**1. Introduction**

Using biodegradable polymers as substitutes for common plastics has attracted immense interest in reducing the environmental impact of plastic waste [1,2]. Nanomaterial additives is one of the effective methods to enhance the mechanical, thermal, and crystallization properties of biodegradable polymers [3,4]. Many studies have been conducted on the effect of nanomaterials on the degradation behavior of biodegradable polymers in the soil, compost, or simulated environments in the laboratory [5–7]. However, the impacts of light, water, and temperature on the degradability of polymers in the natural environment require further investigation. These factors usually deteriorate the properties of biodegradable polymers and affect their lifetime. For polymeric materials that are often exposed to outdoors, sunlight is the main cause of photodegradation and performance loss [8,9]. The photodegradation behavior of several biodegradable polymers, including poly(l-lactide) (PLA), poly(butylene adipate-co-terephthalate) (PBAT), and poly(butylene succinate-co-adipate) (PBSA) have been reported [10–12]. The photodegradation feature of the polymer itself and the influence of nanomaterials as fillers have received extensive attention to increase the lifetime of biodegradable polymers under sunlight exposure. Chen et al. prepared a nanocomposite by PBAT and clay, and showed that inorganic particles can absorb or reflect photon energy and reduce the intensity of the light, inhibiting the photodegradation of the polymer [13]. Zhang et al. discussed the effect of ZnO on the photodegradation of PBSA [14] and showed that ZnO hinders the photodegradation of PBSA, but does not significantly change the photodegradation mechanism of PBSA. The ultraviolet light with high energy UV-C (wavelengths 100–280 nm) is absorbed by

**Citation:** Wang, J.-M.; Wang, H.; Chen, E.-C.; Chen, Y.-J.; Wu, T.-M. Role of Organically-Modified Zn-Ti Layered Double Hydroxides in Poly(Butylene Succinate-Co-Adipate) Composites: Enhanced Material Properties and Photodegradation Protection. *Polymers* **2021**, *13*, 2181. https://doi.org/10.3390/ polym13132181

Academic Editors: José Miguel Ferri, Vicent Fombuena Borràs and Miguel Fernando Aldás Carrasco

Received: 16 June 2021 Accepted: 29 June 2021 Published: 30 June 2021

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**Copyright:** © 2021 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/).

the earth's atmosphere while allowing the transmission of UV-B (280–320 nm) and UV-A (320–400 nm) [15]. Therefore, UV-B and UV-A contribute to the photodegradation of most polymers that are used outdoors. To reduce the photodegradation of biodegradable polymers, the UV-B and UV-A absorption capacity of nanomaterials should be further investigated [9,16].

The layered double hydroxide (LDH) conventionally prepared by bivalent and trivalent cations has been well-known for its ability to affect mechanical, crystallization, thermal, and biodegradation properties of biodegradable polymers [17,18]. The general formula for these LDHs is (M2+ <sup>1</sup>−xM3+ <sup>x</sup>(OH)2)(An−)x/n · mH2O, where M2+ and M3+ are divalent (Mg2+, Zn2+, Cu2+) and trivalent (Al3+, Cr3+, Fe3+) cations, and An<sup>−</sup> represents interlamellar anions [19]. The distance between two adjacent layers, which depends mainly on the nature of the interlayer species and their electrostatic interactions with the main layers, can be adjusted by introducing anionic compounds into the interlayer by ion exchange to replace its native anions [20]. The reports also pointed out that by modifying the LDH with a hydrophobic aliphatic carbon chain, the compatibility between originally hydrophilic LDH and the hydrophobic polymer can be improved [21]. The eco-friendliness and biocompatibility of LDHs have been demonstrated [22]. Zn-Ti LDH, which consists of bivalent (Zn2+) and tetravalent (Ti4+) cations, was developed by Saber et al. [23]. Compared to LDHs with other metal ions (e.g., Mg-Al LDH and Zn-Al LDH), Zn-Ti LDH can provide better protection in broadband UV [24]. Wang et al. also found that Zn-Ti LDH is a safe UV-shielding material as it has lower photocatalytic activity than TiO<sup>2</sup> and ZnO [15]. Ekambaram et al. also gave same discussion and indicated its ability to shield UV radiation [25]. In the study of Naseem et al., Zn-Ti LDH as a UV-absorbing nanomaterial provided a method for protecting polypropylene from UV-vis degradation [26].

PBSA is an aliphatic biodegradable copolyester, which synthesized via polycondensation of 1,4-butanediol in the presence of succinic and adipic acids [27]. It is worth noting that 1,4-butanediol and succinic acid can not only be extracted from oil, but also via fermentation [28]. The photodegradation reaction of PBSA produces carboxyl end groups (C=O) and chain scission. At the same time, the peak of C=O in the FTIR spectrum can be used to study the evolution of photodegradation. [14,29]. Due to the appropriate degradation rate, thermal stability, mechanical property, and good processability, practical applications of PBSA can be found in mulch films, where the photodegradation stability is a crucial property. The characteristics of Zn-Ti LDH, such as excellent biocompatibility, broadband UV protection, and lower photocatalytic activity, makes it suitable as a UVprotection additive in the PBSA matrix [14,29]. Reviewing past literature regarding PBSA nanocomposites, many studies focus on the addition of nanomaterial and its modification to achieve a better dispersion in the PBSA matrix [5,17,18]. Therefore, to improve the dispersion of Zn-Ti LDHs in PBSA, biocompatible and nontoxic stearic acid (SA) was used to modify in this study [30]. The changes in the physical properties of PBSA composites at various photodegradation periods were investigated. The crystallization, rheology, and thermal and mechanical properties of the PBSA composites were evaluated for their practical applications.
