## 3 Results and Discussion
### Characterization of (PAA-_co_-PAM)-Modified Deproteinized Natural Rubber  
#### 3.2.1 Chemical Structure of the (PAA-_co_-PAM)-Modified DPNR  
The chemical structure and functional groups of DPNR and (PAA-_co_-PAM)-modified DPNR were examined by FTIR to confirm graft copolymerization. Figure 4a shows the FTIR spectra of the (PAA-_co_-PAM)-modified DPNR with different monomer contents after extraction to separate the ungrafted PAA-_co_-PAM and compare it to the original DPNR. The FTIR spectrum of the DPNR shows a peak at 1664 cm\({}^{-1}\), which corresponds to C = C stretching vibration. The peaks at 1446 cm\({}^{-1}\) and 1376 cm\({}^{-1}\) are assigned to -CH\({}_{2}\) and -CH\({}_{3}\) stretching, respectively. The peak appearing at 841 cm\({}^{-1}\) corresponds to =CH bending out of plane. These peaks indicate the chemical structure of polyisoprene in natural rubber [43,44]. In the case of (PAA-_co_-PAM)-DPNR at different monomer concentrations after extraction, its spectra showed the important characteristics of natural rubber. Moreover, a broad  
Figure 3: (**a**) Effect of initiator contents; (**b**) effect of monomer contents on the monomer conversion, grafting efficiency and grafting percentage.  
peak was found at approximately 3500-3200 cm\({}^{-1}\), indicating NH and OH stretching. The C = O and C-O stretching vibrations appeared at 1663 and 1240 cm\({}^{-1}\), respectively. The peak at 1563 cm\({}^{-1}\) was assigned to carboxylate (-COO\({}^{-}\)), obtained by the partial neutralization of the acrylic acid with a basic compound [45]. A peak at 1613 cm\({}^{-1}\) for NH bending was also observed. These correspond to the functional groups of polyacrylic acid and polyacrylamide [46]. This can indicate the successful grafting of polyacrylic acid and polyacrylamide onto natural rubber. The grafting was also confirmed by solid-state NMR. The CP/MAS \({}^{13}\)C-NMR spectrum of P30-DPNR (Figure S2, see Supplementary Information) shows characteristic signals both of natural rubber and grafted polymer. The signals at 136.4, 126.9, 34.0, 28.3, and 25.1 ppm were assigned to the isoprene units of natural rubber [47]. Moreover, peaks at around 182.9 ppm (C=O) and 45.5 ppm (CH\({}_{2}\) and CH) of polyacrylic acid and polyacrylamide also appeared [48]. Furthermore, it was observed that the C = O stretching (1663 cm\({}^{-1}\)) of the (PAA-_co_-PAM)-DPNR from the FTIR spectra was shifted from both PAA-DPNR and PAM-DPNR. As can be seen in Figure S3 (see Supplementary Information), the PAA-DPNR spectrum showed a signal at 1704 cm\({}^{-1}\)[49], which was attributed to C = O stretching in the carboxylic acid. The signal of C = O in the PAM-DPNR occurred at 1656 cm\({}^{-1}\)[50]. For the (PAA-_co_-PAM)-DPNR at a monomer ratio of 50:50 by weight percentage, the C = O shifted to 1663 cm\({}^{-1}\). These changes indicate the occurrence of hydrogen bonding interactions between polyacrylic and polyacrylamide in the system [51]. Moreover, the peak intensity ratios between the C = O stretching of PAA-_co_-PAM at 1663 cm\({}^{-1}\) and the -CH\({}_{3}\) stretching of natural rubber at 1376 cm\({}^{-1}\) were analyzed, as shown in Figure 4b. From the FTIR spectra, although the peak representing the C = C (1664 cm\({}^{-1}\)) of natural rubber overlapped with the peak of the C = O, the peak intensity ratio of the modified DPNR was higher than that of the DPNR. This demonstrated the presence of grafted PAA-_co_-PAM. The intensity ratios also increased with increasing monomer content. These confirmed the increase in polymer grafted onto the natural rubber, which corresponded to the results calculated from the gravimetric analysis.