## 3 Results and Discussion
### The Effect of the BA/BI Ratio of PBABA Copolymers with an EDTA Concentration of 0.1 mole%
An increase in the storage modulus (\(E^{\prime}\)) was observed below the \(T_{\text{g}}\) (glassy state) in Figure 5b, indicating that the molecular chain movement was fixed, and the physical property was dominated by raw chemicals in the glassy state, resulting from a more rigid double-bond within the IA. The transition behavior of E' appeared near \(T_{\text{g}}\); \(E^{\prime}\) decreased when the temperature was raised above \(T_{\text{g}}\) (rubbery state), because the double-bond within IA could be induced to disturb and damage the main-chain molecules' symmetry and regularity in the amorphous regime, to decrease the \(E^{\prime}\) in the rubbery state [51]. The X-ray diffraction (XRD) of PBABI copolymers in the 20 of 10-40\({}^{\circ}\), is shown in Figure 6. The XRD patterns of the PBABI had similar patterns to that of the PBA (BA/BI = 10/0), with a BI ratio of 30 mole%, demonstrating that the existence of the BI unit did not affect the crystal behavior of the BA unit. This was also observed in the poly(butylene succinate-\(\infty\)-butylene itaconate) system [65]. The feature peak of XRD was carried out at the 20 values of 21.61\({}^{\circ}\), 22.31\({}^{\circ}\), and 23.96\({}^{\circ}\), for the (110), (020), and (020) crystal lattices, respectively, which were associated with the \(\alpha\)-phase of the neat PBA [38; 66]. The (110) and (020) crystal lattices decreased as the IA concentration increased to BA/BI = 8/2; however, the intensity grew when the concentration increased to BA/BI = 7/3, implying that IA could participate in and enhance the chain packing into the ordered state, to increase the d-spacing. The percentage crystallinities of synthesized polyesters determined using XRD are listed in Table 3. PBABI copolymers displayed crystallinity values within the range of 34.1% to 39.4%. Generally, the crystallinities of the PBABI copolymers did not exceed 40%, which specified that the crystal formation was favored by the overall van der Waals interaction and by EDTA, to form cross-linked polyesters. PBABI copolymers displayed an increase of chain flexibility with increasing BI content. This increased chain flexibility might have promoted a more-effective rearrangement of the polymer chains, permitting van der Waals interactions to contribute a more significant degree towards the crystalline domains formation, and might have increased the crystallinity. Generally speaking, the calculated crystallinities obtained via DSC were lower than those from XRD results; although, there were similar trends, suggesting a good correlation between the two measurements. In the NMR results in Figure 1, the concentration of the C=C bond of IA, within the PBABI copolymers was maintained near 50%, in each ratio of the copolymers, implying that 50% C=C of IA might be converted into saturated C-C, to support the cross-linking agent in forming a 3D network structure. [PERSON] et al. studied the poly(butylene  
Figure 5: (a) Tan 8 and (b) storage modulus of the PBABI copolymers, via the dynamic mechanical analyzer. itaconate-_co_-butylene succinate) (PBIBSu) system [50] via FT-IR analysis, which indicated the degree of curing of the PBIBSu copolymersets could be located around 50%-75%, with various ratios of IA and SuA. They hypothesized that the IA is not reactive enough under melt polymerization and the conversion of C=C in cross-linking operation could be achieved at 75%. At a 7/3 BA/BI ratio, the preserved C=C concentration was maintained at 15.01%, and the intensity of XRD increased in the (110) and (020) crystal lattices. Hence, staying near a 15% ratio for the C=C concentration of IA was critical for enhancing the unsaturated C=C to saturated C-C, to form a higher partial network architecture, which could decrease the main-chain motion to make the molecular chain easy to pack into the ordered state, thereby, increasing the crystal region. Furthermore, the (021) crystal lattice disappeared in the BI unit of 30 mole%, due to a higher degree of cross-linking.