reaction time of 360 min, a reaction temperature of 50 \({}^{\circ}\)C, and an initiator concentration of 1.0 phr. Monomer conversion was found in the range of 91.9-94.1%. The percentage of grating efficiency and grafting percentage were approximately 20.8-38.9% and 2.1-9.9%, respectively. The PAA-co-PAM covered the DPNR surface, as observed by TEM, bringing colloidal stability to the DPNR particles via electrosteric stabilization. The modification of the DPNR can enhance hydrophilicity, which was investigated by water contact angle measurement. The contact angle of the natural rubber decreased from 95.5\({}^{\circ}\) in the DPNR to  
Figure 17: (**a**) Storage modulus: inset; storage modulus of silica-filled P10-DPNR as a function of silica content; (**b**) loss tangent (tan \(\delta\)) of silica-filled natural rubber composites as a function of temperature.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Samples** & **DPNR** & **DPNR-10 Si** & **P10-DPNR** & **P10-DPNR-10 Si** & **P10-DPNR-20 Si** & **P30-DPNR** \\ \hline Tg (\({}^{\circ}\)C) & \(-\)48.06 & \(-\)49.98 & \(-\)47.87 & \(-\)45.47 & \(-\)45.49 & \(-\)47.60 \\ Tan \(\delta\) peak height & 2.73 & 2.33 & 2.65 & 2.18 & 1.98 & 1.26 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Viscoelastic properties of vulcanaries.  
35.2\({}^{\circ}\) in the P30-DPNR, indicating enhanced polarity. It is worth mentioning that increasing polarity may improve the compatibility between the silica and rubber, which, consequently, improves the tensile strength, tear strength, and hardness. Moreover, the swelling of the silica-filled (PAA-_co_-PAM)-DPNR in a nonpolar solvent decreased due to the H-bonding interaction with silica. In addition, the composite made from grafting PAA-_co_-PAM onto the DPNR exhibited an increase in storage modulus and a decrease in tan \(\delta\) peak, indicating that the silica-filled (PAA-_co_-PAM)-DPNR composites had more thermal stability when compared to the DPNR. The modified natural rubber composites in this study showed superior mechanical properties, which can be employed in many rubber applications.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/polym14214602/s1](https://www.mdpi.com/article/10.3390/polym14214602/s1)]([https://www.mdpi.com/article/10.3390/polym14214602/s1](https://www.mdpi.com/article/10.3390/polym14214602/s1)), Figure S1: Creating index of (PAA-_co_-PAM)-DPNR prepared by using weight ratio of acrylic acid and acrylamide at 50:50 compared to PAA-DPNR and PAM-DPNR; Figure S2: Solid \({}^{13}\)C-NMR spectrum of P30-DPNR; Figure S3; FTIR of (PAA-_co_-PAM)-DPNR at different weight ratio of acrylic acid and acrylamide.  
Conceptualization, C.R. and Y.R.; Data curation, S.I.; Formal analysis, S.I. and C.R.; Funding acquisition, C.R. and Y.R.; Investigation, S.I. and N.B.; Methodology, S.I. and C.R.; Project administration, C.R. and Y.R.; Resources, C.R.; Supervision, C.R.; Validation, K.J., P.C. and C.R.; Visualization, C.R.; Writing--original draft, S.I.; Writing--review & editing, S.I., K.J., P.C., C.R. and Y.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Thailand Science Research and Innovation (TSRI), Grant No. 160344.  
Not applicable.  
The authors are grateful to Suranaree University of Technology (SUT), Thailand Science Research and Innovation (TSRI), the National Science, Research and Innovation Fund (NSRF), and the Research Center for Biocomposite Materials for the Medical Industry and Agricultural and Food Industry for the financial support.  
The authors declare no conflict of interest.