Thermally Stable Electromagnetic Interference

Thermally Stable Electromagnetic InterferenceShielding Material From Polysulfone Nanocomposites:Comparison on Carbon Nanotube and NanofiberReinforcement

Lalatendu Nayak,1,2 Ranjan R. Pradhan,1 Dipak Khastgir,2 Tapan K. Chaki21CIIR, CV Raman College of Engineering, Bhubaneswar, India

2Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India

The present investigation aims to develop thermallystable electromagnetic interference shielding materialsfrom polysulfone (PSU) nanocomposites filled withmultiwall carbon nanotubes (MWCNT) or carbon nano-fibers (CNF). The effect of filler type and their struc-tural features such as aspect ratio (length/diameter)and wall integrity on the different properties of nano-composites has been investigated. Nanocompositefilled with MWCNT/CNF exhibits higher thermal stabil-ity compared with the neat PSU matrix. The onset deg-radation temperature of PSU at 532�C enhances to 537and 538�C at 3 wt% MWCNT and 3 wt% CNF loading,respectively. CNFs filled nanocomposite shows higherelectromagnetic interference shielding effectiveness(EMISE) compared with MWCNT filled one at the samefiller loading. Compared with MWCNT, CNF impartslower electrical percolation threshold. Nanocompositefilled with MWCNTs possesses percolation threshold at1.5 wt%, whereas nanocomposite filled with CNFs pos-sesses the same at 0.9 wt%. The EMISE of 20–45 dBare obtained from only 1 mm thick CNF filled nano-composites from the filler loading 3 to 10 wt%. Thisvalue of EMISE above 40 dB suggests that the pre-pared nanocomposite can be used as an effectivelightweight EMI shielding material for high frequency(8.2–12.4 GHz) applications, where high thermal stabil-ity is required. POLYM. COMPOS., 36:566–575, 2015.VC 2014 Society of Plastics Engineers

INTRODUCTION

There has been a tremendous growth of high perform-

ance electrical and electronics devices in the field of

medical, defense, aerospace, automobile, and communica-

tion. The electromagnetic interference (EMI) has been a

great obstacle for electrical and electronic devices to

function properly. The electromagnetic waves produced

from one electronic instrument have an adverse effect on

the performance of other nearby electronic instruments

[1]. In order to alleviate this problem, the electronic

instruments that produce EM radiation should be shielded

by EMI shielding materials. Generally, metals like steel,

copper, and aluminum are used for EMI shielding due to

their high conductivity [2, 3]. However, their high den-

sity, prone to oxidation, limited physical/mechanical flexi-

bility, complex, and uneconomic processing has limited

their practical applications [4]. To avoid the above diffi-

culties of metals as EMI shielding materials, there is a

need to develop an efficient microwave-absorbing mate-

rial that is of low cost, relatively lightweight, flexible,

and efficient in absorption in a wide band frequency

range.

Generally polymer composites filled with carbon fillers

like graphite, carbon black, and carbon fibers are used for

EMI shielding purpose, but the concentration of these

micro fillers required to provide adequate EMISE is sub-

stantially high and adversely affect the mechanical prop-

erties of composites. This problem can be overcome by

the use of different conductive nanofillers like MWCNT

and CNF, which have received great attention to prepare

EMI shielding materials due to their high intrinsic con-

ductivity and high aspect ratio [5, 6]. Recently, MWCNTs

have received significant research interest compared with

CNFs due to their better mechanical properties (due to

less microstructural defects), smaller diameters as well as

lower density. However, the low cost, high availability

and better prone to functionalization (due to no. of broken

side walls of CNF) have led CNF as more preferable con-

ductive filler than MWCNT [7].

Different properties of a composite are affected by the

shape and structure of the incorporated filler particles. So,

it is important to investigate the effect of filler type and

Correspondence to: Tapan K. Chaki; e-mail: [email protected] or

[email protected]

DOI 10.1002/pc.22973

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2014 Society of Plastics Engineers

POLYMER COMPOSITES—2015

their structural features like aspect ratio and wall integrity

on the different properties of the polymer composites. But

the literature dealing with the usage of CNT and CNF

separately in a single matrix polymer and the comparative

analysis of their effectiveness on the different properties

of composites is scanty.

