Surface modification of nanofibrillated bacterial cellulose (NFBC) with methyltrimethoxysilane was performed in water to hydrophobize its surface. A series of cellulose samples with different degrees of molar substitution of methyltrimethoxysilane was prepared by changing the NFBC:methyltrimethoxysilane ratio in the preparation. Detailed structural characterization of these samples, including their degrees of molar substitution and crystallinity, was conducted by X-ray diffraction, Fourier-transform infrared spectroscopy, and solid-state NMR spectroscopy, and their molecular dynamics were studied by solid NMR relaxation measurements. Furthermore, the effects of degrees of molar substitution on their dispersibility in organic solvents and fiber morphology were evaluated. These analyses revealed that the solidstate relaxation behaviors of the cellulose and polysiloxane domains formed on the NFBC surface strongly correlate with their dispersibility in chloroform. The optimal degree of molar substitution for NFBC was found to be 0.66. The surface-modified NFBC with the degree of molar substitution of 0.66 showed the almost equivalent molecular mobility as NFBC and could be uniformly dispersed in chloroform. Therefore, it was revealed that the solid-state NMR could provide extremely valuable information on the mobility and surface chemical property of the surface-modified NFBC. In addition, the surface-modified NFBC could be an excellent filler for fiber-reinforced nanocomposite resins.

The development of eco-friendly fiber-reinforced composite resins is an important objective from an environmental perspective, and nanofibrillated bacterial cellulose (NFBC), with extremely long high-aspect-ratio fibers, is a filler material with high potential for use in such composite resins. In this study, we investigated chemical modification of the surfaces of NFBC fibers by coupling with (3-aminopropyl)­trimethoxysilane and fabricated nanocomposite materials using the prepared surface-modified NFBC. The product prepared by the one-pot reaction of (3-aminopropyl)­trimethoxysilane with NFBC microfibrils dispersed in aqueous acid retained the same nanofibril structure as the intact NFBC. The degree of molar substitution and the silicon states on the surface of the product depended on the NFBC/(3-aminopropyl)­trimethoxysilane ratio. The thermal analysis revealed that the thermal degradation temperature of the products increases with an increase of degree of molar substitution. Highly transparent (78–89% at 600 nm) poly­(methyl methacrylate)-based nanocomposites were prepared by solvent casting; the nanocomposite containing 1.0 wt % (3-aminopropyl)­trimethoxysilylated NFBC was only 8% less transparent than neat poly­(methyl methacrylate) at 600 nm. In addition, the tensile strength of the nanocomposite was more than twice that of neat poly­(methyl methacrylate) when 1 wt % of the surface-modified NFBC was added. The surface-modified NFBC is expected to be a reinforcing nanofiber material that imparts excellent physical properties to fiber-reinforced resins.

Nanofibrillated bacterial cellulose (NFBC), a type of cellulose nanofiber biosynthesized by Gluconacetobacter sp., has extremely long (i.e., high-aspect-ratio) fibers that are expected to be useful as nanofillers for fiber-reinforced composite resins. In this study, we investigated a composite of NFBC and poly(methyl methacrylate) (PMMA), a highly transparent resin, with the aim of improving the mechanical properties of the latter. The abundant hydroxyl groups on the NFBC surface were silylated using 3-(methacryloyloxy)propyltrimethoxysilane (MPTMS), a silane coupling agent bearing a methacryloyl group as the organic functional group. The surface-modified NFBC was homogeneously dispersed in chloroform, mixed with neat PMMA, and converted into PMMA composites using a simple solvent-casting method. The tensile strength and Young’s modulus of the composite increased by factors of 1.6 and 1.8, respectively, when only 0.10 wt% of the surface-modified NFBC was added, without sacrificing the maximum elongation rate. In addition, the composite maintained the high transparency of PMMA, highlighting that the addition of MPTMS-modified NFBC easily reinforce PMMA. Furthermore, interactions involving the organic functional groups of MPTMS were found to be very important for reinforcing PMMA.