Fabrication of a stretchable electromagnetic interference shielding silver nanoparticle/elastomeric polymer composite

Abstract

Highly conductive and stretchable electromagnetic interference shielding (EMI) materials were developed from a silver nanoparticle/elastomeric polymer (NP/SBS) composite. This material shows superior conductivity and EMI shielding efficiency due to highly incorporated silver NPs forming an electrical percolation. In addition, this composite can endure hundreds of cycles of stretching tests with moderate conductivity and EMI shielding efficiency.

---

The widespread interest of the electromagnetic effect on human health or near electronic devices has triggered a vigorous expansion in the development of high performance electromagnetic interference (EMI) shielding materials. In particular, flexible and lightweight EMI shielding materials are required in the field of aircraft, automobile, and flexible electronics such as wearable devices. Although conventional metal based EMI shielding materials have high performance due to high conductivity, their brittleness and heavy weight are limiting for flexible electronics. Therefore, many researchers have been studying soft or flexible EMI shielding composites composed of conductive materials and polymers.^1,2

Among several conductive materials, carbon based materials such as carbon nanotubes (CNTs) and graphene or metal based materials such as silver or copper have been used as conductive fillers of electrically conductive polymer composites for EMI shielding.^3–8 Gupta et al. showed CNT–polystyrene foam composites⁹ and Ren et al. showed lightweight and flexible graphene foam composites for EMI shielding.¹⁰ As metal based materials, silver, which is one of the most important conductive materials, has been of interest as EMI shielding materials. In particular, silver nanowire polymer composites have been widely investigated as EMI shielding materials, because their high aspect ratio shape is advantageous to make a high percolated network structure for high conductivity.^11–14 Li et al. demonstrated flexible EMI shielding materials using silver nanowires on a flexible polymer substrate.¹⁵ We believe that these electrically conductive polymer composites are promising EMI shielding materials for flexible electronics due to their high conductivity and flexibility.

When we try to make a wearable device in a shape of fabric form, flexibility and stretchability of EMI shielding materials are getting more important in the final textile product. And so far, there has been no published work reporting stretchable EMI shielding materials with metal nanoparticles (NPs)/polymer composites. Most studies regarding flexible EMI shielding materials have worked on their lightweight and flexibility.

In this study, we report stretchable EMI shielding materials through fabrication of silver NP/polymer composites and EMI shielding characteristics on their mechanical deformation (Fig. 1). A stretchable property can be achieved by poly(styrene-b-butadiene-b-styrene) (SBS) and silver NPs can be decorated on the SBS polymer by chemical reduction of silver precursors. The chemical reduction process of the silver precursor solution has not only a low cost and simple manufacturing process but also a fast way for obtaining high conductivity by a highly interconnected and packed structure. Previously, Park et al. fabricated stretchable composite material by chemical reduction of silver NPs on electrospun SBS fiber mats and investigated the electrical and physical property as electronic circuits.¹⁶ To make a channel for infiltration of silver precursors inside the polymer matrix and form large amounts of silver NPs with a large surface area, we also prepared a SBS composite polymer with a porous structure. These porous electrical conductive polymers are useful for increasing of the EMI shielding efficiency because of multiple internal reflections of electromagnetic waves in empty pores.^17–19

Fig. 1 Schematic of the procedure for fabricating silver NPs/SBS.

To demonstrate our idea, a microporous SBS polymer was obtained from sugar templates, which is a low cost and eco-friendly material. Also, they are easily prepared by the fabrication process of sugar powder. Choi et al. have already verified experimentally that microporous PDMS can be achieved from a sugar template.²⁰ So, microporous SBS polymer films were prepared by drop casting on a sugar template using a 20% poly(styrene-b-butadiene-b-styrene) (M_w = 140 000 g mol⁻¹, weight fraction of styrene = 30%, purchased from Sigma-Aldrich) solution in toluene and subsequently drying the solvent and dissolving the template in de-ionized water. The composite of silver NPs was prepared by reduction of silver trifluoroacetate (AgCF₃COO) used as a precursor for silver NPs. Microporous SBS rubber film was dipped in a 15 wt% AgCF₃COO ethanol solution for 30 min, and then dried in air for three hours. Infiltrated silver precursors were reduced by dropping of a hydrazine hydrate solution (50% in ethanol/water mixture). The final silver NP/SBS composite was obtained after rinsing out several times by de-ionized water and drying the films in air.

Prepared silver NP/SBS composites show high flexibility and elasticity because of the rubbery characteristics of SBS and web structure with porosity (Fig. 2a). Fig. 2b–e presents scanning electron microscopy (SEM) images of the silver NP/SBS composite before (Fig. 2b) and after (Fig. 2c and d) chemical reduction of AgCF₃COO. It can be seen clearly that silver NPs are deposited over the entire SBS polymer surface compared to before reduction of the silver precursor solution. Also, silver NPs inside the polymer matrix are clearly revealed in the cross sectional SEM images (Fig. 2e). This suggests that the chemical reduction of silver precursors has occurred not only at the surface but also in the interior of the swelled polymer matrix, which might contribute electrical conductivity by networked percolation.

