(763b) 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°Á25°Á11.6 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¦¸∙cm
or lower than 10⁶¦¸∙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 ¨C 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.

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