**2. Synthesis of PLA/Silica Composites**

The direct melt-mixing, solution mixing, sol–gel process, and in situ polymerization methods are considered as the main methods utilized for the synthesis of PLA/silica composites. The melt-mixing method involves the direct mixing of PLA with silica nanoparticles, while the sol–gel process can be performed either in the existence of PLA or simultaneously during the polymerization of lactic acid monomers [25]. The solution mixing method starts from the dissolution of polymers in a suitable solvent with nanoparticles together with the evaporation of the solvent, or precipitation [25]. As for the in situ polymerization method, the silica nanoparticles should be distributed in the monomers before polymerization [25]. In addition, it is worth mentioning that the freezing-drying process would also be a promising method to fabricate PLA composites. In the freezing-drying process, the colloidal dispersion of the aerogel precursors is frozen, with the liquid component freezing into different morphologies depending on a variety of factors, such as the precursor concentration, type of liquid, temperature of freezing, and freezing container [25]. However, a lack of works was found on the utilization of this method to fabricate PLA/silica composites.

The surface modification by physical or chemical methods is a common procedure to increase the compatibility between PLA and silica nanoparticles [26–30]. The functionalization of hydrophilic silica nanoparticles, which is usually conducted on the reactive silanol end-groups, would improve the hydrophobicity of silica nanoparticles, improving their dispersion in the PLA matrix. This process, i.e., the formation of hydrophobic-fumed silica, can be obtained by chemical treatment of hydrophilic silica with silanes or siloxanes. As a result of the improved dispersion of silica particles in the PLA matrix, the rheological properties of the composites are improved. In general, the large surface area and the smooth surface of silica nanoparticles could increase the interactions not only between silica nanoparticles but also between PLA and silica nanoparticles. Thus, good physical interactions between the silica and PLA matrix can be achieved, leading to significant enhancements in composites properties. Several functionalization (coupling) agents, such as tetraethyloxysilane (TEOS) and γ-glycidoxypropyltrimethoxysilanes (GOPTMS) were used to improve the dispersibility of silica nanoparticles in the PLA matrix. In the study of Hakim et al. [26], a reactive extrusion method was utilized for the melt-mixing of PLA with 2.5 wt.% of silica nanoparticles. The schematic illustrations of SiO<sup>2</sup> before and after surface modification by organic chains are displayed in Figure 1. As shown in Figure 1b, the surface -OH groups of both surface-modified silica types were mostly substituted by functionalized organic chains. Thus, many PLA chains could be grafted on one silica nanoparticle, which would have risen the local shear field applied to the agglomerated nanoparticles during mixing, thereby improving the dispersibility of silica nanoparticles in the polymer matrix. However, transmission electron microscopy (TEM) observations implied that the uniformity and dispersibility of the nanoparticles under the experimental processing conditions were found not to be affected by the enhanced interfacial interaction of both surface-treated silica types (Figure 1c–e).

*Polymers* **2021**, *13*, x FOR PEER REVIEW 3 of 20

**Figure 1.** Schematic representation of the (**a**) surface-unmodified silica and (**b**) surface-modified silica aggregate. (**c**–**e**) TEM images of the PLA composites containing (**c**) un-modified SiO2, (**d**) SiO2- A, and (**e**) SiO2-E [26]. **Figure 1.** Schematic representation of the (**a**) surface-unmodified silica and (**b**) surface-modified silica aggregate. (**c**–**e**) TEM images of the PLA composites containing (**c**) un-modified SiO<sup>2</sup> , (**d**) SiO<sup>2</sup> -A, and (**e**) SiO<sup>2</sup> -E [26].

The fabrication of PLA/silica composites via the sol–gel method was first reported by Yan et al. [31], who aimed to obtain plasticized composites. The fabrication process of the composites involved an in situ synthesis of silica nanoparticles via condensation reactions of TEOS and GOPTMS in the presence of PLA and polyethylene glycol as a plasticizer in tetrahydrofuran. Hydrochloric acid was utilized as a catalyst during the synthesis method. The infrared results proved the formation of a silica network structure within the PLA matrix, while the results of mechanical tests indicated that the incorporation of 4 wt.% of silica nanoparticles would increase the tensile strength of PLA from 15 to 18 MPa. In another work, Yan et al. [32] polymerized L-lactic acid (PLLA) in the presence of silica without the use of catalysts, but in solution. In this case, the silica surface was not organically treated. The polycondensation was carried out in toluene to remove the water formed by azeotropic dehydration. The result is silica grafted with PLLA oligomers. The authors were able to witness the grafting by infrared spectroscopy characterization (unfortunately, the molar masses are not specified). The grafted silica was then dispersed in PLLA and its good dispersion helped to improve mechanical properties, as compared to The fabrication of PLA/silica composites via the sol–gel method was first reported by Yan et al. [31], who aimed to obtain plasticized composites. The fabrication process of the composites involved an in situ synthesis of silica nanoparticles via condensation reactions of TEOS and GOPTMS in the presence of PLA and polyethylene glycol as a plasticizer in tetrahydrofuran. Hydrochloric acid was utilized as a catalyst during the synthesis method. The infrared results proved the formation of a silica network structure within the PLA matrix, while the results of mechanical tests indicated that the incorporation of 4 wt.% of silica nanoparticles would increase the tensile strength of PLA from 15 to 18 MPa. In another work, Yan et al. [32] polymerized L-lactic acid (PLLA) in the presence of silica without the use of catalysts, but in solution. In this case, the silica surface was not organically treated. The polycondensation was carried out in toluene to remove the water formed by azeotropic dehydration. The result is silica grafted with PLLA oligomers. The authors were able to witness the grafting by infrared spectroscopy characterization (unfortunately, the molar masses are not specified). The grafted silica was then dispersed in PLLA and its good dispersion helped to improve mechanical properties, as compared to the PLLA composites with non-grafted silica.

