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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24217−24223
Healable and Foldable Carbon Nanotube/Wax Conductive Composite Tso-Hsuan Chen,† Yu-Chi Yeh,† and Ying-Chih Liao*,†,‡ †
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Advanced Research Center of Green Materials Science & Technology, Taipei 10617, Taiwan
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S Supporting Information *
ABSTRACT: In this study, a composite material with healable and foldable features is formulated to print conductive patterns on rough surfaces, such as paper, cloth, and three-dimensional (3D) printed objects. Carbon nanotubes (CNTs) are mixed with wax to formulate a solid composite for pen writing. The composite has a low percolation threshold of 2.5 wt % CNTs and can be written on various rough substrates, such as paper and cloth, to create conductive patterns for electronic conductors. Because of the strong infrared (IR) absorption of CNTs, the printed patterns can be selectively sintered by noncontact IR radiation efficiently to show great electrical conductivity. The electrical resistance of the written patterns on paper also show an insignificant increase after bending, folding, and crumpling. Furthermore, the conductive composite exhibits great healability after destructive damages. The conductivity of the damaged patterns after severe folding or knife cutting recovers to its original value with thermal or IR heating. Several examples, such as conductive tracks on paper, cloth, or 3D printed objects, are also demonstrated to show the potential of this healable conductive composite for electronic applications. KEYWORDS: foldable conductive patterns, healable material, carbon nanotubes, wax, direct writing
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INTRODUCTION Pen writing is a simple direct printing approach for pattern formation and has the characteristics of compact, fast period coating, and easy accessibility.1 With these advantages, penwriting techniques have been utilized to fabricate various functional electronic devices.2−4 Typical pen-writing techniques include brush pen,3 fountain pen,5 ball pen,6 and pencil.7 These pens use mostly solution-based inks and thus need gravity or capillary forces for writing.1 Take fountain pen as an example; when the nib is upward, the ink cannot be transferred to the substrate surfaces because of the reverse ink flow against gravity. Unlike ink-based writing methods, pencil writing depends only on frictional forces between the solid pen composites and substrates and provides an alternative option to avoid gravity effects.1 The frictional forces rub off the pencil lead composite on the substrate to form patterns.8,9 Thus, pencils can write patterns in any direction even when the nib is upward against gravity. Pen-writing techniques have also been widely applied in printing electronic devices to provide bendable, elastic, lightweight, and nonbreakable features.5,10,11 For example, Tai’s group fabricated paper-based flexible electronics by writing conductive nanosilver ink with a fountain pen.11 However, regular printed flexible electronic devices could only afford low mechanical bending stress. Under folding conditions, permanent creases12 caused by serious deformation © 2018 American Chemical Society
are observed in the printed conductive patterns. These folding creases remain after unfolding. Therefore, a common way to manufacture foldable electronics is by using both foldable substrates and foldable conductors13 that could withstand deformation,12 maintain stable contact with each other,13 and preserve their operating characteristics after folding. The foldable conductors are usually synthesized by mixing conductive fillers with elastic polymers. Because silver nanomaterials provide both great mechanical flexibility and electrical conductivity, silver nanoparticles, nanowires, or nanosheets have been widely used in foldable conductors.12,14−16 However, silver fillers are mostly applied in solution-based pen-writing materials. Because conductive solid composites with high silver concentrations are brittle,17 pencils with silver fillers have little ductility for writing patterns. On the other hand, carbon nanotubes (CNTs) provide a better choice for conductive filler in pencils. CNTs have high electrical conductivity and excellent mechanical properties to recover from severe bending and compressing.18 Moreover, CNTs have also been widely applied in the fabrication of foldable electronics. For example, Honda et al.19 used CNT/ polymer inks as raw materials to fabricate foldable electronics Received: May 20, 2018 Accepted: June 22, 2018 Published: June 22, 2018 24217
DOI: 10.1021/acsami.8b08310 ACS Appl. Mater. Interfaces 2018, 10, 24217−24223
Research Article
