*3.6. 2D-WAXS Patterns of Biaxial Stretching Films*

Figure 6 shows the 2D-WAXS images of the PBAT-1.0 casting films under different biaxial stretching ratios. It is observable that there are four crystal planes (111), (100), (110), and (010) in the PBAT film (1 × 1) in Figure 6a, and these crystal planes belong to the PBAT phase [10]. With the increase of the stretching ratio in the machine direction (MD), the crystal planes (111), (100), (110), and (010) in the PBAT composite films (Figure 6b,c) had more obvious orientation. In addition, the larger the stretching ratio, the more obvious the orientation effect, which indicates that uniaxial stretching can promote the orientation of the PBAT/Ti3C2TX biaxial stretching films' crystal form along the MD. In Figure 6d,e, there is no obvious crystal orientation in the 2D-WAXS diffraction pattern in the biaxially stretched PBAT/Ti3C2TX films, indicating that the biaxial stretching will not cause the film to have an obvious crystal orientation in a certain direction. The crystal orientation of PBAT/Ti3C2TX composite films further confirms that the biaxially oriented PBAT/Ti3C2TX film has excellent isotropy.

**Figure 6.** The films of PBAT-1.0 under different biaxial stretching ratio (transverse direction × machine direction). (**a**) 1 × 1, (**b**) 1 × 2, (**c**) 1 × 3, (**d**) 1.5 × 1.5, and (**e**) 2 × 2.

#### *3.7. Gas Barrier Properties of Biaxial Stretching Films*

The gas barrier properties of PBAT/Ti3C2TX nanocomposite casting films are shown in Figure 7. In Figure 7a, the OTR of PBAT nanocomposite casting films shows a decreasing trend with the increase of Ti3C2TX content. The lowest OTR was achieved for PBAT-1.0, which decreased from 1030 to 782 cc/m2·day. Similarly, the water vapor transmission rate (WVTR) of PBAT/Ti3C2TX nanocomposite casting films decreased as the Ti3C2TX content increased in the PBAT matrix. In Figure 7b, the WVTR for PBAT-0, PBAT-0.5, PBAT-1.0, and PBAT-2.0 is determined to be 14.3, 12.7, 10.2, and 11.7 g/m2·day, respectively. It is speculated that the addition of Ti3C2TX nanosheets can serve as a barrier to form a tortuous path, increasing the effective diffusion path length. Furthermore, the abundant hydroxyl groups on the surface of Ti3C2TX will contribute to the interactions with water molecules, delaying the diffusion to some extent. However, the aggregation of Ti3C2TX will result in a deterioration of the gas barrier performance when the content of Ti3C2TX is increased by up to 2.0 wt%.

To investigate the effects of the stretching ratio on the gas barrier performance of PBAT/Ti3C2TX nanocomposite stretching films, the OTR and WVTR data of PBAT-1.0 stretching film under different stretching ratios are shown in Figure 8. In Figure 8a, it is clear that the OTR of PBAT-1.0 stretching film decreased from 782 to 732 cc/m2·day with the stretching ratio increasing to 3 under uniaxial stretching. This can be attributed to the enhanced orientation of PBAT crystallites formed during the uniaxial stretching process, which is demonstrated in 2D-WAXS patterns (Figure 6). Meanwhile, the WVTR for PBAT-1.0 stretching film achieved the lowest value of 6.5 g/m2·day under 2 × 2 biaxial stretching condition, shown in Figure 8b. This is because the biaxial stretching process can contribute to the formation of an amorphous phase of PBAT and the exfoliation of Ti3C2TX sheets. However, the barrier effect of Ti3C2TX sheets is more profound than the effect of the PBAT

amorphous phase, resulting in a further decrease in OTR and WVTR. The combination of two-dimensional, inorganic nanofillers with the biaxial stretching process paves the way for the preparation of a biodegradable polymer with enhanced gas barrier performance.

**Figure 7.** The gas barrier properties of PBAT/Ti3C2TX nanocomposite casting films. (**a**) OTR, (**b**) WVTR.

**Figure 8.** The gas barrier properties of PBAT-1.0 films under different biaxial stretching ratio. (**a**) OTR, (**b**) WVTR.
