Crystallite Size Effect on Thermal Conductive Properties of Nonwoven

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Crystallite Size Effect on Thermal Conductive Properties of Nonwoven Nanocellulose Sheets Kojiro Uetani, Takumi Okada, and Hideko T. Oyama* College of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan ABSTRACT: The thermal conductive properties, including the thermal diffusivity and resultant thermal conductivity, of nonwoven nanocellulose sheets were investigated by separately measuring the thermal diffusivity of the sheets in the in-plane and thickness directions with a periodic heating method. The cross-sectional area (or width) of the cellulose crystallites was the main determinant of the thermal conductive properties. Thus, the results strongly indicate that there is a crystallite size effect on phonon conduction within the nanocellulose sheets. The results also indicated that there is a large interfacial thermal resistance between the nanocellulose surfaces. The phonon propagation velocity (i.e., the sound velocity) within the nanocellulose sheets was estimated to be ∼800 m/s based on the relationship between the thermal diffusivities and crystallite widths. The resulting in-plane thermal conductivity of the tunicate nanocellulose sheet was calculated to be ∼2.5 W/mK, markedly higher than other plastic films available for flexible electronic devices.

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INTRODUCTION Heat management has become a significant problem in thin electronic devices that require transparency or flexibility, such as electronic paper, solar cells, printed-circuit boards, wearable devices, and flexible displays. Installation of metal heat sinks is generally effective for protecting integrated circuits and lightemitting diode arrays. However, this approach cannot be applied to thin electronics. Therefore, the transparent or flexible base materials need to have high thermal conductivity in the in-plane direction to alleviate local heating. Polymeric films are often used as the base materials for transparent and flexible devices. However, the low thermal conductivities of bulk polymer films (ca. 0.1 W/mK)1 are not sufficient to cool devices with ever-increasing heat generation. The development of transparent and/or flexible base materials with high thermal conductivity remains an important challenge. In the present study, nanocellulose (NC) sheets were used as candidate materials for the base of thin electronics with “high” thermal conductivity. NC sheets, which are composed of an extended-chain crystal of cellulose with strong hydrogen bonds between NC surfaces, have attracted attention as transparent, flexible, and foldable materials for next-generation electronic devices because of their high mechanical strengths and low thermal expansion coefficients.2,3 In general, cellulose fibers are widely used as thermal insulation materials in the building industry, because these materials are sponge-like with low bulk density and have very low thermal conductivity. Although NC aerogels have been reported to have good thermal insulation properties,4,5 there have been no reports of thermally conductive NCs. However, Shimazaki et al. reported the enhanced thermal conductivity of a cellulose nanofiber−epoxy resin composite (1.1 W/mK) compared with the thermal conductivity of the pure epoxy resin (0.15 W/mK).6 The © XXXX American Chemical Society

enhancement of the thermal conductivity of polymer matrices by mixing NCs6,7 demonstrates the potential of NCs as “high” thermal conductivity materials. Naturally derived NCs potentially have three main advantages in thermal conduction over existing plastics. First, the extended-chain crystals of NCs form a close to ideal structure with minimal defects, which is beneficial for thermal conduction. In general, polymers are thermal insulators because they contain amorphous regions, voids, chain ends, and entanglements that act as defects or interfaces and hinder thermal conduction.8 Several techniques have been used to enhance the thermal conductivity of polymers, such as ultraorientation,9 ultradrawing,10 and stretching.11,12 Heat can effectively propagate within polymeric extended-chain crystals by means of lattice vibration with minimal phonon scattering. Second, the tensile modulus of polymeric superfibers is positively correlated with thermal conductivity.9,10,13 In particular, tunicate NCs have a high tensile moduli of ∼150 GPa,14 which is similar to that of aramid fibers. Third, the polymeric materials with enhanced thermal conductivity described above were only formed as one-dimensional fibers. Two-dimensional thin films with strong connections are difficult to form using ultradrawn plastic fibers without deforming the crystalline structure. However, by simple filtration of NC suspensions, similar to traditional paper making processes, NCs can form a “nanopaper”, having strong hydrogen bonding between NC surfaces.15 It is therefore crucial to systematically investigate the thermal conductive properties of NC sheets. Received: May 8, 2015 Revised: June 23, 2015

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DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

