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Journal of Materials ScienceFull Set - Includes `Journal of MaterialsScience Letters' ISSN 0022-2461 J Mater SciDOI 10.1007/s10853-018-2072-3
Self-sensing and mechanical performanceof CNT/GNP/UHMWPE biocompatiblenanocomposites
Tejendra K. Gupta, M. Choosri,K. M. Varadarajan & S. Kumar
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BIOMATERIALS
Self-sensing and mechanical performance of CNT/GNP/
UHMWPE biocompatible nanocomposites
Tejendra K. Gupta1 , M. Choosri1, K. M. Varadarajan2,3 , and S. Kumar1,*
1Department of Mechanical and Materials Engineering, Masdar Institute, Khalifa University of Science and Technology,
Masdar City, P.O. Box 54224, Abu Dhabi, UAE2Harris Orthopaedics Laboratory, Department of Orthopaedics Surgery, Massachusetts General Hospital, 55 Fruit St, Boston, MA,
USA3Department of Orthopaedic Surgery, Harvard Medical School, A-111, 25 Shattuck Street, Boston, USA
Received: 12 December 2017
Accepted: 23 January 2018
� Springer Science+Business
Media, LLC, part of Springer
Nature 2018
ABSTRACT
Ultra-high molecular weight polyethylene (UHMWPE)-based conductive
nanocomposites with reduced percolation and tunable piezoresistive behavior
were prepared via solution mixing followed by compression molding using
carbon nanotubes (CNT) and graphene nanoplatelets (GNP). The effect of
varying wt% of GNP with fixed CNT content (0.1 wt%) on the mechanical,
electrical, thermal and piezoresistive properties of UHMWPE nanocomposites
was evaluated. The combination of CNT and GNP enhanced the dispersion in
UHMWPE matrix and lowered the probability of CNT aggregation as GNP
acted as a spacer to separate the entanglement of CNT with each other. This has
allowed the formation of an effective conductive path between GNP and CNT in
UHMWPE matrix. The thermal conductivity, degree of crystallinity and
degradation temperature of the nanocomposites increased with increasing GNP
content. The elastic modulus and yield strength of the nanocomposites were
improved by 37% and 33%, respectively, for 0.1/0.3 wt% of CNT/GNP com-
pared to neat UHMWPE. The electrical conductivity was measured using four-
probe method, and the lowest electrical percolation threshold was achieved at
0.1/0.1 wt% of CNT/GNP forming a nearly two-dimensional conductive net-
work (critical value, t = 1.20). Such improvements in mechanical and electrical
properties are attributed to the synergistic effect of the two-dimensional GNP
and one-dimensional CNT which limits aggregation of CNTs enabling a more
efficient conductive network at low wt% of fillers. These hybrid nanocomposites
exhibited strong piezoresistive response with sensitivity factor of 6.2, 15.93 and
557.44 in the linear elastic, inelastic I and inelastic II regimes, respectively, for
0.1/0.5 wt% of CNT/GNP. This study demonstrates the fabrication method and
the self-sensing performance of CNT/GNP/UHMWPE nanocomposites with
improved properties useful for orthopedic implants.
Address correspondence to E-mail: [email protected]
https://doi.org/10.1007/s10853-018-2072-3
J Mater Sci
Biomaterials
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Introduction
Ultra-high molecular weight polyethylene
(UHMWPE) is a biomaterial widely used in hip, knee
and spine implants. It was first introduced clinically
as a bearing material in total joint replacement (TJR)
in the early 1960s by Sir John Charnley owing to its
extraordinary physical and mechanical properties [1].
However, wear and structural failure are the major
factors limiting the lifetime of implanted UHMWPE
components. Therefore, monitoring and assessment
of such damage in implanted components during
their in vivo service are necessary to better under-
stand the factors affecting the longevity of these
devices, and develop improved implant designs and
materials. Currently we have no way of measuring
device performance once it is implanted in the
patient. Such damage can be monitored by using
external sensors such as strain gauges, ultrasonic
sensors and piezoelectric sensors that are embedded
in the structure or attached on the surface of the
structure. However, embedding a separate sensor
into an implant is associated with several challenges
including high cost, introduction of new failure
modes, limited space available for such sensors, etc.
[2]. These disadvantages could potentially be over-
come with self-sensing materials which provide
information on damage state in real time without the
need for embedded sensors. Advantages of self-
sensing sensors as compared to the traditional
embedded devices are low production cost, high
durability and absence of mechanical property loss
[3].
The introduction of conductive fillers in UHMWPE
can induce the self-sensing capability via the
piezoresistive phenomenon [2, 4, 5]. These conduc-
tive nanofillers therefore could be used to produce
orthopedic biomaterials that can sense in vivo envi-
ronment and measure in vivo device performance,
without the need for incorporating separate sensors.
These conductive filler-based nanocomposites have
revealed extraordinary potential for self-sensing
owing to their fast response to applied stress or
strain. The fast response is denoted by the resistance
variation upon applying an external strain, resulting
from the changes in the conductive network or tun-
neling distance between distributed conductive fillers
[6].
