Optical assessment of lignin-containing nanocellulose films under extended sunlight exposure

This study investigates the stability of various cellulose substrates and their UV-blocking properties from the perspective of optoelectronic applications that are obliged to endure prolonged periods under sunlight. The application of cellulose substrates to optoelectronic applications, such as solar cells, has gained traction thanks to cellulose films’ ability to function as a sustainable alternative that can be modified and functionalized optically in various ways. Here, the films of cellulose nanofibrils (CNF) and TEMPO-oxidized cellulose nanofibrils (TOCNF), with and without lignin, were tested under 1000 hours of exposure to artificial sunlight. The literature to date provides no quantitative analysis of such films’ stability. As such, this contribution examines how the films age with respect to their optical and mechanical performance and their retention of their UV protective qualities. While UV-blocking properties of lignin are well known, films containing residual lignin (LignoCNF and LignoTOCNF), and lignin nanoparticles (CNF-LNP and TOCNF-LNP) demonstrated remarkable UV-blocking stability; even after the aging transmittance of LignoCNF and CNF-LNP films remained lower than 1% below 390 nm. Most lignin-containing films exhibited increased transmittance between 400 and 600 nm after aging, except for LignoTOCNF, which showed a decrease in transmittance that was comparable to that displayed by non-lignin films. Mechanical testing revealed increased brittleness in both CNF and LignoCNF, while films with lignin nanoparticles exhibited reduced strain at the break. The observed changes were linked to the potential oxidation of COO- groups and structural modifications in both cellulose and lignin. Color changes were also observed, with TOCNF films darkening significantly after aging, while the lignin-containing films exhibited varied behaviors: while LignoCNF, CNF-LNP, and TOCNF-LNP became lighter, LignoTOCNF darkened. Overall, the incorporation of lignin into nanocellulose films enhances their durability, UV protection, and mechanical stability, making them promising candidates for sustainable optoelectronic applications.