Improvement of Gas Barrier Properties for Biodegradable Poly(butylene adipate-co-terephthalate) Nanocomposites with MXene Nanosheets via Biaxial Stretching

In order to ease the white pollution problem, biodegradable packaging materials are highly demanded. In this work, the biodegradable poly (butylene adipate-co-terephthalate)/MXene (PBAT/Ti3C2TX) composite casting films were fabricated by melt mixing. Then, the obtained PBAT/Ti3C2TX composite casting films were biaxially stretched at different stretching ratios so as to reduce the water vapor permeability rate (WVPR) and oxygen transmission rate (OTR). It was expected that the combination of Ti3C2TX nanosheets and biaxial stretching could improve the water vapor and oxygen barrier performance of PBAT films. The scanning electron microscope (SEM) observation showed that the Ti3C2TX nanosheets had good compatibility with the PBAT matrix. The presence of Ti3C2TX acted as a nucleating agent to promote the crystallinity when the content was lower than 2 wt%. The mechanical tests showed that the incorporation of 1.0 wt% Ti3C2TX improved the tensile stress, elongation at break, and Young’s modulus of the PBAT/Ti3C2TX nanocomposite simultaneously, as compared with those of pure PBAT. The mechanical dynamical tests showed that the presence of Ti3C2TX significantly improved the storage modulus of the PBAT nanocomposite in a glassy state. Compared with pure PBAT, PBAT-1.0 with 1.0 wt% Ti3C2TX exhibited the lowest OTR of 782 cc/m2·day and 10.2 g/m2·day. The enhancement in gas barrier properties can be attributed to the presence of Ti3C2TX nanosheets, which can increase the effective diffusion path length for gases. With the biaxial stretching, the OTR and WVPR of PBAT-1.0 were further reduced to 732 cc/m2·day and 6.5 g/m2·day, respectively. The PBAT composite films with enhanced water vapor and water barrier performance exhibit a potential application in green packaging.


Introduction
Green packaging materials are highly demanded in the recent years because of the everincreasing plastic pollution problem. The traditional packaging materials, such as polyethylene (PE) and poly(vinyl chloride) (PVC), have been gradually replaced by biodegradable polymers such as poly(lactic acid) (PLA), poly(butylene adipate-co-terephthalate) (PBAT), and poly(butylene succinate) (PBS) [1][2][3][4]. Compared with other biodegradable polyesters, PBAT has adjustable properties due to the copolymerization of 1,4-butanediol, adipic acid, and terephthalic acid [5]. In addition, it has good ductility, good thermal resistance, and high impact performance, which is similar to PE. However, it was reported that the oxygen transmission rate (OTR) of PBAT under ambient conditions was around 1050 cc/m 2 ·day, whereas the water vapor permeability rate (WVPR) was 3.3 × 10 −11 g·m/m 2 ·s·Pa, which made it difficult to meet the requirements for packaging applications [6][7][8]. The poor oxygen and water vapor barrier performances limit the broad applications of PBAT in packaging. Therefore, it is necessary to improve the oxygen and water vapor barrier performance of PBAT so as to prolong the shelf life and maintain good quality of food.
It is widely accepted that the incorporation of nanofillers is a simple and effective method to reduce the OTR and WVPR of polymer films [9][10][11][12] because the presence of fillers can have barrier effects that increase the escape distance of oxygen and water molecules [13,14]. Li et al. reported that the well-aligned graphene nanosheets simultaneously reduced the oxygen permeability and enhanced the aging resistance of the PBAT composite film [15]. The oxygen-containing groups on graphene nanosheets enhanced the interactions between water molecules and altered the diffusion paths of water molecules. Mondal et al. found that the WVPR of PBAT could be reduced by 25% with the addition of 4 wt% organically modified montmorillonite (OMMT) [16]. This was because the impermeable OMMT in the PBAT matrix increased the tortuosity of the path for water molecules. Li et al. mixed graphene nanosheets with PBAT by the solution casting method [15]. The presence of graphene resulted in an 80% reduction in water permeation and a 99% reduction in oxygen transmission of PBAT nanocomposite films, which was ascribed to the fact that the graphene nanosheets enlarged the effective diffusion path length of water and oxygen across the films.
MXene is a novel family of (2D) transition metal carbides and/or nitrides [17][18][19][20][21]. The abundance of functional groups on the surface of MXene, such as oxygen (=O), hydroxyl (-OH), or fluorine (-F), endows it with good compatibility with many polar polymer matrices [22]. MXene has attracted considerable research interest for various applications, such as energy storage, sensors, electromagnetic shielding, and so on [23][24][25][26]. Ti 3 C 2 T X nanosheets have high stiffness and strength, which can serve as effective, reinforced fillers to improve the mechanical properties of polymer/Ti 3 C 2 T X nanocomposites. In addition, Wu et al. demonstrated that a small amount of Ti 3 C 2 T X improved the complex viscosity and storage modulus of PVDF nanocomposites significantly [22]. The ultrahigh molecular weight polyethylene (UMWPE) composites containing 0.75 wt% Ti 3 C 2 T X had the best creep performance [27]. With the addition of 1.9 vol% of MXene, the Ti 3 C 2 T X /polystyrene nanocomposites exhibited a 54% higher storage modulus than that of neat polystyrene [28]. Yu et al. demonstrated that the addition of MXene improved the thermal stability and fire safety of polystyrene [29]. However, to the best of our knowledge, the PBAT/Ti 3 C 2 T X nanocomposites have not been reported on yet. It is expected that the impermeable Ti 3 C 2 T X nanosheets in the PBAT matrix via biaxial stretching can not only improve the gas barrier performance, but also enhance the thermal stability and stiffness of the nanocomposites.
Biaxial stretching processing is the process of stretching hot polymeric films in the cross-machine direction. It is also reported that the biaxial stretching process endows the polymer matrix with an ordered structure and improved gas barrier properties [30][31][32]. This is because the biaxial stretching can help to reduce the free volume of the amorphous region of the polymers, resulting in an enhancement in gas barrier performance [33]. In addition, the biaxial stretching can help the 2D filler in the polymer matrices form an orientation structure, which can benefit for the enhancement of the gas barrier performance [34]. Li et al. reported that the orientated OMMT in the PBAT matrix prepared by biaxial stretching significantly reduced the WVPR and caused a 99% reduction in OTR with an enhancement in elongation at break [15]. Yoksan et al. demonstrated that the stretched PLA/PBAT/thermoplastic starch composite films had stacked-layer planar morphology, which contributed to the improvement in crystallinity, impact strength, water vapor, and oxygen barrier properties [35].