Among various conductive nanofillers evaluated
by researchers to be used for polymer composites,
graphene and their tubular variants such as carbon
nanotubes are the promising carbon-based nano-
fillers owing to their excellent electrical [7], mechan-
ical [8, 9] and thermal [10, 11] properties and high
specific surface area. Carbon nanotubes have hexag-
onal sp2 carbon layer structure that bestows them
with unusual properties by imposing additional
quantum confinement and topological constraints in
the radial direction [12]. With the ability to conduct
electricity with low resistance [13, 14] and biocom-
patibility, CNT is suitable for devices and sensors,
especially for biomedical applications. Few
researchers have prepared nanocomposites of
UHMWPE reinforced with CNT and studied their
mechanical properties. Samad et al. [15] used
nanoindentation method to evaluate the nanome-
chanical properties of UHMWPE/single-walled car-
bon nanotube (SWCNT) composites and they
reported 58 and 66% improvement in elastic modulus
and hardness, respectively, for UHMWPE/SWCNT
composite compared to neat UHMWPE. Bakshi et al.
[16] also showed enhancement in the elastic modulus
by 82% by the addition of 5 wt% multiwalled carbon
nanotubes (MWCNTs) in UHMWPE matrix; how-
ever, the strain to failure was reduced drastically due
to the formation of MWCNT cluster.
Along with other carbon allotropes, graphene has
also attracts a huge interest owing to its outstanding
properties since their first discovery by Novoselov
et al. [17] using micromechanical cleavage technique.
The experimental and theoretical study of graphene
is a rapidly growing field of condensed matter
research, and graphene has the potential for diverse
applications on account of its excellent mechanical,
electrical, thermal and barrier [18] properties as well
as high surface area. These properties make it
attractive reinforcement for polymer nanocomposites
[19]. An addition of only 0.1 wt% graphene nano-
platelets to UHMWPE by electrostatic spraying
showed 54% improvement in fracture toughness and
71% increase in tensile strength [20]. Storage modulus
with respect to temperature was also explored with
the addition of 3 wt% GNP and resulted in an
improvement of 170% compared to the pure
UHMWPE [21]. Besides the excellent mechanical
properties with the incorporation of both CNT and
GNP nanofillers, the modification of electrical prop-
erties also arises. The addition of conductive filler
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leads to a transition from a non-conducting to con-
ducting state at particular threshold volume fraction
of the fillers. Due to intrinsic properties of CNT
including high aspect ratio and specific surface area,
at very low concentration of CNT within
0.025–4 wt%, the composite could reach the percola-
tion threshold [22, 23].
Theoretical and computational analyses in the
exploration of basic physical phenomena such as
percolation and conductivity in novel nanocompos-
ites play an important role. Hu et al. [24] predicted
the electrical properties of CNT nanocomposites, at
and after percolation based on three-dimensional
(3D) statistical percolation and 3D resistor network
modeling. Numerical techniques such as molecular
dynamics simulation [25], Monte Carlo modeling [29]
and micromechanical modeling [26–28] are fre-
quently used to investigate the effects of aspect ratio,
electrical conductivity, thermal conductivity, CNT
concentration, aggregation and the shape of CNTs on
the mechanical, electrical and thermal properties of
the nanocomposites.
No attempt has thus far been made to develop
UHMWPE nanocomposites by using two types of
reinforcing components, i.e., CNT and GNP as bi-
nanofillers to improve the mechanical, electrical and
self-sensing properties. In the present study,
UHMWPE-based conductive hybrid nanocomposites
comprising a combination of CNT and GNP were
fabricated and the electrical and thermal properties
were studied. GNP could act as spacer to reduce
entanglement of CNT from each other, and the CNT
could bridge the gap between individual GNP. This
may be beneficial for dispersion of CNT and forma-
tion of effective conductive paths, leading to better
electrical conductivity at low wt%. The piezoresistive
behavior of CNT/GNP/UHMWPE nanocomposites
was investigated to study the synergistic effect of
CNT and GNP on the self-sensing behavior of
UHMWPE nanocomposites.
Experimental
Materials
Two conductive nanofillers, namely multiwalled
carbon nanotubes (MWCNTs) and graphene nano-
platelets (GNP), used in this study were purchased
from Applied Nanostructured Solutions, LLC, and
Advanced Chemicals Supplier (ACS Material, LLC),
respectively. MWCNTs consist of aligned bundles
with average outer diameter of 10–20 nm and length
of 30–50 lm. According to the supplier’s specifica-
tion, the average particle thickness of graphene
nanoplatelets was 2 to 10 nm. The host polymer
matrix used in this study was medical-grade GUR
1020 UHMWPE procured from Celanese, USA, with
density of 0.935 g/cm3 and average molecular weight
of 3.6 9 106 g/mol.
