Fabrication of Highly Stretchable Conductors Based on 3D Printed

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Fabrication of Highly Stretchable Conductors Based on 3D Printed Porous Poly(dimethylsiloxane) and Conductive Carbon Nanotubes/Graphene Network Shasha Duan, Ke Yang , Zhihui Wang, Mengting Chen, Ling Zhang, Hongbo Zhang, and Chunzhong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10791 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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ACS Applied Materials & Interfaces

Fabrication of Highly Stretchable Conductors Based on 3D Printed Porous Poly(dimethylsiloxane) and Conductive Carbon Nanotubes/Graphene Network Shasha Duan,§,† Ke Yang, §,‡ Zhihui Wang, † Mengting Chen,† Ling Zhang,*,† Hongbo Zhang,*,‡ Chunzhong Li*,† †

School of Materials Science and Engineering, Key Laboratory for Ultrafine Materials of

Ministry of Education, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China ‡

School of Mechanical and Power Engineering, Complex and Intelligent Research Centre, East

China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China KEYWORDS. three-dimensional printing, porous PDMS, graphene, carbon nanotubes, stretchable, conductive

ABSTRACT. The combination of carbon nano-material with three-dimensional (3D) porous polymer substrates has been demonstrated to be an effective approach to manufacture highperformance stretchable conductive materials (SCMs). However, it remains a challenge to fabricate 3D-structured SCMs with outstanding electrical conductivity capability under large strain in a facile way. In this work, 3D printing technique was employed to prepare 3D porous

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poly(dimethylsiloxane) (O-PDMS) which was then integrated with carbon nanotubes and graphene conductive network resulted in highly stretchable conductors (OPCG). Two types of OPCG were prepared and it has been demonstrated that the OPCG with split-level structure exhibited both higher electrical conductivity and superior retention capability under deformations, which was illustrated by using a finite element method. The specially designed split-level OPCG is capable of sustaining both large strain and repeated deformations showing huge potential in the application of next-generation stretchable electronics.

INTRODUCTION In the wake of future electronics moving toward wearable and flexible, stretchable conductive materials (SCMs), substitutes of traditional rigid wafer-based conductors, have been extensively studied in the applications of next-generation smart electronics, such as E-skins, wearable electronics, strain sensors and so on.1-9 In order to achieve both outstanding stretchability and electrical conductivity, a general and effective approach to fabricate high-performance SCMs is to incorporate conductive components into flexible substrates.10-13 Metal nanowires and carbon nano-materials are two common conductive components to fabricate SCMs.14 One-dimensional (1D) metal nanowires percolation network has been demonstrated to be advantageous in accommodating deformations by altering the network shape.15-25 On other other hand, featuring outstanding mechanical strength, elastic modulus as well as stability, 1D carbon nanotubes (CNTs) and two-dimensional (2D) graphene have been widely employed in the manufacture of carbon nano-materials-based SCMs.26-34 Though the solution-based process endows a relative facile way to integrate CNTs and graphene with elastomeric matrices, the large π-π interactions and high aspect ratio makes it a challenge to disperse CNTs and graphene in matrices uniformly.35-36 Introducing 3D structure into carbon nano-materials-based SCMs has been


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demonstrated to be a promising method to solve the problem.37-38 For instance, the construction of 3D CNTs/graphene aerogel brought the uniform dispersion of graphene and CNTs. However, both of the complicated fabrication process and the poor deformation capability restricted its application in high-performance SCMs.37 Though the integration of CNTs/graphene conductive components with porous poly(dimethylsiloxane) (PDMS) not only overcame the problem of uneven dispersion but also improved the retention of electrical conductivity under large strains, the etching process of nickel (Ni) foam is likely to cause structural defects of porous PDMS, which may limited its degree of deformation.38 Fortunately, a 3D porous nanostructured PDMS was particularly designed by J. Park et al. using a photolithography technique and attributed to the rotation of bridging elements the special porous PDMS exhibited both enhanced stretchability and elongation compared to solid PDMS.39 However, the thickness of this kind of 3D SCMs was limited to micrometre scale imposing restrictions on its applications. Hence, exploring novel strategies to fabricate SCMs based on carbon nano-materials and 3D porous PDMS (O-PDMS) shows significant potential. Here, 3D printing technique is creatively employed to prepare highly stretchable and conductive carbon nano-materials-based SCMs. Split-level and aligned 3D porous polylactic acid (O-PLA) were patterned by 3D printing to serve as templates of O-PDMS. When introducing CNTs and graphene into O-PDMS, the carbon nano-fillers coated on the 3D skeleton resulting in connective and effective conductive network on the 3D skeleton.

