Lignin Nanoparticles as an Interfacial Modulator in Tough and Multi-Resistant Cellulose–Polycaprolactone Nanocomposites Based on a Pickering Emulsions Strategy

Free-standing nanocellulosic films (nanopapers) emerge as attractive sustainable materials to replace traditional plastics. However, the moisture sensitivity of cellulose and its poor dispersion in hydrophobic polymers are challenges to its widespread application. Harnessing the inherent properties of cellulose, lignin, and polycaprolactone, a Pickering emulsion approach is proposed to produce multifunctional cellulose nanofibril (CNF) nanocomposite films. Aqueous CNF dispersion is combined with hydrophobic polycaprolactone (PCL) using colloidal lignin nanoparticles (CLPs) as the emulsion stabilizer. CNF–PCL nanocomposite films with over 134% increase in dry strength compared to nanocomposites without CLPs are fabricated. This interfacial engineering strategy results in a CNF-based nanocomposite with wet strength up to 87 MPa without any chemical modification or crosslinking agents. The mechanism behind the achieved excellent dry and wet strength and water resistance is investigated and it is suggested that it is due to the amphiphilic CLPs that are able to form non-covalent bonds with both cellulose and PCL, thus binding these together. Furthermore, the nanocomposite films’ protection against UV and oxidation is significantly enhanced by increasing the CLPs content. Our proposed interfacial engineering strategy can be generically applied to other polymer systems and shows a great potential to pave the way toward replacing fossil-based plastics.