Fiber-content dependency of the optical transparency and thermal expansion of bacterial nanofiber reinforced composites

Abstract

We produced transparent nanocomposite reinforced with bacterial cellulose having a wide range of fiber contents, from 7.4 to 66.1 wt %, by the combination of heat drying and organic solvent exchange methods. The addition of only 7.4 wt % of bacterial cellulose nanofibers, which deteriorated light transmittance by only 2.4%, was able to reduce the coefficient of thermal expansion of acrylic resin from 86×10–6 to 38×10–6 K–1. As such, the nanofiber network of bacterial cellulose has an extraordinary potential as a reinforcement to obtain optically transparent and low thermal expansion materials.

Full text

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Title
Fiber-content dependency of the optical transparency and
thermal expansion of bacterial nanofiber reinforced composites
Author(s)
Nogi, Masaya; Ifuku, Shinsuke; Abe, Kentaro; Handa, Keishin;
Nakagaito, Antonio Norio; Yano, Hiroyuki
CitationApplied Physics Letters (2006), 88(13)
Issue Date2006-03
URLhttp://hdl.handle.net/2433/84903
Rightc 2006 American Institute of Physics.
TypeJournal Article
Textversionpublisher
KURENAI : Kyoto University Research Information Repository
Kyoto University

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Fiber-content dependency of the optical transparency and thermal
expansion of bacterial nanofiber reinforced composites
Masaya Nogi,a?Shinsuke Ifuku, and Kentaro Abe
International Innovation Center, Kyoto University, Nishikyo-ku, Kyoto 606-8501, Japan
Keishin Handa
Mitsubishi Chemical Group Science and Technology Research Center, Mitsubishi Chemical Corporation,
Yokohama, Kanagawa 227-8502, Japan
Antonio Norio Nakagaito and Hiroyuki Yano
Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto 611-0011, Japan
?Received 30 October 2005; accepted 7 March 2006; published online 31 March 2006?
We produced transparent nanocomposite reinforced with bacterial cellulose having a wide range of
fiber contents, from 7.4 to 66.1 wt %, by the combination of heat drying and organic solvent
exchange methods. The addition of only 7.4 wt % of bacterial cellulose nanofibers, which
deteriorated light transmittance by only 2.4%, was able to reduce the coefficient of thermal
expansion of acrylic resin from 86?10−6to 38?10−6K−1. As such, the nanofiber network of
bacterial cellulose has an extraordinary potential as a reinforcement to obtain optically transparent
and low thermal expansion materials. © 2006 American Institute of Physics.
?DOI: 10.1063/1.2191667?
Bacterial cellulose ?BC?, cellulosic nanofibers produced
by bacteria, has extraordinary potential as a reinforcement to
obtain optically transparent materials. Since bacterial cellu-
lose nanofiber is a ribbon-shaped fiber of 10?50 nm,1–4it
can be used to reinforce transparent plastics with less than
10% loss of light transmittance, even at fiber contents as high
as 70 wt %.5Because the nanofibers are made up of bundles
of semicrystalline extended cellulose chains, their Young’s
modulus and tensile strength are 138 GPa ?Ref. 6? and at
least 2 GPa,7respectively, and surprisingly, the thermal ex-
pansion in the axial direction is less than 0.1?10−6K−1.8
Hence, this nanofiber reinforced composite showed incred-
ibly low thermal expansion ?3?10−6K−1? and high tensile
strength ?325 MPa?, while maintaining the flexibility and
ductility of many plastics.5Besides, due to the nanofiber size
effect, high transparency was obtained against a wide distri-
bution of resin refractive indexes, from 1.492 to 1.636 at
20 °C.9The optical transparency was also insensitive to tem-
perature increases up to 80 °C.9These nanofiber-network-
reinforced polymer composites should lead the way to a
wider use of optically transparent polymers in optoelectronic
devices such as substrates for flexible displays and compo-
nents for precision optical devices, which demand high trans-
parency and low thermal expansion.
However, to further enhance the transparency of BC
nanocomposites, maintaining their low thermal expansion
and high strength, their optimum fiber-content must be elu-
cidated based on the fiber-content dependency of the proper-
ties. Hence, in this Letter, we developed a method to produce
BC composites in a wider fiber-content range and clarified
the changes of regular light transmittance and the coefficient
of thermal expansion ?CTE? of the composites against fiber
content.
BC pellicles with a thickness of 10 mm consisting of
1 vol % BC nanofibers and 99 vol % water were used as the
starting material.5,9When the pellicles were pressed to one-
tenth of their original thickness to squeeze out the water and
then dried, the BC sheets shrank during the drying and their
density became 1.0–1.2 g/cm3. As the density of the cellu-
lose microfibrils is 1.6 g/cm3, the interstitial cavities in the
dried sheets were estimated to account for 1/3–1/4 of the
volume. Thus, when these cavities were completely filled by
impregnating the sheets with neat acrylic resin ?tricyclode-
cane dimethanol dimethacrylate ?TCDDMA?? under a re-
duced pressure and cured by UV light,5,9the fiber content of
the BC nanocomposites was restricted to a range of
52.4–66.1 wt %.
To produce BC nanocomposites with lower fiber con-
tents, it is necessary to prevent cohesion of BC nanofibers
during the evaporation of water. Thus, organic solvents that
are soluble in both water and acrylic resin were applied for
the removal of water from BC pellicles instead of drying.
That is, the BC pellicles were compressed, adjusting the
thickness by a cold press to control the fiber content, and
were dipped into a mixture of water and acetone, the con-
centration of which was increased from 50% to 100% step
by step, to prevent nanofiber cohesion. Afterwards, the
solvent-replaced sheets were impregnated with neat acrylic
resin under reduced pressure and UV cured. The acetone was
completely evaporated during the resin impregnation under
reduced pressure. This method enabled the production of
nanocomposites with lower fiber contents in a range of
19.0–49.0 wt %. The fiber contents of BC nanocomposites
were determined by elemental analysis.
When ethanol was used as a solvent, due to its higher
polarity, the thickness adjusted BC sheets swelled during the
impregnation of acrylic resin. As a result, BC nanocompos-
ites with fiber content in the range of 7.4–35.2 wt % were
obtained. Consequently, the study of the fiber-content depen-
dency of the optical transparency and thermal expansion of
bacterial nanofiber reinforced composites became possible in
a wider fiber-content range, from 7.4 to 66.1 wt %. The re-
a?Electronic mail: nogi@rish.kyoto-u.ac.jp
APPLIED PHYSICS LETTERS 88, 133124 ?2006?
0003-6951/2006/88?13?/133124/3/$23.00
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© 2006 American Institute of Physics
88, 133124-1