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polymer pellets (COP, Zeonor 1430R, Zeon) were manually hotpressed at 260 °C to form clear films. Analysis. Transmission electron microscopy (TEM, JEM 1230, JEOL, Japan) was performed at an accelerating voltage of 80 kV to observe the morphologies of the NCs after negative staining in 2% phosphotungstic acid aqueous solution. The fiber dimensions were determined from the TEM images using ImageJ (NIH, USA). Raman spectrometry (NRS-5100, JASCO, Japan) was performed with a 532 nm polarized green laser for excitation. The laser was focused to a spot size (diameter) of 4 μm with a 20× lens and to 15 μm with a 5× lens for the cross-sectional and 360° rotation measurements, respectively. For the TOSNF sheet, to decrease the fluorescence intensity, laser spot sizes of 1 μm (with a 100× lens) and 4 μm (with a 20× lens) were used for the cross-sectional and 360° rotation measurements, respectively. The laser power was set to 10.1 mW, except for the TOSNF sheet where a power of 5.4 mW was used. The exposure time for a single scan was automatically set between 12 and 123 s, although a fixed time was used within each sample and each measurement to directly compare the peak intensity. The accumulation number was set to two for all of the measurements. Data were obtained at least three times for each measurement. For the crosssectional measurements, the samples were irradiated with the laser for 10 min before each measurement to decrease the fluorescence background. For the 360° rotation measurements, the samples were placed at the center of rotation and irradiated for 10 min before measurements were taken. The Raman spectra from a single spot with different azimuthal angles were then measured at 10° intervals without preirradiation. Preirradiation was not necessary for the TOSNF sheets because the fluorescence intensity was sufficiently reduced by changing the laser power and spot sizes. Wavenumber correction using a polypropylene standard and fluorescence correction with a regular parameter were performed using Spectra Manager version 2 (JASCO). X-ray diffraction (XRD) was performed by the reflection method using a RINT 2000 X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation generated at 40 kV and 40 mA. The sample was scanned in the 2θ range 2−40° in 0.02° steps. The full width at half-maximum (fwhm) of the diffraction peaks (fitted by a pseudo-Voigt function) was used to calculate the crystallite width D of each diffraction plane (hkl) using the Scherrer’s equation: Dhkl = Kλ/β cos θ, where K = 0.9, λ is the wavelength (1.5418 Å), β is the fwhm of the peak, and θ is the Bragg angle. The relative degree of crystallinity XC was estimated from the area ratios of the crystalline peaks to the sum of the separated peaks. The thermal diffusivity was measured using a periodic heating method with a thermowave analyzer (TA3, Bethel, Japan).16−19 Both sides of the samples were blackened with graphite spray (FC-142, Fine Chemical Japan, Japan) to avoid permeation of the heating laser to the samples and maintain the emissivity constant for detection of the temperature change. The heating laser (beam diameter ∼150 μm) was focused on the sample and heated the surface sinusoidally. The radiation thermometer measures temperature changes on the backside of the samples by detecting the phase Θ as follows:

In this study, we investigated the thermal conductive properties of nonwoven NC sheets by focusing on the crystalline structure of cellulose. Seven types of NC material were prepared to investigate the main factor that affects the thermal conductivity. The thermal diffusivity was measured using the periodic heating method, which is best suited to separately measure the thermal diffusivity in the in-plane and thickness directions of the sheets. Through detailed analysis of the orientation and crystalline structures of the NC sheets using Raman spectroscopy and X-ray diffraction, the main factor that affects the phonon conduction was identified.