Fig. 2 (a) Photograph of silver NP/SBS composites, showing its high elasticity. SEM images of the microporous SBS film before chemical reduction of AgCF₃COO (b), after chemical reduction of AgCF₃COO (c) and (d), cross-sectional view of the silver NP/SBS composite (e).

The dependence of electrical conductivity of the silver NP/SBS composite on the silver NP weight fraction is shown in Fig. 3a. Various silver NP weight fraction samples can be prepared by control of the silver precursor solution concentration from 1.5 wt% to 22.5 wt% and a real amount of silver NPs in the prepared composite was checked by Thermogravimetric Analysis (TGA). When the weight fraction of silver NPs is increased, the electrical conductivity is also gradually increased; but at a certain silver NP weight fraction of 30% to 50%, electrical conductivity is dramatically increased. This curve represents typical electrical percolation behavior.^21,22 This means that the increased density of silver NPs contributes to form an electrical pathway for enhancement of conductivity.

Fig. 3 (a) Conductivity of silver NP/SBS composites with different silver NP content. (b) EMI shielding effectiveness of silver NP/SBS composites with different silver NP content at a frequency of 10 GHz.

We also investigated the typical EMI shielding effectiveness of silver NP/SBS composites with various silver NP weight fractions. As expected, samples with a higher silver NP weight fraction show higher EMI shielding efficiency which is attributed to high conductivity by high loading of silver NPs.^23–25 Fig. 3b shows the EMI shielding efficiency versus frequency with various silver NP/SBS composite samples. Significantly, an average EMI shielding efficiency of 69 dB was obtained at a frequency range from 8 GHz to 12 GHz with silver NPs of 66.5 wt%. As the weight fraction of silver NPs increase, the EMI shielding efficiency is also gradually increased, which means electrical conductivity is the key factor of EMI shielding efficiency (Fig. 3b).

The stability of the EMI shielding property under the stretching motion of prepared silver NP/SBS composites is an important issue also; we thus investigated the stability of electrical conductivity and EMI shielding efficiency of the silver NP/SBS composite under a stretching condition.² Upon elongation of the silver NP/SBS composite, we observed using SEM images that crack formations of silver NPs occurred randomly on the SBS polymer surface because of different hardnesses between two materials (Fig. 4a–c). For elongation with ε = 0.1, lots of cracks were clearly observed on the surface of the silver NP/SBS composite. As the strain reached ε = 0.5, more crack formations were generated and a large part of the bare SBS polymer surface was exposed (Fig. 4c). Because conductivity is related to percolated silver NPs, the conductivity of the silver NP/SBS composite is decreased with increasing strain.

Fig. 4 SEM images of silver NPs/SBS composites (a) before elongation and after elongation at (b) ε = 0.1 and (c) ε = 0.5.

Nevertheless, it is interesting that silver NP/SBS composites still maintain moderate electrical conductivity and EMI shielding efficiency even after the cyclic elongation test. Fig. 5a shows electrical conductivity of the elongated sample versus number of stretches after the strain is released. Electrical conductivity is decreased by crack formation of the silver NPs on the surface of a highly elongated sample. Also, the composite can’t be completely recovered after releasing the strain at high strains because of accumulated stress. However, the silver NP/SBS composite still maintains moderate electrical conductivity stably during repeated stretching with high strain. The conductivity shows a small diminution after repeated stretching with ε = 0.2, 0.4, and 0.6. Although conductivity was decreased during the first 50 times of stretching, there was only a slight change in conductivity between stretching 50 times and 300 times . These properties coincide with results of EMI shielding efficiency before and after stretching of the silver NP/SBS composite (Fig. 5b). The EMI shielding effectiveness of the silver NP/SBS composite was tested after stretching 10, 100, and 300 times with ε = 0.6, respectively. Though there were small decreases of EMI shielding during the first 10 times of stretching, silver NP/SBS composites can keep moderate EMI shielding efficiency until stretching 300 times. This result means that the silver NP connection in the SBS polymer was reconstructed when the strain was released even if it was ruptured when stretched. The reconstruction of the silver NP connection was not as perfect as the pristine state before elongation, but it still had and maintained moderate conductivity and EMI shielding efficiency during continuous stretching. These experimental results clearly demonstrate that these superior flexible and stretchable properties of the silver NP/SBS composite can be easily applicable to EMI shielding materials of next generation electronics such as wearable electronics and printed electronics.

Fig. 5 (a) Conductivity changes of silver NP/SBS composites after repeatedly stretching with ε = 0.2, ε = 0.4, and ε = 0.6. Conductivity is measured after releasing strain. (b) EMI shielding effectiveness change of silver NP/SBS composites after stretching 10 to 300 times with ε = 0.6 in frequency ranges of 8–12 GHz.

