Synthesis of Superabsorbent Polyacrylic Acid-Grafted Cellulose Nanofibers via Silver-Promoted Decarboxylative Radical Polymerization

Graft polymerization has been widely used to produce new cellulosic nanomaterials with unique and smart properties. Silver-promoted decarboxylative polymerization is a novel and green method to polymerize vinyl monomers from the surface of carboxylated nanocellulose. Here, we report a new family of polyacrylic acid-grafted cellulose nanofibers (PAA-g-CNFs) synthesized using this polymerization method. The polymerization of acrylic acid (AA) monomers is surface-initiated by the oxidative decarboxylation of TEMPO-oxidized CNFs in the presence of persulfate and catalytic silver(I). This reaction also promotes polymer branching as the carboxylic acid groups from acrylic acid can undergo further decarboxylation, introducing a high density of sites for branching and polymer propagation. Small-molecule model experiments, Fourier transform infrared spectroscopy, and thermogravimetric analysis showed the successful grafting of PAA from CNFs. 1H NMR analysis also showed that the polymerization of AA from CNFs is rapid and is completed at 4 h, along with the formation of ungrafted PAA. Results reveal that the Ag(I) concentration in the reaction affects the chemical and water-retention properties of PAA-g-CNFs. The carboxylate content of PAA-g-CNFs increases with Ag(I) concentration due to the increase of initiation sites for polymer propagation. Small-angle neutron scattering, along with dynamic light scattering and surface charge analyses, reveals that polymerization reactions with low Ag(I) catalyst concentrations (0.1–0.25 equiv) form more linear and open structures, while those with high Ag(I) concentrations (>0.25 equiv) form hyperbranched and compact structures. The differences in the architecture of PAA grafted from CNFs directly affect the water-retention properties. PAA-g-CNFs with lower levels of branching (low Ag polymerization) display excellent water-retention properties, while PAA-g-CNFs with a hyperbranched and compact structure (high Ag polymerization) can poorly hold water. Ultimately, this study validates a novel approach to graft PAA from CNFs and demonstrates that the Ag(I) catalyst loading can modulate the degree of branching, leading to new superabsorbent polymers for a wide range of applications.