Polysulfone is a high performance amorphous engi-

neering thermoplastic. It has tremendous applications

especially in medical, food processing equipment, electri-

cal and electronics components due to its excellent prop-

erties, such as high glass transition temperature (Tg

�185�C), high mechanical strength, flexibility and excel-

lent thermal stability. Hence in our study, the addition of

MWCNTs/CNFs into PSU matrix is expected to achieve

thermally stable EMI shielding material for different elec-

trical and electronic applications.

The objective of this investigation is to develop ther-

mally stable EMI shielding material from very low con-

centration of MWCNT or CNF and also to present a

comparative study on the effect of MWCNT and CNF on

different material properties of nanocomposites.

EXPERIMENTAL

Materials

Polysulfone (Grade: Udel P-1700) used in this study

was supplied from Solvay Advanced Polymers, India.

Highly graphitic vapor grown carbon nanofibers (CNF,

trade name PR-24-XT-HHT, average diameter 100 nm,

and length on the order of 50 to 200 lm, purity >98%)

was procured from Pyrograf Products, Inc. an affiliate

Applied Science. MWCNT (average diameter 50 nm and

length on the order of 0.5 to 40 lm, purity >95%) used

in this study were purchased from Helix Material Solu-

tion. Tetrahydrofuran (THF) used as solvent was pur-

chased from Merck Specialties Private Limited, India. To

improve interfacial adhesion and dispersion of fillers in

the polymer matrix, both MWCNT and CNF were func-

tionalized by air oxidation process according to the

method described in our previous publication [8]. Using

this process some polar groups were introduced on the

surface of CNTs/CNFs without hampering their aspect

ratio. At first, fillers were weighted and placed in a

designed quartz tube (two end narrow and middle wide).

Then the quartz tube was placed in a cylindrical furnace.

The oxygen required for oxidation of fillers was provided

using an air flow of 40 mL/min into the furnace. Heat

treatment was carried out at 450�C for 2 h. The quartz

tube was rotated at 30 min time interval to expose all the

filler particles to air for proper oxidation.

Preparation of Nanocomposites

Nanocomposite films were prepared by solution mix-

ing process. MWCNTs/CNFs were dispersed in THF by

sonication for one hour in a horn type sonicator

(frequency: 22.5 kHz, power: 100 W). Then required

amount of PSU was added to MWCNT/CNF dispersed

solvent and the solution was stirred for 30 min and then

sonicated and stirred for 16 min in 2 min interval. Films

were prepared by casting the solution and evaporating the

solvent at room temperature for overnight. Then the dried

films were further heated for 2 h at 60�C, for 1 h at

120�C, and for 1 h at 150�C to remove entrapped solvent

completely. Test specimens of each composite were pre-

pared by compression molding of stacked films at a tem-

perature of 280�C.

Characterization

To analyze the functional groups attached to oxidised

MWCNT/CNF, Fourier transform-infrared (FTIR) spec-

trophotometer was used (model 870, Thermo Nicolet cor-

poration). To evaluate the interfacial interaction between

oxidized MWCNT/CNF and PSU, FTIR spectrometer

equipped with an attenuated total reflectance (ATR) probe

attachment was used. The state of dispersion of

MWCNT/CNF into the PSU matrix was studied using a

high resolution transmission electron microscope

(HRTEM, JEM 2100, JEOL Limited, Tokyo, Japan)

attached with charge couple device (CCD) camera (Gatan,

Inc., CA). The thermal decomposition behaviour of nano-

composites was studied using thermogravimetric analysis

(TGA Q50 V6.1 series, TA Instruments) under a nitrogen

atmosphere from room temperature to 650�C, operated at

a heating rate of 20�C/min, taking samples weight about

8 to 9 mg. Tensile properties of samples with size 70 3

10 3 0.1 mm (ASTM D-882) was carried out at room

temperature at a crosshead speed of 1 mm/min using a

Universal Testing Machine (Model 5980, Instron India

Pvt. Ltd., India).