Sample preparation
UHMWPE-based CNT–GNP nanocomposites were
prepared by solution mixing followed by compres-
sion molding technique as shown in Fig. 1. CNT and
GNP both together were added to ethanol solution,
and dispersion was achieved via 30 min of ultra-
sonication. The UHMWPE granules were also dis-
persed separately in the same solvent, and then, the
suspension of CNT/GNP was mixed with the solu-
tion of UHMWPE via magnetic stirring at a rate of
400 rpm and under 110 �C until ethanol completely
evaporated. During the stirring process, GNP and
CNT were coated on the UHMWPE granules. Finally,
the mixed powder of CNT/GNP/UHMWPE at dif-
ferent weight fraction was compressed between the
Teflon sheets with a hot press for 15 min, at 145 �C to
obtain thin composite films of approximately 1 mm
thickness. The obtained hybrid nanocomposites were
denoted as CNT0.1/GNPx/UHMWPE, where
x = 0.1–1.0 wt% of GNP.
Characterizations
The morphologies of CNT, GNP and the fracture
surfaces of CNT/GNP/UHMWPE nanocomposites
were analyzed by scanning electron microscopy
(SEM Quanta 250, UK). A high-resolution transmis-
sion microscopy (HRTEM) study of CNT and GNP
was also carried out using a Tecnai TF20, 200 kV
instrument. Raman spectra were obtained using
WITec confocal Raman spectrometer with 532-nm
argon ion laser. A spot size of between 1 and 2 lmwas obtained using X100 magnification objective lens
with a long working distance. Raman system was
calibrated using 520 cm-1 band of silicon, and spec-
tra were obtained using 10% laser power.
Differential scanning calorimetry (DSC) was per-
formed via high-temperature DSC 400, NETZSCH, to
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study the crystallinity and melting behavior of CNT/
GNP/UHMWPE nanocomposites. Each sample was
subjected to two heating cycles in a nitrogen atmo-
sphere (20 ml/min). The samples were scanned in
the first cycle by heating from room temperature to
200 �C at a rate of 10 �C/min, then held isothermally
at 200 �C for 3 min before cooling down to 20 �C and
then held isothermally for another 1 min to remove
all the thermal history before repeating the process in
the second heating cycle. The degree of crystallinity
obtained by DSC was then calculated by
X ¼ DHf=DH�f � 100, where DHf is the heat of fusion
of the composites obtained from DSC and DH�f is the
fusion enthalpy of 100% crystalline polymer (for
UHMWPE, DH�f = 289.6 J/g) [16].
Thermal gravimetric analysis (TGA) was carried
out to measure the amount of weight loss as a func-
tion of temperature. Material decomposition and
thermal stability were investigated by performing
TGA with high-temperature TGA NETZSCH instru-
ment. Samples were tested under nitrogen atmo-
sphere (20 ml/min) by heating from room
temperature to 700 �C at a heating rate of 10 �C/min.
The thermal diffusivity of CNT/GNP/UHMWPE
nanocomposites was determined via laser flash
thermal diffusivity technique using Flash Diffusivity
TA DXF-EM900 operated with xenon at room tem-
perature, 60, 90 and 110 �C. Thermal conductivity
was then calculated from the acquired thermal dif-
fusivity using the following equation:
k ¼ aqCp ð1Þ
where k is the thermal conductivity, a is the thermal
diffusivity, q is the density and Cp is the specific heat
of the composite at specific pressure.
The mechanical behavior of CNT/GNP/
UHMWPE nanocomposites was evaluated by Zwick
Roell universal testing machine as per ISO 527-3
standard. Tensile tests were conducted on the com-
posite films at a strain rate of 5 mm/min at room
temperature. The fracture surface of these composites
was observed by scanning electron microscopy to
investigate the morphological features.
The electrical resistivity of CNT/GNP/UHMWPE
nanocomposites was measured with the LakeShore
7607 instrument in a four-probe measurement con-
figuration. The 5 9 5 mm2 specimens were coated
with silver paste at each corner and dried for 12 h to
ensure a good contact between tungsten probes and
samples.
The piezoresistive behavior of the conductive
polymer composites under tensile loading was
Dissolved UHMWPE granules
Magne�c S�rring
Dispersed CNTs+GNPs
Mixed solu�on of UHMWPE and
CNTs+GNPs
Hot pressingComposite film
Evapora�on of Solvent
Mixed powder of UHMWPE and CNT+GNP
Figure 1 Schematic showing the fabrication procedure of CNT/GNP/UHMWPE nanocomposites.
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characterized by using a Tektronix DMM 4050 elec-
trometer in a four-wire resistance measurement con-
figuration along with a Zwick Roell universal
machine as per ISO 527-3 standard for tensile testing.
Two copper wire electrodes were glued to the spec-
imen with conductive epoxy (Chemtronics CW2400)
over the gage section keeping 15 mm distance
between electrodes. The piezoresistivity of the com-
posites was observed under tensile loading at a strain
rate of 5 mm/min.
Results and discussion
Morphological characterization
Surface morphological characterizations of CNT,
GNP and CNT/GNP/UHMWPE powder were car-
ried out using SEM and TEM techniques, and these
are discussed in supplementary information (Fig-
ure S1 and Figure S2).