ACS Applied Materials & Interfaces by screen printing. Darabi’s20 group developed a stretchable and foldable sensor based on CNTs and chewing gum membrane. Sekitani et al. also demonstrated the feasibility of applying CNTs in the formulation of stretchable and foldable conductors.21 On the basis of the aforementioned studies, CNTs with high conductivity have shown great potential as candidate conductive fillers in foldable or stretchable conductive composites. However, these examples used solution-based liquid inks for pen writing, and thus one would encounter the aforementioned flow problems when printed under gravity. To address this issue, a new formulation of solid CNT composite pencil is needed. For pen writing with solid composites, wax has been widely used in the formulation because of its great flowability under strong frictional forces. For example, crayons are made with wax and colorants and can be drawn on paper, clothes, or walls. Using similar formulation as crayon, a solid wax/CNT composite material can be easily made for pencil material to draw conductive tracks with precise patterns. As a commonly used additive of hot-melt adhesive, wax has a low boiling point (∼65 °C)22 and a low viscosity.23 Thus, wax composites can be easily processed through simple heating to form compact solids with good filler connectivity or electrical conductivity. Besides, wax also shows a thermally healable property24 because of its low surface tension25 and great spreading ability to move toward an uncovered area.26 In this study, we developed a crayon formulation by mixing CNT and beeswax. This composite can be used to print a conductive pattern by direct pen writing on foldable substrates or rough surfaces such as three-dimensional (3D) printed objects. The printed conductive pattern can sustain strong strains such as bending, folding, and even twisting. The printed patterns also exhibit a healable property, and one could repair the damaged conductive tracks by simply heating it with infrared (IR) radiation or a hair dryer. Several examples will also be demonstrated to show the potential of this composite material in electronic applications.
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3D printer (D-Force Co., Taiwan) using a polylactic acid (PLA) material.
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RESULTS AND DISCUSSION Percolation and Conductivity of MWCNTs. The shapes of MWCNTs strongly affect the conductive network or the percolation threshold in the MWCNT/wax composite.27 Thus, selection of MWCNTs with good percolative microstructures is crucial to the composite conductivity. Figure 1 shows the
Figure 1. TEM images of (a) c-MWCNT and (b) s-MWCNT.
transmission electron microscopy (TEM) images of two types of MWCNTs. The c-MWCNT has a curly appearance with a cylindrical diameter of 30 nm. On the other hand, the sMWCNT is straight with a larger diameter of 50 nm. Apparently, although the c-MWCNT has more branching points, the s-MWCNT has a longer length and theoretically can form more connective points for network percolation in the MWCNT/wax mixture. To evaluate the effects of percolation on conductivity, the composites were rubbed onto a glass slide to create a square solid film of 15.0 × 15.0 × 0.09 mm (length × width × thickness) dimensions to measure the corresponding electrical conductivity. The variations in the conductivity of the CNT/wax films at different MWCNT weight fractions are summarized in Figure 2. With the same
EXPERIMENTAL SECTION
Beeswax pellet with a melting point of 65 °C was obtained from First Chemical Co. Ltd. Two types of multiwalled carbon nanotubes (MWCNTs) were purchased from Golden Innovation Business Co. Ltd.: highly-directional MWCNTs (s-MWCNT, length between 10 and 50 μm) and curly MWCNTs (c-MWCNT, length between 10 and 30 μm). Toluene (>99.8%, Alfa Aesar Chemical) and ethanol (>99.8%, Sigma-Aldrich) were used as purchased without further purification. A solvent mixture containing 30 g of toluene and 10 g of ethanol was first prepared in a glass bottle. Three grams of wax with proper amount of CNT powders were then added into the bottle. The bottle containing the CNT/wax mixture was capped and placed in a hot water bath at 90 °C under strong stirring for 2 h. After that, the bottle was opened and placed in a hot bath at 80 °C under slow stirring for 5 h to evaporate the solvent. After drying at atmospheric pressure, the remnant was further put into a vacuum oven at 85 °C for 5 h to remove the rest of the solvent residues. The dried CNT/wax composite material was then place in a mold, heated under 80 °C, and compressed to form a pen. The pen in the mold was cooled under room temperature for 15 min and was used to write conductive patterns on substrates. IR light (Philips IR-175R-PAR, 175 W) was used as the radiative heating source for the printed or written CNT/ wax patterns. Morphology of the CNT was examined by using a transmission electron microscope (Tecnai F30, Philips). Four-point probe (GW Instek GDM-8261A) was used to measure the sheet resistance of the written patterns. The line resistance was measured by a multimeter (HOLA DM-2690TU). 3D objects were printed with a
Figure 2. Conductivity of the MWCNT/wax composite at different weight fractions of s-MWCNT and c-MWCNT.