Preparation of PBAT/Ti 3 C 2 T X Nanocomposite Biaxial Stretching Films
The preparation diagram of PBAT/Ti 3 C 2 T X nanocomposite biaxial stretching films is shown in Scheme 1. To achieve a better dispersion of Ti 3 C 2 T X nanosheets in the PBAT matrix, Ti 3 C 2 T X was first mixed with PBAT pellets by melt compounding via a twin extruder to obtain composite casting films. Then the PBAT/Ti 3 C 2 T X nanocomposite casting films were biaxially stretched. The effects of Ti 3 C 2 T X content on the morphology, thermal stability, and crystallization behavior, as well as mechanical properties of PBAT/Ti 3 C 2 T X nanocomposites, were comprehensively evaluated. The PBAT nanocomposite films containing the optimized 1 wt% Ti 3 C 2 T X were biaxially stretched under different parameters. Scheme 1. Schematic diagram of preparation of PBAT/Ti 3 C 2 T X nanocomposite biaxial stretching films.
The PBAT/Ti 3 C 2 T X nanocomposite biaxial stretching films were prepared by two steps. Prior to the experiment, the PBAT pellets were dried in a vacuum oven at 80 • C for 12 h. Firstly, the PBAT/Ti 3 C 2 T X nanocomposite casting film was prepared by extrusion casting (FDHU-35, Potpo, Guangzhou, China) so that the Ti 3 C 2 T X could have a better dispersion in the PBAT matrix. The temperatures from the hopper to the nozzle were set at 130-150-170-170-170-150 • C. The screw speed was set at 60 rpm. The thickness of the obtained PBAT/Ti 3 C 2 T X nanocomposite casting films was approximately 100 µm. The code of the samples was abbreviated as PBAT-X, where X represents the weight ratio of Ti 3 C 2 T X in the PBAT/Ti 3 C 2 T X nanocomposites.
The extruded PBAT nanocomposite casting films containing 1 wt% Ti 3 C 2 T X nanosheets were cut into squares (80 mm side length) for biaxial stretching. The biaxially oriented films were then prepared at different stretching ratios on a KARO 5.0 (Brückner Maschi-nenbauGmbH & Co., Siegsdorf, Germany) equipped with mechanically driven clamps. The stretched films were thermally annealed at a temperature of 110 • C for 60 s. Finally, the biaxially oriented nanocomposite films were obtained to characterize the crystal orientation and the gas barrier properties.

