PLA-*g*-Polyesters

For the same reasons as described with MA, PLA/PBAT and PLA/PBS blends were compatibilized by the addition of PLA-*g*-GMA, with the main results of improved mechanical and thermal properties [69]. Specifically, the presence of PLA-*g*-GMA in a blend PLA/PBAT led to a decrease in crystallization rate, an increase in melt strength and viscosity, an improvement of tensile strength, and elongation at break, which are dependent on the proportion of PLA-*g*-GMA [78]. In a typical operating procedure, PLA, PBS or PBAT and PLA-*g*-GMA were melt blended together in a twin-screw extruder at a rotational speed of 30 rpm for several minutes. The temperature was between 150 and 170 ◦C [79]. The PLA/PBAT/PLA-*g*-GMA blends were successfully printed by 3D printing. A reaction mechanism between PBAT and PLA-*g*-GMA is described in Figure 15 [78]. With 10 wt.% of compatibilizer, the viscosity of the PBAT/PLA blend increased, and there was no longer a crystalline region of PBAT, showing an improved compatibility of PLA and PBAT.

**Figure 15.** Mechanism of the reaction between PLA-*g*-GPA and PBAT (from Lyu et al. [78], copyright Elsevier, reproduced with permission).

PLA/cassava pulp/PBS ternary biocomposites were also compatibilized by PLA-*g*-GPA, with the mechanical properties of the PLA/cassava pulp/PBS composites being improved with the addition of PLA-*g*-GMA [80]. Similar to polyesters, PLA/thermoplastic polyurethane (TPU) blends were also compatibilized in the presence of PLA-*g*-GMA [81]. In this example, PLA-*g*-TPU acted as a compatibilizer for the blend PLA/TPU.

## 2.2.3. PLA-*g*-Acrylic Acid (PLA-*g*-AA)

Another functionalization of the PLA chain is used in the field of the compatibilization of PLA-based blends, namely the grafting of acrylic acid (AA) to give a PLA-*g*-AA graft copolymer. Typically, AA grafting is performed under free radical conditions, by adding a mixture of AA and BPO to molten PLA in a mixer at 95 ◦C for a period of6h[82]. A PLA*g*-PAA copolymer is formed, which can then react with alcohol functions of the cellulosic derivatives by esterifying the alcohol functions of the cellulosic compound (Figure 16).

**Figure 16.** Reaction scheme for the grafting of acrylic acid on PLA and reaction with starch (from Wu, [82], copyright Wiley-VCH GmbH. Reproduced with permission).

As with PLA-*g*-MA, PLA-*g*-AA can compatibilize blends with natural cellulosic compounds, such as sisal fiber [31], wood flour [83], corn starch [84], rice husk [85], and hyaluronic acid [86]. In all cases, improvements in mechanical properties and/or biodegradation are obtained in the compatibilized mixtures. The compatibilization effect is shown by the size of the corn starch (CS) phase in PLA/CS and PLA/CS/PLA-*g*-corn starch. In a PLA/CS blend (50/50 *w*/*w*), the CS phase size decreased from 17.5 μm to 7.3 μm when PLA-*g*-corn starch was added in the blend [84].

Acrylic acid can also be graft-polymerized onto PLA chains in a solution using a photoinitiator, typically benzophenone, under UV irradiation at 254 nm [87]. Finally, the grafting of PAA onto the PLA backbone was also obtained by a free-radical reaction of BPO onto a solution of PLA in chloroform, followed by a reaction and polymerization of AA at 100 ◦C for 10 min under pressure. A drastic decrease in toughness and an increase in tensile modulus were observed in PLA-*g*-PAA as compared to PLA [88].

Inorganic–organic hybrid composites, based on mixtures of PLA and SiO2 [89] and TiO2 [90] generated via a sol–gel process, also showed improved mechanical and thermal properties when PLA was replaced by PLA-*g*-AA. This was attributed to stronger interfacial forces between carboxylic acid groups of PLA-*g*-AA and the residual Si-OH and Ti-OH groups [89].
