*3.2. Characterization of PBSA and Its Composites*

Figure 3a shows the TEM images of PBSA/m-LDH-5. It can be seen that the stacking layers of m-LDH dispersed in PBSA matrix. Figure 3b shows the XRD diffraction patterns of PBSA and its composites. The XRD experimental data of PBSA shows three diffraction peaks at 2θ = 19.4◦ , 21.7◦ , and 22.4◦ , corresponding to (020), (021), and (110) crystallographic planes of monoclinic PBS, respectively [17,18]. The XRD results show that the addition of m-LDH did not change the crystal structure of PBSA. In addition, by increasing m-LDH content, the peak positions of (003), (006), and (009) planes for m-LDH remained unchanged, and the intensities of these peaks increased. These results indicate that the initial layer stacking structure of m-LDH is still present in PBSA.

**Figure 3.** TEM image of (**a**) PBSA/m-LDH-5 and (**b**) XRD, (**c**) DMA, and (**d**) UV-vis result of PBSA and its composites.

The mechanical properties, represented by the storage modulus (E0 ), of the PBSA and its composites evaluated by DMA are shown in Figure 3c. The phenomenon of E' dropping rapidly at approximately −40 ◦C contributes to the occurrence of glass transition of PBSA [27]. The effect of m-LDH on the mechanical properties of PBSA can be seen from E<sup>0</sup> at the temperature below Tg. The value of E<sup>0</sup> at −80 ◦C increases significantly with increasing filler content. Detailed data are presented in Table 1. The enhancement of mechanical properties for the PBSA composites may be attributed to the reinforcement effect of the additional stiffness of LDH [5]. In addition, the PBSA/m-LDH-5 composites exhibit the highest mechanical properties at 25 ◦C, as shown in Figure 3c and Table 1.



E0−<sup>80</sup> and E0<sup>25</sup> : storage modulus at −80 and 25 ◦C, respectively, measurement by DMA. Tc: crystallization temperature during 1st cooling trace, measurement by DSC. Tm: crystalline melting temperature during 2nd heating trace, measurement by DSC. Xc: crystallinity, which obtained by the following equation, Xc(%) = ∆Hf/ h (<sup>1</sup> <sup>−</sup> <sup>ϕ</sup>)∆H<sup>0</sup> f i <sup>×</sup> 100%, where <sup>∆</sup>H<sup>0</sup> f = 117.2 Jg−<sup>1</sup> for PBSA with 72% BS group ratio, and ϕ is the weight fraction of the filler in the composites.

> The UV–vis absorbance spectrum of PBSA and its composites are shown in Figure 3d. A relatively weak absorption of PBSA was observed in the range from 250 to 320 nm, which is attributed to the absorption of the carbonyl group [14]. By adding m-LDH into the PBSA

polymer matrix, the composites show an improvement in UV-absorbing character in the whole UV-B and UV-A region. The absorption intensity of PBSA composites increases with increasing m-LDH content. This result indicates that excellent UV absorption property of m-LDH can be used as a UV protecting additive for PBSA.

The effect of the chemical modification of nanofiller on polymer composite has been reported by Zhang et al., which shows an increasing T<sup>c</sup> with modified nanofiller but a decreasing T<sup>c</sup> by an unmodified nanofiller [33]. In this study, the crystallization behavior of PBSA and its composites were analyzed using 1st cooling and 2nd heating of DSC measurement. The crystallization temperature (Tc) of PBSA/m-LDH-1 shown in Figure 4a is higher than that of PBSA. With increasing m-LDH content, T<sup>c</sup> gradually decreased but was still higher than that of PBSA. The detailed data are presented in Table 1. The increase in T<sup>c</sup> is due to the heterogeneous nucleation caused by m-LDH. Further, increasing m-LDH content may hinder the chain motion of PBSA during crystallization, thus, leading to a decrease in T<sup>c</sup> [3,20].

**Figure 4.** (**a**) DSC 1st cooling and (**b**) 2nd heating curves of PBSA and its composites.

Figure 4b shows the melting behavior in the 2nd heating of PBSA and its composites. The melting temperatures (Tm) of PBSA and its composites are almost the same in all samples, showing that the melting behavior of the PBSA crystallite was not affected by adding fillers. The detailed data are presented in Table 1. An obvious difference observed in the composites with m-LDH revealed a small melting peak appearing before the main peak of samples. These small melting peaks might be attributed to the melt–recrystallization– remelt phenomenon of the PBSA crystallite. The crystallinity of polymer can be calculated by dividing the observed ∆H<sup>f</sup> by the theoretical value (∆H<sup>0</sup> f ) for perfectly (100%) crystalline polymer. The theoretical ∆H<sup>0</sup> f for polybutylene succinate (PBS) and polybutylene adipate (PBA) are 110.3 and 135.0 J/g, respectively. The theoretical ∆H<sup>0</sup> f for PBSA can be calculated via the basis of the butylene succinate (BS)/butylene adipate (BA) group contribution method [34]. In this study, the PBSA material has 72% BS composition (determined via <sup>1</sup>H nuclear magnetic resonance, shown in Figure A1). The crystallinity of all samples are presented in Table 1. By adding m-LDH into PBSA, the crystallinity increases from 27.4% for PBSA to 31.0, 29.5, and 28.6% for PBSA/m-LDH-1, PBSA/m-LDH-3, and PBSA/m-LDH-5, respectively. The increase in crystallinity might be due to the effect of m-LDH via heterogeneous nucleation. Compared to PBSA, the higher crystallization temperature of PBSA/m-LDH-1 leads to better chain motion during crystal growth at previous cooling process. Higher m-LDH content hinders chain motion during crystallization, which leads to the crystallinity decrease with increasing m-LDH content. At the same time, the lower crystallization temperature compared to PBSA/m-LDH-1 is not conducive to chain motion. Higher m-LDH content hinders chain motion during crystallization, which leads to a decrease in crystallinity with increasing m-LDH content.