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EXPERIMENTAL DETAILS

Sample Preparation. To prepare tunicate nanowhiskers (TNWs), wet tunicate of ascidian (Halocynthia roretzi) was cut into ∼1 cm squares and purified in 1% NaClO2 aqueous solution at pH ≈ 4 (achieved by adding acetic acid) for 3 h at 80 °C until the product became white. After washing the product with a large amount of distilled water, 2 g of the dried product was hydrolyzed in 55% H2SO4 aqueous solution at 60 °C for 20 min. The suspension was washed by repeated filtration and dilution with distilled water until the pH reached ∼4, and it was then ultrasonicated for 30 min. The supernatant was centrifuged at ∼4400g for 30 min to obtain the TNWs. After removing the TNW suspension, the residue was agitated using a high-speed blender for 30 min and centrifuged at ∼4400g for 30 min to obtain tunicate nanofibers (TNFs). To prepare cotton nanowhiskers (CNWs), 5 g of cotton filter papers (No. 101, Advantec, Japan) were cut into ∼1 cm squares and hydrolyzed in 55% H2SO4 solution at 60 °C for 5 min. The product was then thoroughly washed and ultrasonicated for 30 min, followed by centrifugation at ∼4400g for 30 min to yield the CNWs. Purified bacterial cellulose nanofiber (BNF) gels were prepared from the food product “nata de coco” (Seven & i Holdings with Maruha Nichiro, Japan), which was washed several times with distilled water followed by dialysis for 3 days. 5mesh powder of Japanese cedar “Sugi” (Cryptomeria japonica) was obtained after delignification using the Wise method. The Sugi powders were treated in 1% NaClO2 aqueous solution under acidic conditions at 80 °C for 1 h, and the treatment was cycled eight times to prepare the bleached pulp. After washing with distilled water, the bleached pulp was gently stirred in 3% NaOH aqueous solution at room temperature for 24 h to remove hemicellulose. After washing, the never-dried, purified pulp was obtained with an α-cellulose content of 71.2%. The purified pulp was agitated in a high-speed blender for 60 min to give Sugi nanofibers (SNFs). To prepare Sugi nanowhiskers (SNWs), 5 g of dried purified pulp was hydrolyzed in 55% H2SO4 solution at 60 °C for 2 h. The product was thoroughly washed and ultrasonicated for 30 min, followed by centrifugation at ∼4400g for 30 min to give the SNWs. To prepare TEMPO-oxidized Sugi cellulose nanofibers (TOSNF), 1.56 g (dry weight) never-dried, bleached pulp was dispersed in 200 mL distilled water and 0.031 g (0.2 mmol) 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) with the addition of 0.20 g (2 mmol) NaBr. Oxidation was initiated by adding 14.81 g (10 mmol) NaClO, and the pH was kept constant at ∼10 by adding 0.5 M NaOH. After reacting for 1 h, the product was washed until the pH reached ∼7, and it was then agitated in a high-speed blender for 20 min to disintegrate the TOSNFs. For all of the NC samples, except for BNF, the aqueous suspensions were filtered through a membrane filter. The obtained wet mats were sandwiched between metal meshes (300 mesh) and hot-pressed at 110 °C to prepare highly packed and nonwoven NC sheets 50−100 μm thick. To obtain the BNF sheet, wet BNF gel was directly hot-pressed between metal meshes. We compared the thermal conductive properties with the following plastic films: polyimide (PI, Kapton, Du Pont-Toray), polyethylene terephthalate (PET, Lumirror, Toray), polyphenylene sulfide (PPS, Torelina, Toray), polyamide (PA, Trogamid CX7323, Daicel-Evonik), and poly(ether sulfone) (PES, Sekisui Seikei). All of the plastics were used as received. Cyclo-olefin

Θ=−

πf ·r α

(1)

where f is the heating frequency, α is the thermal diffusivity of the sample, and r is the distance from the heating spot. When the radiation is detected directly behind the heating spot, α in eq 1 corresponds to the thermal diffusivity in the thickness direction (αT). By plotting the measured Θ against √f, the slope a of the linearly decaying part is defined as a = (π/αT)1/2 · d. Therefore, αT is calculated as αT = πd2/a2. We changed the heating frequency from 5.1 to 100.1 Hz in steps of 2.0 Hz for the determination of αT. When the radiation thermometer detects Θ at a distance l from the heating spot, α in eq 1 corresponds to the thermal diffusivity of the sample in the in-plane direction (αI). By plotting l as a function of Θ, the slope a is defined as a = −(π f/ αI)1/2. Therefore, αI is calculated as αI = πf/a2. Different heating frequencies were applied (0.2, 0.3, 0.5, 0.6, 0.7, 0.9, and 1.2 Hz), and l was varied from 0.0 to 2.0 mm in 0.1 mm steps. The obtained plots B

DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

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Figure 1. Preparation of NC sheets. (a) TEM images of each NC. (b) Typical Raman spectra of the NC sheet cross sections. The directional axes of the NC sheets in the measurement system geometry were set parallel to the x and y axes, and the laser was polarized in the y direction. (c) Orientational distributions of NCs within the sheet planes determined by the relative intensities of the Raman peaks at ∼1095 cm−1 as a function of the rotation angle with respect to the laser polarization axis, which was set to 0°. (d) XRD profiles for each NC. The black lines are the experimental data, and the lines of best fit (yellow) are the sums of the separated crystalline peaks (green) and the amorphous broad peaks (red). (e) Crosssectional structures of the elementary crystallites of the NCs estimated from the separated crystalline peaks in the XRD profiles. showing linear decay were used to determine α at each value of f. From the plot of α as a function of 1/√f, the values of α in the flat region were averaged to determine αI. The measurements were repeated three times and the median values were used. The thermal conductivity κ was calculated using the relation κ = α · Cp · ρ, where Cp is the specific heat capacity and ρ is the bulk density of the sample. Cp was measured with a temperature-modulated differential scanning calorimeter (TM-DSC, Q-200, TA Instruments, USA) with a sample weight of 3−6 mg in the temperature range −30

to 150 °C at a heating rate of 10 °C/min. The period and amplitude of temperature modulation were 50 s and 1 °C, respectively. The TMDSC separately measured the reversing heat flow, from which the Cp of the NC samples was determined, and the nonreversing heat flow derived from absorbed water. During heating of the NC samples, interference from a large amount of absorbed moisture that evaporated caused incorrect reversing heat capacities to be obtained. Therefore, after heating the samples to 150 °C, the temperature was decreased to 30 °C at a rate of −20 °C/min and the weights of the samples C

DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules Table 1. Crystal and Thermal Conductive Properties of NCs

TNW TNF BNF CNW SNW SNF TOSNF

91.6 88.4 87.9 87.4 79.7 74.5 67.9

XC

minimum width Dmin

average crosssectional area

Cp at 25 °C

ρ

αT

αI

κT

κI

%

nm

nm2

J/gK

g/cm3

mm2/s

mm2/s

W/mK

W/mK

1.11 1.11 1.11 1.12 1.18 1.22 1.20

1.09 1.06 0.98 1.46 1.25 1.35 1.10

± ± ± ± ± ± ±

3.4 0.83 0.97 0.91 1.2 3.4 5.7

7.1 6.6 4.6 4.6 3.0 2.7 2.4

64.0 52.0 26.5 21.1 10.4 9.8 5.7

± ± ± ± ± ± ±

5.2 1.7 0.4 0.2 0.3 0.4 0.6

0.235 0.350 0.339 0.274 0.318 0.224 0.275

± ± ± ± ± ± ±

0.012 0.016 0.006 0.003 0.021 0.020 0.012

2.04 1.43 1.19 0.901 0.768 0.682 0.480

± ± ± ± ± ± ±

0.048 0.106 0.094 0.040 0.005 0.040 0.032

0.29 0.41 0.37 0.45 0.47 0.37 0.36

± ± ± ± ± ± ±

0.015 0.019 0.006 0.005 0.030 0.033 0.016

2.47 1.68 1.29 1.47 1.13 1.12 0.635

± ± ± ± ± ± ±

0.058 0.125 0.102 0.065 0.008 0.066 0.042

Figure 2. Thermal conductive properties of the NC sheets. (a) Schematic illustration of the thermal diffusivity measuring system. (b) Typical experimental data of TNW sheets in the thickness direction. The data shown in red were used for linear fitting to determine a. (c) Typical temperature response for TNW sheets in the in-plane direction. The inset shows the calculated αI values as a function of 1/√f. The average value of the αI values shown in red was 2.04 mm2/s. (d) Heat capacity Cp at 25 °C as a function of the degree of crystallinity XC. (e) Thermal conductivities in the in-plane and thickness directions (κI and κT) for all of the NC sheets.

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remaining in the aluminum pan were determined. Immediately after placing the sample in the DSC chamber, a second measurement was carried out with the same protocol. The reversing heat flow of the second heat scan showed a linear Cp profile for all of the NC samples. The reversing Cp profiles of the second and third heat scans were similar because the sample weights were not significantly different. Therefore, we used the second measurement to determine Cp for all of the NCs. The Cp values of the dried plastic films were measured for the first heat scan to avoid changes in the thermal history. Infrared thermography (CPA-E40, FLIR systems, USA) was used to visualize the steady-state temperature distribution of the ∼1-cm-wide sheet-like materials, which were blackened with graphite spray. One end of each sample was secured to a polyimide film with tape, and they were then sandwiched between metal plates and hot-pressed at 180 °C for ∼1 h to achieve a constant temperature distribution. The thermal images were taken manually from directly above the samples.

