**6. Biodegradbility and Other Properties**

In general, significant research still needs to be performed to achieve the final target of ideal biodegradable PLA/silica composites that exhibit high performance and easy biodegradability when their roles are completed. The hydrophilic nature of silica nanoparticles is expected to affect the degradation of PLA. The hydroxyl groups in silica are bound together by hydrogen bonds and can assist the hydrogen bonding interaction with the functional groups in PLA or a covalent bonding with a macromolecular chain [87]. Thus, silica nanoparticles are expected to facilitate the hydrolysis or enzymatic attacks of ester groups of PLA, leading to a fast biodegradation rate. For instance, Li et al. [50] reported that the weight loss during the biodegradation process of PLA/silica composites, fabricated by melt-mixing method, was larger than that in neat PLA. Indeed, the incorporation of 9 wt.% of silica nanoparticles into the PLA matrix led to a biodegradation rate of 0.36 mg cm−<sup>2</sup> h −1 , which was 6.5 times higher than that of neat PLA. Figure 7a shows the fast biodegradation of the PLA composites compared to neat PLA. This result was confirmed by the DSC curves shown in Figure 7b. As shown in Figure 7b, all samples were amorphous at a T<sup>g</sup> of 60 ◦C. The improvements in the degradation rate were attributed to the easy release of silica particles from the PLA matrix. The hydrophilic silica facilitated the hydrolysis and enzymatic attack of ester groups of PLA chains. The biodegradation was more pronounced in the composite membrane after 2 months of in vitro tests [88]. On the other hand, it was reported that the flame retardant properties of PLA could be improved greatly by adding treated silica nanoparticles into the PLA matrix [64]. Indeed, the weight loss ratios of PLA/silica composites at different burning time intervals tended to decrease with the increase in the silica content, which was linked to the excellent dispersion of particles in the PLA matrix. So, the larger specific surface area in the composites provided a better

effect of thermal insulation [64]. SiO2-fluorinated PLA composites can be used as reversible and highly hydrophobic coatings to protect the exterior of buildings [89]. Similarly, it was reported that the flame retardancy of PLA could be improved by the addition of fumed silica and Ni2O<sup>3</sup> into the polymer matrix [90]. reversible and highly hydrophobic coatings to protect the exterior of buildings [89]. Similarly, it was reported that the flame retardancy of PLA could be improved by the addition of fumed silica and Ni2O3 into the polymer matrix [90].

other hand, it was reported that the flame retardant properties of PLA could be improved greatly by adding treated silica nanoparticles into the PLA matrix [64]. Indeed, the weight loss ratios of PLA/silica composites at different burning time intervals tended to decrease with the increase in the silica content, which was linked to the excellent dispersion of particles in the PLA matrix. So, the larger specific surface area in the composites provided a better effect of thermal insulation [64]. SiO2-fluorinated PLA composites can be used as

*Polymers* **2021**, *13*, x FOR PEER REVIEW 13 of 20

**Figure 7.** (**a**) The effect of silica content on the biodegradation rate of PLA/silica composites. (**b**) The DSC curves of the PLA/silica composites before and after degradation of 24 h [50]. **Figure 7.** (**a**) The effect of silica content on the biodegradation rate of PLA/silica composites. (**b**) The DSC curves of the PLA/silica composites before and after degradation of 24 h [50].