The direct current (DC) resistivity of the nanocompo-

site having the resistivity greater than 106 X.cm was

measured using Agilent 4339B (High Resistance Multim-

eter coupled with Agilent 16008B Resistivity Cell). DC

resistivity of the nanocomposites having the resistivity

less than 106 X.cm was measured using DC milli-ohm

meter (model no.GOM-802, Goodwill Instek Co., Tai-

wan). The electrical data reported in this paper were aver-

age value of five measurements for each formulation.

Percentage error associated with DC conductivity experi-

ments were within 63%. The EMI shielding effectiveness

of different samples was estimated using a Scalar Net-

work Analyzer (HP 8757C, Hewlett Packard) coupled

with a sweep oscillator (HP 8350B, Hewlett Packard) in

the X-band frequency range (8–12 GHz). Reported

EMISE data of each composition was obtained from the

mean value of three samples of 1 mm thickness whereas

the standard deviation was less than 6%. As we know

that for commercial application, minimum EMI SE of a

shielding material should be 20 dB. So in this article we

have stated MWCNT/CNF loading above 3 wt%, because

at 3 wt% EMI SE of nanocomposite is �18 dB.

DOI 10.1002/pc POLYMER COMPOSITES—2015 567

RESULTS AND DISCUSSION

FTIR Study

The functionality of oxidized MWCNT/CNF and their

interaction with PSU matrix have been confirmed from

FTIR study (Fig. 1). In the oxidized MWCNT (Fig. 1a),

the peaks at around 1,722 cm21 and 3,410 cm21 corre-

spond to carbonyl group (>C5O) and hydroxyl group,

respectively [9]. Similarly in oxidized CNF (Fig. 1b), the

peak at 1,729 cm21 confirms the presence of carboxyl

group and the peak at 3,437 cm21 represents the exis-

tence of hydroxyl functional groups. The possibility of

occurrence of any interaction between MWCNT/CNF and

PSU matrix (Fig. 1c) has been investigated through FTIR

spectrometer equipped with an attenuated total reflectance

(ATR) probe attachment. The observed peaks for polysul-

fone appear at 1,232 cm21, 1,145 cm21, and 1,322 cm21

which correspond to asymmetric C-O-C stretching of aryl

ether group, symmetric and asymmetric O5S5O stretch-

ing of sulfone group, respectively. After the addition of

MWCNT, the peak of PSU matrix at 1,232 cm21 is

shifted to 1,238 cm21. Similarly after the addition of

CNF, the peak at 1,232 cm21 of PSU matrix is also

shifted to 1,236 cm21. So there is possibility of intermo-

lecular hydrogen bonding interaction between carboxylic

or hydroxyl groups on the surface of oxidized MWCNT/

CNF and aryl ether groups of polysulfone [10].

HRTEM Study

Low electrical percolation threshold for a composite

depends on the state of dispersion of conducting filler

particles and their interconnected continuous network for-

mation in the polymer matrix, but high degree of EMI SE

depends on the close packed conductive networks of filler

particles. To observe the state of dispersion and intercon-

nected conductive network formation of MWCNT and

CNF in the PSU matrix, HRTEM images of nanocompo-

sites were investigated. Figure 2a–c and Fig. 2d–f repre-

sent the HRTEM micrographs of PSU/MWCNT and

PSU/CNF nanocomposites, respectively. Individual dis-

persion of CNT/CNF is observed throughout the samples

filled with low nanofiller concentration, whereas some

agglomerates are observed at high filler concentration.

These agglomerates are due to van der Waals force of

attraction among MWCNTs/CNFs. When MWCNTs/

CNFs concentration increases from 1 to 10 wt%, three

important factors are observed. These factors are (i)

increase in number of interconnected conductive net-

works, (ii) decrease in mesh size, and (iii) increase in

number of agglomerates. Composite filled with 10 wt%

FIG. 1. FTIR spectra of (a) oxidized MWCNT, (b) oxidized CNF, and (c) PSU nanocomposites filled with

1 wt% MWCNT/CNF. [Color figure can be viewed in the online issue, which is available at wileyonlineli-

brary.com.]