Raman spectroscopy
Raman spectroscopy was used to analyze the struc-
tural and chemical properties of carbon-based nano-
fillers. The study showed that the D, G and 2D peaks
of both CNT and GNP are in the same range of
wavenumber (cm-1) and bear a lot of similarities as
depicted in Fig. 2a. The G mode represents the bond
stretching or vibration character of all pairs of sp2
carbon atoms which originate from a first-order
Raman scattering process [29, 30]. The G band resides
at 1577 and 1582 cm-1 for CNT and GNP, respec-
tively. The value of GNP, G band is slightly higher
than that of CNT meaning that GNP have a higher
energy of the sp2 bonds than CNT. This results in
pushing the vibrational frequency of the bonds and
band in the spectrum to a higher frequency. The D
and 2D bands arise from a second-order double res-
onant process. The 2D band or G0 band is resided at
2672 and 2706 cm-1 for CNT and GNP, respectively.
The position of the bands is different where peak
shift in GNP is a result of higher interactions between
graphene stacked layers. Lastly, D band sits at
1342 cm-1 for MWCNT, while it shows small inten-
sity at 1340 cm-1 for GNP. This D band is referred to
the disorder band or defect band which proceeds
from a hybridized vibrational mode correlated with
graphene edge. The more remarkable D band in CNT
is owing to their structure since multilayer configu-
ration could indicate more disorder in the structure.
Additionally, the spectra with different wt% of
GNP were obtained for CNT/GNP/UHMWPE
nanocomposite as illustrated in Fig. 2b. The peak
positions corresponding to UHMWPE powder are C–
C asymmetric mode, B1g (1060 cm-1) and symmetric
mode, A1g (1130 cm-1) [20]. In the present work, the
peak positions are shifted to 1065 and 1132 cm-1.
These peak positions are very sensitive to strains
present in the films where the shift to higher
wavenumbers corresponding to compressive strains
present in the composite films originated during the
compression molding process.
(a) (b)
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900
0.1CNT + 0.1GNP
0.1CNT + 0.3GNP
0.1CNT + 1.0GNP
0.1CNT + 0.5GNP
Ram
an In
tens
ity (a
.u.)
Raman Wavenumber (cm-1)
1065 1132 15781344
UHMWPE
0 500 1000 1500 2000 2500 3000 3500
765
770
775
780
785
790
795
CNT
GNP
2D (2672 cm-1)G (1579 cm-1)
D (1342 cm-1)
D (1340 cm-1)
Ram
an In
tens
ity (a
.u.)
Raman shift (cm-1)
2D (2706 cm-1)
G (1582 cm-1)
Figure 2 Raman spectra of a CNT and GNP (b) CNT/GNP/UHMWPE nanocomposites.
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Thermal properties
Differential scanning calorimetry (DSC)
Differential scanning calorimetry analysis was per-
formed to investigate the melting temperature (Tm)
and assess changes in the crystallization behavior of
the nanocomposite by observing the heat of fusion
(DHf) during the non-isothermal second heating
process. Melting temperature (Tm) and degree of
crystallinity of CNT/GNP/UHMWPE nanocompos-
ites are shown in Fig. 3a and presented in Table 1.
The melting temperature for all nanocomposites was
relatively similar to that of pure UHMWPE (141 �C)which reveals good dispersion of hybrid CNT–GNP
nanofillers in the composites. Thus, these nanofillers
did not act as impurities to affect the temperature
during crystallization.
The degree of crystallinity of CNT/GNP/
UHMWPE nanocomposites was calculated as dis-
cussed before. The result shows a clear trend where
increasing the GNP content increased the degree of
crystallinity for all composites (Fig. 3b). The maxi-
mum degree of crystallinity (42.5%) was achieved at
0.1 wt% CNT ? 1.0 wt% GNP with which repre-
sented an increase of 7% from the pure UHMWPE.
This means that the presence of both CNT and GNP
as hybrid nanofillers can enhance the degree of
crystallinity by acting as heterogeneous nucleation
agents that promote the nucleation process during
crystallization.
0.0 0.2 0.4 0.6 0.8 1.0
39.5
40.0
40.5
41.0
41.5
42.0
42.5
43.0
Crs
ytal
linity
(%)
GNP Concentration (wt%)80 100 120 140 160 180 200
0.1CNT + 0.1GNP
0.1CNT + 0.3GNP
0.1CNT + 0.5GNP
0.1CNT + 1.0GNP
Endo
ther
mal
hea
t flo
w (m
W/g
)
Temperature ( C)
UHMWPE
0 100 200 300 400 500 600 700
0
20
40
60
80
100
120
Wei
ght (
%)
Temperature ( C)
UHMWPE 0.1CNT + 0.1GNP 0.1CNT + 0.3GNP 0.1CNT + 0.5GNP 0.1CNT + 1.0GNP
300 320 340 360 3800.35
0.40
0.45
0.50
0.55
Ther
mal
Con
duct
ivity
(W/m
K)
Temperature (K)
0.1CNT + 0.1GNP 0.1CNT + 0.3GNP 0.1CNT + 0.5GNP 0.1CNT + 1.0GNP
(a) (b)
(c) (d)
Figure 3 a DSC second cycle heating scan. b Degree of crystallinity (%). c TGA results. d Thermal conductivity as a function of
temperature of CNT0.1/GNPx/UHMWPE nanocomposites.