A study has been reported by Zhao et al. [91], where *N*-halamine precursor with epoxy and hydantoin structures, 3-(4′-epoxyethyl-benzyl)-5, 5-dimethylhydantoin (EB-DMH) was utilized for the *N*-halamine-modified silica nanoparticles. EBDMH was fabricated and immobilized onto aminofunctionalized silica nanoparticles to form EBDMH– SiO2 nanoparticles (Figure 8a). PLA was mixed with EBDMH–SiO2 nanoparticles via the melt-mixing method. The efficiency of the PLA/EBDMH–SiO2 composite as antimicrobial material was evaluated against *S. aureus* and *E. coli*, respectively, and the obtained results are shown in Figure 8b. The PLA/EBDMH–SiO2 composite exhibited excellent bactericidal efficiency. Indeed, the PLA/EBDMH–SiO2 composite with a contact time of 10 min neutralized about 90.2% of *S. aureus* and 89.4% of *E. coli*. When the contact time was increased to 180 min, a kill efficiency of 99.97% and 99.91% against *S. aureus* and *E. coli*, respectively, was obtained. Due to improved biocompatibility between PLA and silica, as well as the excellent antibacterial efficiency, the PLA/EBDMH–SiO2 composite could be utilized for A study has been reported by Zhao et al. [91], where *N*-halamine precursor with epoxy and hydantoin structures, 3-(40 -epoxyethyl-benzyl)-5, 5-dimethylhydantoin (EBDMH) was utilized for the *N*-halamine-modified silica nanoparticles. EBDMH was fabricated and immobilized onto aminofunctionalized silica nanoparticles to form EBDMH–SiO<sup>2</sup> nanoparticles (Figure 8a). PLA was mixed with EBDMH–SiO<sup>2</sup> nanoparticles via the melt-mixing method. The efficiency of the PLA/EBDMH–SiO<sup>2</sup> composite as antimicrobial material was evaluated against *S. aureus* and *E. coli*, respectively, and the obtained results are shown in Figure 8b. The PLA/EBDMH–SiO<sup>2</sup> composite exhibited excellent bactericidal efficiency. Indeed, the PLA/EBDMH–SiO<sup>2</sup> composite with a contact time of 10 min neutralized about 90.2% of *S. aureus* and 89.4% of *E. coli*. When the contact time was increased to 180 min, a kill efficiency of 99.97% and 99.91% against *S. aureus* and *E. coli*, respectively, was obtained. Due to improved biocompatibility between PLA and silica, as well as the excellent antibacterial efficiency, the PLA/EBDMH–SiO<sup>2</sup> composite could be utilized for hygienic product packaging and filters, as well as medical textiles.

hygienic product packaging and filters, as well as medical textiles. The impact of silica nanoparticles on the interfacial tension between PLA and supercritical CO<sup>2</sup> at high temperature and high pressures was examined by Sarikhani and coworkers [92]. The addition of a low loading of silica (less than 2 wt.%) led to reducing the interfacial tension, while the interfacial tension tended to increase when the silica content was larger than 2 wt.%, which was linked to the fact that higher levels of silica originated attractive lateral capillary forces due to the perturbation of the PLA–CO<sup>2</sup> interface by particles. The interfacial interactions between PLA and silica nanoparticles were found to be decreased with an increase in the CO<sup>2</sup> content, which facilitated the adsorption behavior at higher pressures. Based on the experiments carried out by Seng and co-workers [93], it was reported that the addition of 1 wt.% of silanol treated-silica into the PLA matrix led to a decrease of 40% in the hygroscopicity of PLA, while the other loadings of silica caused improvements between 3 and 19% in the reduction in hygroscopicity. Chen et al. [94] reported that the hydrophilicity of PLA/silica composites tended to be improved with increasing the silica content, where the inclusion of 5 wt.% of silica into the PLA matrix led to a decrease in the water contact angle of PLA from 82◦ to 68◦ . In other words, the less silica in the composite, the larger the contact angle is, thereby, the higher the hydrophobicity of the composite surface is. Thus, the hydrolytic degradation ability of PLLA/silica composites was accelerated in the presence of higher loadings of silica in the polymer matrix.

**Figure 8.** (**a**) Synthesis of *N*-halamine precursor-modified silica nanoparticles (EBDMH–SiO2 NPs). (**b**) Antibacterial tests of PLA/EBDMH–SiO2-9 plastic sheets after chlorination against *S. aureus* and *E. coli*. In the inset are photographs showing the bacterial culture plates of *S. aureus* and *E. coli* upon 180 min contact with the control and PLA/EBDMH–SiO2-9 plastic sheets [91]. **Figure 8.** (**a**) Synthesis of *N*-halamine precursor-modified silica nanoparticles (EBDMH–SiO<sup>2</sup> NPs). (**b**) Antibacterial tests of PLA/EBDMH–SiO<sup>2</sup> -9 plastic sheets after chlorination against *S. aureus* and *E. coli*. In the inset are photographs showing the bacterial culture plates of *S. aureus* and *E. coli* upon 180 min contact with the control and PLA/EBDMH–SiO<sup>2</sup> -9 plastic sheets [91].

