*3.3. Characterization of PBSA/m-LDH Composites after Irradiation*

The photodegradation caused by UV could induce the polymer chain scission, which supports the change in molecular weight with increasing irradiation time [14]. Figure 5a shows the change in number average molecular weight (Mn) for PBSA and PBSA/m-LDH composites after a period of irradiation time. These results demonstrate that the photodegradation causes a remarkable reduction in molecular weight in all samples, but the addition of higher m-LDH content could reduce the degradation rates. The result indicates that m-LDH can play a significant role in the photodegradation protection. In addition, the molecular weight of each sample drops sharply in the first week. To understand the difference in the degradation rate of samples on this relatively short irradiation time, the FTIR analysis was applied. As a result of hydroxyl end group oxidation and main chain scission from photolysis at ester linkages, the terminal carboxyl groups are generated. Thus, as irradiation progresses, an increase in C=O peaks is observed in the FTIR spectrum. Therefore, the carbonyl index is defined as [14,29]:

$$(\frac{A^t\_{\mathbb{C}=O}}{A^t\_{\mathbb{C}\to H}}) / (\frac{A^{t0}\_{\mathbb{C}\to O}}{A^{t0}\_{\mathbb{C}\to H}}) \tag{1}$$

where *A t*0 *C*=*O* and *A t C*=*O* are the intensity of carboxyl groups at 1712 cm−<sup>1</sup> before and after irradiation, respectively; *A t*0 *C*−*H* and *A t C*−*H* are assigned to the C–H stretching peak at 2858 cm−<sup>1</sup> , which is used as the reference for calculating the value of the carbonyl index, before and after irradiation, respectively [14,28]. Therefore, the higher carbonyl index indicates the poor photostability of materials. The evolution of C=O at 1712 cm−<sup>1</sup> of PBSA and PBSA/m-LDH composites are shown in Figure A2. Figure 5b shows the carbonyl index of the corresponding samples. The increase of m-LDH content could remarkably reduce the carbonyl index in different irradiation time, which indicates decreased photodegradation of PBSA.

**Figure 5.** (**a**) Molecular weight (Mn) and (**b**) carbonyl index of PBSA and PBSA/m-LDH composites after different irradiation time.

The result of samples after artificial photodegradation test shows the m-LDH is an effective nanomaterial to reduce the photodegradation of PBSA. Based on past literature, Zn-Ti LDH has been shown to have UV absorption ability. This study also showed that Zn-Ti LDH keeps this feature after SA modification. The photodegradation of polymer starts from the surface and then develops along the depth [14]. In addition, destruction via photodegradation can induce the entry of oxygen and promote further degradation. For pure PBSA, UV light could enter the inside of the material without additional hindrance, causing the above degradation reaction. For PBSA/m-LDH composites, UV light can be absorbed by m-LDH, which might decreased the photo intensity enter the inside of the material, causing the less degradation reaction. The morphologies of PBSA and PBSA/m-LDH-5 after different UV irradiation time are shown in Figure 6. Prior to irradiation, both of them exhibited a smooth surface with no significant defects. After irradiation, the

morphology became rough and was characterized with cracks. After 4 weeks, PBSA showed a rougher surface, indicating a stronger photodegradation behavior than PBSA/m-LDH-5.

**Figure 6.** SEM images of surface morphology of PBSA and PBSA/m-LDH-5 after different irradiation time.