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Thermal gravimetric analysis
The non-isothermal decomposition characteristics of
CNT/GNP/UHMWPE nanocomposites are shown in
Fig. 3c. T0.1, the onset degradation temperature that
denotes the initial temperature of weight loss, and
T0.5, the midpoint degradation temperature, are
extracted and presented in Table 2. The thermal
decomposition of CNT/GNP/UHMWPE nanocom-
posites occurred in a single stage from the tempera-
ture of 400 to 500 �C. As indicated in Table 2, the
onset degradation temperature increased with the
presence of nanofillers in the polymer matrix. The
increment for T0.1 is approximately 3–4% for all
nanocomposites. The midpoint degradation temper-
ature also slightly increased with the addition of
CNT–GNP fillers. Generally, better thermal stability
is expected because of good thermal properties of
both CNT and GNP. The high thermal stability has
been reported for SWCNTs and the temperature
stability is estimated up to 2800 �C in a vacuum and
750 �C in the air, while the thermal conductivity of
SWCNTs is approximately 3000 W/m K [14]. These
fillers can be a source to accelerate heat absorption to
the polymer matrix. Moreover, as explained by Bris-
coe et al. [31], the thermal degradation is easier if
molecules have high mobility at the interface. The
inclusion of CNT and GNP fillers can restrict the
motion of the molecular chain of UHMWPE and
therefore resulted in the better thermal stability of the
composite. For the hybrid CNT–GNP, both T0.1 and
T0.5 increased for all concentrations. Therefore, this
reveals the good distribution between CNT and GNP
in the polymer matrix. Additionally, the ash content
present in an only small amount would not signifi-
cantly affect the thermal stability of the system.
Thermal conductivity
Thermal conductivity is investigated to analyze the
ability of heat dissipation by the composites. The
laser flash method is a well-established non-steady-
state measurement technique for measuring the
thermal diffusivity. Thermal conductivity is then
calculated from Eq. 1. Generally, most of the ther-
moplastic polymers are known as insulators; there-
fore, thermal conductivity cannot be observed for the
neat UHMWPE. Referring to Fig. 3d, the thermal
conductivity of the composites depends on two
parameters: filler concentration and temperature. At
room temperature, the reported thermal conductivity
of pure UHMWPE is 0.41 W/m K. The nonlinear
improvement in thermal conductivity is attributed to
the synergistic effect of CNT–GNP hybrid nanofillers
(Fig. 3d) [32–35]. The maximum thermal conductivity
of CNT/GNP/UHMWPE nanocomposite is
0.504 W/m K at 0.1 wt% CNT ? 1.0 wt% GNP. With
further addition of GNP to the polymer matrix, the
more effective thermal conductive networks are
constructed due to contacts between CNT–GNP and
GNP–GNP and a higher degree of crystallinity of the
composites. Consequently, the corresponding ther-
mal conductivity is slightly increased.
Additionally, thermal conductivity significantly
depends on the temperature for every concentration
of CNT–GNP fillers. There were minimal changes in
Table 1 Melting temperature
and degree of crystallinity
(%Xc) of CNT/GNP/
UHMWPE hybrid
nanocomposites from DSC
Tm (�C) DHf (J/g) % Xc
UHMWPE 141.4 114.7 39.69
UHMWPE ? 0.1 wt% CNT ? 0.1 wt% GNP 141.1 118.6 41.04
UHMWPE ? 0.1 wt% CNT ? 0.3 wt% GNP 141.0 121.5 42.04
UHMWPE ? 0.1 wt% CNT ? 0.5 wt% GNP 141.5 121.7 42.11
UHMWPE ? 0.1 wt% CNT ? 1.0 wt% GNP 141.5 122.9 42.52
Table 2 TGA result for the
UHMWPE nanocomposites
with different hybrid CNT0.1/
GNPx concentration
T0.1 (�C) T0.5 (�C) Ash content (%)
UHMWPE 435.0 465.1 0.97
UHMWPE ? 0.1 wt% CNT ? 0.1 wt% GNP 447.8 466.5 0.31
UHMWPE ? 0.1 wt% CNT ? 0.3 wt% GNP 448.3 466.9 1.11
UHMWPE ? 0.1 wt% CNT ? 0.5 wt% GNP 453.3 467.5 2.19
UHMWPE ? 0.1 wt% CNT ? 1.0 wt% GNP 448.5 467.5 2.00
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thermal conductivity till 60 �C (333 K). However,
thermal conductivity drastically dropped beyond
60 �C (333 K) as shown in Fig. 3d. For the semicrys-
talline polymer, the structure is divided into two
regions: crystalline and amorphous phase. The crys-
talline phase, uniformly packed structure, is respon-
sible for the thermal conduction in the composites. As
the temperature is increased to their melting tem-
perature, the crystalline phase is reduced since the
structure of the composite tends to change from solid
to rubbery state so that the thermal conductive net-
work is damaged [36]. Thus, overall thermal con-
ductivity decreases at a higher temperature for all
concentrations.
