## 3 Results and Discussion
### The Effect of Varying EDTA Concentrations at BA/BI = 8/2 of PABBI Copolymers
Figure 13 illustrates the DMA analysis of Tan \(\delta\) and storage modulus in a ratio of BA/BI = 8/2 of PBABI copolyesters, at different concentrations of EDTA. The Tan \(\delta\) of PBABI copolyesters decreased slightly at higher concentrations of EDTA, and the \(T_{\text{g}}\) values were located around \(-\)54.5 \(\pm\) 1.6 \({}^{\circ}\)C. The largest storage modulus was observed at an EDTA ratio of 0.2 mole%, suggesting that the PBABI copolyesters in both glassy and rubbery states tended to be harder, due to the relatively higher proportion of EDTA. A greater concentration of EDTA played a node role within copolyesters to form a tighter and well-dispersed 3D network architecture, which could improve the hardness of the PBABI copolyesters. Figure 11: DSC trace of the PBABI copolyesters with different concentrations of EDTA. (**a**) Curves of heating in the first cycle, (**b**) curves of cooling in the first cycle, and (**c**) curves of heating in the second cycle. Figure 12: Thermogravimetric analysis (TGA) curve of the PBABI copolyesters with different concentrations of EDTA. Thermal property measurements are outlined in Table 6. \(T_{\mathrm{g}}\) was observed between \(-\)52.8 to \(-\)56.1 \({}^{\circ}\)C, with an increase in the EDTA ratio. At an EDTA ratio of 0.05 mole%, \(T_{\mathrm{c}}\) was reached at a temperature of 9.3 \({}^{\circ}\)C, indicating it could crystallize at a higher temperature than other concentrations of EDTA, which was evident in the higher \(\Delta H_{\mathrm{c}}\) value of \(-\)43.8 mJ\(\cdot\)mg\({}^{-1}\). The DSC curve of T\({}_{\mathrm{m}}\) in the reheating procedure had a similar trajectory in the two separate peaks around 30 and 40 \({}^{\circ}\)C, but the \(\Delta H\)m was slightly different. It is interesting that the most significant \(\Delta H_{\mathrm{m}}\) value occurred at an EDTA ratio of 0.05 mole% and then decreased at 0.1 mole%, implying that the optimal concentration of EDTA might be around 0.05-0.1 mole%. The absorption peak of FT-IR spectra in the PBABI copolymers at various ratios of EDTA are displayed in Figure 14. It is not surprising that the FT-IR curve of each ratio of EDTA in BA/BI = 8/2 exhibited similar trends and absorption intensities, because the EDTA concentration was too low to examine the feature peak in FT-IR spectra. All absorption peak positions could be referred to Figure 2.  
The XRD patterns for BA/BI = 8/2 of PBABI copolymersters at EDTA concentrations from 0.001 to 0.2 mole% are shown in Figure 15. The values of 20 of BA/BI = 8/2 with EDTA of 0.001 mole% of PBABI copolymers were identified at 21.46\({}^{\circ}\), 22.13\({}^{\circ}\), and 23.97\({}^{\circ}\), which relates to the crystal lattices of (110), (020), and (020), respectively. The intensity of the crystal lattices at (110) and (020) increased with a ratio of EDTA between 0.001 to 0.05 mole%, which indicated that the degree of crystallization was increased by increasing the concentration of EDTA. The most considerable \(\Delta H_{\mathrm{m}}\) value was also observed at a ratio of 0.05 mole%. However, when the EDTA was increased to 0.1 and 0.2 mole%, the intensity  
Figure 14: FT-IR spectra of the PBABI copolymers with different concentrations of EDTA. Figure 13: (a) Tan \(\delta\) and (b) storage modulus of the PBABI copolymers at different concentrations of EDTA. of feature peaks of (110) and (020) decreased gradually, implying the crystal region was reduced as the EDTA increased. The results demonstrated that EDTA played a crucial role in controlling the enthalpy of crystallization. The lower concentration of EDTA might have decreased the chain rotation to enhance the crystal zone, and the crystal region became smaller with an increase in EDTA ratio to lower the \(\Delta H_{\text{m}}\). Hence, our suggested optimal rate of EDTA as a cross-linking agent of PBABI copolymers is between 0.05-0.1 mole%.