Bacterial cellulose (BC) nanofibers were acetylated to enhance the properties of optically transparent composites of acrylic resin reinforced with the nanofibers. A series of BC nanofibers acetylated from degree-of-substitution (DS) 0 to 1.76 were obtained. X-ray diffraction profiles indicated that acetylation proceeded from the surface to the core of BC nanofibers, and scanning electron microscopy images showed that the volume of nanofibers increases by the bulky acetyl group. Since acetylation decreased the refractive index of cellulose, regular transmittance of composites comprised of 63% BC nanofiber was improved, and deterioration at 580 nm because of fiber reinforcement was suppressed to only 3.4%. Acetylation of nanofibers changed their surface properties and reduced the moisture content of the composite to about one-third that of untreated composite, although excessive acetylation increased hygroscopicity. Furthermore, acetylation was found to reduce the coefficient of thermal expansion of a BC sheet from 3 x 10(-6) to below 1 x 10(-6) 1/K.

Cellulose, (1f4)--D-glucopyranan, was for the first time synthesized by cationic ring-opening polymerization of 3,6-di-O-benzyl-R-D-glucose 1,2,4-orthopivalate (5) into 3,6-di-O-benzyl-2-O-pivaloyl--D-glucopyranan (6) and subsequent removal of the protective groups. Polymerization of the orthoester 5 by triphenyl carbenium tetrafluoroborate gave 3,6-di-O-benzyl-2-O-pivaloyl--D-glucopyranan with [R] D -37.2°and a number-average molecular weight of 8.3 × 10 3 (DP n ) 19.3). Removal of the pivaloyl and benzyl groups and subsequent acetylation gave acetylated (1f4)--D-glucopyranan which was completely identical with cellulose triacetate (CTA) prepared from low molecular weight cellulose. The synthesized CTA was converted by deacetylation to cellulose, which has the cellulose-II crystal structure.

The authors studied acetylation of bacterial cellulose ͑BC͒ nanofibers to widen the applications of BC nanocomposites in optoelectronic devices. The slight acetylation of BC nanofibers significantly reduces the hygroscopicity of BC nanocomposites, while maintaining their high optical transparency and thermal stability. Furthermore, the degradation in optical transparency at elevated temperature ͑200°C͒ was significantly reduced by acetylation treatment. Therefore, the acetylation of bionanofibers has an extraordinary potential as treatment for property enhancement of bionanofiber composites.

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