Characterization
The fracture surfaces of the PBAT/Ti 3 C 2 T X nanocomposite were characterized by scanning electron microscopy (SEM, Quanta 250, FEI, Hillsboro, OR, USA). The samples were fractured in liquid nitrogen for 30 min. They were observed at an accelerating voltage of 5 kV. Prior to the observation, all the samples were coated with a layer of gold.
Thermogravimetric analysis (TGA) of all PBAT/Ti 3 C 2 T X nanocomposites was conducted on a TG-209F1 thermal analyzer (Netzsch, Selb, Germany) to measure the thermal stability under air atmosphere. The samples of about 10 mg were heated from room temperature to 600 • C at a heating rate of 10 • C/min.
The crystallization and melting behaviors of PBAT/Ti 3 C 2 T X nanocomposites were conducted on a DSC-204F1 (Netzsch, Selb, Germany). The samples of approximately 5 mg were first heated to 180 • C at a heating rate of 10 • C/min to establish the thermal history, then cooled to 20 • C at a cooling rate of 10 • C/min and followed by a second heating rate of 10 • C/min to 180 • C. The peak crystallization temperature (T cp ), the peak melting temperature (T mp ), the crystallization enthalpy (∆H c ), and the melting enthalpy (∆H m ) of these samples were summarized. The degree of crystallinity of PBAT (χ c ) was calculated by the following equation: where ∆H 0 is the 100% melting enthalpy of PBAT (114 J/g) [36], and ϕ c presents the weight ratio of Ti 3 C 2 T X in the nanocomposites. The tensile test was performed on an Instron 5566 universal electron tensile machine. The specimens were cut into rectangle shape with a dimension of 1 cm × 8 cm × 100 µm. The tensile speed was fixed at 10 mm/min. The reported results were the average values of at least five successful specimens.
The dynamical mechanical analysis was conducted on a Netzsch DMA 242E (Netzsch, Selb, Germany) analyzer. Tensile measurements were taken on specimens with dimensions of 30 mm at a fixed frequency of 1 Hz and from 90 • C to 70 • C at a ramping rate of 3 • C/min.
The 2D, wide-angle, X-ray scattering (2D-WAXS) measurements were carried out on an X-ray diffractometer (Rigaku UltimaIV, The Woodlands, TX, USA). The data were collected in the scanning range of 10-60 • with a step of 0.02 • .
The  Figure 1 shows the SEM images of the cross-section fracture surfaces of PBAT/Ti 3 C 2 T X nanocomposite films. In Figure 1a,b, PBAT-0 exhibits a relatively smooth surface due to its brittle fracture after immersion in liquid nitrogen [37]. With the addition of Ti 3 C 2 T X nanosheets, the surfaces of the PBAT/Ti 3 C 2 T X nanocomposites become gradually rough in Figure 1c-f when the Ti 3 C 2 T X content is lower than 2.0 wt%. This is due to the presence of Ti 3 C 2 T X , which served as a rigid filler to transfer the stress during facture. In addition, it can be seen that an agglomeration phenomenon existed on the surface of PBAT-2.0 (Figure 1g,h). In Figure 1, no pores and holes are observable between the exposed Ti 3 C 2 T X nanosheets on the surfaces (Figure 1f,h) and the PBAT matrix, indicating that Ti 3 C 2 T X nanosheets have good compatibility with the PBAT matrix. It is attributed to the large number of polar groups of Ti 3 C 2 T X that can react with the polyester groups of PBAT.