Mechanical properties
Figure 4a shows representative stress–strain curves
of UHMWPE and its nanocomposites with different
wt% loading of CNT–GNP hybrid fillers. Figure 4
shows that the mechanical properties were enhanced
significantly for CNT/GNP/UHMWPE nanocom-
posites as compared to neat UHMWPE. Mechanical
properties such as elastic modulus, yield strength
and yield strain, of CNT/GNP/UHMWPE
nanocomposites were also evaluated to see the effect
of hybrid fillers on mechanical performance and are
shown in Fig. 4 and also presented in Table 3. As
shown in Fig. 4b, elastic modulus and yield strength
of UHMWPE with the addition of CNT–GNP hybrid
fillers were improved and followed similar trends.
The maximum improvement in elastic modulus of
these hybrid nanocomposites were found to be 37%
for 0.3 wt% of GNP loading along with 0.1 wt% of
CNT. Further increase in the wt% of GNP reduces the
elastic modulus, likely due to the effect of GNP
aggregation at higher loadings. Nonetheless, the
elastic modulus was increased relative to that of neat
UHMWPE matrix for all tested weight fractions, i.e.,
35 and 34% for 0.5 wt. and 1.0 wt% of GNP loadings,
respectively.
Yield strength also improved by 31, 33, 32 and 30%
for 0.1 wt% CNT with 0.1, 0.3, 0.5 and 1.0 wt% GNP,
respectively, and followed the same trend as the
elastic modulus. Even though elastic modulus and
yield strength were improved at every concentration
as compared to neat UHMWPE, at 0.1 wt%
CNT ? 0.5 wt% GNP and beyond, both values start
to deteriorate.
These variations in mechanical properties are
attributed to the microstructural changes in polymer
matrix. In the elastic regime, as GNP are randomly
dispersed in the polymer matrix, only portion of GNP
which is oriented parallel to the tensile axis is
responsible for the absorption of stress. With a higher
concentration of GNP, there is a higher possibility for
GNP to be oriented along the tensile axis. Thus, it can
sustain higher load transfer or support more stress
[37]. Additionally, increase in the degree of crys-
tallinity with the addition of fillers could also con-
tribute to better mechanical properties. In contrast to
the elastic regime, the plastic regime deformation is
dependent more on an effective load transfer
between the polymer matrix and fillers. GNP ori-
ented in non-axial directions also could affect defor-
mation in this case.
pureUHMWPE
0.1CNT +0.1GNP
0.1CNT +0.3GNP
0.1CNT +0.5GNP
0.1CNT +1.0GNP
550
600
650
700
750
800
850
E (M
Pa)
12
13
14
15
16
17
18
19
y (M
Pa)
0 1 2 3 4 50
10
20
30
40
Stre
ss (M
Pa)
Strain
UHMWPE 0.1CNT + 0.1GNP 0.1CNT + 0.3GNP 0.1CNT + 0.5GNP 0.1CNT + 1.0GNP
(a) (b)
Figure 4 Representative stress–strain curves (a) and elastic modulus and yield strength (b) of CNT/GNP/UHMWPE nanocomposites as a
function of CNT0.1–GNPx wt%.
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For the 0.1 wt% CNT ? 0.1 wt% GNP composite,
only CNT is present on the fracture surface, while
GNP are absent (Fig. 5a) because GNP are coated on
the surface of UHMWPE granules. Some CNT pull-
outs were also seen at higher magnification as shown
in Fig. 5b.
As shown in Fig. 5c, the fracture surface of
UHMWPE with 0.1 wt% CNT ? 1 wt% GNP shows
large clusters of GNP and pullout of CNT from the
surface. The agglomerations of both CNT and GNP at
higher concentration do not allow for uniform
embedment within the polymer matrix (Fig. 5d).
During the tensile loadings, poor bonding at the
interface would cause these fillers to be pulled out of
the surface and the fillers may act as stress
concentrators causing an ineffective load transfer.
Moreover, the presence of these clusters may allow
for easy sliding between the graphene particles under
shear forces. Hence, easy sliding at weak interfaces
could restrict the elongation of the composites,
resulting in small strain to failure (Fig. 4a).
Electrical properties
Electrical conductivity
UHMWPE is a thermoplastic polymer known to be
an insulator and has a very high volume resistivity,
reported to be more than 1015 X cm [38]. Poly-
ethylene shows insulating properties as all valence
Table 3 Mechanical properties of UHMWPE/CNT0.1–GNPx nanocomposites
GNP (wt%) Elastic modulus (MPa) (% change) Yield strength (MPa) (% change) Yield strain (%)
GNP0 590.9 ± 17.4 12.6 ± 1.2 2.14
GNP0.1 768.6 ± 12.4 (? 30.1) 16.5 ± 1.6 (? 30.7) 2.15
GNP0.3 811.6 ± 10.7 (? 37.3) 16.8 ± 1.1 (? 33.4) 2.07
GNP0.5 795.4 ± 30.3 (? 34.6) 16.6 ± 0.4 (? 31.6) 2.09
GNP1.0 794.2 ± 24.8 (? 34.3) 16.5 ± 0.8 (? 30.4) 2.07
(a) (b)
(c) (d)
CNT
GNP
CNT
CNT pull out
CNT pull out
0.1 wt.% CNT + 1.0 wt.% GNP 0.1 wt.% CNT + 1.0 wt.% GNP
0.1 wt.% CNT + 0.1 wt.% GNP 0.1 wt.% CNT + 0.1 wt.% GNPFigure 5 Fractured surface of
CNT0.1/GNPx/UHMWPE
nanocomposites with 0.1 wt%
GNP (a, b) and 1 wt% GNP
(c, d) at lower and higher
magnifications, respectively.