RESULTS AND DISCUSSION The main factor that affects the thermal conductive properties of most polymers is the crystalline structure, because phonon propagation is predominant. Therefore, we prepared seven types of NC to vary the crystallite size and degree of crystallinity. The morphologies and dimensions of the NCs were different, as shown in Figure 1a. The morphologies observed for all of the NCs were similar to those previously reported.20 The NCs were concentrated and hot-pressed to form highly packed nonwoven sheets with bulk densities ρ of ∼1.0−1.4 g/cm3, as listed in Table 1. The pore sizes in the NC networks might vary depending on the fiber dimensions. The BNFs were thick bundles with 29.6 nm median width and the BNF sheet had the lowest ρ of 0.98 g/cm3. Well-disintegrated thick fibers of TNWs and TNFs (12.7 and 10.8 nm in median width) formed the sheet with the second lowest ρ of ∼1.1 g/ cm3. The short rod-like CNWs and SNWs (6.54 and 5.52 nm D

DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

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NCs were layered in the thickness direction of the sheets for all of the samples. Interestingly, the peaks in the Raman spectra at ∼1100 cm−1 broadened with decreasing degree of crystallinity. In the 360° rotation measurements on the sheet planes, the Raman peaks became narrower because of reducing fluorescence. The NC orientation within the sheet planes was directly measured by rotating the sheets in steps of 10° with respect to the polarization angle, which was set to ∼0°. Figure 1c shows that the intensity distribution of the peak at ∼1095 cm−1 was point-symmetric with only small differences in the relative intensities between samples. This indicates that most NCs were randomly oriented within the sheet planes for all samples. The SNW and SNF sheets, in particular, showed an ellipsoidal shape, indicating a slight degree of orientation. The thin straight rod-like SNWs might partially form nematic order by condensation during filtration. Some fibrils in the SNF sheet were possibly not fully disintegrated by high-speed blending, and the lamella orientation of the pulp fibers might remain within the small area irradiated by the laser. Even allowing for these fibrils, the sheet materials used in this study had similar fiber distributions for NCs layered in the thickness direction and almost randomly oriented in the plane. From the XRD profiles shown in Figure 1d, the spectra for all of the NCs have peaks at similar diffraction angles with some variation in the peak intensities, and the peaks are attributed to cellulose I type crystals. The pseudo-Voigt function was used to separate the individual peaks, and the original profiles were fitted with a determination coefficient R2 > 0.99. We calculated the average cross-sectional structures of the cellulose crystals using the crystalline widths and assuming the angle δ between (11̅0) and (110) to be 90°. When the d-spacings of (110), (11̅0), and (200) are 0.53, 0.62, and 0.39 nm, respectively,25 δ ≈ 95°. The cross-sectional area of crystals with δ = 95° was then calculated by multiplying the area with δ = 90° by cos(95−90°). However, cos(5°) = 0.996 ≈ 1 and both areas become almost equal. The γ values for unit cells of cellulose Iβ slightly differ depending on the plant species.26 However, it was difficult to determine the unit cell parameters for each cellulose from the different species used in this study. Because X-ray diffraction gives only averaged information about the crystallites, we decided to estimate the average cross-sectional area of the cellulose crystals by assuming δ = 90°. The NC crystallites had cross-sectional structures typical of thick and hexagonal tunicate cellulose,27 relatively flat-shaped bacterial cellulose,28 square-shaped cotton cellulose,27 and thin wood cellulose,29 as shown in Figure 1e. The cross-sectional areas varied from 6 to 64 nm2 for cellulose I. We believe it is significant that heat conducted through the nanometer-sized heat pipes of cellulose crystallites in the in-plane direction within the sheets. The thermal diffusivities (α) in the thickness and in-plane directions (αT and αI) were successfully measured by the periodic heating method, and are shown in Figure 2a. The value of αT for the TNW sheet was determined to be 0.235 mm2/s by fitting the linear plot of Θ as a function of √f in Figure 2b. The linearity of these curves confirms that the heating laser did not penetrate the sheets. In contrast, the αI value for the TNW sheet was determined to be as high as 2.04 mm2/s, as shown in Figure 2c. The temperature responses of all of the NC sheets were consistent with those theoretically predicted, and the resultant diffusivity showed αI > αT for all of the samples (see Table 1). The fiber orientation anisotropy directly affected the propagation rate of the temperature.

