In-Plane Anisotropic Thermally Conductive Nanopapers by Drawing

Mar 20, 2017 - We developed flexible polymeric “heat-guiding materials” by simply drawing bacterial cellulose (BC) hydrogels to align the cellulos...
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In-Plane Anisotropic Thermally Conductive Nanopapers by Drawing Bacterial Cellulose Hydrogels Kojiro Uetani,* Takumi Okada, and Hideko T. Oyama* Department of Chemistry, College of Science, Rikkyo University, 3-34-1 Nishi-ikebukuro, Toshima-ku, Tokyo 171-8501, Japan S Supporting Information *

ABSTRACT: We developed flexible polymeric “heat-guiding materials” by simply drawing bacterial cellulose (BC) hydrogels to align the cellulose nanofibers and form “nanopapers” with anisotropic thermal conductivity. The in-plane anisotropy of thermal conductivity between the drawn and transverse directions increased as the draw ratio increased. For the drawn BC nanopapers, the coefficient of thermal expansion was found to be inversely correlated with the thermal diffusivity. We fabricated a planar spiral sheet by assembling the drawn BC strips to visualize the “heat flux controllability”. The coexistence of heat-diffusing and heat-insulating capacities within the single nanopaper plane could help to cool future thin electronics.

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network of NCs, expressing the unique texture of nata de coco, shows resistance against elongation breakage by deformation of the NC network structures to make the fibrils align. We stretched the BC pellicles to form drawn BC nanopapers with highly orientated NCs. This nanopaper has κ = 2.1 W m−1 K−1 in the drawing direction (machine direction, MD) and κ = 0.94 W m−1 K−1 in the transverse direction (TD) with κ anisotropy of 220%. Although stretching treatment of polymer nanocomposites containing planar boron nitride nanosheets has been shown to isotropically enhance the thermal conductivity,8 the coexistence of heat-diffusing capacity in the MD, and heatinsulating capacity in the TD within the sheet plane by stretching is unprecedented and could help to cool future thin electronics. The key technique of our approach is mechanical drawing of BC hydrogels, allowing control of the draw ratio (DR) to vary the orientation degree of the NC fibers. We set the DR at 0, 5, 10, 15, 20, or 25% to draw the BC hydrogels (see Figure 1a,b), where DR (%) is defined as DR (%) = (ldrawn − l0)/l0 × 100, where ldrawn is the length of the BC hydrogel after drawing and l0 is the initial length. BC pellicles were drawn roughly in proportion to the tensile stress until a DR of 25%, and they started to break at larger DRs (see Figure S1). When the elongation stress was removed after drawing, the BC pellicles did not shrink or further deform to change the DR values. The drawing strains were almost entirely converted to NC network deformation, and the internal stress was thought to be rarely stored after drawing. After hot pressing the drawn BC hydrogels, we used only the central uniform part with a constant thickness as the drawn BC nanopapers. Mechanical drawing is a simple method whereby NC fibers align along the MD,9 as shown in Figure 1c. Because of the

eat-spreading substrate materials for thin paper electronics, such as electronic paper and organic lightemitting diodes, have attracted attention for effective cooling to avoid thermal failure rather than installing bulky heat sinks.1 When the heat-generating components are placed close to the heat-sensitive components to downsize the devices, the in-plane anisotropic thermally conductive substrate is thought to be essential to insulate between both components and diffuse their heat to another in-plane direction. However, conventional flexible substrates made of plastic films have low isotropic thermal conductivities (of the order of 0.1 W m−1 K−1) in the film plane. These plastics are often highly stretched to enhance the thermal conductivity and its anisotropy,2,3 but it is difficult to form two-dimensional paper-like materials from the obtained fibrous materials because of their poor self-adhesiveness. To realize next-generation paper electronics, development of cooling substrate films with in-plane anisotropic thermal conductivity remains a great challenge. Our recent study revealed that nonwoven sheets of naturally produced nanocellulose (NC) fibers have high thermal conductivities and very large anisotropy of thermal conductivity κ, where the maximum κ in the in-plane direction is 8× greater than that in the thickness direction.4 The single NC fibers were suggested to have high anisotropy of κ, and the fiber orientation affected the κ anisotropy of the NC sheets. In addition, hydrophilic NCs with many hydroxyl groups on the fiber surfaces allow formation of mechanically tough “nanopaper” owing to their strong hydrogen bonding after drying.5,6 Therefore, NCs are thought to be an optimum material for in-plane anisotropic thermally conductive substrates by orientation control. Here, we investigated formation of anisotropic NC sheets from bacterial cellulose (BC) pellicles by drawing treatment. BC pellicles, which are known as “nata de coco” fruits, are densely networked NC hydrogels synthesized by acetobacters with fine NC layers in the thickness direction resembling the traditional French puff pastry millefeuille.7 The in-plane tight © XXXX American Chemical Society