Finally, split-level O-

PDMS/CNTs/graphene (S-OPCG) and aligned O-PDMS/CNTs/graphene (A-OPCG) with high stretchability and conductivity are obtained. What’s more, due to the extraordinary split-level structure, S-OPCG exhibits significant advantages over A-OPCG in both electrical conductivity and deformation capability. Under 100% uniaxial stretching, conductivity retention of S-OPCG


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is 40% while that of A-OPCG is only 25%. Especially, the electrical conductivity of S-OPCG remains constant after 5000 times of bending cycles and exhibits a slight decrease after 100 times of 50% stretching-releasing process. Afterwards, finite element method (FEM) is applied to analyse the strain distributions of S-OPCG and A-OPCG.

EXPERIMENTAL METHODS Fabrication of O-PLA. The 3D scaffold of PLA was patterned using the 4th 3-D Bioplotte (EnvisionTEC GmbH, Germany). PLA granules were first housed in a syringe followed by heating up to 150℃. A 200-µm nozzle was used to extrude molten PLA onto substrate with an applied pressure of 100 kPa at a speed of 15 mm/s. By designing the model with the 3-D Bioplotter software, fibrous PLA paste was extruded layer by layer and 3D porous PLA (O-PLA) with aligned and split-level structures with 12 layers were manufactured. The PDMS prepolymer was prepared by mixing PDMS base agent with curing agent in a weight ratio of 10:1. Then the PLA framework was backfilled with PDMS prepolymer with a vacuum suction method we mentioned in previous work.37 After curing at 50℃ for 5h, PLA was etched by dichloromethane (CH2Cl2). The composite was first dipped in CH2Cl2 followed by washed with ethanol and white filament precipitated out. Repeated the above-mentioned process until white filament hardly appeared and a sample close to transparent was obtained, which indicated that PLA was removed completely. After being dried at room temperature, O-PDMS was prepared. Preparations of S-OPCG and A-OPCG. The graphene oxide (GO) and acid-treated MWCNTs (t-MWCNTs) were prepared as we reported before.37 To prepare a conductive solution (2.5mg/ml), GO and t-MWCNTs were mixed with a mass ratio of 1:1 followed by dispersing in a ultrasonic cell disruptor (model JY96-II, 15 sonic power ~150 W, frequency ~20 kHz, Ningbo Haishu Kesheng Ultrasonic Equipment Co., Ltd., Ningbo, China) for 2h. The GO/t-MWCNTs


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mixed solution was added to prestrained O-PDMS dropwise and dried under vacuum at 120℃. Repeated the above two steps until desired weight content conductive filler was loaded on splitlevel/aligned O-PDMS. After reduced by HI at 60℃ for 2h, the highly stretchable conductive SOPCG/A-OPCG was obtained. To demonstrate the reproducibility of the process, another 3D printer (FFS-MDJ. Fuchi. Ltd) was used to print O-PLA, which was then prepared into a highly stretchable electrode with the same preparation as S-OPCG/A-OPCG (see Figure S6 and S7 in the Supporting Information). Charactization. The morphologies of O-PLA, O-PDMS, S-OPCG and A-OPCG were examined by a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), operating at 15kV. The electrical conductivity of A-OPCG and S-OPCG was measured by four-point probes (model RTS-8, Guangzhou 4Probes Tech Industrial Co., Ltd., Guangzhou, China) contact direct current (dc) conductivity measurement method at room temperature. The tensile tests were performed using a universal testing machine (CMT4204, Sansi Co., Ltd., China).

RESULTS AND DISSUSSION The preparation process of S-OPCG is illustrated in Figure 1. Here, PLA was selected to print the 3D skeleton because it is eco-friendly and easy to remove by a certain solvent. Then a 200µm syringe needle was used to print O-PLA and the spacing between neighbouring skeletons was also set to 200 µm. Through controlling the program and path of 3D printer, structured OPLA scaffolds with split-level and aligned structure were obtained. Figure 2a and c are respectively the cross-section FE-SEM images of aligned and split-level O-PLA. Obviously, skeletons of aligned O-PLA in different layers are in a straight line while those of split-level OPLA are interlaced. After backfilling PDMS into the pores and dissolving PLA, PDMS perfectly replicated the porous structures of O-PLA resulting in aligned and split-level O-PDMS. For O-


