Silica aerogels, which consist of a three-dimensional network of 10−20 nm silica nanoparticles with a porosity of 90−98%, have attracted much attention as a transparent insulator.1Althou
Trang 1Chitosan Aerogels: Transparent, Flexible Thermal Insulators
Satoru Takeshita * and Satoshi Yoda
Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
*S Supporting Information
Thermal insulation is a key technology for energy
conservation in the 21st century One of the major issues
of the current thermal insulation technology is heat flow
through transparent parts such as windows and glass walls
Silica aerogels, which consist of a three-dimensional network of
10−20 nm silica nanoparticles with a porosity of 90−98%, have
attracted much attention as a transparent insulator.1Although
silica aerogels have high transparency in the visible region and
extremely low thermal conductivity, their commercial uses are
limited by their brittleness and fragile nature A variety of
approaches to reinforcing the mechanical properties of silica
aerogels have been reported, e.g., coating polymers at the silica
surface,2,3 incorporating organic functional groups,4,5
penetrat-ing fibrous polymers into silica aerogel matrices,6 , 7
and embedding silica aerogel particles in polymer matrices.8
However, increasing organic contents inevitably increases
structural inhomogeneity at the nanoscale, which results in
low transparency and high thermal conductivity A new
category of thermal insulating materials, which has a
combination of transparency, flexibility, and a low thermal
conductivity, has long been required in thisfield
In contrast to traditional aerogels consisting of nanoparticle
skeletons, some researchers have focused on nanofibrous
aerogels composed of entangled nanofibers of soft
poly-mers.9−11 These aerogels are expected to have flexibility and
become suitable candidates forflexible thermal insulators Most
of the nanofibrous aerogels reported so far were opaque
because they had structural inhomogeneity at the scale of >∼
100 nm Thus, the synthesis of transparent nanofibrous
aerogels has been a major challenge in this field A recent
breakthrough was made by Isogai’s group, who reported a
translucent aerogel of oriented cellulose nanofibers.12
This aerogel had a homogeneous porous structure at the nanoscale
and hence a low thermal conductivity However, the use of
oxidative catalysts, e.g., 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO), in the production of cellulose nanofibers becomes
an industrial problem because of its high cost and hazardous
properties An alternative approach is therefore needed to
synthesize transparent nanofibrous aerogels without using any
high-cost catalysts or oxidants
Chitosan is a biomass-derived polysaccaride abundant in
many raw materials, with a low cost, yet is environmentally
friendly, and has high biocompatibility Several gelation
approaches to producing chitosan aerogels, such as physical
gelation in basic solutions,13,14cross-linking with hemicellulose
citrate,15 and cross-linking with aldehydes,16−19 have been
reported Despite the range of available synthetic methods,
these previous reports have dealt only with opaque aerogels
with high densities The chitosan aerogels have been proposed
as potential catalysts,13,14 absorbers,16,20,21 and scaffolds for biomedical applications,17−19 but they have not been considered as potential transparent thermal insulators
In this work, we try to open up the application field of thermal insulators by proposing a translucent and ultralight chitosan aerogel prepared without using any catalysts or oxidants The key to achieving transparency in the visible region, i.e., a structural homogeneity at the nanoscale, is the cross-linking conditions employed Chitosan powder wasfirst dissolved in acetic acid to form a homogeneous solution at the molecular level, and then regenerated to form a gel through cross-linking We used formaldehyde as a mild cross-linker Formaldehyde reacts with a NH2group to form a Schiff base,
−NCH2, which reacts with another NH2group to form an
NCN cross-linking bond,22 , 23
as shown in Figure 1a
Preparation conditions and physical properties of the chitosan aerogels obtained are summarized in Table 1 The concen-tration of chitosan in thefinal gelling solution varied from 4 to
16 g L−1, whereas that of formaldehyde was 1.8 or 7.2 wt % The sample C4F2, which was prepared using 4 g L−1chitosan and 1.8 wt % formaldehyde solutions, did not form any solid gel, probably because of an insufficient amount of cross-links Received: September 14, 2015
Revised: October 22, 2015 Published: November 6, 2015
Figure 1 Preparation of chitosan aerogels (a) Synthesis procedure Photographs of (b) methanogels and (c) aerogels Thicknesses of methanogels and aerogels: 6.2 and 3.9 mm (C16F7), 8.5 and 4.2 mm (C16F2), 7.3 and 4.0 mm (C8F7), 8.6 and 4.8 mm (C8F2), and 8.4 and 3.7 mm (C4F7).
