Aging of Phosphorylated Cellulose Nanofibers under Moist-Heat Conditions

Understanding the long-term stability of cellulose nanofibers is critical for their practical application in advanced functional materials. In this study, we investigated the aging behavior of phosphorylated cellulose nanofibers (PCNFs) sheets under moist-heat accelerated aging conditions (80 °C, 65% relative humidity (RH)) for up to 42 days. PCNFs with different phosphate group densities were prepared by controlling the phosphorylation time, and their chemical and morphological changes were systematically analyzed. Liquid-sta³¹P NMR revealed a progressive transformation of surface phosphate esters into inorganic phosphate salts during aging. This dephosphorylation was thought to lead to a decrease in the pH within the sheets, which in turn promoted hydrolysis of the cellulose backbone. The resulting degradation manifested as decreases in the degree of polymerization (DP) and fibril length, particularly in PCNFs with higher surface charge. Conversely, the lateral crystallite size of the cellulose increased. These findings provide insights into PCNF aging and highlight the importance of controlling the initial phosphate ester structure and environmental conditions to increase the stability of PCNF-based materials in practical applications.