## 2 Results
### Direct Functionalization of PLA Backbone: Towards PLA-Based Graft Copolymers
#### 2.2.1 PLA-\(g\)-Maleic Anhydride (PLA-\(g\)-MA)  
The most widely used reagent for functionalizing the PLA chain is maleic anhydride (MA) due to its good chemical reactivity, low toxicity and good stability to the experimental synthesis conditions [21; 22; 23]. The grafting of MA onto the PLA backbone results in a reactive compatibilizer due to the presence of the anhydride function. PLA graft copolymers are therefore obtained by a reaction of the anhydride group with some reactive functions of the blended polymer. Functionalization with MA is typically carried out between 120 \({}^{\circ}\)C and 200 \({}^{\circ}\)C with stirring at 50-200 rpm in an extruder in the presence of a radical initiators, such as dicumyl peroxide (DCP) or benzyl peroxide (BPO). A typical reaction scheme is shown in Figure 5[21]. The role of different reaction parameters (MA concentration, nature of the initiator, temperature, molar mass of PLA) on the percentage of grafting is also described [23]. It was found that the molecular weights of PLA decreased during the reaction due to chain sessions. The grafting percentage of low-molecular-weight PLA is greater than that of the high-molecular-weight PLA because of steric hindrance (Table 1). It should be noted that the grafting percentage of MA increased with the initial concentration of MA, but remained low (<1.25%) regardless of the different parameters of the reaction, which shows the low efficiency of these radical reactions [23]. If one wishes to graft at least two molecules of anhydride per PLA chain, the molar mass of the latter must be higher than 14,500 g.mole\({}^{-1}\) if substitution degree = 1%, which is not always the case. Nevertheless, the results obtained for the compatibility of various PLA-based blends are significant. Examples of variations in mechanical properties are shown in Table 2 in blends of PLA/cellulose nanofibers (CNF) [24]. Figure 5: Reaction scheme for the grafting of MA on PLA (from [PERSON] et al. [21], copyright Taylor and Francis, reproduced with permission). The following paragraphs show the main PLA graft copolymers obtained from PLA-\(g\)-MA with their principal properties and applications. PLA-\(g\)-Cellulosic Derivatives  
Cellulose fibers derived from renewable biomass have attracted interest as microscale reinforcements in composite materials. Natural fibers have many advantages--low density, low cost, renewability, biodegradability--that make them excellent candidates for the design of biodegradable materials. They can advantageously replace mineral reinforcements in PLA matrices [3; 4]. However, a poor compatibility between the fiber and the polymer matrix leads to materials with poor performances. In particular, nanocelluloses, due to their polar surfaces, are difficult to uniformly disperse in a non-polar medium. The consequences of this poor interfacial compatibility between polymer and filler are poor properties of the final blend. The compatibilization of PLA/cellulosic derivatives blends to improve many of the blends' properties, especially mechanical properties, while maintaining a natural source, are the most widely described in the literature [24; 25]. Because PLA-\(g\)-MA acts as a reactive compatibilizer, a PLA-\(g\)-cellulose copolymer is formed as a result of the reaction between the anhydride of PLA-\(g\)-MA, and alcohol functions of the cellulosic derivative (Figure 6) [21]. The presence of a very low percentage of PLA-\(g\)-MA (<1%) in the original blend significantly improved the properties of the blend [24]. Tensile strength, tensile modulus and strain at break were increased by 55.3%, 15.45% and 30.4%, respectively, over neat PLA by adding 5 wt.% of cellulose nanofibers (CNFs), and by 169.2%, 36.3% and 139.1%, respectively, by adding 5% of PLA-\(g\)-MA to the blend PLA/CNF.