J Mater Sci
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electrons are used in covalent bond formation which
leads to a very large band gap between the valence
and the conduction band. As UHMWPE is reinforced
with conductive filler forming a composite material,
resistivity drops dramatically when the conductive
filler concentration exceeds a critical value called
percolation threshold, leading to a transition from a
non-conducting to conducting state [39]. The sudden
rise in electrical conductivity proceeds via two dif-
ferent mechanisms: (i) conductive path created
between conductive particles and (ii) electron tun-
neling effects. As stated in quantum mechanical
tunneling theory, electrical current can flow under
specific conditions through an insulator where a pair
of conductive filler is electrically connected by a
resistor formed by the matrix, so the electrons can
pass from one filler to the adjacent one [40, 41].
As discussed earlier, CNT content was fixed at
0.1 wt% and electrical conductivity was measured
with increasing wt% of GNP as shown in Fig. 6. The
use of hybrid CNT–GNP is expected to lower the
percolation threshold and increase the conductivity
of the composites since the presence of GNP is ben-
eficial in restricting CNT from getting attracted to
each other and forming aggregates [23].
The percolation threshold was achieved at 0.1 wt%
CNT ? 0.1 wt% GNP with the sudden rise of con-
ductivity from 10-15 S cm-1 for pure UHMWPE to
10-5 S cm-1. The electrical conductivity increased
continuously and reached a value of 1.6 10-4 S cm-1
with increasing wt% up to 1.0 wt% which is 9 orders
of magnitude higher than that of neat UHMWPE.
[42].
The conductivity of the composites in classical
percolation theory is given by the following equation:
r ¼ r0 u� ucð Þt; u�uc ð2Þ
where r is the conductivity of the composite, r0 is a
constant related to the intrinsic conductivity of con-
ductive filler, u is the weight percentage of filler, uc is
the weight percentage of filler at percolation thresh-
old and t is the critical exponent. The dimensionality
of the conductive system can be identified from the
critical exponent. In a two-dimensional system, t is
between 1 and 1.3, while in a three-dimensional
system, t is approximately 1.6–2.0 [43]. The critical
exponent t in the present study was estimated to be
1.20 which implies the presence of a two-dimensional
conductive network for these hybrid composite
systems.
Piezoresistivity
CNT/GNP/UHMWPE nanocomposites with differ-
ent loading of GNP and 0.1 wt% CNT were tested
under monotonic tension to analyze the relationship
between changes in electrical resistance with
mechanical strain. CNT/GNP/UHMWPE nanocom-
posite samples exhibit change in electrical resistance
up to a strain level of 400% as shown in Fig. 7a.
Figure 7b shows piezoresistive response of these
nanocomposites in linear elastic regime (up to 2.1%
strain), while Fig. 7c shows stronger piezoresistive
response in the inelastic regime (2.1–400% strain
level). Note that the uniaxial yield strain (ey) for the
tested nanocomposites is * 2.1% (as shown in
Table 3).
Under no load condition, the nanocomposites with
high GNP loading have more number of electrical
paths where a distinct electrical contact is established
since many GNP are in contact and CNT could easily
bridge adjacent two-dimensional graphene nanopla-
telets and form more efficient conductive system.
Geometry, orientation and concentration of nano-
fillers (GNP, CNT) are the main parameters which
influence the electrical resistance. Within elastic
regime where e\ 2:1%, the change in electrical
resistance with strain is linear for all concentrations
(Fig. 7b). However, in inelastic regime (e [ 2:1%),
the response exhibits a nonlinear characteristic
(Fig. 7c). During the initial loading (elastic regime),
0.0 0.2 0.4 0.6 0.8 1.010-16
10-14
10-12
10-10
10 -8
10 -6
10 -4
Con
duct
ivity
(Scm
-1)
GNP Concentration (wt%)
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0-4.5
-4.4
-4.3
-4.2
-4.1
-4.0
-3.9
-3.8
-3.7
log
(Con
duct
ivity
(Scm
-1))
log ( c)
0.1
Figure 6 Electrical conductivity of CNT0.1/GNPx/UHMWPE
nanocomposites.
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applied mechanical strain causes small changes in
electrical resistance due to good electrical contact
between nanofillers and large surface area of GNP.
