## 3 Results and Discussion
### The Effect of the BA/BI Ratio of PBABA Copolymers with an EDTA Concentration of 0.1 mole%
A simulation model has been designed and constructed with a center molecule of the EDTA, to investigate the rigidity of EDTA as a node within PBABI copolymers. The EDTA has four COOH groups, which connect with four 1,4-BDOs with OH groups, to extend the length of the molecular chain, and then link to the three AA units and one IA unit, for our artificial initial geometrical conformation (as denoted by EDTA-BDO-BA/BI). This model for the BA/BI ratio was associated with a ratio of 75/25. The geometric conformation for the EDTA-BDO-BA/BI model was built in a tetrahedral shape. In Figure 9, the tetrahedral structure was maintained well at 298 K, after 1 ns molecular dynamics (MD) simulation, indicating that EDTA played a crucial role in supporting the stereo conformation. When the temperature increased, the stereo conformation became distorted and twisted, but the tetrahedral architecture was preserved by the central formation of the EDTA molecule, indicating that EDTA was not a sufficiently flexible or rigid molecule, and exhibited a semi-rigid property to play a node role within the cross-linking network. The torsional angle distribution of N-C-C-N, within the EDTA and of N-C-C-O between the EDTA and the 1,4-BDO, as a function of time, were calculated at various temperatures, including \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Yield Strength** & **Elongation** & **Young’s Modulus** \\  & **(MPa)** & **(\%)** & **(MPa)** \\ \hline BA/BI = 10/0 & 13.21 \(\pm\) 1.24 & 531.76 \(\pm\) 55.43 & 128.65 \(\pm\) 12.23 \\ BA/BI = 9/1 & 9.33 \(\pm\) 1.03 & 33.67 \(\pm\) 4.23 & 94.09 \(\pm\) 8.37 \\ BA/BI = 8/2 & 4.73 \(\pm\) 0.45 & 26.24 \(\pm\) 2.46 & 49.83 \(\pm\) 2.44 \\ BA/BI = 7/3 \({}^{*}\) & 0.27 \(\pm\) 0.07 & 71.04 \(\pm\) 10.45 & 0.77 \(\pm\) 0.12 \\ \hline \hline \end{tabular} \({}^{*}\) The \(T_{\rm m}\) of BA/BI = 7/3 is below room temperature.  
\end{table} 
Table 5: Mechanical properties of the PBABI copolymers. Figure 9: The structural deviation of the node point (EDTA-BDO-AA/IA) at various temperatures. (Color key—gray: carbon, red: oxygen, and blue: nitrogen, white: hydrogen (emitted)). 298 K, 333 K, and 373 K (as shown in Figure 10). The torsional angle of N-C-C-N was seen at 180\({}^{\circ}\) with a sharp peak, demonstrating that the center of EDTA between the two nitrogen atoms tended to form the coplanar and trans-conformation, even at higher temperatures of 333 K and 373 K [68]. Meanwhile, the torsional angle of N-C-C-O was maintained at 70\({}^{\circ}\), 180\({}^{\circ}\), and 250\({}^{\circ}\), implying the geometric architecture exhibited a tetrahedral conformation. As the temperature increased, the peaks of N-C-C-O distribution stayed around 70\({}^{\circ}\), 180\({}^{\circ}\), and 250\({}^{\circ}\), with a broader distribution, implying that the stereo conformation of tetrahedral shapes was distorted by thermal treatment, but was not disrupted into a disordered state [69]. In the process, EDTA as a cross-linking node tended to represent a semi-rigid molecule. When the EDTA's semi-rigid characteristic was investigated, the chain motion near the EDTA could be restricted to form a nucleation site, which could make the main-chain near EDTA easier to pack into the ordered state to form crystal regions. In other words, the molecular chain linked with EDTA of PABBI copolyesters, could be rotated to reach a suitable steric conformation, which could be stacked into a requested state, as a result of the semi-rigid properties of the EDTA as a node.