Received: February 7, 2017 Accepted: March 17, 2017

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DOI: 10.1021/acsmacrolett.7b00087 ACS Macro Lett. 2017, 6, 345−349

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ACS Macro Letters

Figure 1. Structural characterization of the drawn BC nanopapers. (a) Wet BC pellicles were drawn to a certain draw ratio (DR) in the machine direction (MD) and then (b) dried by hot pressing to fix the structures (DR = 0, 5, 10, 15, 20, and 25% from left to right). (c) FESEM images of drawn BC nanopapers with DR = 0 and 25%. (d) WAXS images in the normal direction (ND) views for each nanopaper. (e) Azimuthal intensity profiles at 2θ = 22.8°, indicating that the (200) reflection of the cellulose I crystals increases as the DR increases. The red lines are the curves fitted by pseudo-Voigt functions for each profile. (f) Relationship between the orientational order parameter S and the DR. The inset shows the dimensional definition of the drawn BC nanopapers. (g) Relationships between the bulk density of the nanopapers ρ and the specific heat capacity Cp with S.

increases (see Figure 1g). The maximum anisotropy of κ is 220% at S = 0.4 with κ = 2.1 W m−1 K−1 in the MD (α = 1.43 mm2 s−1) and κ = 0.94 W m−1 K−1 in the TD (α = 0.65 mm2 s−1) with ρ = 1.22 g cm−3 and Cp = 1.19 J g−1 K−1. κ = 0.94 W m−1 K−1 in the TD is as low as the in-plane κ of polyimide film,4 while κ = 2.1 W m−1 K−1 in the MD is almost as high as that of a nonwoven tunicate NC sheet (κ = 2.5 W m−1 K−1).1,4 The linear relationships of κ in both the MD and TD allow rough extrapolation of the hypothetical κ values at S = 1 where NCs perfectly align, although the limited range of the plots prevents an exact prediction. According to the fitted lines, the κ values in the MD and TD reach ∼3.4 and ∼0.37 W m−1 K−1, respectively. Assuming that the κ anisotropy of the nanopaper reflects the κ anisotropy of the single NC, NC fibers consisting of BC are suggested to have 10× greater κ in the length direction than in the width direction. This rough prediction agrees well with the previously predicted κ anisotropy for single cellulose nanocrystals (CNCs) with the same cellulose I crystals obtained by molecular dynamics simulations.12 The absolute κ values for nanopapers are also affected by the crystallite size effect4 and indistinct interfacial thermal resistance. To highlight the uniqueness of the anisotropic κ of the nanopaper, we radially set 12 strips of the maximally drawn BC hydrogels with DR = 25% (∼8 mm × ∼70 mm), manually curled them in a spiral shape to press against each other, and then hot-pressed them to produce a planar spiral sheet, as shown in Figure 2b. This shape was inspired by the thermal inverter,13 which controls the heat flux by the artificial pattern of isotropic materials. When the planar spiral sheet was

microscale heterogeneity of the fiber density within the BC hydrogels owing to bacteria production, the NC orientation varies according to the location within the single nanopaper, as observed by field-emission scanning electron microscopy (FESEM). To evaluate the average orientation degree of NCs, wide-angle X-ray scattering (WAXS) images were taken by irradiating X-rays in the normal direction (ND) of the nanopapers. Figure 1d shows that the diffraction spots derived from cellulose I crystals appear in increments as the DR increases. Particularly strong (200) diffraction was used to quantify the degree of orientation of the NCs. The azimuthal intensity profiles of the (200) peaks against the azimuthal angle ϕ were fitted by pseudo-Voigt functions10,11 (Figure 1e) and the orientational order parameter S was calculated. In this study, the values of S range from 0 to 1, where 0 represents random alignment and 1 represents perfect alignment of the NCs along the MD. As the BC hydrogel were drawn, the S value linearly increases to ∼0.4 at DR = 25%, as shown in Figure 1f. Although the heterogeneity of the fiber density and fiber entanglement inhibited perfect alignment of the NCs, BC nanopapers with clearly different NC alignment were successfully prepared by simple mechanical drawing. The thermal conductivity κ of the isotropic BC nanopaper without drawing is ∼1.3 W m−1 K−1 in both the MD and TD, as shown in Figure 2a. As S increases, the κ anisotropy increases, in which κ in the MD linearly increases whereas that in the TD decreases. These κ values directly reflect the thermal diffusivity α because the specific heat capacity Cp remains constant and the bulk density ρ monotonically increases as S 346