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PDMS, its skeletons are exactly the pores of O-PLA and its pores are the skeletons of O-PLA. As shown in Figure 2b and d, continuous pores are distributed in PDMS evenly and the surface of O-PDMS is relatively smooth despite of the etching process. The special 3D structure is proved to be beneficial for the mechanical performance of PDMS. Compared to solid PDMS, the breaking elongation of split-level O-PDMS gets 24% improvement (see Figure S2 in the Supporting Information). Afterwards, a conductive solution mixed by CNTs and GO was added into O-PDMS framework dropwise which was stretched to 50% strain in advance. With a vacuum degassing process, conductive components coated the O-PDMS scaffold sufficiently. After being dried, the prestretched O-PDMS was released followed by reduction process in hydroiodic acid, in which GO was transferred into graphene to exhibit electrical conductivity. With 1 wt% CNTs/graphene, the conductivity of S-OPCG can reach 5.12 S/m while that of A-OPCG is only 0.34 S/m, 15-fold smaller than the former. The distinct difference of electrical properties between the two composites probably derived from the structural difference. Featuring interlaced porous structure, conductive solution can permeate the whole skeleton of split-level O-PDMS slower with enough residence time and coat it tightly with the help of vacuum. The micro-morphologies of S-OPCG are shown in Figure 3a and c, and it can be observed that a compact conductive layer formed on the skeletons of O-PDMS (see Figure S3 in the Supporting Information). What's more, both of graphene sheets and CNTs present uniform distributions and no agglomeration or stacking can be found, which is attributed to the synergistic effect between the carbon nanotubes and graphene sheets under the optimized mass ration (Figure 3e). However, for aligned OPDMS, pores in different layers are connected and when adding conductive solution into it, conductive solution flowed through it relatively quickly resulting in incomplete and


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unconsolidated coating (Figure 3b and d). Uneven dispersion of CNTs and graphene sheets can also be observed on the crevice of A-OPCG (Figure 3f). Hence, the loose conductive coating led to poorer electrical conductivity of A-OPCG compared to S-OPCG. Also, the geometric parameter of O-PDMS has an effect on the electrical property of S-OPCG (see Table S1 in the Supporting Information). With the same content of CNTs/graphene, thinner PDMS skeleton and smaller pitch can both result in higher electrical conductivity of S-OPCG. Besides, the prestretching process and the addition of conductive solution drop by drop also make contributions to the excellent electrical conductivity of the products, which bring easy and full infiltration of conductive components. Finally, the obtained products inherited excellent stretchability and electrical conductivity from O-PDMS and CNTs/graphene respectively. The mechanical and electrical properties of S-OPCG and A-OPCG were demonstrated by measuring their electrical conductivity (σ) as a function of the applied tensile strain (see Figure 4a). σ0 is defined as the initial conductivity of each composite at zero strain and σ/σ0 represents conductivity variations under a certain strain. Conductivity of these two composites presents a downward trend along with increasing strain. When strain is smaller than 20%, both S-OPCG and A-OPCG show little change of conductivity, which is presumably because of the effect of pre-stretching. After that, their conductivity gets decreased gradually upon increasing the applied strain. At the largest strain of 100%, σ/σ0 of S-OPCG is about 40% while that of A-OPCG is only 25%. During all the stretching process, the electrical conductivity retention rate of S-OPCG is invariably higher than that of A-OPCG under the same strain, which means that the stretchable conductor with split-level structure possesses superior deformation capability. The superior durable capability of S-OPCG should be attributed to its special split-level structure. To prove the advantage of S-OPCG in bearing strains, finite element method (FEM)


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was used to analyse the true strain distributions of the two types of O-PDMS. The models used in FEA are unites from the structures of split-level O-PDMS and aligned O-PDMS (see Figure S4 in the Supporting Information). And then, the deformations of the two models under 20%, 40%, 60%, 80% and 100% stretching were respectively simulated (see Figure S5 in the Supporting Information). The strain distributions of aligned and split-level O-PDMS with applied strain of 100% are shown in Figure 4b and c. Most regions of split-level O-PDMS exhibit true strain lower than 100%. Though a part of strain distributes between 90% and 100%, a considerable area possesses strain smaller than 32% and only several tiny areas present strain above 100%. In other words, the special structured O-PDMS is capable of eliminating part of the loading resulted in less actual strain on the conductive layer. However, the strain on aligned O-PDMS is higher than 100% in almost half of the regions, and the strain under 50% is far less than split-level OPDMS. Hence, the split-level O-PDMS shows superior ability to bear loading than the aligned one, which imparts S-OPCG with ascendant deformation capability and makes it a highperformance stretchable conductor. Furthermore, the micro-morphologies of S-OPCG after 100% stretching were investigated. It can be observed that even sustained such a large deformation, the conductive shell on A-OPCG still coated on the framework tightly and didn’t fall off (Figure 5a). Cracks emerged inevitably and for larger carks (Figure 5b), no connections exist while for smaller cracks, conductive components are still connected in the edge of the crevice, which kept the conductive paths consecutive (Figure 5c and d). Hence, the S-OPCG composite is still electrically conductive under large degree of stretching. To further testify the performance of S-OPCG as a stretchable conductor, its electrical conductivity retention capability under cyclic deformations was also investigated. As shown in