568 POLYMER COMPOSITES—2015 DOI 10.1002/pc

MWCNT/CNF (Fig. 2c and f) shows more dense packed

conductive networks with smaller mesh size compared

with the composite filled with 1 and 3 wt% MWCNTs/

CNFs.

Mechanical Properties

The mechanical properties of PSU matrix and its nano-

composites filled with different wt% of MWNT and CNF

are shown in Fig. 3. It is revealed that the tensile strength

of PSU nanocomposite is a function of filler concentra-

tion. The tensile strength and modulus of nanocomposite

increase with increasing concentration of both MWCNT

and CNF up to 3 wt%. The addition of 3 wt% CNF leads

to an increase in tensile strength from 64 MPa to 78 MPa

and tensile modulus from 1820 MPa to 2650 MPa, indi-

cating an improvement of 21.8% in strength and that of

46% in modulus. Similarly at 3 wt% MWCNT concentra-

tion, tensile strength increases by 15.6% and tensile mod-

ulus by 44%. This enhancement in tensile strength and

modulus at low filler concentration may be attributed to

the good dispersion of fillers in nanocomposite and also

good interfacial adhesion between fillers and PSU matrix,

which result a good load transfer from PSU matrix to fil-

ler. Another factor for the enhancement in tensile strength

may be due to the entanglement of polymer chains. This

entanglement of polymer chains with filler particles plays

a role of physical cross-linking, which enhances the

strength of the nanocomposite. Further addition of

MWCNT/CNF above 3 wt% decreases the tensile strength

and modulus of nanocomposites, but these values are still

higher than that of neat PSU matrix. This decrease in

mechanical properties at higher filler concentration (>3

wt% MWCNT/CNF) can be attributed to the poor disper-

sion and formation of agglomerates of MWCNT/CNF in

the PSU matrix, which act as flaws and regions of stress

concentration under dynamic loading and finally result in

a premature failure.

An interesting fact observed in this study is that CNF

filled composite shows higher tensile strength and modu-

lus compared with MWCNT filled one at equal filler

loading. CNTs have higher mechanical strength compared

with CNFs, but reinforcing effect of fillers depends on

their geometrical features. CNFs have higher rough surfa-

ces compared with MWCNTs due to their stacked-cup

structure. This rough surface entangles/interlocks more

polymer chains and helps to transfer more stress from

FIG. 2. HRTEM images of PSU nanocomposites filled with (a) 1 wt%

MWCNT, (b) 3 wt% MWCNT, (c) 10 wt% MWCNT, (d) 1 wt% CNF,

(e) 3 wt% CNF, and (f) 10 wt% CNF. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

FIG. 3. Tensile strength and tensile modulus of PSU and its nanocom-

posites as a function of (a) MWCNT and (b) CNFs concentration. [Color

figure can be viewed in the online issue, which is available at wileyonli-

nelibrary.com.]


PSU matrix to CNFs, leading to the higher tensile strength

and modulus of the composite. A schematic presentation ofpolymer chain entanglement around CNTs and CNFs is

shown in Fig. 4. The “A” type of polymer entanglement isonly due to wrapping of polymer chains around fibers, butthe “B” type of entanglement is mainly due to the rough

surface of fibers which hooks the polymer chain. CNTshave nearly smooth surfaces, because concentric graphenelayers are continuous and parallel to the fiber axis. Hence

only “A” type of polymer entanglement (wrapping of poly-mer chains) is possible for CNT filled composites (Fig. 4b),

which enhances the tensile strength. Graphene layers ofCNFs are not continuous and are concentrically nested atcertain angle to the fiber axis (as shown in Fig. 4a). Due to

the staking of graphene layers at certain angle to the fiberaxis, CNF possesses rough surface. Rough surfaces ofCNFs increase the frictional resistance to slide and act as

hooks to the polymer chain, by which more stress is trans-ferred from PSU to CNFs. Hence reinforcing effect of CNF

is contributed from both “A” and “B” type of entanglement.

