## 3 Results and Discussion
### The Effect of the BA/BI Ratio of PBABA Copolymers with an EDTA Concentration of 0.1 mole%
Shore D as a function of UV curing time of the PABBI copolymersets was measured; shown in Figure 7. The highest hardness was observed at a ratio of BA/BI =10/0. The Shore D of synthesized PABBI copolymersets decreased as the IA concentration increased, due to the disruption of the crystal region. However, the Shore D values decreased gradually, when UV curing time through ultraviolet irradiation increased to more than 1 min, at a ratio of BA/BI = 10/0. At BA/BI = 9/1 and 8/2, the Shore D increased slightly, within 2 min, and then showed a smooth decline after 2 min of UV curing. It was notable that the Shore D of BA/BI = 7/3 increased within 5 min, during UV curing, which was attributed to the effectiveness of the C=C, in IA concentration, with regard to enhancing the degree of partial cross-linking, to improve the hardness of the PABBI copolymersets. The Shore D decreased slightly, after a UV curing time of 10 min, due to disruption of the structure from a higher thermal energy with the UV-Vis light. The results indicated that the cross-linking behavior of IA, occurring at BA/BI = 7/3, was triggered by the post-polymerization modification of the UV curing operation, resulting from sufficient C=C in IA concentration, at a value of 15 mole%, for cross-linking. The cured IA-based copolymers were observed to enhance the \(T_{\text{g}}\) and storage modulus, suggesting that the C=C of IA could improve the thermal and mechanical characteristics, through UV curing procedures [50; 67]. All specific Shore D values have been summarized in Table 4. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Cure Time (min)** & **BA/BI = 10/0** & **BA/BI = 9/1** & **BA/BI = 8/2** & **BA/BI = 7/3** \\ \hline
0 & \(62.0\pm 3.52\) & \(56.5\pm 3.27\) & \(52.5\pm 2.88\) & \(49.5\pm 2.43\) \\
1 & \(61.9\pm 3.78\) & \(56.3\pm 3.06\) & \(52.8\pm 2.53\) & \(53.6\pm 2.37\) \\
2 & \(58.2\pm 5.99\) & \(56.7\pm 5.14\) & \(52.7\pm 2.71\) & \(53.8\pm 1.99\) \\
5 & \(55.0\pm 3.65\) & \(52.2\pm 5.28\) & \(49.0\pm 3.30\) & \(55.8\pm 2.20\) \\
10 & \(51.3\pm 3.16\) & \(47.1\pm 1.91\) & \(46.2\pm 4.29\) & \(54.8\pm 3.77\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The values of Shore D with standard deviation as a function of curing time for the PABBI copolymersets. Figure 6: X-ray diffraction of the PABBI copolymersets. Stress-strain measurements were performed to investigate the mechanical properties, including the yield strength, elongation at break, and Young's modulus of the PBABI copolyesters. Those results were extracted from Figure 8 and are summarized in Table 5. In the tensile test, the macroscopic deformation of BA/BI = 10/0 (pure PBA) was examined at the highest strength (13.205 MPa) and elongation at break (\(\sim\) 531.755%). All of the mechanical properties decreased and displayed a brittle characteristic when the IA ratio increased above 10 mole%. The yield strength, elongation, and Young's modulus in BA/BI = 9/1 are shown for 9.328 MPa, 33.665%, and 94.093 MPa at room temperature measurement, respectively. [PERSON] et al. (2016) studied whether an increase in the length and concentration of the itaconate ester group could worsen mechanical properties and hinder the packing behavior of the molecular chain. The tensile test could not be measured accurately at BA/BI = 8/2 and 7/3, because their \(T_{\rm m}\) values were observed at 33.8 and 21.1 \({}^{\circ}\)C, respectively, which was too close to or below RT (25 \({}^{\circ}\)C). Since BA/BI = 8/2 and 7/3 exhibited soft and viscose properties, respectively, the tensile test could not be performed at RT, through a universal tension machine. The real stress-strain curve could be tested at a lower temperature, to obtain the unique mechanical properties related to the storage modulus, implying that the PBABI copolyesters in low-temperature conditions might manifest an excellent mechanical behavior. Figure 8: The stress–strain curves of the PBABI copolyesters. Figure 7: Shore D values at different UV curing times of the PBABI copolyesters.