PLA-*g*-Polyester

Other degradable and flexible polyesters are now being developed on an industrial scale, such as polybutylene adipate-co-terephthalate (PBAT), polybutylene succinate (PBS) or poly <sup>ε</sup>−caprolactone (PCL); the first two examples are derived from renewable resources. Blends of PLA with these polyesters are good candidates for improving the brittleness and toughness of PLA while maintaining the biodegradability of the blends. Unfortunately, these blends are largely incompatible, especially with PBAT, due to its structure, which contains aromatic rings [48]. As with PLA/NR blends, PLA-*g*-MA was used as a reactive compatibilizer whose possible reaction of the alcohol chain end of PBAT with the grafted anhydride is shown in Figure 11.

Typically, the reaction is carried out in a co-rotating twin screw extruder at a temperature between 160 and 180 ◦C and a screw speed of 25–100 rpm [49,50]. Effective grafting of PBAT was demonstrated by a drastic increase in the molecular mass of the copolymer, from 10 kDa to ca. 20 kDa. The mechanical properties of the PLA/PBAT blend were only slightly improved despite the incorporation of PLA-*g*-MA. However, the improvement of the interfacial adhesion between PLA and PBAT was evidenced by SEM micrographs. A very limited compatibility effect of the addition of PLA-*g*-MA in PLA/PBAT/TiO2 blends was also observed, with TiO2 acting as a nucleating agen<sup>t</sup> [51]. The addition of CaCO3 to PLA/PBAT/PLA-*g*-MA blends resulted in an increase in Young's modulus [52].

**Figure 11.** Possible reaction between PLA-*g*-MA and PBAT (from Rigolin et al. [50], copyright Elsevier, reproduced with permission).

Phetwarotai et al. studied PLA and PBS blends in the presence of PLA-*g*-MA or toluene diisocyante (TDI) as compatibilizers [53]. They showed that TDI is a more effective compatibilizer than PLA-*g*-MA for PLA/PBS blend films, their failure mode changed from brittle to ductile due to the improved compatibility. The effect of PLA-*g*-MA was also studied on PLA/PBAT/thermoplastic starch (TPS) ternary blend films. The thermal stability, tensile properties and compatibility of the PLA, PBAT, and TPS blends were slightly improved with the addition of the compatibilizer [54].

In the case of PLA/PCL with non-compatible blends, a reaction of MA with PCL is more likely to occur between the anhydride and OH groups on the PCL chain ends. As in the case of PBAT, it is the terminal alcohol of the PCL that reacts with the grafted anhydride of the PLA chain (Figure 12A) [55]. According to the authors, the compatibility effect of PLA-*g*-MA was shown by SEM micrographs (Figure 12B) and by a significant increase in the elongation at break of the material, from 7% in PLA or in a mixture of PLA and PCL to 53% in a compatibilized blend.

**Figure 12.** (**A**) Proposed reaction scheme between PLA-*g*-MA and PCL (**B**) SEM micrographs of PLA/PCL blends (**a**) without and (**b**) with 10% PLA-*g*-MA (adapted from Gardella et al. [55], copyright Springer, reproduced with permission).

By reactive extrusion of the bio-based poly(glycerol succinate-co-maleate) (PGSMA) with PLA at 150–180 ◦C in the presence of a free radical initiator, a PLA-*g*-PGSMA graft copolymer was obtained that acts as an interfacial compatibilizer due to the double bond of the maleate unit in PGSMA [56]. The tensile strength of PLA/PGSMA blends was improved by almost 400% compared to that of pure PLA. This increase was caused by (i) the in situ formation of PLA-*g*-PGSMA graft copolymers and (ii) the crosslinking of PGSMA within the PLA matrix, which act as interfacial compatibilizers. Two-dimensional NMR and FTIR confirmed the formation of PLA-*g*-PGSMA, but the substitution degree on the PLA backbone was not evaluated. It is important to note that this work was performed with very-low-molecular-weight PGSMA (Mn < 1200 Da).

PLA-*g*-MA is also used to compatibilize PLA with thermoplastic polyurethane elastomers (TPU) and thermoplastic polyester elastomers (TPE). Charpy impact, toughness and fracture toughness of brittle PLA were improved when the blends were compatibilized by addition of PLA-*g*-MA without adversely damaging effects on other mechanical and thermal properties of the PLA blends [57].

## Other PLA-Based Blends

PLA-*g*-MA serves as reactive compatibilizer for other polymer blends by reacting the anhydride functions on the second constituent of the blend. Examples include polyamides 11 and 12, where anhydride function reacts in the amine groups [58,59]. The ductility and impact strength of the compatibilized blends were increased by a factor two compared to non-compatibilized blends. A crosslinking agen<sup>t</sup> (trimethylolpropane trimethacrylate) was also used to enhance the impact strength of the PLA-*g*-MA-containing blend. PLA/Polyamide6 (PA6) blends were compatibilized with PLA-*g*-IA (IA = itaconic anhydride), a compound similar to PLA-*g*-MA [60]. It was reported that IA can react with amine and amide functions of PA6. Unfortunately, a weak compatibility effect was obtained, likely due to the low concentration of IA moieties grafted onto PLA backbone (0.4 wt.%). PLA-*g*-IA is also a compatibilizer of PLA/Novatein (a protein-based thermoplastic) as shown in the SEM micrographs of Figure 13 [61].

**Figure 13.** SEM micrographs of PLA/novatein 90/10 blends (**a**) uncompatibilized, (**b**) compatibilized (adapted from Walallavita et al. [61], copyright Wiley-VCH GmbH. Reproduced with permission).

Compatibilizing effects of PLA-*g*-MA on the properties of PLA/Soy Protein concentrate (SPC) blends were demonstrated by a 19% increase in tensile strength compared to the non-compatibilized blend [62]. Finally, PLA-*g*-MA also has the ability to provide some compatibility to PLA/mineral mixtures such as PLA/carbon nanotubes [22], PLA/Titanium oxide [63], PLA/halloysite [64], PLA/hydroxyapatite [65], PLA/talc [66], PLA polyhedral oligomeric silsesquioxane (POSS) [67].

## 2.2.2. PLA-*g*-Glycidyl Methacrylate (PLA-*g*-GMA)

A second type of compound that allows for the reactive functionalization of the PLA chain is glycidyl methacrylate (GMA). Grafting is a free-radical reaction performed in a mixer (80 rpm) at 160 ◦C for 12 min in the presence of BPO [68]. A proposed scheme of the reaction is shown in Figure 14 [69].

**Figure 14.** Reaction scheme for the grafting of GMA on PLA backbone (from Gu et al. [69], copyright Springer Nature, reproduced with permission).

The possibility of further functionalization is ensured by the presence of the new oxirane group on the grafted chain. As for PLA-*g*-MA, the main application lies in the compatibility of polymer blends. The graft content increased from 1.8 to 11.0 wt.% as the GMA concentration in the feed is varied from 5 to 20 wt.%. [68]. The degree of substitution, which is significantly higher than that of PLA-*g*-MA, is therefore the main advantage of GMA over MA. The characterization and properties of PLA-*g*-GMA (crystallization, characteristics, tensile stress, stress–strain curve, brittleness, thermal properties) compared to the one of PLA, are described by Kangwanwatthanasiri et al. [70].

The main graft copolymers obtained from PLA-*g*-GMA and their principal properties and applications are described in the following paragraphs. We found almost the same types of structures as those obtained with PLA-*g*-MA; therefore, they will not be described in detail.