Thermal Stability

Thermogravimetric analysis of PSU/MWCNT and PSU/

CNF nanocomposites has been carried out to evaluate the

effect of MWCNT and CNF on the thermal stability of

PSU matrix. From the TGA/DTG plots given in Fig. 5a

and b and Table 1, it can be said that the thermal stability

of PSU nanocomposite at each concentration of filler is

higher than that of neat PSU matrix and the onset degrada-

tion temperature (5% weight loss) for both MWCNT and

CNF filled nanocomposites increases with the increase in

nanofiller concentration up to 3 wt%. The onset degrada-

tion temperature of neat PSU matrix is 532�C, which

increases to 539�C and 544�C at 3 wt% MWCNT and 3

wt% CNF concentrations, respectively. However, further

increase in nanofiller concentration has only marginal

effect on the thermal stability. This improvement in ther-

mal stability may be attributed to the following major

aspects: (i) MWCNTs/CNFs act as heat sinks and absorb

some of the heat energy available for the decomposition of

polymer molecules. Hence in the presence of MWCNT/

CNF, more energy is required for the thermal decomposi-

tion of PSU matrix and (ii) high thermal conductivity of

MWCNT/CNF reduces the interfacial thermal resistance

between PSU and MWCNT/CNF, which is responsible for

the smooth heat transfer from PSU matrix to MWCNT/

CNF. Furthermore this facilitates the uniform heat distri-

bution throughout the nanocomposites without assembling

of excess heat on the composite surface and results an

increase in thermal decomposition temperature [11].

The rate of degradation is also slowed after the addi-

tion of nanofillers. From DTG graph, a shift in Tmax (tem-

perature corresponding to the maximum rate of

decomposition) to higher value with the increase in both

MWCNT and CNF content in PSU matrix is observed.

FIG. 4. Schematic presentation of polymer wrapping on (a) CNF and

(b) MWCNT. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

FIG. 5. TGA and DTG curves of (a) PSU/MWCNT nanocomposites

and (b) PSU/CNF nanocomposites. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]


This implies that the incorporated MWCNT/CNF in PSU

matrix act as thermal degradation inhibitor. Higher ther-

mal stability of composites compared with neat PSU

matrix was confirmed from the analysis of residual mass

at 640�C. The residual mass of composite increases by 11

and 13% at 3 wt% MWCNT and CNF loading, respec-

tively, compared with neat PSU matrix, which may be

due to the protective thermal barrier effect of char residue

to the undegraded part of composite.

At the same filler loading, a very marginal difference in

thermal stability between MWCNT and CNF filled compo-

sites is observed. Due to highly graphitic nature of CNF,

thermal stability of its composite is somewhat higher, but

CNTs used in this study possess some quantity of amor-

phous carbon, which degrades at lower temperature and

lowers the thermal stability of the composite.

Effect of Filler Type on Electrical Percolation

The geometrical factors of conducting fillers like

shape, size, aspect ratio, and dispersion in the polymer

matrix are the critical issues for the enhancement in elec-

trical conductivity of a composite. Fillers having higher

aspect ratio impart higher conductivity at lower loading.

A comparative study on the effect of MWCNT and CNF

loading on the conductivity of PSU matrix is presented in

Fig. 6a and b. Like most polymers, PSU is insulating in

nature having conductivity 6.01 3 10218 mho cm21. A

sharp increase in conductivity of PSU up to 1.53 3 1025

mho cm21 at 1.5 wt% MWCNT loading and 4.74 3

1025 mho cm21 at 0.9 wt% CNF loading is observed, but

with further addition of MWCNT/CNF, the change of

conductivity is marginal.

An insulating material is converted to conductive one

by the incorporation of a critical concentration of conduc-

tive additive (called percolation threshold). At this critical

filler concentration, a continuous conducting path of filler

particles is formed across the volume of the composite

[12–14]. In order to estimate the percolation threshold of

MWCNTs and CNFs in PSU nanocomposites, the experi-

mental conductivity data have been fitted to the following

power law equation of percolation theory [15, 16].

r / ðu2ucÞt for u > uc (1)

where r is the conductivity of nanocomposite, uc is the

critical filler volume fraction at which percolation take

place, and t is the critical exponent of conductivity. The

TABLE 1. TGA data of neat PSU/MWCNT and PSU/CNF nanocomposites.