the PLLA composites with non-grafted silica. Wu et al. [32] also polycondensed L-lactic acid in the presence of silica nanoparticles, but this time in bulk. The authors first mixed an aqueous solution of L-Lactic acid (LA) with an acidic silica sol containing silica particles of 12 nm. The mixture was dehydrated Wu et al. [32] also polycondensed L-lactic acid in the presence of silica nanoparticles, but this time in bulk. The authors first mixed an aqueous solution of L-Lactic acid (LA) with an acidic silica sol containing silica particles of 12 nm. The mixture was dehydrated under

under vacuum with sonication treatment to well disperse the particles. After complete

vacuum with sonication treatment to well disperse the particles. After complete drying of the mixture, the polycondensation was performed under vacuum conditions to remove the water formed. The grafting occurred as above, with the polycondensation using the SiOH groups on the surface of the silica nanoparticles. The results showed that the molar mass of the grafted PLLA was about 31,100 g·mol−<sup>1</sup> . Liu et al. [33] fabricated PLA/silica composites through ring-opening polymerization of lactide initiated by modified silica nanoparticles in the presence of stannous octoate as a catalyst. The experimental conditions, such as the weight ratio of silica to the lactide monomer, reaction temperature, and reaction time were optimized to be 1:20, 140 ◦C, and 72 h, respectively. The morphological observations revealed that silica nanoparticles tended to be distributed uniformly within the PLA matrix, improving compatibility between PLA and silica nanoparticles. Accordingly, the thermal and mechanical properties of the composites were improved significantly in comparison to that of pure PLA. In other works [24,34,35], the melt-mixing method to fabricate PLA/silica composites was carried out at 175 ◦C. In comparison to neat PLA, the PLA/silica composites containing various contents of silica (from 1 to 10 wt.%) exhibited significant improvements in the thermal stability and the barrier properties against nitrogen and oxygen gases, which were connected with the establishment of the silica network structure as approved by Fourier-transform infrared (FTIR) and rheological results. In the work of Liu and co-workers [36], amino-functionalized nano-SiO<sup>2</sup> (m@g-SiO2) was first prepared through a coupling reaction on the surface of silica nanoparticles before its melt-mixing with PLA. Molecular dynamics simulation was carried out to discover the correlation between PLA and silica nanoparticles before and after the organic functionalization of silica. The crystallization behavior and the mechanical properties of PLA exhibited significant improvements after the melt-mixing with functionalized silica nanoparticles which were attributed to the fact the organic modification of silica nanoparticles would lead to enhancements in the interaction energy and mobility of PLA chains.

To improve the dispersion of silica in the PLA matrix, an in situ melt condensation of l-lactic acid was carried out in the presence of silica nanoparticles [37]. However, when the content of silica exceeded 10 wt.%, some agglomerated nanoparticles appeared in the TEM images of the prepared composites. The agglomeration of silica nanoparticles would be attributed to the strong van der Waals forces, which tended to reduce the physical properties of the obtained composites. Thus, it is believed that the physical mixtures of PLA and organo-modified SiO<sup>2</sup> resulted in the separation in discrete phases, leading to inferior mechanical properties [38,39]. Zou and coworkers tried to mix silica nanoparticles with a copolymer made of PLA and epoxidized soybean oil [40]. The FTIR and thermogravimetric analysis (TGA) results confirmed the reactions between silica and epoxidized soybean oil, which, in turn, led to improve crystallization behavior and mechanical properties of PLA, while Sepulveda et al. [41] demonstrated that the direct grafting of L-lactic acid oligomer onto the silica surface through its silanol groups was a good strategy to enhance the physical, thermal, and mechanical properties of PLA/silica composites. As such, Zhu et al. [42] found that the surface functionalization of fumed silica nanoparticles oleic acid by oleic acid would help to improve the rheological, thermal, and mechanical properties of PLA/silica composites prepared via the melt-mixing method, which was linked to the good interfacial adhesion in the composites containing functionalized-silica nanoparticles.