weight percentage in wax, the s-MWCNT films always show a much lower resistance than the c-MWCNT films. This difference in film electrical conductivity is attributed to the MWCNT percolation. As shown in Figure 1, the s-MWCNT is much longer than c-MWCNT and is easier to provide percolative contacts.28,29 As a result, the s-MWCNTs can form a dense percolating network with more connections and exhibit a lower percolation threshold of ∼5 wt %. For the sake of electrical conductivity, s-MWCNT is used as the conductive filler in the following sections. 24218
DOI: 10.1021/acsami.8b08310 ACS Appl. Mater. Interfaces 2018, 10, 24217−24223
Research Article
ACS Applied Materials & Interfaces A classical percolation scaling law30 can be used to explain the conductivity σ in the percolating s-MWCNT network in wax σ = σ0(p − pc )t
27% decrease in the volume. The removal of voids results in more connection between conductive CNT fillers, and therefore, the sheet resistance drops to ∼200 Ω, which is only one-third of its original value. Although thermal post-treatment can be easily applied to papers using a hot plate, it is difficult to apply on a thick bulk material, such as 3D printed objects. Alternatively, noncontact IR radiative heating is adopted here to provide surface heating instead of using hot plates. Moreover, both wax and CNTs can absorb IR radiation effectively,32,33 and the MWCNT/wax patterns can be selectively heated. As shown in Figure 4, the
(1)
where σ0 is a constant related to the intrinsic conductivity of the filler, pc is the critical concentration of the filler, p is the MWCNT weight fraction, and t is the conductivity exponent. The best-fitted values after data regression (Figure 2) using eq 1 show a critical concentration pc of 4.850% with an exponent t = 1.902, which falls in the regular range between 1.3 and 2.0 for CNT percolation.31 The conductivity of the s-MWCNT/ wax composite continues to rise as the concentration of the sMWCNT increases. However, a lower MWCNT weight fraction can yield a softer pencil for writing at a lower pressure. In addition, a lower wax ratio in the composite will lead to weaker healability. Therefore, 15 wt % MWCNT/wax composite is used in the following sections. Frictional Force and Printed Amount. The amount of MWCNT/wax composite printed on paper depends strongly on the applied frictional forces. To quantitatively describe the relationship, a slender cylinder (5 mm in diameter and 3 cm long) made of MWCNT/wax composite is placed vertically perpendicular to a piece of paper. After applying a fixed vertical force by a force gauge, the paper substrate is pulled in the horizontal direction for 6.5 cm at a fixed speed of 3 cm/s. The weight of the written MWCNT/wax composite on the paper is measured, as shown in Figure S1. The result shows that the conductive composite can be printed on a paper substrate easily at low normal pressure. Because 3D printed objects have a much rougher surface than paper, this composite can be printed more easily and applied on the surface for conductive pattern formation. Moreover, the linear relationship between the applied pressure and the coating density allows one to control the printed composite amount on the substrates by simply adjusting the applied forces. Post-treatment. The resistance of the written MWCNT/ wax patterns can be decreased after thermal treatment. Because the MWCNT/wax patterns are transferred onto substrates by frictional forces, the as-prepared patterns are not compact and the porous MWCNT/wax microstructure reduces the effective pathways for electron transfer. The MWCNT/wax pattern can be solidified by melting the wax. After heating at 80 °C for 10 min, a compact solid film with a reduced resistance is obtained (Figure 3). From the cross-sectional examination, the thickness of the written film decreases from 90 to 65 μm, representing a
Figure 4. Temperature profile of the s-MWCNT/wax pattern under IR radiation: (a) top-view image, (b) IR radiation setup, (c) heating process, and (d) thermal profile after 5 min IR radiation.
temperature of the written MWCNT/wax pattern reaches ∼70 °C (above the melting point of the wax) after radiative heating for 2 min, whereas the surface temperature of the 3D printed substrate maintains at less than 50 °C. This IR radiative process can therefore provide an effective local heating on the MWCNT/wax pattern, decrease the voids in the written conductive tracks, and decrease the electrical resistance without causing thermal deformation on the substrate. The resistance reduction ratio, R/R0, of the conductive MWCNT/ wax composite shows that a 5 min exposure under an IR lamp can provide satisfactory heating (Figure 5). In particular, the resistance of the composite decreases over 90% on paper and ∼70% on other solid surfaces. This higher resistance reduction on paper is possibly due to the wax absorption in porous paper
Figure 3. Cross-sectional images (a) before and (b) after the thermal postprocess. (c) Top-view images (c) before and (d) after the thermal postprocess. The MWCNT/wax was printed on a glass substrate and heated on a hot plate at 80 °C for 10 min.