Nanofiller (wt%)

PSU/MWCNT nanocomposites PSU/CNF nanocomposites

T5% loss (�C) Tamax(�C) WtR

b (%) T 5% loss (�C) Tamax(�C) WtR

b (%)

0 532 6 0.09c 563 6 0.3 36.41 6 0.04 532 6 0.09 563 6 0.3 36.41 6 0.04

1 536 6 0.05 565 6 0.1 38.25 6 0.02 539 6 0.3 569 6 1 38.76 6 0.0 3

3 539 6 0.1 570 6 0.5 40 .36 6 0.05 544 6 0.7 570 6 1.5 41 .21 6 0.04

5 537 6 0.6 568 6 1 41.82 60.07 538 6 1 571 6 2 42.63 60.07

7 537 6 0.7 566 6 0.3 44.37 6 0.1 536 6 0.4 569 6 0.5 45.5 6 0.01

10 535 6 1 565 6 1.1 46.02 6 0.03 535 6 0.2 568 6 0.7 46.8 6 0.2

aTemperature at maximum degradation rate.bWeight percentage of residue at 640�C.cStandard deviation.

FIG. 6. DC conductivity of (a) PSU/MWCNT nanocomposites and (b)

PSU/CNF nanocomposites. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]


inset of Fig. 6a and b for log(r) versus log((u 2 uc)

plots shows a very well agreement between the experi-

mental conductivity data of PSU nanocomposite and the

percolation behavior predicted by Eq. 1. The best linear

fit of the conductivity data to the log-log plot of power

law gives uc5 0.014 volume fraction (1.5 wt%) and t 5

1.62 (inset of Fig. 6a) for PSU/MWCNT nanocomposite,

and uc5 0.0079 volume fraction (0.9 wt%) and t 5 1.73

(inset of Fig. 6b) for PSU/CNF nanocomposite. The

obtained values of critical exponent “t” in both MWCNT

and CNF filled nanocomposite systems are found to be in

good agreement with the theoretical results (t 5 1.6–2)

for a three dimensional system [17]. Higher value of trepresents better efficiency of a filler to form a continu-

ous conductive network in a composite system. Therefore,

the efficiency of conductive network formation is higher

for CNF compared with MWCNT; 1.5 wt% MWCNT

concentration exhibits an increase in conductivity by thir-

teen orders of magnitude compared with the neat PSU

matrix, whereas the same order of magnitude (�1013)

increase in conductivity is exhibited by 0.9 wt% of CNF

concentration. The percolation threshold for electrical

conductivity depends strongly on the geometry of the

conducting fillers mainly aspect ratio. The higher the

aspect ratio, the lower the percolation threshold necessary

for the formation of conducting network. In some litera-

tures it was reported that the percolation threshold of

MWCNT in a composite was lower than that of CNF [18,

19]. However, in the present study the percolation thresh-

old of CNF in PSU nanocomposite was lower than that of

MWCNT because of the higher aspect ratio and higher

graphitic nature of CNF. Similar results were also stated

by many researchers. George et al. [20] studied the effect

of expanded graphite, CNF, and MWCNT on the conduc-

tivity of ethylene vinyl acetate and observed the lower

percolation threshold from CNF filled composite com-

pared with MWCNT filled one.

Electromagnetic Interference Shielding Effectiveness

Electromagnetic interference is one kind of artificial

environmental pollution, which causes performance degra-

dation of an electronic system by inducing spurious volt-

age and current in the electronic circuits. Shielding

materials are used to reduce such pollution. When electro-

magnetic waves incident on shielding material, they are

split into four parts: a reflected wave, an absorbed wave,

an internal reflected wave, and a transmitted wave as

shown in Fig. 7 and attenuated by the shielding material

through three major mechanisms, namely: reflection,

absorption, and multiple reflection. SE due to reflection

of EM wave from the surface of shielding material is

called reflection loss (SER), SE due to absorption of EM

wave within the shielding material is called absorption

loss (SEA), and SE due to internal reflection of the EM

wave within the shielding material is called internal

reflection loss (SEM). When SEA � 10 dB, most of the

re-reflected waves are absorbed within the shield [21, 22].

