Dodecylated lignin-g-PLA for effective toughening of PLA

Abstract

A green approach to an effective toughening agent of the popular green plastic, polylactide (PLA), is established. Firstly an aqueous approach is developed to selectively graft dodecane to 100% of phenolic hydroxyl (OH) and 77% of carboxylic OH of alkaline lignin. The selective alkylation is characterized using ¹H NMR, ³¹P NMR, FTIR and DSC. Then PLA is grafted from the remaining OH groups of dodecylated lignin. The grafting of PLA is characterized using ¹H NMR, ³¹P NMR, and GPC. Note that M_n of the grafted PLA chains can reach more than 10 K Da. Subsequently lignin/PLA blends are prepared by mixing PLA and dodecylated lignin-g-PLA. On the basis of DSC results, it is suggested that there are three phases composed of bulk PLA, mixed bulk PLA and the grafted PLA, and dodecylated lignin, resepectively, in the blends. In comparison with PLA, the blends have similar thermal stability as reflected by TGA, are more hydrophobic as reflected by higher contact angles, and block UV light better. Impressively the elongation of the blends can be more than 40 times higher than PLA with certain improvements in Young's modulus and tensile strength. This should be due to the well integrated dodecylated lignin units in the bulk PLA being able to disperse energy. This understanding can guide the development of new PLA toughening approaches.