Figure 5. Relative resistance of the s-MWCNT/wax composite pattern after IR radiation process on a paper and a 3D printed PLA plate. The pattern is 20 × 5 (mm). 24219
DOI: 10.1021/acsami.8b08310 ACS Appl. Mater. Interfaces 2018, 10, 24217−24223
Research Article
ACS Applied Materials & Interfaces
images (Figure S4), the protruding fibers break the upper conductive layer and hence leads to a large resistance increase. Nevertheless, the result reveals that the MWCNT/wax can resist strong tensile deformation for more than 1000 folding cycles and can be used as a foldable conductor (Figure S5). In addition, after writing on clothes and IR post-treatment, the MWCNT/wax composite remains conductive after being twisted and/or crumpling (Figure S6). These results show that the MWCNT/wax composite has strong mechanical strength and stability under various deformation conditions. Healable Characteristics. The damages on the MWCNT/wax composite after folding or cutting can be healed with re-exposure of IR radiation. Because of the low melting point and good flowability of liquid wax,24 the written MWCNT/wax composite pattern can recover its conductivity by IR heating after suffering deformational or destructive damages. Two examples are given here. As shown in Figure 7,
structures, as shown by the scanning electron microscopy (SEM) image in Figure S2, leading to a higher MWCNT lateral density on the paper surface. Nevertheless, the IR posttreatment can provide much better local heating effects even on 3D surfaces; and thus in the following sections, IR postprocess with exposure time of 5 min is used to improve the conductivity of the MWCNT/wax composite. Scratch Resistance and Heat Stability. The scratch resistance of the printed composite thin films on paper after post-treatment is tested by using ASTM-D-3363 standard. The scratch resistance or the hardness of the composite thin films is determined by comparing with the hardness of pencil leads. The experiment results show that pure wax has a rally low scratch resistance and is a soft material with pencil hardness of 9B. The hardness increases with increasing MWCNT percentage and is up to 2B when 20% MWCNT is added (Figure S3a), indicating that a higher ratio of MWCNT could enhance the mechanical strength of the composite coatings and improve the scratch resistance. The MWCNT/wax composite has great thermal stability, and the printed patterns exhibit stable electrical resistance for a long time. To evaluate the heat stability of the MWCNT/wax conductive composite, a rectangular pattern of 1 cm × 4 cm is first printed on a 3D printed PLA plate. The samples are then kept in an oven at 60 °C, and the resistance of the patterns is recorded each day. As shown in Figure S3b, there is insignificant change in the electrical resistance for 20 days, indicating that the conductive pattern made of MWCNT/wax composite is thermally stable. Foldable and Twistable Characteristics. After the IR post-treatment, the MWCNT/wax conductive tracks on paper has a strong mechanical strength under folding or twisting conditions. As shown in the inset figure of Figure 6, the
Figure 7. Images and resistance (a,b) before and (c,d) after crumpling. (e,f) Image and resistance after IR radiation.
Figure 6. Relative resistance of printed s-MWCNT/wax tracks on paper after repetitive folding process at various folding angles. The inset figure shows the schematic diagram of folding angles and the relative resistance after the first folding cycle.
the written conductive pattern after multiple folding shows an increase in the resistance to three times of its original value. After applying IR heating on the folded paper, the resistance nearly reduces to its original value because the melting MWCNT/wax composite refills the folding creases and reinforces the conductive networks. The fluidity of the composite at moderate temperatures also provides healability for cut tracks. As shown in Figure 8a, the written conductive pattern on a 3D printed plate is cut with a scalpel to open the circuit. After heating the damaged track area, because of the higher surface free energy of the PLA substrate (contact angle of 52°, Figure S7a) than the melted composite (contact angle of 111°, Figure S7b), the melt composite slowly covers the cut area on the substrate and results in a healed pattern. As shown in Figure 8c, the cut heals after heating with a hair dryer for 15 min, and the resistance restores to near its initial value (Figure 8b). The healing time dependence for electrical resistance
resistance increases slightly (
