## 2 Results
### Direct Functionalization of PLA Backbone: Towards PLA-Based Graft Copolymers
#### 2.2.1 PLA-\(g\)-Maleic Anhydride (PLA-\(g\)-MA)
\begin{table}
\begin{tabular}{c c c} \hline \hline
**PLA** & **PLA-\(g\)-MA** & **Grafting \%** \\
**Molecular Weight** & **Molecular Weight** & **Grafting \%** \\ \hline
294,000 & 196,000 & 0.65 \\ \hline
92,000 & 76,000 & 1.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variations in molecular weight and grafting percentage during functionalization of PLA by MA (extract of data from [PERSON] et al. [23]). Figure 6: Reaction scheme for the grafting of a cellulosic fiber on PLA-\(g\)-MA (from [PERSON] et al. [21], copyright [PERSON] and Francis, reproduced with permission). Many cellulosic derivatives were introduced into PLA matrices in the presence of PLA-\(g\)-MA to improve the mechanical properties of blends such as Luffa [26], flax [27], coffee grounds [28], wood flour or rice husk [29; 30], sisal fibers [31], straw [32], bamboo fiber [33], cassava starch [34; 35], starch [36], and lemongrass fiber [37]. For example, the use of PLA-\(g\)-MA in a PLA/ cassava starch blend has a significant impact on elongation at break but not on Young's modulus or tensile strength. However, it was noted that PLA-\(g\)-MA with a higher proportion of grafted MA (0.52 wt.%) had a lower molecular weight and higher dispersity value, showing some degradation of the polymer backbone [34; 38]. Many other properties are improved in PLA-\(g\)-MA compatible blends, such as morphological, rheological, thermal, tensile and moisture sorption properties as well as thermal degradation [39; 40; 41]. For example, Figure 7 shows SEM micrographs of PLA/TPS (ther-moplastic starch) blends (70/30 \(w/w\)) without (Figure 7a) and with two parts per hundred rubber (phr) PLA-\(g\)-MA (Figure 7b), highlighting the compatibilization of the blend of PLA-\(g\)-starch formed in situ. To obtain a PLA-\(g\)-starch copolymer, another method is to react maleic anhydride with starch to obtain a maleated thermoplastic starch (MTPS), which is then mixed with PLA in the presence of Luperox 101 (2,5-bis(tert-butylperoxy)-2,5-dimethylhexane) in a Brabender at 180 \({}^{\circ}\)C for 5 min. The reaction scheme is shown in Figure 8[42]. Direct grafting of cellulose nanocrystals (CNC) on PLA, without the addition of PLA-\(g\)-MA, is also described, following the reaction scheme of Figure 9[43]. could his DCP was sprayed onto PLA beads, and the DCP-coated PLA pellets were mixed with CNC and extruded in a twin-screw extruder at 180 \({}^{\circ}\)C at 50 rpm for 5 min. The effective grafting of CNC onto PLA was identified by SEC, FTIR and NMR, but NMR showed a very low proportion of CNC in the copolymer. Some mechanical and structural properties were significantly impacted (increased Young modulus, decreased elongation, increased crystallinity). Figure 7: SEM micrographs of PLA/TPS blends (70/30 \(w/w\)) (**a**) without and (**b**) with 2 phr PLA-\(g\)-MA (adapted from [PERSON] et al. [39], copyright Springer Science, reproduced with permission). PLA-_g_-Natural Rubber (PLA-_g_-NR)  
Among the drawbacks of PLA materials, we can also highlight their fragility. Natural rubbers (NR), on the other hand, are highly flexible, environmentally friendly and derived from a renewable resource. They are good toughness agents due to their high molecular weight and very low glass transition temperature. However, due to the non-polarity of NRs, PLA/NR blends are immiscible and not compatible. To improve the interfacial  
Figure 8: Reaction scheme of the MTPS formation and coupling to PLA (from [PERSON] et al. [42], copyright Wiley-VCH GmbH. Reproduced with permission). Figure 9: Reaction scheme for the synthesis of PLA-_g_-CNC (from [PERSON] et al. [43], copyright Elsevier, reproduced with permission). interaction between PLA and NR, the reactive compatibilizer PLA-\(g\)-MA is used to form a graft copolymer PLA-\(g\)-NR [44; 45]. Typically, the compatibilized blend is made in a twin screw extruder at a temperature between 160 and 180 \({}^{\circ}\)C and a screw speed of around 30 rpm.