Electrical Conductivity and Electromagnetic Interference (EMI) Shielding Properties of Copper Nanowire/Polypropylene Composites

Abstract

Nanocomposites of copper nanowires synthesized by AC electrodeposition and then used as filler with polypropylene as the matrix were prepared by miscible mixing and precipitation (MSMP) method. Electrical conductivity and electromagnetic interference (EMI) shielding properties were studied. A plateau was observed between 0.8 vol. % and 1.7 vol. % on the conductivity curve. The different behavior observed for copper nanowire/polypropylene and copper nanowire/polystyrene composite for electrical conductivity and EMI shielding properties are discussed.

Introduction

Conductive polymer composites (CPCs) have been studied intensively due to their many advantages, such as good processability, corrosion resistance, and comparatively low weight and cost. High EMI shielding performance gives CPCs the potential to be used widely in laptops, cell phones, aircraft electronics, and medical device housings [1]. Highly conductive copper nanowire/polystyrene (CuNW/PS) nanocomposite with high EMI shielding effectiveness at low CuNW concentration by MSMP method was demonstrated. CuNW/PS composites were highly conductive and had a percolation threshold at 0.67 vol. % CuNW [2].

In this study, we are extending our studies to copper nanowire/polypropylene (CuNW/PP) composites. As one of the most commercially used polymers, PP exhibits excellent mechanical, thermal and electric properties and most importantly, low cost. Therefore, it is a good choice for the matrix. Herein, we report the electrical conductivity and EMI shielding properties of CuNW/PP composites prepared using MSMP method [2].

Experimental

Homopolymer Polypropylene H0500HN with a melt flow rate of 5g∙(10min)^-1 (ASTM D1238) purchased from Flint Hills Resources (Longview, Texas, US) was used as matrix.

Copper Nanowires (CuNWs) were synthesized by AC electrodeposition; synthesis details can be found elsewhere [3]. The procedure includes using 25 V 8 hours anodized Al electrodes (AlfaAesar, 99.99+%) as templates, then applying 10 Vrms continuous sine wave on the aluminum template placed between two copper plates in an electrolyte solution for 10 min, and then by liberating CuNW in 0.1M NaOH solution.

The CuNW/PP composite powder was prepared by MSMP method. PP was dissolved into xylene solution at 120 ºC. Then different volumes of 3.34 mg CuNW/ml methanol solution were added into the PP in a 80 ºC ultrasound bath with 120 W output power. The mixture was filtered out and placed in a fume hood followed by 2hours in a vacuum oven at 40 ºC. The dry mixing powder was annealed into 0.87偭25偭11.6 mm³ samples by Carver compression molder at 190ºC, 4500psi for 15mins.

Electrical resistivity measurements were conducted using two different electrometers. Keithley 6517A electrometer (Keithley Instruments, USA) and Loresta GP resistivity meter (Mitsubishi Chemical Co., Japan) were used for resistivity higher than 10⁶次∙cm or lower than 10⁶次∙cm, respectively. The voltage applied to measure resistivity was 90V.

The EMI SE and permittivity measurements were carried out with Agilent Vector Network Analyzer (Model 8719 ES) in X-band frequency range (8.2 每 12.4 GHz). 140mm sample holders were placed between two wave guides and connected to separate ports of the analyzer.

Results and Discussion

Electrically conductive polymer nanocomposites were made by adding conductive fillers into the polymer matrix to form a continuous network. The minimum loading of the conductive filler which can make the composites conductive is known as the percolation threshold. The aspect ratio, concentration and surface properties of the fillers, dispersion, distribution and alignment of the filler in the polymer matrix will affect the percolation threshold and electrical conductivity [4].

The electrical conductivity increased about 18 orders by adding 3 vol. % CuNW compared with pure PP (Figure 1). A plateau was found on the conductivity curve around 10^-7 S/cm at CuNW concentration from 0.8 vol. % to 1.7 vol. %. This is very different than the typical percolation curve reported in other studies [2], where the conductivity increased dramatically near the percolation threshold. Instead, it only reached high conductivity of 1 S/cm above 1.7vol. %. The plateau between 0.8 and 1.7 vol. % shows a much wider percolation threshold region. This wider percolation concentration window gives a potential for the composite to be applied in charge storage devices [4].

This plateau phenomenon is believed to be due to agglomeration of CuNWs in the PP matrix as shown in the SEM image (Figure 2). As mentioned previously, percolation can be achieved once the conductive fillers form an effective network. But in PP/CuNW composites, the conductivity of PP composites is first controlled by the distribution of CuNW agglomeration instead of CuNW dispersion inside the matrix. When a certain concentration (0.7vol. %) of filler is reached, excess fillers tend to join the clump in the formation rather than distribute inside the polymer. When the distributed clumps and dispersed fillers form an effective network, the composite becomes conductive.

