Surface modification of TEMPO-oxidized cellulose nanofibers, and properties of their acrylate and epoxy resin composite films

The carboxy groups abundantly and densely present on 2,2,6,6-tetramehylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TEMPO-CNFs) have been chemically modified to prepare resin composites with high thermal dimensional stability. A primary-amine-terminated polyalkylene glycol (PAG) with number-average degree of polymerization of 2222 was reacted with the carboxy groups in the TEMPO-CNFs through amide bonds at PAG amidation degrees of 22%, 44%, and 76% of the carboxy groups in the TEMPO-CNFs by changing the reaction conditions. These surface PAG-amidated TEMPO-CNFs were nanodispersed in methyl ethyl ketone (MEK), mixed with the polyurethane acrylate monomer, and PAG-amidated TEMPO-CNF/polyacrylate resin composites were prepared by ultraviolet-light curing. When PAG-amidated TEMPO-CNFs with amidation degree of 44% were used, the tensile modulus and thermal dimensional stability of the PAG-amidated TEMPO-CNF/polyacrylate resin composite remarkably improved. This was because the TEMPO-CNF elements were homogeneously distributed in the polyacrylate resin matrix, which was confirmed by atomic force microscopy observation and percolation theory. In contrast, when epoxy resin was used as the composite matrix, the above PAG-amidated TEMPO-CNFs did not have high thermal dimensional stability. However, when a dual surface-modified TEMPO-CNF sample containing phenyl amides and PAG-ammonium/carboxylate salt structures at amidation and amine/carboxylate salt formation degrees of 80% and 6% of the carboxy groups in the TEMPO-CNFs, respectively, the thermal dimensional stability of the neat epoxy resin remarkably improved. Thus, the carboxy groups on the TEMPO-CNF surfaces have to be suitably modified, such as by PAG amide or phenyl amide/PAG-ammonium salt dual structures, depending on the matrix resin used.