C. K. Kum et al.
458 Macromol. Res., Vol. 14, No. 4, 2006
Figure 2 shows the EMI SE of the PC/MWNT composites
with the MWNT content (0.5 to 7.0 wt%). The sample of the
PC/MWNT composites with 15.0 wt% MWNT content was
not able to prepare since the PC/MWNT (15.0 wt%) composite
was too brittle. The EMI SE is defined in terms of the
ratio of power of incident and transmitted EM wave as follows:
16
EMI SE = 10 log (PI/PT) (1)
where PI and PT are the power of incident and transmitted
EM waves, respectively. The unit of the EMI SE is given in
dB. Theoretical value of the EMI SE can be calculated by
applying the electrical conductivity to eq. (2):15
EMI SE = (2)
where σ is the electrical conductivity, d is the sample thickness,
and Zo is the free space impedance (constant: 377 S-1).
In Table II shows the experimental and theoretical values
of the EMI SE of the PC/MWNT composites with MWNT
content. From Figure 2 and Table II, the theoretical values
of the EMI SE are in good agreement with the experimental
values of the EMI SE with the MWNT content. Eq. (2) will
valid when the uniform distribution of the conductive filler in
the polymer composites is obtained.15 In this case, a reliable
value of the average electrical conductivity will be obtained.
For the PC/MWNT composites at 7.0 wt% MWNT content,
both the experimental and theoretical values of the EMI SE
show about 15 dB. At 15.0 wt% MWNT content, the theoretical
value of the EMI SE is about 30 dB. The comparison
between the experimental and theoretical values of the EMI
SE of the PC/MWNT composites shown in Table II is the
first experimental study of the PC/MWNT composites.
Morphology. Figure 3 shows the scanning electron micrographs
of the cryogenically fractured cross-sectional surfaces
20 1
1
2
+ --σdZo
log
Table II. Theoretical and Experimental EMI SE of the PC/
MWNT Composites with MWNT Content
MWNT
Content (wt%)
Theoretical
EMI SE (dB)
Experimental
EMI SE at
0.6 GHz (dB)
Experimental
EMI SE at
1.2 GHz (dB)
0 0 0 0
0.5 2.0 * 10-9 2.0 * 10-2 2.1 * 10-1
1.5 0.08 0.59 0.83
2.5 1.28 1.94 2.04
4.5 4.03 4.13 4.40
7.0 15.49 15.82 15.95
15.0 29.69 - -
Figure 2. Electromagnetic-radio frequency interference shielding
of the PC/MWNT composites with the MWNT content.
Figure 3. Scanning electron micrographs of the cryogenically
fractured cross-section surfaces for the PC/MWNT without O2
plasma etching for the 2.5 and 15.0 wt% MWNT content: (a) 2.5
wt% MWNT and (b) 15.0 wt% MWNT.