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sulting thickness of the composites was distributed from
56 to 580 ?m.
Regular luminous transmittances were measured at
wavelengths from 190 to 1000 nm using a UV-visible spec-
trometer with an integrating sphere 60 mm in diameter
?U-4100, Hitachi High-Tech. Corp.?. Regular transmittance
was measured by placing the specimens 25 cm from the
entrance port of the integrating sphere. The regular trans-
mittances of BC nanocomposite sheets against various fiber
contents are shown in Fig. 1. In Fig. 1?a?, the regular trans-
mittance spectra of acrylic resin and the BC nanocomposite
produced by heat drying or by ethanol exchange are com-
pared. All the composites absorb light below 400 nm af-
fected by the optical characteristics of the acrylic resin and
the transmittance gradually increases with increasing wave-
length. Despite the wide distribution of fiber content and
sample thickness as described above, all the BC nanocom-
posites transmitted more than 75% of the light in the wave-
length of 500–800 nm, including Fresnel’s reflection. How-
ever, careful comparison of the transmittance spectra against
various fiber contents revealed an unexpected dependency;
that is, the transmittance decreased in spite of the decrease in
fiber content.
To investigate the dependency more precisely, the regu-
lar transmittances of BC nanocomposite at 590 nm were
compared, as shown in Fig. 1?b?. It was discovered that de-
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creasing the fiber content from 66.1 to 7.4 wt % leads to a
reduction of the regular transmittance from 84.1% to 79.5%.
In the production of BC composites, fiber contents were
adjusted by changing the thickness of the composite while
maintaining the same amount of bacterial cellulose. The
transmittances are influenced not only by their fiber contents
but also by their thicknesses. Thus, to discuss the fiber-
content dependency of the optical transparency excluding the
effect of sample thickness, the regular transmittances at
590 nm of BC nanocomposites with 100 ?m thickness were
calculated by defining the regular transmittance of acrylic
resin as 100%.10As shown in Fig. 2, it was found that the
normalized transmittance of BC nanocomposites increased
against the decrease of fiber content linearly and regardless
of the sample preparation methods.
Haraguchi and Usami reported that the light transmit-
tance of a transparent phenolic resin sheet 100 ?m thick de-
teriorated by 15% when 20 nm silica nanoparticles were well
dispersed by 10 wt % using the sol-gel method.11Contrary to
this, with BC nanocomposites, the reduction of the regular
transmittance at the fiber content of 11.7 wt % was only
3.3% ?Fig. 2?. Even at fiber contents as high as 66.1 wt %,
the loss of transparency was a mere 13.7% ?Fig. 2?. This
clearly indicates that the bacterial cellulose has an extraordi-
nary potential as a reinforcement to obtain optically transpar-
ent materials.
As mentioned above, lower fiber content is preferable
from the viewpoint of high light transmittance of BC com-
posites; however, decreasing fiber content could detract from
the low CTE of BC composites. Hence, the CTEs of BC
composites were evaluated against the fiber content. The
CTEs were measured by a thermomechanical analyzer
?TMA/SS6100, SII Nanotechnology Inc.?. Specimens were
25 mm long and 3 mm wide with a 20 mm span. After heat-
ing at 180 °C for 2 h in a nitrogen atmosphere to postcure
the acrylic resin, the measurements were carried out three
times with a heating rate of 5 °C/min in a nitrogen atmo-
sphere in tensile mode under a negligible load of 3 mg to
detect the thermal strain. The CTE values were determined
as the mean values at 20–150 °C in the second run.
Surprisingly, the addition of only 7.4 wt % of BC
nanofibers, which deteriorated light transmittance only by
2.4% ?Fig. 2?, could reduce the CTE of acrylic resin from
86?10−6to 38?10−6K−1?Fig. 3?. This is an excellent re-
FIG. 1. Regular transmittance spectra ?a? and the regular transmittance at
590 nm ?b? of BC nanocomposite of various fiber contents ?wt %? and
thicknesses ??m?.
FIG. 2. Normalized regular transmittance at 590 nm and 100 ?m thickness
of BC nanocomposite against fiber content.
133124-2 Nogi et al.Appl. Phys. Lett. 88, 133124 ?2006?

