**1. 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) [1–4]. 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/m2·day, whereas the water vapor permeability rate (WVPR) was 3.3 × <sup>10</sup>−<sup>11</sup> <sup>g</sup>·m/m2·s·Pa, which made it difficult to meet the requirements for packaging applications [6–8]. 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.

**Citation:** Wang, X.; Li, X.; Cui, L.; Liu, Y.; Fan, S. Improvement of Gas Barrier Properties for Biodegradable Poly(butylene adipate-co-terephthalate) Nanocomposites with MXene Nanosheets via Biaxial Stretching. *Polymers* **2022**, *14*, 480. https:// doi.org/10.3390/polym14030480

Academic Editors: Wei Wu, Hao-Yang Mi, Chongxing Huang, Hui Zhao, Tao Liu and Alexey Iordanskii

Received: 3 December 2021 Accepted: 23 January 2022 Published: 25 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

It is widely accepted that the incorporation of nanofillers is a simple and effective method to reduce the OTR and WVPR of polymer films [9–12] 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 [17–21]. 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 [23–26]. Ti3C2TX nanosheets have high stiffness and strength, which can serve as effective, reinforced fillers to improve the mechanical properties of polymer/Ti3C2TX nanocomposites. In addition, Wu et al. demonstrated that a small amount of Ti3C2TX improved the complex viscosity and storage modulus of PVDF nanocomposites significantly [22]. The ultrahigh molecular weight polyethylene (UMWPE) composites containing 0.75 wt% Ti3C2TX had the best creep performance [27]. With the addition of 1.9 vol% of MXene, the Ti3C2TX/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/Ti3C2TX nanocomposites have not been reported on yet. It is expected that the impermeable Ti3C2TX 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 [30–32]. 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].