So SEM can be ignored for practical purposes and the

total SE can be expressed as the sum of SEA and SER in

most shielding environments [23, 24].

In a two port network analysis system, SER and SEA

of the shielding material are determined from the scatter-

ing parameters (S), i.e. S11 (S22) and S12 (S21) using fol-

lowing Eq. 2 and 3 [24].

SER5210 logð12jS11j2Þ (2)

where jSijj2 represents the power transmitted from port

i to port j.

Effect of Reflection and Absorption Loss on Total

EMI SE. Maximum polymers are insulating in nature

and transparent to electromagnetic radiation. The incorpo-

ration of any conducting filler enhances the EMISE of

polymers. The incorporated filler forms a conductive

mesh like structure in the polymer matrix. This conduc-

tive mesh interacts with incident electromagnetic radiation

and accounts for increase in EMI SE of the system.

The contribution of absorption loss (SEA) and reflec-

tion loss (SER) to the total SE of MWCNT and CNF

filled nanocomposites in the frequency range of 8.2 to

12.4 GHz are presented in Fig. 8a–d. It is observed that

SER of PSU/MWCNT nanocomposite increases slightly

from 1.47 dB to 4 dB at 10 GHz with the increase in

MWCNT content from 3 to 10 wt%, whereas SER of

PSU/CNF nanocomposite increases from 3.76 dB to 5.35

dB with the increase in CNF content from 3 to 10 wt%.

This enhancement in SER with the increase in MWCNT

and CNF loading may be due to the increase in conduc-

tivity of the composites. The increase in conductivity

increases the interaction between charge carriers present

in composite (i.e., either free electron or vacancy) and

EM radiation leading to the increase in SER [25]. At the

same filler loading, CNFs filled composite shows higher

SER value compared with MWCNT filled one which is

FIG. 7. Schematic view of EM wave interaction with shielding mate-

rial. [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com.]


mainly due to the higher conductivity of CNF filled com-

posite ones as mentioned in Fig. 6.

The variation of SEA of different composition of PSU/

CNF and PSU/MWCNT nanocomposite against frequency

is almost linear in nature over the entire frequency range

of measurement (Fig 8b and d). Interfacial boundary

charges are formed by the incorporation of conducting

fillers in an insulating polymer matrix. Under the EM

field, these bound charges are subjected to polarization

(restricted movement of bound charges) leading to the

absorption of electrical energy by the system. This

absorption of electrical energy by the conductive system

is the main cause of SEA. For MWCNT filled nanocom-

posite, SEA increases from 10.9 dB to 32.4 dB with the

increase in MWCNT loading from 3 to 10 wt%, similarly

SEA of CNF filled nanocomposite also increases from 16

dB to 39.6 dB with the same increase in CNF loading. It

is evident from the experimental results that the absorp-

tion is the primary shielding mechanism and the reflection

is the secondary shielding mechanism for both MWCNT

and CNF filled nanocomposites.

EMI SE as a Function of Nanofiller Loading. The

EMISE as a function of nanofiller loading for PSU/

MWCNT and PSU/CNF nanocomposites of 1 mm thick-

ness at a fixed frequency of 10 GHz is presented in

Fig. 9. A progressive increase in SE with the increase in

filler loading is observed. The SE of PSU/MWCNT

nanocomposite is found to increase from 12 to 36 dB

with the increase in MWCNT loading from 3 to 10 wt%,

whereas SE of PSU/CNF nanocomposite increases from

19.8 to 45 dB with the same increase in CNF loading.

The SE of a composite material is due to the intercep-

tion of interconnecting conductive networks with incident

EM radiation and it mainly depends on the number of

mobile charge carriers and the mesh size of the conduct-

ing networks formed by the filler particles in composite.