Figure 3. Size effect on the thermal conduction in NC sheets. (a) Thermal diffusivity in the in-plane direction αI as a function of the degree of crystallinity XC. The inset shows the relationship between the cross-sectional area and XC. (b) Relationships between the crosssectional area and αI and αT. (c) αI as a function of the minimum crystallite width Dmin for all of the NCs. The gradient of the linear slope is 0.27, which corresponded to v/3.

in median width, 345 and 309 nm in median length) increased the ρ of the sheets by highly dense packing. The lengths of the SNFs and TOSNFs (5.07 and 3.08 nm in median width) were difficult to accurately measure, but their length and flexibility surely caused the relatively high ρ of the sheets. The tendency that a smaller width and shorter length resulted in a higher ρ and smaller pore size in the nonwoven sheets agrees with theoretical prediction.21 Orientation of the crystalline axis also affects the thermal conductivity of polymeric materials. Therefore, the NC orientations in the nonwoven sheets were investigated by Raman spectroscopy. The 532 nm excitation laser polarized parallel to the y axis (as shown in Figure 1b) allowed the carbonyl (C−O) stretching mode to be detected along the chain direction of cellulose by the relative intensity of the Raman shift at ∼1095 cm−1.22,23 The strong peak at ∼1095 cm−1 indicates the existence of highly oriented cellulose molecules in the polarization direction of the laser.24 The Raman spectra of the cross sections of all of the samples showed similar characteristics, having stronger peak intensities at 1095 cm−1 when the sheets were placed parallel to the y axis than when they were placed perpendicular to the y axis. The E

DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

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Figure 4. Comparison of the TNW sheet with other plastic films. (a) Thermal image of a polyimide (PI) film and the TNW sheet, both of which were ∼50 μm thick. (b) Contour map of the temperature distribution determined from the thermal image of (a) with 160 × 120 pixels. The x pixel numbers of the central lines for the PI film and the TNW sheet were 51 and 97, respectively. The bar denotes the temperature (°C). (c) Temperature profiles of the PI and TNW sheet determined from the temperature data of the central lines. The temperature of the PI film was subtracted from that of the TNW sheet to show the temperature difference. (d) Comparison of the thermal conductivity of the TNW sheet with six types of plastic film currently available for transparent, flexible devices.

The specific heat capacity Cp of the materials was inversely proportional to the degree of crystallinity XC (see Figure 2d). This trend is similar to that previously reported.30 By multiplying α by Cp and ρ, the thermal conductivity in the in-plane direction (κI) was calculated to be 2.5 W/mK in the TNW sheet. Figure 2e shows that the κI values widely varied between the different NCs, whereas the thermal conductivity in the thickness direction (κT) was relatively constant. Similar to the trend in α, for all of the samples, κI > κT, which indicates that the thermal conductivity is strongly influenced by the fiber orientation. Furthermore, the similar κT values for the different NCs suggest the existence of large interfacial thermal resistance between the NC fiber surfaces. To identify the main determining factor of the thermal conductive properties, we plotted the thermal diffusivity in the in-plane direction as a function of the degree of crystallinity for all of the NC samples, as shown in Figure 3a. αI increased with increasing XC with an inflection point at XC ≈ 87%. The same trend was observed for the cross-sectional area with increasing XC (see inset of Figure 3a). XC represents the relative proportion of the crystalline part. Therefore, XC was considered to have an indirect effect on αI. We found that the crosssectional area of cellulose crystallites is linearly correlated with αI with R2 = 0.93, as shown in Figure 3b. Cellulose crystallites transported heat more effectively with increasing cross-sectional area, and thus the cross-sectional area is the dominant factor with respect to the in-plane thermal conductivity of the NC sheets. In other words, there is a “size effect” of the nanometersized crystallites in the in-plane direction of sheets on the thermal conduction. In contrast, no relationship was found between αT and the cross-sectional area. This result is thought to reflect the existence of strong interfacial thermal resistance at