DOI: 10.1021/acsmacrolett.7b00087 ACS Macro Lett. 2017, 6, 345−349

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ACS Macro Letters

Figure 3. Thermal expansion behavior of the drawn BC nanopapers. (a) Change of the coefficient of thermal expansion (CTE) in the MD and TD with respect to the orientational order parameter S. (b) Inverse relationship between CTE and the thermal diffusivity. Figure 2. In-plane anisotropic thermal conductive properties of the drawn BC nanopapers. (a) Thermal conductivity in the three directions of the nanopaper as a function of the order parameter S. (b) Manual formation of the planar spiral shape by assembling the drawn BC hydrogel strips with DR = 25%. The bottom thermograph shows that the heat flow is guided depending on the strip direction indicated by the double-headed arrows.

nanopapers. In other words, the dimensional instability might sensitively affect scattering of thermal phonons and decrease κ. The transparency is important for the practical use of the nanopaper as an electronic substrate. We mixed transparent acrylic resin with the BC nanopapers by the membrane-assisted method,1 which prevents formation of heat-insulating resin layers on the film surface. Figure 4 shows that the composite

cantilevered between the hot plates of the press machine, a asymmetrical temperature distribution was clearly observed. Heat smoothly and circularly transferred along the MD of the stripes from the edge of the hot plate (left side of the spiral sheet), while the stripes in the TD hardly conducted heat (right side of the sheet). Although the interface between the stripes might also interfere with heat transfer, the κ anisotropy of the drawn BC nanopaper is clearly expressed in this image. In addition, this shows that heat flux controlling materials can be produced by assembling pieces of drawn BC hydrogels before drying. With increasing S, the coefficient of thermal expansion (CTE) in the MD decreases to around 0, while that in the TD linearly increases (see Figure 3a). The CTE trends in each direction with S agree well with the previously reported tendency for short rod-like CNC films measured in a contactfree fashion14 and clearly reflects the CTE anisotropy of single NC fibers. The relatively small CTE values of the BC nanopapers compared with those of cellulose I crystals15 is thought to be caused by structural effects, including complex entanglement of long fibers and the effect of expansion in the thickness direction owing to the negative Poisson’s ratios of highly networked nanopapers16 under tensile stress using the mechanically contact method to measure the CTE. Interestingly, the thermal diffusivity α shows an inverse linear relationship with CTE regardless of the direction, as shown in Figure 3b. This Ashby plot clearly indicates that heat becomes less conductible in the directions with larger CTEs within the

Figure 4. Transparent materials based on the drawn BC nanopapers. (a) Transmittance spectra of the nanopaper with DR = 25% before and after resin impregnation by the membrane-assisted method. Appearance of the materials (∼1 cm × ∼2 cm) (b) without polarizing plates and (c) under crossed nicols. The white arrows indicate the polarizing directions.

film based on the nanopaper with DR = 25% turned translucent (transmittance of ∼50% at a wavelength 600 nm) compared with the white nanopaper before mixing with the resin (transmittance of ∼7% at a wavelength of 600 nm). The composite nanopaper with DR = 25% shows clear birefringence with interference colors (Figure 4c), clearly indicating the inplane anisotropic fiber orientation. Moreover, the DR = 25% nanopaper composite shows the largest in-plane κ anisotropy of ∼500% with κ = 2.5 ± 0.07 and 0.5 ± 0.03 W m−1 K−1 in the MD and TD, respectively, derived from the large variation of 347

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ACS Macro Letters NC orientation degree. The intrinsic κ of skeletal drawn BC sheets is fully expressed after mixing with resin. Simultaneous expression of anisotropic κ and transparency is significant for widely producing translucent thermal-guiding films. In summary, we have developed in-plane anisotropic thermally conductive nanopapers by simply drawing BC pellicles. The 220% κ anisotropy allows production of “heatguiding materials” by assembling the drawn BC pieces. For the BC nanopapers, the CTE was found to have an inverse relation with the thermal diffusivity. We envision that sophisticated polymeric heat-guiding substrates without any thermally conductive fillers could be realized by simultaneously exploiting the thermally conductive, mechanical and optical features of NC to efficiently cool next-generation thin devices.