pubs.acs.org/cm
Trang 2The other samples formed transparent hydrogels and
methanogels (Figure 1b) After extraction with supercritical
CO2, the obtained aerogels (Figure 1c) were translucent with a
yellowish color that originated from −NCH2 groups.22,23
The color became darker with increasing chitosan and
formaldehyde concentrations, reflecting the increase in the
amount of cross-links According to the Fourier-transform
infrared spectroscopy and thermogravimetry (Figures S1 and
S2), the aerogel samples did not contain a detectable amount of
free formaldehyde The apparent density of the aerogels
decreased from 0.175 to 0.042 g cm−3in the order of C16F7,
C16F2, C8F7, C8F2, and C4F7 These results mean that the
apparent density depends on the amount of cross-links, which
can be controlled by changing the chitosan and formaldehyde
concentrations
As shown in the scanning electron microscopy (SEM)
images (Figures 2 and S3−S8), the cross-linked chitosan
aerogels consist of entangled nanofibers of 5−10 nm in
diameter with mesopores of 10−50 nm in size These pores are
small enough to cause negligible scattering of visible light The
area of pores observed in the SEM images increases as the
apparent density decreases The specific surface area
determined by the Brunauer−Emmett−Teller method is 545
m2g−1for C4F7 (Figure S9) This value is 4−30 times larger
than those of chitosan cryogels24,25and comparative to those of nanocellulose aerogels.12 Assuming a cylindrical shape, the nanofibers are calculated to be 5.2 nm in diameter from the specific surface area This is consistent with the SEM observations, showing the small contact areas between nanofibers The X-ray diffraction profiles (Figure S10) show that chitosan nanofibers in the aerogels are almost amorphous The UV−visible transmission spectra of the chitosan aerogels (Figure 3a) consist of two wavelength regions: a broad
absorption in the 300−500 nm blue region corresponding to
−NCH2bonds and a gradual increase in transmittance in the
500−800 nm visible region corresponding to the light scattering The apparent optical densities per mm (Figure 3b) were calculated from these spectra with an assumption of negligible surface reflection The optical density at 400 nm roughly increases with increasing density, indicating that the amount of −NCH2 bonds increases as the cross-links increase On the contrary, the optical density at 800 nm decreases with increasing density We suggest that the microstructure of chitosan aerogels has a small inhomogeneity
at the nanoscale, which causes light scattering, particularly in low density samples The largest optical density at 800 nm per
mm, 0.15, of C4F7 corresponds to 71% in transmittance This value is still sufficiently high to be used in transparent thermal insulation applications
The thermal insulation property of the chitosan aerogels was evaluated based on a thermal conductivity measurement using the axial heatflow method The thermal conductivities of C4F7 and C8F2 are 0.022 and 0.029 W m−1K−1, respectively These values are lower than those of commercial flexible thermal insulators, e.g., mineral wools (0.033−0.05 W m−1 K−1), cellulose foams (0.046−0.054 W m−1 K−1), and polystyrene foams (0.03−0.04 W m−1K−1),26,27and comparative to those
of nanocellulose aerogels (0.018−0.038 W m−1 K−1).12 In
Table 1 Preparation Conditions and Physical Properties of Chitosan Aerogels
Sample chitosan [g L−1] formaldehyde [wt %] ρ a [g cm−3] P b [%] OD c [mm−1] k d [W m−1K−1] E e [MPa]
a Density.bPorosity.cOptical density at 800 nm.dThermal conductivity.eElastic modulus.
Figure 2 SEM images of chitosan aerogels: (a) C16F7, (b) C16F2,
(c) C8F7, (d) C8F2, and (e,f) C4F7.
Figure 3 Optical properties of chitosan aerogels (a) Transmission spectra with thicknesses of 4.0 mm (C16F7), 4.4 mm (C16F2), 3.5
mm (C8F7), 4.6 mm (C8F2), and 4.6 mm (C4F7) (b) Optical densities per 1 mm thick at 800 and 400 nm.