The increase in filler loading increases the number of

FIG. 8. (a) SER and (b) SEA as a function of frequency for PSU/MWCNT composite; (c) SER and (d) SEA

as a function of frequency for PSU/CNF composite. [Color figure can be viewed in the online issue, which

is available at wileyonlinelibrary.com.]

FIG. 9. EMISE as a function of nanofillers loading for PSU/MWCNT

and PSU/CNF nanocomposites. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]


mobile charge carriers in the composite, which reflect

back the incident EM wave and provide more EMISE.

Increase in filler loading also reduces the mesh size of

conductive networks formed in the shielding material by

which the incident EM radiation is more efficiently inter-

cepted by the conductive networks leading to an increase

in SE through absorption mechanism.

The gap in conductive mesh is the way to pass EM

radiation through composites. Hence the gap size in con-

ductive mesh is an important factor for the determination

of SE of a shielding material. The densely packed con-

ductive networks with finer mesh size provide better

EMISE [26]. The effect of filler loading on SE can be

clearly explained using a schematic presentation given in

Fig. 10. A continuous increase in degree of interception

of EM radiation with the increase in filler loading is

observed from Fig. 10a to c. At lower filler loading the

gap size of conducting mesh is larger, which allows more

radiations to pass through the composite and account for

lower value of SE (as shown in Fig. 10a). The mesh of

conducting fillers becomes more and more dense (Fig.

10c) with increasing filler loading and closed pack struc-

ture of networks is formed which prevent the passage of

EM radiation and provides higher value of EMI SE.

From Fig. 9, it is also revealed that at the same filler

concentration, CNF filled nanocomposite shows higher

EMISE compared with MWCNT filled one. SE of a com-

posite depends on its conductivity and the close packed

network of fillers. This network formation and conductiv-

ity is more pronounced by the filler having higher con-

ductivity and aspect ratio. As in this study, CNF has

higher conductivity and aspect ratio compared with

MWCNT, composite filled with CNF possesses higher

conductivity and more dense packed conductive networks

leading to more EMISE.

In fact, the value of EMISE of a material indicates the

fraction of incident EM radiation blocked by the shielding

material. 20 dB EMISE means 99%, 30 and 40 dB means

99.9 and 99.99%, respectively, of incident EM radiation

is blocked. For PSU nanocomposite film containing 10

wt% of MWCNT or CNF with 1 mm thickness, the SE

shows more than 99.9% shielding of electromagnetic radi-

ation over the entire frequency range of measurement.

Hence the prepared nanocomposites filled with 5, 7, and

10 wt% MWCNT or CNF can provide excellent shielding

effectiveness for EM radiation in X-band region and can

meet the requirement of commercial applications.

CONCLUSIONS

The important findings of the present investigation can

be presented as:

i. Very low percolation thresholds were obtained for both

MWCNT and CNF filled nanocomposite systems; uc 5

0.0079 vol. fraction (0.9 wt%) for PSU/CNF nanocompo-

site and uc 5 0.014 vol. fraction (1.5 wt%) for PSU/

MWCNT nanocomposite.

ii. CNF filled nanocomposite exhibited higher mechanical,

thermal, and electrical properties, and also higher EMISE

compared with MWCNT filled one at the same filler concen-

tration. At 10 wt% filler concentration, SE was 45 dB for

CNF filled composite of 1 mm thickness, whereas SE was

36 dB for MWCNT filled composite of same thickness.

iii. The SE due to absorption was the primary mechanism,

whereas SE due to reflection was the secondary mecha-

nism for the determination of total EMI SE of PSU

nanocomposites.

iv. The formation of conductive networks by nanofillers in

the PSU matrix was the key reason for the improvement

of EMISE and conductivity.

The mechanical strength above 65 MPa, thermal stability

(onset degradation) above 535�C, and EMISE above 40 dB

suggest that the prepared composite can be used as promis-

ing thermally stable EMI shielding/absorbing material for

different electrical and electronic applications, where high

thermal stability and mechanical strength are required.

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Thermally Stable Electromagnetic Interference