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Figure 6a and b, the electrical conductivity of S-OPCG remains constant after bending to a curvature of 2 mm for 5000 times; reduces 32% of initial conductivity after the preliminary 10 times of 50% stretching-releasing process and keeps nearly unchanged during the later 90 times cycles. The brightness of the LED lamps got slight decrease after stretched to 50% while kept unchanged after bending, which also supported the above results. The outstanding durability of S-OPCG should be attributed to the flexible CNTs/graphene coating and the pre-stretching process. The special inherent structure imparts graphene and CNTs superior ability to accommodate deformations compared to other rigid conductive fillers. After releasing the prestretched S-OPCG, the conductive fillers moved together along with slipping of graphene sheets and crinkling of CNTs, which flattened gradually upon applying stretch again. Thus, S-OPCG is capable of sustaining times of 50% stretching without great changes of electrical conductivity. Meanwhile, the conductive shell loaded on O-PDMS tightly, because of which it hardly dropped off during cycles of deformations. In one word, 3D conductive network composing of graphene and CNTs on O-PDMS could keep their conductive path intact under repeated deformations, which renders the special-designed polymer composites promising candidates for the development of SCMs.

CONCLUSIONS In conclusion, highly stretchable conductive S-OPCG and A-OPCG were manufactured by introducing CNTs/graphene into flexible polymer matrix with 3D porous structure. The 3D porous polymer is prepared by a novel strategy, 3D printing technique, with O-PLA as an intermediate. Two types of O-PLA are patterned resulting in to two different structured products and the special split-level structure renders S-OPCG higher conductivity compared to that with aligned structure. With the same content of conductive filler, the conductivity of S-OPCG is 15


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times larger than that of A-OPCG. What’s more, under uniaxial stretching to 100% strain, SOPCG gets 40% retention of conductivity while A-OPCG could only retain 25%. The difference of electrical conductivity retention capability is probably caused by different strain distributions of the 3D scaffolds, which is demonstrated by FEM. Besides, electrical conductivity of S-OPCG remains stable after 5000 times of bending cycles and exhibits a slight decrease after 100 times stretching of 50% strain. The remarkable performance of the special structured S-OPCG endows it huge prospects in the application of the next-generation stretchable electronics.

Figure 1. Schematic illustration of S-OPCG preparation.


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Figure 2. Cross-section FE-SEM images of (a) aligned O-PLA, (b) aligned O-PDMS (c) splitlevel O-PLA and (d) split-level O-PDMS.

Figure 3. Cross-section FE-SEM images of (a, c) S-OPCG and (b, d) A-OPCG; (e) and (f) are respectively magnified FE-SEM images of (c) and (d).


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Figure 4. (a) Normalized conductivity of S-OPCG and A-OPCG as a function of tensile strain; and strain distributions of (b) aligned O-PDMS and (c) split-level O-PDMS models under 100% stretching.


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Figure 5. (a-d) FE-SEM images of S-OPCG after 100% stretching.

Figure 6. Normalized electrical conductivity of the S-OPCG as a function of (a) Stretchingreleasing cycles with 50% strain and (b) the number of bends. The brightness of LED lamps depending on the strains and bends are shown below the curves.

Corresponding Author *

Tel.: +86 21 64252055. Email: [email protected]; [email protected];

[email protected] Author Contributions §

These two authors contributed equally to this work.

ACKNOWLEDGMENT


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The authors are grateful to the National Natural Science Foundation of China (51173043, 21136006, 2123600, 21322607), the Special Projects for Nanotechnology of Shanghai (12nm0502700), the Basic Research Program of Shanghai (13JC1408100), the Key Scientific and Technological Program of Shanghai (14521100800), Program for New Century Excellent Talents in University (NCET-11-0641), the Fundamental Research Funds for the Central Universities. REFERENCES 1.

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TOC GRAPHIC The special-designed split-level porous PDMS/CNTs/graphene (S-OPCG) exhibits outstanding electrical performance, including higher electrical conductivity and better electrical conductivity retention capability than that with aligned structure; the finite element method explained superior deformation durability of S-OPCG well. 233x187mm (300 x 300 DPI)


Fabrication of Highly Stretchable Conductors Based on 3D Printed

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