Thermal Stability
The thermal stability of PBAT/Ti 3 C 2 T X nanocomposites is shown in Figure 2, and the corresponding data, including the temperatures at 10% weight loss (T 10 ), the temperatures at the maximum weight loss rate (T max ), and the char yields at 600 • C, are listed in Table 1. In Figure 2a, two thermal decomposition stages are observable for all the samples. The first stage between 300 • C and 420 • C can be attributed to the random, main-chain scission and thermo-oxidative reactions of PBAT [38]. The second stage, in the range of 420-550 • C, corresponds to cis-elimination and thermo-oxidative reactions [32]. In Table 1, it can be seen that the values of T 10 showed an increasing trend with the increase of Ti 3 C 2 T X content. When compared to PBAT-0, the T 10 of PBAT-2.0 dropped from 373.5 • C to 379.2 • C. In addition, the T p of PBAT-2.0 gradually increased to 412.5 • C with the addition of 2 wt% Ti 3 C 2 T X . That is because the presence of Ti 3 C 2 T X rapidly catalyzed the formation of a char layer that served as a thermal barrier to protect the underlying polymer matrices [39]. The improvement of char yield benefited from the isolating of volatile gases and oxygen; therefore, improving the thermal stability of PBAT. Furthermore, the char yield at 600 • C for PBAT-0, PBAT-0.5, PBAT-1.0, and PBAT-2.0 was 0.7%, 1.0%, 1.4%, and 1.7%, respectively. It was mainly due to the introduction of Ti 3 C 2 T X, which promoted charring, and partial polymers could not be completely thermally decomposed, resulting in enhanced char residues [40].

Crystallization and Melting Behavior
The DSC curves of the PBAT/Ti 3 C 2 T X nanocomposites are shown in Figure 3. In Figure 3a, the onset crystallization temperatures of PBAT nanocomposites exhibit an increasing trend with the increase of Ti 3 C 2 T X content. In addition, the values of T cp for PBAT nanocomposites in Table 2 are 72.1, 73.1, 73.7, and 75.2 • C, respectively. The increase in T cp indicated that the presence of Ti 3 C 2 T X had a heterogeneous nucleation effect, accelerating the formation of crystallites in the PBAT matrix during cooling [41]. It was noted that the values of ∆H m were lower than ∆H c for the PBAT/Ti 3 C 2 T X composites, which can be ascribed to the fast cooling rate. In Figure 3b, the values of T mp for PBAT nanocomposites are 119.3, 121.0, 121.6, and 121.0 • C, respectively. The increase in T mp suggests that the filling Ti 3 C 2 T X contributes to the formation of perfect crystallinity of PBAT during the cooling procedure. Moreover, the crystallization degree of PBAT-1.0 had the highest value of 13.4% with the addition of 1.0 wt% Ti 3 C 2 T X . When the content of Ti 3 C 2 T X was further increased up to 2.0 wt%, the crystallization degree of PBAT-2.0 showed a slight decrease. This may be due to the excessive addition of Ti 3 C 2 T X , which led to agglomeration, to some extent. On the other hand, the inhibition effects of the excessive Ti 3 C 2 T X nanosheets were more profound than the nucleating effect that led to smaller crystallization and decreased crystallinity [42].   Figure 4 shows the typical stress-strain curves for pure PBAT and PBAT/Ti 3 C 2 T X nanocomposite casting films, and the corresponding data are summarized in Table 3. It was observed that PBAT-0 exhibited a high ductility (elongation at break~1442%) but low tensile strength (~22.6 MPa), which is consistent with previous report [14]. With the addition of 0.5 wt% Ti 3 C 2 T X , the tensile strength of the PBAT/Ti 3 C 2 T X nanocomposite increased by 19.8% with a slight increase in the elongation at break. As depicted in Figure 1, the Ti 3 C 2 T X had good interfacial interaction with the PBAT matrix; therefore, the addition of Ti 3 C 2 T X nanosheets can transform the stress during the tensile process. When the Ti 3 C 2 T X content was increased to 1.0%, the PBAT/Ti 3 C 2 T X nanocomposite had the maximum tensile strength (31.6 MPa). This enhancement can be ascribed to the reinforcement effects of the nanofillers and the interaction between the stress concentration zones around the Ti 3 C 2 T X nanosheets [43,44]. It is worth noting that PBAT-2.0 showed a decreasing tendency in both tensile stress and elongation at break as compared with PBAT-1.0. This may be due to the aggregation of Ti 3 C 2 T X nanosheets in the PBAT matrix.