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2-hydroxy-2-methylpropiophenone curing agent, Tokyo Chemical Industry Co., Ltd.) by the membrane-assisted method.1 The parallel beam transmittance was evaluated using an ultraviolet−visible spectrophotometer (V-630, JASCO Corp., Japan) with a film holder attachment.

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* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00087. Figure S1: Typical stress-strain curve of drawing the BC pellicle (PDF).

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EXPERIMENTAL SECTION

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kojiro Uetani: 0000-0003-3245-6929 Hideko T. Oyama: 0000-0002-7904-7818 Author Contributions

K.U. conceived the concept of this study; T.O. performed the experiments and analyzed the data with K.U; and K.U. wrote the manuscript with contributions from all the authors. Funding

This study was partially supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013−2017, the Kao Foundation for Arts and Sciences, the Sekisui Chemical Grant Program for Research on Manufacturing Based on Innovations Inspired by Nature, the Mazda Foundation, and the Foundation for Technology Promotion of Electronic Circuit Board. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.U. thanks Dr. K. Hatori, Hudson Laboratory, Bethel Co., Ltd., for cooperation in measuring the thermal diffusivities. This study was performed under the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices”.

3⟨cos2 ϕc , z⟩ − 1 2

⟨cos2 ϕc , z⟩ = 1 − 2⟨cos2 ϕ200, z⟩

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q

⟨cos2 ϕ200, z⟩ =

∑ p I(ϕ) sin ϕ cos2 ϕ q ∑ p I(ϕ)

AUTHOR INFORMATION

Corresponding Authors

Bacterial cellulose pellicles obtained by incubating Acetobacter xylinum were purchased from Fijicco Co., Ltd. (Japan). The ∼15 mm thick pellicles were boiled in 2 wt % NaOH aqueous solution at 90 °C for 2 h, followed by thorough washing with distilled water for 5 days to remove the bacterial cell debris. After purifying, BC hydrogels (∼3 cm × ∼13 cm, ∼15 mm thick) with fiber content of ∼0.67 wt % were compressed by a press machine without heating to remove the extra water (fiber content ∼6 wt %, ∼2 mm thick) and then drawn at a tensile rate of 10 mm min−1 with a tensile testing machine (Strograph VES5D, Toyo Seiki Seisaku-sho, Ltd., Japan) to each draw ratio (DR, 0−25%). The drawn BC hydrogels were dried by hot pressing at 110 °C for 20 min to fix the fiber structures. Drawn BC nanopapers with thicknesses of 50−70 μm were obtained. A field emission scanning electron microscope (SU8020, Hitachi High-Tech. Corp.) was used to image the nanopaper surfaces at an accelerating voltage of 2 kV with a Pt coating (ca. 1 nm) applied by ion sputtering. The WAXS measurements were outsourced to EAG Inc. (U.S.A.). They were carried out using a Siemens/Bruker GADDS system with a Hi-Star detector and Huber goniometer at 50 kV and 40 mA with Cu Kα radiation (λ = 1.54059 Å). The two-dimensional diffraction frames acquired five times for 200 s per frame were summed and averaged for better statistics without detector saturation issues. The orientational order parameter S was calculated from the fitted intensity of the Debye−Scherrer ring I at 2θ = 22.8° with respect to the azimuthal angle ϕ using following equations:17,18

S=

ASSOCIATED CONTENT

S

REFERENCES

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sin ϕ

where p and q were set to 190° and 350°, respectively, in this study. The thermal diffusivity α was measured at three points in the machine direction (MD), transverse direction (TD), and normal direction (ND) by the periodic heating method using a thermowave analyzer (TA3, Bethel Co., Ltd., Japan). The thermal conductivity κ was calculated by κ = αρCp, where ρ is the bulk density and Cp is the specific heat capacity measured by a temperature-modulated differential scanning calorimeter (Q-200, TA Instruments, U.S.A.). Infrared thermography (CPA-E40, FLIR systems, Inc., U.S.A.) was used to visualize the steady-state temperature distribution of the sample blackened with graphite spray. The coefficient of thermal expansion (CTE) was measured by the dimensional change in the range from 20 to 120 °C for the 24 mm (in MD) and 8 mm (in TD) length specimens applying a constant tensile stress of 0.12 N with a thermomechanical analyzer (Q-400, TA Instruments, U.S.A.). The nanopaper was mixed with transparent acrylic resin (A-PTMG-65, kindly supplied by Shin Nakamura Chemical Co., Ltd., mixed with 5% 348

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