Trang 3general, the thermal conductivity of a porous material, k, is
given by the following equation,28,29
k kg,cond kg,conv ks,cond krad (1)
where kg,cond, kg,conv, ks,cond, and kradare the thermal conductivity
factors for gas conduction, gas convection, solid conduction,
and radiation, respectively Because the gas convection, kg,conv, is
negligible for pores smaller than 3 mm,28 the total thermal
conductivity strongly depends on the gas and solid
con-ductions The gas conduction, kg,cond, can be dramatically
suppressed when pores are smaller than the average mean free
paths of air components, e.g., 68 nm for N2 Such a nanoporous
structure is called a “nano insulation material (NIM)”,29
and enables us to achieve a lower thermal conductivity than that of
stationary air, 0.0262 W m−1K−1at 300 K.30 As the sample
C4F7 has a thermal conductivity of 0.022 W m−1 K−1, the
entangled structure of chitosan nanofibers acts like a NIM On
the other hand, the solid conduction, ks,cond, is proportional to
the square of the density, because the area of solid in a cross
section increases with increasing density This explains the large
thermal conductivity of 0.029 W m−1 K−1 for C8F2 with a
density of 0.081 g cm−3 We also note that the thermal
conductivity of chitosan aerogels is much lower than those of
nanocellulose aerogels when they are compared at the same
apparent density, although their true densities are almost the
same.31,32 This can be attributed to the difference in
microstructures Cross-linked chitosan aerogels consist of
entangled nanofibers with a random orientation In contrast,
the cellulose nanofibers tend to form oriented domains.12
Such
an oriented structure probably has a longer continuous length
of pores along the orientation, which makes the gas conduction
easier along that direction
According to a thermal stability test using a see-through
heating cell (Figures S11 and S12), C4F7 did not show any
change in appearance during heating from room temperature to
175 °C This result means that the chitosan aerogel has a
sufficient thermal stability for residential and vehicular
applications, such as house windows, glass walls of buildings,
and car windows
Figure 4a shows the compression stress−strain curves for the
chitosan aerogels They were compressed without forming
fractures or cracks up to∼95% strain The compression curves
are similar to those of nonbrittle cellular polymer foams,33and
consist of the following three parts Thefirst linear region up to
∼15% strain is attributed to elastic deformation The elastic
region is followed by a gradual increase in the stress with a large
strain up to 60−90% This is probably attributable to a plastic
deformation through breaking the cross-links After the second
region, a steep increase in the stress is observed at strain >60−
90% This might be attributable to elimination of pores through
densification of nanofibers We point out two mechanical
characteristics of the chitosan aerogels: (i) The entangled
nanofibrous structure has high mechanical toughness compared
to nanoparticle skeletons, such as silica aerogels, which usually
break at low strains∼10%.34
(ii) The nanofibrous structure also shows a strong density dependence in elasticity, e.g., the elastic
modulus, yield stress, and stress at 50% strain (Figure 4b) This
might be attributed to the increase in the cross-links, which
harden the structure The thin sample of C4F7 aerogel (Figure
4c) is even bendable by hand without a fracture Judging from
these results, the entangled nanofibrous structure of the
chitosan aerogels has a great advantage in terms of mechanical
toughness andflexibility, and these aerogels have the potential
to be used asflexible thermal insulators
In conclusion, we have prepared translucent chitosan aerogels by the cross-linking gelation method without using any catalysts or oxidants The aerogels consisted of entangled nanofibers with a high porosity up to ∼97% The aerogel microstructure exhibits visible transparency and a low thermal conductivity of∼0.022 W m−1K−1, which is lower than that of stationary air The aerogels also showed flexibility and higher mechanical toughness than conventional silica aerogels The characteristic mechanical properties associated with a low thermal conductivity enable us to realize a new category of thermal insulators: environmentally friendly, transparent, and flexible
■ ASSOCIATED CONTENT
*S Supporting Information The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-ter.5b03610
Detailed experimental procedure, FT-IR spectra, TG profiles, SEM images, BET and XRD profiles, and a thermal stability test (PDF)
■ AUTHOR INFORMATION
Corresponding Author
*S Takeshita E-mail:s.takeshita@aist.go.jp Notes
The authors declare no competingfinancial interest
■ ACKNOWLEDGMENTS
The mechanical measurement was performed by Global Application Development Center of Shimadzu Corporation
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