Dynamic Mechanical Analysis
The storage modulus as a function of temperature for PBAT/Ti 3 C 2 T X nanocomposite casting films is shown in Figure 5a. It is clear that the storage modulus of the PBAT nanocomposite reinforced with Ti 3 C 2 T X was higher than that of PBAT-0 in the glassy states. In addition, the reinforcement effect was more obvious with the increase of Ti 3 C 2 T X content. When compared to PBAT-0, the storage modulus of PBAT-2.0 at 80 • C increased from 1220 MPa to 2342 MPa. This is due to the stiffening effect of rigid Ti 3 C 2 T X nanosheets. Aside from this, the polar groups on the surface of Ti 3 C 2 T X may have had intramolecular interaction with the PBAT. matrix, which may have also improved the storage modulus of the PBAT/Ti 3 C 2 T X nanocomposites [43]. The loss factor peak (tanδ) is usually defined as the glass transition temperature (T g ). It is observable from Figure 5b that the T g shifted to a lower temperature when the PBAT matrix was incorporated with Ti 3 C 2 T X . With the addition of 2 wt% Ti 3 C 2 T X , the PBAT-2.0 shifted from −11.9 • C to −15.0 • C, as compared with that of PBAT-0. It can be attributed to the incorporation of Ti 3 C 2 T X , which can improve the chain mobility of the amorphous regions of PBAT due to the liberation effect of Ti 3 C 2 T X . In addition, the height of tanδ also showed a slight increase, indicating that an increase in Ti 3 C 2 T X content will result in higher dissipative energy [45].    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/Ti 3 C 2 T X 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/Ti 3 C 2 T X 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/Ti 3 C 2 T X composite films further confirms that the biaxially oriented PBAT/Ti 3 C 2 T X film has excellent isotropy.

Gas Barrier Properties of Biaxial Stretching Films
The gas barrier properties of PBAT/Ti 3 C 2 T X 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 Ti 3 C 2 T X content. The lowest OTR was achieved for PBAT-1.0, which decreased from 1030 to 782 cc/m 2 ·day. Similarly, the water vapor transmission rate (WVTR) of PBAT/Ti 3 C 2 T X nanocomposite casting films decreased as the Ti 3 C 2 T X 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/m 2 ·day, respectively. It is speculated that the addition of Ti 3 C 2 T X 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 Ti 3 C 2 T X will contribute to the interactions with water molecules, delaying the diffusion to some extent. However, the aggregation of Ti 3 C 2 T X will result in a deterioration of the gas barrier performance when the content of Ti 3 C 2 T X is increased by up to 2.0 wt%.
To investigate the effects of the stretching ratio on the gas barrier performance of PBAT/Ti 3 C 2 T X 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/m 2 ·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/m 2 ·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 Ti 3 C 2 T X sheets. However, the barrier effect of Ti 3 C 2 T X 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.

Conclusions
In this work, two-dimensional MXene (Ti 3 C 2 T X ) nanosheets were mixed with PBAT by melt compounding. The effects of Ti 3 C 2 T X content on the morphology, thermal stability, crystallization behavior, and gas barrier performance of PBAT were investigated. Furthermore, the effects of the biaxial stretching ratio on the gas barrier properties were further discussed. The TGA results showed that the addition of Ti 3 C 2 T X improved the thermal stability of the PBAT nanocomposite. In addition, the tensile tests showed that the addition of 1.0 wt% Ti 3 C 2 T X improved the maximum tensile stress without losing ductility. The storage modulus of PBAT was significantly improved in the glassy state with the addition of Ti 3 C 2 T X . After biaxial stretching, the PBAT-1.0 film (1 × 3) exhibited an oxygen transmission rate of 732 cc/m 2 ·day, which was 28.9% lower than that of pure PBAT casting film. When the stretching ratio was 2 × 2, the WVTR of PBAT-1.0 biaxial stretching film was 6.5 g/m 2 ·day, which was 36.3% lower than that of 1 × 1 PBAT-1.0 film. The enhancement in gas barrier properties can be attributed to the presence of Ti 3 C 2 T X nanosheets, which can increase the effective diffusion path length for gases. The results of this work indicate the need for further studies on the influence of the orientation and surface functionalization of Ti 3 C 2 T X nanosheets, as well as the incorporation of compatbilizers in the PBAT composite films for packaging applications.
Author Contributions: Investigation, X.W., X.L., L.C. and S.F.; writing-original draft preparation, X.W. and X.L.; methodology, X.W. and X.L.; writing-review and editing, Y.L. and S.F.; supervision, Y.L. and S.F. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Natural Science Foundation of Hunan Province (no. 2019JJ50132) and the Innovation Platform Open Fund of Hunan Province (no. 18K079).

Institutional Review Board Statement:
This study did not involve humans or animals.
Informed Consent Statement: This study did not involve patients.

Data Availability Statement:
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Conflicts of Interest:
The authors declare no conflict of interest.