## 2 Results
### Direct Functionalization of PLA Backbone: Towards PLA-Based Graft Copolymers
#### 2.2.2 PLA-\(g\)-Glycidyll Metachrylate (PLA-\(g\)-GMA)  
A second type of compound that allows for the reactive functionalization of the PLA chain is glycidyll methacrylate (GMA). Grafting is a free-radical reaction performed in a mixer (80 rpm) at 160 \({}^{\circ}\)C for 12 min in the presence of BPO [68]. A proposed scheme of the reaction is shown in Figure 14 [69].  
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 [PERSON] 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.  
PLA-\(g\)-Cellulosic Derivatives  
PLA-\(g\)-GA is blended with cellulosic derivatives, fully renewable and degradable resources, to obtain a fully biodegradable product. For example, when used with starch, a small percentage of PLA-\(g\)-GMA plays the role of a compatibilizer, which is incorporated into both PLA and starch phases [68]. Essentially, when PLA-\(g\)-GMA is incorporated into PLA/cellulosic blends, the mechanical properties are improved: the starch tensile strength at break increased from 18.6 \(\pm\) 3.8 to 29.3 \(\pm\) 5.8 MPa, the tensile modulus from 510 \(\pm\) 62 to 901 \(\pm\) 62 MPa, and elongation at break from 1.8 \(\pm\) 0.4 to 3.4 \(\pm\) 0.6% [68].  
Based on the reaction with PLA-\(g\)-GMA, the various PLA/cellulosic derivatives blends that were compatibilized are PLA-starch copolymers [68], PLA-treated arrowroot fiber [71], PLA-lignin [72; 73] PLA-cassava pulp [74], PLA-cellulose [75], PLA-bamboo flour [76], PLA-rice straw fiber [77].  
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 viscos  
Figure 14: Reaction scheme for the grafting of GMA on PLA backbone (from [PERSON] et al. [69], copyright Springer Nature, reproduced with permission).  
ity, 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 \({}^{\circ}\)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.  
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.