Self-Healing Properties of Lignin-Containing Nanocomposite: Synthesis of Lignin-graft-poly(5-acetylaminopentyl acrylate) via RAFT and Click Chemistry

Lignin can be an important source of synthetic commodity materials owing to its abundance in nature and low production cost. The current use of lignin as a raw material, however, is very limited and focused only on cheap and poorly defined nonfunctional materials. Herein we report a new lignin-containing functional polymer, lignin-graft-poly(5-acetylaminopentyl acrylate) (lignin-graft-PAA), which has been prepared by the covalent linkage of chemically modified lignin with PAA, which is an end-group functionalized polymer. This work makes two significant advances in the study of lignin-containing polymers: (1) lignin-graft-PAA is the first example of lignin being modified by a polymer with sophisticated structure, and (2) lignin-graft-PAA shows a special performance, autonomic self-healing properties, which have not yet been seen in lignin-containing polymers. The key synthetic step in this process utilizes a copper-catalyzed azide–alkyne cycloaddition or “click” reaction in order to join together the lignin and polymer moieties. The polymer, PAA, was itself prepared via reversible addition–fragmentation chain transfer (RAFT) polymerization of monomers containing multiple hydrogen-bonding sites on their pendants in the form of acetylamino functional groups. The selected RAFT agents also resulted in a polymer with an azide group at its terminus, which is necessary for the desired click reaction. Separately, biomass lignin was chemically modified by 5-hexynoic acid to introduce an alkyne functionality onto lignin. The azide terminus of the polymer was joined to the alkyne group of lignin to form a covalent bond. The lignin-graft-PAA possesses a well-dispersed multiphase nanostructure, a rigid lignin phase and a soft-PAA phase, which has a rubber-like flexibility. The mechanical properties of the newly synthesized lignin-graft-PAA can be readily controlled by the mass ratio of lignin and polymer during synthesis. In this study, the mass ratio was varied by either polymer length or weight percentage of lignin. It was revealed that a lignin-graft-PAA composite with 15–20 wt % lignin demonstrated the most optimal rubber-like, flexible property. Thanks to the high degree of hydrogen bonding from the acetylamino functionalities, lignin-graft-PAA also showed self-healing properties, recovering up to 93% of its original maximum stress before fracture.