The impact of silica nanoparticles on the interfacial tension between PLA and supercritical CO2 at high temperature and high pressures was examined by Sarikhani and coworkers [92]. The addition of a low loading of silica (less than 2 wt.%) led to reducing the interfacial tension, while the interfacial tension tended to increase when the silica content was larger than 2 wt.%, which was linked to the fact that higher levels of silica originated attractive lateral capillary forces due to the perturbation of the PLA–CO2 interface by particles. The interfacial interactions between PLA and silica nanoparticles were found to be decreased with an increase in the CO2 content, which facilitated the adsorption behavior at higher pressures. Based on the experiments carried out by Seng and co-workers [93], it was reported that the addition of 1 wt.% of silanol treated-silica into the PLA matrix led to a decrease of 40% in the hygroscopicity of PLA, while the other loadings of silica caused improvements between 3 and 19% in the reduction in hygroscopicity. Chen et al. Kosowska and Szatkowski [95] examined the impacts of silica addition on the ultraviolet aging of PLA nonwovens obtained by the electrospinning technique. The inclusion of silica nanoparticles greatly increased the percentages of crystalline and amorphous phases in the fabricated films and altered the photodegradation mechanism. In the work of Jin-Bo [96], a novel porous membrane composed of PLA with the addition of SiO2–CaO by the sol–gel method was designed. The surface potentials of the composite membranes became more negative with higher SiO2–CaO contents. Apatite with an orderly ring structure nucleated and grew on the surface of the composite membranes after immersion in SBF for 7 days, implying that the incorporation of SiO2–CaO significantly improves the bioactivity of PLA. Based on the dielectric spectroscopy study [97], it was reported that the lifetime and thermal stability of the electric state in PLA can be increased with the addition of SiO2, where the optimal concentration of silica for negative corona electrets was found to be 4 wt.%.

#### [94] reported that the hydrophilicity of PLA/silica composites tended to be improved with **7. Potential Applications of PLA/Silica Composites**

increasing the silica content, where the inclusion of 5 wt.% of silica into the PLA matrix led to a decrease in the water contact angle of PLA from 82° to 68°. In other words, the less silica in the composite, the larger the contact angle is, thereby, the higher the hydrophobicity of the composite surface is. Thus, the hydrolytic degradation ability of PLLA/silica composites was accelerated in the presence of higher loadings of silica in the polymer matrix. Kosowska and Szatkowski [95] examined the impacts of silica addition on the ultra-Owing to its high resistance to heat and good thermal stability, silica has received considerable attention in 3D-printing applications. The enhanced melt viscosity of PLA/silica composites could facilitate the molding and processing of these materials which could lead to a variety of applications [54]. Since the usage of PLA used in the bone tissue applications would not be able to withstand the high-load resistance associated with such applications, the incorporation of silica would make PLA a suitable candidate in these applications, as the flexural modulus and the tensile strength of PLA tended to be improved

violet aging of PLA nonwovens obtained by the electrospinning technique. The inclusion

significantly with the incorporation of a silica filler. For this reason, it was reported that the biorenewable PLA/SiO<sup>2</sup> composites can be used as promising materials in the bone repair processes in animal models [98]. As indicated by cell-culture measurements conducted by Abe et al. [99], PLA/silica composite could exhibit excellent cytocompatibility. Interestingly, the ROS-responsive LDLR peptides-conjugated PLA-coated mesoporous silica nanoparticles were reported to have an important opportunity for oxidative stress therapy in the central nervous system [100]. Moreover, the good thermal stability and mechanical properties of PLA/silica composites would make these materials suitable for structural applications where high thermal stability and high mechanical strength are needed [71]. Thanks to their physical, thermal, rheological, and mechanical properties, the PLA/silica composites can be used in 3D-printing applications, as indicated by Thongsang and coworkers [101]. Moreover, the PLA/silica composites have a great opportunity to be used as smart and active packaging. According to Jaikaew [102], the CO2/O<sup>2</sup> permeability ratio of PLA/silica composite films can be tuned by varying the types of silica particles and their compositions. Light transmission reduction in both the UV and visible regions is achieved in PLA/modified-silica bio-composite films. As approved by Opaprakasit et al. [103], the films obtained from PLA/modified silica composites have strong potential to be used as biodegradable packaging materials with tunable gas permeability.