Page 4

inforcement
?10−6K−1?/silica nanohybrids, which resulted in just 20%
reduction of CTE at a filler content of 5 wt %, accompanied
by 30% degradation in transparency.12,13Even when micro-
sized silica particles with extremely low CTE of 0.5
?10−6K−1were mixed with cyanate ester resin ?68
?10−6K−1? up to 70 wt %, the reduction of CTE against
filler content was linear and resulted in 34% of the CTE of
the matrix.14On the other hand, BC nanocomposites’ CTEs
drastically decreased to 15?10−6K−1up at around 30 wt %
fiber content and then gradually declined to 10?10−6K−1
above 50 wt % fiber content, which is only 12% of the origi-
nal acrylic resin ?Fig. 3?. It should be pointed out that the
nanofiber network of BC might play an important role in the
drastic reduction of CTE.
In conclusion, we produced bacterial cellulose nano-
composites with a wide range of fiber contents, from
7.4 to 66.1 wt %, by the combination of heat drying and or-
effectifcomparedtopolyimide
?59
ganic solvent exchange methods. The addition of bacterial
cellulose nanofibers linearly decreased the regular transmit-
tance of the nanocomposites; however, the deterioration of
light transmittance was limited to just 13.7% at a fiber con-
tent of 66.1 wt %. The coefficient of thermal expansion was
drastically suppressed by the addition of the nanofibers. The
addition of only 7.4 wt % of BC nanofibers, which deterio-
rated light transmittance by only 2.4%, could reduce the CTE
of acrylic resin from 86?10−6to 38?10−6K−1, and a coef-
ficient of thermal expansion of 15?10−6K−1was attained at
30.8 wt % fiber content.
The BC pellicles were a kind gift from Dr. Y. Kuwana
?Fujicco Co., Ltd.?. The authors thank Professor Dr. L. Ber-
glund for helpful discussion. This work was supported by a
grant-in-aid from the International Innovation Center, Kyoto
University.
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FIG. 3. The coefficients of thermal expansion of BC nanocomposites with
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