Conclusions

In summary, we have fabricated stretchable EMI shielding materials with high EMI shielding efficiency. The material consists of a microporous SBS polymer with a high concentration of silver NPs, which form an electrical percolation. This new composite has great potential for flexible electronic applications. In particular, since the silver NP/SBS composite has flexible and stretchable properties with moderate EMI shielding efficiency, we believe that these results may open the possibilities for application to EMI shielding materials of wearable electronics. Also, we are currently investigating stretchable EMI shielding materials with other metal nanoparticles such as Cu, Ni and Au. We propose that this method could provide a simple avenue to fabricating stretchable EMI shielding metal nanoparticle/polymer composites.

Acknowledgements

We would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea (Grant B551179-13-01-03).

Notes and references

1. M. Chen, L. Zhang, S. Duan, S. Jing, H. Jiang, M. Luo and C. Li, Nanoscale, 2014, 6, 3796.
2. A. Fletcher and M. C. Gupta, J. Compos. Mater., 2014, 48, 1261.
3. W.-L. Song, J. Wang, L.-Z. Fan, Y. Li, C.-Y. Wang and M.-S. Cao, ACS Appl. Mater. Interfaces, 2014, 6, 10516.
4. N. Li, Y. Huang, F. Du, X. He, X. Lin, H. Gao, Y. Ma, F. Li, Y. Chen and P. C. Eklund, Nano Lett., 2006, 6, 1141.
5. Y. Yang, M. C. Gupta, K. L. Dudley and R. W. Lawrence, Adv. Mater., 2005, 17, 1999.
6. H.-B. Zhang, Q. Yan, W.-G. Zheng, Z. He and Z.-Z. Yu, ACS Appl. Mater. Interfaces, 2011, 3, 918.
7. B. Shen, W. Zhai, M. Tao, J. Ling and W. Zheng, ACS Appl. Mater. Interfaces, 2013, 5, 11383.
8. N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, B. Tang, M. Chan and J. K. Kim, Adv. Mater., 2014, 26, 5480.
9. Y. Yang, M. C. Gupta, K. L. Dudley and R. W. Lawrence, Nano Lett., 2005, 5, 2131.
10. Z. Chen, C. Xu, C. Ma, W. Ren and H. M. Cheng, Adv. Mater., 2013, 25, 1296.
11. P. Lee, J. Ham, J. Lee, S. Hong, S. Han, Y. D. Suh, S. E. Lee, J. Yeo, S. S. Lee and D. Lee, Adv. Funct. Mater., 2014, 24, 5671.
12. Y. Li, P. Cui, L. Wang, H. Lee, K. Lee and H. Lee, ACS Appl. Mater. Interfaces, 2013, 5, 9155.
13. Y.-H. Yu, C.-C. M. Ma, C.-C. Teng, Y.-L. Huang, S.-H. Lee, I. Wang and M.-H. Wei, Mater. Chem. Phys., 2012, 136, 334.
14. J. Ma, M. Zhan and K. Wang, ACS Appl. Mater. Interfaces, 2014, 7, 563.
15. M. Hu, J. Gao, Y. Dong, K. Li, G. Shan, S. Yang and R. K.-Y. Li, Langmuir, 2012, 28, 7101.
16. M. Park, J. Im, M. Shin, Y. Min, J. Park, H. Cho, S. Park, M.-B. Shim, S. Jeon and D.-Y. Chung, Nat. Nanotechnol., 2012, 7, 803.
17. G. A. Gelves, M. H. Al-Saleh and U. Sundararaj, J. Mater. Chem., 2011, 21, 829.
18. D. X. Yan, H. Pang, B. Li, R. Vajtai, L. Xu, P. G. Ren, J. H. Wang and Z. M. Li, Adv. Funct. Mater., 2015, 25, 559.
19. D.-X. Yan, P.-G. Ren, H. Pang, Q. Fu, M.-B. Yang and Z.-M. Li, J. Mater.Chem., 2012, 22, 18772.
20. S.-J. Choi, T.-H. Kwon, H. Im, D.-I. Moon, D. J. Baek, M.-L. Seol, J. P. Duarte and Y.-K. Choi, ACS Appl. Mater. Interfaces, 2011, 3, 4552.
21. G. A. Gelves, B. Lin, U. Sundararaj and J. A. Haber, Adv. Funct. Mater., 2006, 16, 2423.
22. S. I. White, R. M. Mutiso, P. M. Vora, D. Jahnke, S. Hsu, J. M. Kikkawa, J. Li, J. E. Fischer and K. I. Winey, Adv. Funct. Mater., 2010, 20, 2709.
23. A. Joshi, A. Bajaj, R. Singh, P. Alegaonkar, K. Balasubramanian and S. Datar, Nanotechnology, 2013, 24, 455705.
24. M. H. Al-Saleh and U. Sundararaj, J. Phys. D: Appl. Phys., 2012, 46, 035304.
25. W.-L. Song, M.-S. Cao, Z.-L. Hou, M.-M. Lu, C.-Y. Wang, J. Yuan and L.-Z. Fan, Appl. Phys. A: Mater. Sci. Process., 2014, 116, 1779.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04765c

---