the contact points of the NCs in addition to the anisotropy of the NC orientation. When the phonon mean free path l approaches the width of the sample, it becomes limited by the width. This size effect on thermal conduction is usually observed in the pure metal crystals at very low temperatures.31 Interestingly, our nanometer-sized cellulose crystallites showed a similar size-dependent effect with respect to both α and κ at room temperature. Combining the relation κ = α · Cp · ρ and the well-known kinetic model κ = Cvl/3, where C is the volumetric specific heat (J/cm3K) and v is the phonon velocity (i.e., the sound velocity; m/s),31 α can be written as α = vl/3. The phonon mean free path is limited and approaches the width of the sample when the size effect becomes dominant.31 We assumed the minimum crystallite width Dmin between D1−10, D110, and D200 of each NC to be the limiting width. Thus, α can be rewritten as α ≈ vDmin/ 3. Considering that the sound velocity is constant within a given cellulose I sample, α was expected to be a linear function of Dmin. The experimental data of αI as a function of Dmin in Figure 3c are consistent with this prediction. The gradient of the linear fitted line is 0.27, which corresponds to v/3. Importantly, the NC length did not influence phonon propagation within the sheets because l ≈ Dmin. The sound velocity v within the NC sheets in the in-plane direction was calculated from the αI data in Figure 3c to be 810 m/s, which is considerably smaller than the reported sound velocities in microfibrillated cellulose sheets (ca. 3000 m/s)15 and bacterial cellulose sheets (ca. 5000 m/s).32 In these reports, the Young’s modulus E of the sheets was directly converted to the sound velocity using the relationship v = (E/ρ)0.5 by assuming the NC sheets to be uniform bulk materials without any interfaces. Although NC sheets generally have high Young’s F

DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS The authors thank Dr. K. Hatori, Hudson Laboratory, Bethel, Co. Ltd. for assistance with the thermal diffusivity measurements. This work was supported by the JSPS KAKENHI (grant number 26892027), the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013− 2017, and the Rikkyo University Special Fund for Research 2014.

moduli because of the strong hydrogen bonds between their surfaces, our results clearly indicate that the interfacial thermal resistance was large enough to impede thermal conduction and sound propagation. This resulted in small αT and κT values, as shown in Figures 2e and 3b, and a low sound velocity calculated from the αI data in Figure 3c. The pores in the NC sheets trap the air. Air has a low thermal conductivity of ∼0.026 W/mK, and the mean free path of gas molecules in air is ca. 70 nm at ordinary temperatures and pressures.5 Considering the fiber widths, however, the pores in our materials were presumed to be smaller than 70 nm based on the theoretical prediction.21 We believe that the thermal conductivity of the air in the pores is further decreased below ∼0.026 W/mK, as indicated in the functional aerogels having a lower thermal conductivity than the air.5 The effective thermal conductive properties of the NC sheets were therefore mainly determined by the NC orientation, the width and crosssectional area of the cellulose crystallites, small pores, and the large interfacial thermal resistance between NC surfaces. The excellent thermal conductivity of the NC sheet was demonstrated by comparing the TNW sheet and a PI film currently used in flexible electronic devices, as shown in Figure 4. The TNW sheet has a higher temperature than the PI film at a given distance from the metal plate. To quantitatively assess the difference, we converted the thermal images to contour maps, as shown in Figure 4b. The relatively horizontal contour lines for both samples are indicative of uniform heat propagation. The central lines in the direction of heat conduction were then determined to extract the temperature data at the center of each sample, as shown in Figure 4c. The TNW sheet with κI ≈ 2.5 W/mK had a maximum temperature 6 °C higher than that of the general-purpose PI film with κI ≈ 0.9 W/mK. From the comparative data in Figure 4d, it is evident that the in-plane thermal conductivity of the TNW sheet greatly surpasses that of other plastic films available for transparent, flexible devices.

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CONCLUSION

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AUTHOR INFORMATION

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REFERENCES

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The width (or cross-sectional area) of crystalline cellulose was found to be the main factor determining the thermal conductive properties of nonwoven NC sheets. The similar αT or κT values for all of the NC sheets suggested the existence of large interfacial thermal resistance between NC surfaces. The phonon propagation velocity in the in-plane direction was estimated from the plot of αI against Dmin to be as low as ∼800 m/s. The resulting thermal conductivity for the TNW sheet was determined to be ∼2.5 W/mK, which is markedly higher than that of other plastic films currently available for flexible electronic devices. We believe that our findings could provide the basis for using naturally derived nanocelluloses in nextgeneration thin electronics by advanced heat management.

Corresponding Author

*E-mail: [email protected], Tel: +81-3-3985-2363. Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.biomac.5b00617 Biomacromolecules XXXX, XXX, XXX−XXX