Comparison of mechanical reinforcement effects of surface-modified cellulose nanofibrils and carbon nanotubes in PLLA composites

Cellulose nanofibrils were prepared using 2,2,6,6-tetramethylpiperidiniyl-1-oxyl (TEMPO)-mediated oxidation of wood cellulose, and the surfaces of the resulting TEMPO-oxidized cellulose nanofibrils (TOCNs) were grafted with amine-terminated polyethylene glycol (PEG) chains via ionic bonds. PEG-TOCN/PLLA composite films were prepared by casting mixtures of various ratios of PEG-TOCN/chloroform dispersion and PLLA/chloroform solution. The mechanical reinforcement properties of PEG-TOCNs prepared using PEG-NH2 of two different PEG chain lengths were evaluated using tensile tests and compared against those of PEG-grafted single-walled carbon nanotubes (PEG-SWCNT). PEG-TOCNs were individually dispersed in the PLLA matrix so that the Young's modulus increased with PEG-TOCN content, following theoretically predicted values. In contrast, the PEG-SWCNT formed aggregates in the PLLA matrix, probably owing to heterogeneous distribution of carboxyl groups. As a result, PEG-TOCN/PLLA composites showed superior properties to those of PEG-SWCNT/PLLA composites in terms of optical transparency, strength, and toughness.