© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 231 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.de different materials used will arise. When cellulose nanoﬁbers are dispersed uniformly enough throughout a transparent polymer matrix, they can strengthen the polymer while the resulting composite material retains its transparency. [8–10] Fol- lowing that, Okahisa et al. [11] have succeeded in combining organic light-emitting diodes (OLEDs) with optically trans- parent wood cellulose nanocomposites. However, nanoﬁbers production is high in energy consump- tion. Hence, we recently successfully fabricated transparent nanocomposites using heterogeneous micro-scale chitin pow- ders consist of uniform chitin nanoﬁbers, in the ﬁrst report of nanostructured particle reinforced composites. [12,13] The resulting material displayed high toughness and thermal sta- bility at elevated temperatures. Encouraged by these ﬁndings, we herein developed a low thermally expanded transparent ﬁlm by exploiting wood ﬁbers that can be considered to be nano- structured ﬁbers in which individual nanoﬁbers do not signiﬁ- cantly agglomerate and are orientated parallel to the ﬁber direc- tion in S2 layer which accounts for 70–80% of wood ﬁbers. We thereby demonstrate that paper, used since ancient times, will be a next-gen optical material. The cell wall of wood has a fascinating 3D hierarchical structure designed to maximize the stability and durability of the tree. Wood ﬁber is made up of semi-crystalline cellulose nanoﬁbrils ( Figure 1), random amorphous hemicelluloses, and lignin that cross-link the different polysaccharides in wood to form a strong and durable structure. [14] Drying after the removal of a matrix such as hemicellulose and lignin gen- erates strong hydrogen bonding, causing aggregation between the ﬁber bundles because the cellulose polymer molecules have a linear chain structure consisting of repeating glucose units with three hydroxyl groups. In the natural state, the deligniﬁed ﬁber wall has a speciﬁc surface area of around 100 m 2 /g. The ﬁber wall collapses during drying, decreasing the speciﬁc sur- face area to around 1 m 2 /g. [14] This resulted in an opaque com- posite when the matrix-removed wood pulp sheet was impreg- nated into acrylic resin, due to light scattering. Hence, generation of hydrogen bonding among cellulose nanoﬁbers in the wood pulp must be avoided. [15] In this study, the wood pulps were acetylated with much care taken to main- tain a never dried state. The degree of substitution (DS) value of the wood pulp by the acetylation was 1.25 and the moisture content of untreated wood pulp sheet at 23 °C and 50% relative humidity (RH) decreased from 6.4% to 2.8% due to the treat- ment. Acetylation also accelerated the ﬁltration speed when producing sheets due to the increased hydrophobicity of the acetylated pulp ﬁbers. The dried acetylated pulp sheets were impregnated in acrylic resin (ABPE 10, refractive index 1.536 at 590 nm, Shin-Nakamura Chemical Co. Ltd) under vacuum at a pressure of 0.01 MPa for 12 h. Both the acetylated and the Wood Pulp-Based Optically Transparent Film: A Paradigm from Nanoﬁbers to Nanostructured Fibers Hiroyuki Yano,* Shouzou Sasaki, Md. Iftekhar Shams, Kentaro Abe, and Takeshi Date Prof. H. Yano, S. Sasaki, Prof. Md. I. Shams, Dr. K. Abe, T. Date Research Institute for Sustainable Humanosphere Kyoto University Uji, Gokasho, Kyoto, 611–0011, Japan E-mail: yano@rish.kyoto-u.ac.jp Prof. Md. I. Shams Forestry and Wood Technology Discipline, Khulna University Khulna, 9208, Bangladesh T. Date Nippon Paper Industries Co., Ltd. 5–21–1 Oji, Kita-ku, Tokyo 114–0002, Japan DOI: 10.1002/adom.201300444 In the digital era, ample information is being exchanged through electronic media and the demand for high-quality, smart, and portable digital devices is increasing sharply. Because of its very low coefﬁcient of thermal expansion, 7–10 ppm K -1[1] and transparency, glass has up to now been used extensively in the substrates of electronic devices such as displays and solar cells. However, in modern ﬂat panel display (FPD) technology and solar cell production, continuous “roll- to-roll” (RTR) processing using ﬂexible plastic substrates is expected to take over from conventional “batch” processing of glass substrates. Plastic is no doubt a good choice for ﬂexible displays and solar cells because it allows sufﬁcient mechan- ical and optical performance for these applications. However, practical RTR processing has so far been prevented by the high coefﬁcient of thermal expansion (CTE) of the plastics to be used. One possible way to reduce thermal expansion while only causing a low loss in transparency would be to use ﬁllers with a diameter signiﬁcantly smaller than the wavelength of visible light. [2] Nanoﬁbers are believed to have the potential to substantially improve the mechanical properties of polymers because their large interfacial area enables an applied load to be transferred through ﬁller/matrix interface while maintaining transparency. [3,4] It is well known that different varieties of nanoﬁbers can be found in nature. Among them, cellulose nanoﬁbers 4 to 20 nm in width, the major component of the plant cell wall and the most abundant bio resources on earth, have received great interest because of their outstanding mechanical proper- ties due to their high molar mass and highly ordered extended chain polysaccharide nanoﬁber structures. The maximum macroscopic Young's modulus of plant cellulose is 128 GPa, [5] and the elastic modulus of the crystalline regions of cellulose I is 138 GPa. [6] Surprisingly, its coefﬁcient of thermal expansion in the axial direction is as low (0.1 ppm K -1 ) as that of quartz. [7] This makes it a preferable candidate for RTR manufacturing processes because no discrepancy between the CTEs of the Adv. Optical Mater. 2014, 2, 231–234