Electromagnetic interference (EMI) shielding effectiveness (SE) is the ability of a material to block or reduce the influence of the incident energy which is radiated or conducted. EMI may impair the performance of the devices. In this experiment, EMI SE is the logarithm of the ratio of the incident energy field to the transmitted energy field and is reported in the unit of dB.

The EMI SE and real/imaginary permittivity of CuNW/PP composites in the X-band frequency range are investigated as shown in Figure 3 (a) and (b). The SE remains below 5 dB when the concentration is less than 1.7 vol. %, whereas the CuNW/PS showed seven times more SE (about 40 dB) [2]. At the same time, real permittivity increases with the increase of CuNW concentration while imaginary permittivity remains around zero before 1.7 vol. % and both permittivity rise quickly above 1.7 vol. %. The increasing concentration of CuNWs could lead to the enhancement of real permittivity because of the increased amount of conductive filler. This would also occur for imaginary permittivity since the increase in the amount of mobile charge carriers (Ohmic loss) and the number of nanocapacitors (polarization loss) increase. But the imaginary permittivity remains unchanged below a concentration of 1.7 vol. % which can be related to inferior network formation due to the agglomeration of CuNWs. The agglomeration has two main effects on the imaginary permittivity: a decrease of interfacial loss and a relatively larger thickness of insulative gaps. Both of these can result in lower imaginary permittivity. This can also lead to network formation: first CuNWs distributing as agglomerations and then forming an effective network after reaching a certain loading of conductive filler.

There may be several reasons for the agglomeration of CuNWs in PP matrix. First, the cloud point of PP in xylene is easily reached. Cloud point shows when phase separation occurs and there is precipitation of the polymer; it depends on the solvent/non-solvent ratio, and temperature of the system [5]. During MSMP, for PP and CuNW systems, temperature drops quickly since the ultrasound bath temperature (80 ºC) is lower than the PP solution (120 ºC). At the same time, room temperature CuNW-Methanol solution is added to the solution. Evaporation of Methanol caused additional heat transfer. Once the cloud point is reached, PP starts to precipitate out of the system without good dispersion of CuNWs. In addition, van der Waals forces between CuNWs are strong and dominant due to the large surface area compared to weak interfacial attraction between the nanowire surface and polymer chains in solution. This stops nanowires from homogeneous distribution during the polymer precipitation process. Other possible reasons for the worse dispersion of CuNW in PP matrix would be the PP (Flint Hills H0500HN) which has higher crystallinity and higher viscosity than PS (Styron 666D). Crystallinity of a polymer can influence the dispersion of fillers when the filler has certain tendency to either amorphous or crystal region of a semi-crystalline polymer [6]. With increasing viscosity, it becomes more difficult to disperse the fillers homogeneously in the polymer matrix.

Multiwall carbon nanotube/PP composites were studied and a similar plateau was found from the conductivity curve and more details will be explored in the future. 

Conclusion

CuNW/PP composites were synthesized by MSMP method and the conductivity and EMI shielding properties were studied and compared with CuNW/PS composites. The conductivity curve of CuNW/PP composites has a plateau during the percolation and the wider percolation concentration window gives the composite potential to be applied in charge storage. CuNW/PP composites also showed lower EMI shielding effectiveness than CuNW/PS composites. Both of these results can be explained by the agglomeration phenomenon, i.e. poor CuNW dispersion in the PP matrix.

References

[1] M. H. Al-Saleh, U. Sundararaj. Carbon, 2009, 47: 2每22

[2] G. A. Gelves, M. H. Al-Saleh and U. Sundararaj, Journal of Materials Chemistry, 2011, 21: 829每836

[3] G. A. Gelves, Z. T. M. Murakami, M. J. Krantz and J. A. Haber, Journal of Materials, 2006, 16(30): 3075每3083

[4] M. Arjmand, M. Mahmoodi, S. Park, U. Sundararaj, Composites Science and Technology, 2013, 78: 24每29

[5] T. Macko, R. Br邦ll, H. Pasch, Chromatographia Supplement, 2003, 57: S39 每S43

[6] F. Gubbels, R. Jerome, Ph. Teyssie, E. Vanlathem, R. Deltour, A. Calderone, V. Parente, J. L. Bredas, Macromolecules, 1994, 27 (7): 1972每1974