With increase in loading to inelastic regime, both
GNP and CNT inter-particular distance increases
causing the separation of conductive paths and thus
0.000 0.005 0.010 0.015 0.020 0.025 0.030-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25 0.1CNT + 0.1GNP 0.1CNT + 0.3GNP 0.1CNT + 0.5GNP 0.1CNT + 1.0GNP
R/R
0
Strain
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400
500 0.1CNT + 0.1GNP 0.1CNT + 0.3GNP 0.1CNT + 0.5GNP 0.1CNT + 1.0GNP
R/R
0
Strain
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5-100
0100200300400500600700800900
1000110012001300
0.1CNT + 0.1GNP 0.1CNT + 0.3GNP 0.1CNT + 0.5GNP 0.1CNT + 1.0GNP
R/R
0
Strain
Linear elas�c regime
Inelas�c
regime
Linear elas�c
regime
Non-linear piezoresis�veresponse
Linear piezoresis�veresponse
Inelas�c regime Linear elas�c
regime
0.0 0.2 0.4 0.6 0.8 1.00
2
4
6
8
10
12
14
16
18
Sens
itivi
ty fa
ctor
GNP (wt. %)
0.0 0.2 0.4 0.6 0.8 1.00
1
2
3
4
5
6
Gau
ge fa
ctor
GNP (wt. %)
0.0 0.2 0.4 0.6 0.8 1.00
100
200
300
400
500
600
Sens
itivi
ty fa
ctor
GNP (wt. %)
Inelas�c regime I Inelas�c regime II
(a)
(d)
(e) (f)
(b)
(c)
Figure 7 a Piezoresistivity, b elastic regime, c plastic regime I, d gauge factor in elastic regime and plastic regime I, e gauge factor in
plastic regime II for CNT0.1/GNPx/UHMWPE nanocomposites.
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large change in resistance. When inter-particular
distance increases, the changes in piezoresistive
behavior are dominated by electron tunneling and
hopping mechanisms. The mean free path of electron
decreases with increasing load leading to a higher
scattering of electrons. Higher scattering of electrons
reduces the conductivity.
The sensitivity of the material to strain can be
determined by the gauge factor (k). The gauge factor
at any instant of stretching can be defined as
k ¼ 1
R0
dR
deð3Þ
where R0 is the initial resistance and dR/de is the rate
of change in resistance with respect to strain. For the
composite films undergoing uniaxial tensile strain, k
at any e such that 0\e\ey is given by
k ¼ 1þ 2mð Þ þ 1
q0
dqde
þ q� q0ð Þq0
1þ 2mð Þ ð4Þ
where dqde is the rate of change in resistivity at any e, q
is the resistivity at any e and q0 is the initial resis-
tivity. The sensitivity factor at any e such that
ey � e� ef is given by
k ¼ 1
q0
dqde
1þ eð Þ1þ eyy� �2 þ
q
1þ eyy� �2 � 2q
1þ eð Þ1þ eyy� �3
deyyde
" #
ð5Þ
where ef is the failure strain of the composite films
and eyy is the lateral strain. From Eq. (4), it can be
seen that the Poisson’s effect and the coupling
between mechanical deformation and resistivity and
its gradient become stronger at higher strains. From
Eqs. (3) and (4), we could also determine the resis-
tivity as a function of strain for different composite
samples using the change in resistance data obtained
from experiments. Gauge factor was calculated for all
nanocomposites in different regimes of strain (linear
elastic and inelastic regimes) to investigate the sen-
sitivity of these nanocomposites to the applied strain
(Fig. 7d, e and f). In linear elastic regime, the value of
gauge factor was determined to be 0.56, 0.58, 6.19 and
1.56 for 0.1, 0.3, 0.5 and 1.0 wt% of GNP (each cou-
pled with 0.1 wt% of CNT loading), respectively, and
is shown in Fig. 7d. Interestingly, the peak gauge
factor in the elastic regime occurred for 0.3 wt%
GNP, while for other GNP weight fractions gauge
factor was substantially lower.
In the inelastic regime, nanocomposites exhibited
linear and nonlinear trends of piezoresistive behav-
ior. In first inelastic regime (2.1–80% strain), a linear
trend of piezoresistivity was observed, with gauge
factor values of 3.54, 3.76, 15.93 and 13.45 corre-
sponding to GNP concentrations of (0.1, 0.3, 0.5 and
1.0 wt%) as shown in Fig. 7c, e. In the second
inelastic regime (80–400% strain), a nonlinear trend of
piezoresistivity was observed with gauge factor val-
ues of 390.57, 397.11, 557.44 and 523.01 corresponding
to GNP concentrations of (0.1, 0.3, 0.5 and 1.0 wt%)
and is shown in Fig. 7c, f. The large increase in gauge
factor between linear elastic and inelastic regimes
may be due to dominance of electron tunneling and
hopping mechanisms at higher strains giving raise to
nonlinear piezoresistive behavior of these
nanocomposites.
Conclusion
Conductive CNT/GNP/UHMWPE nanocomposites
were fabricated by solution mixing followed by
compression molding technique, and their mechani-
cal and piezoresistive properties were investigated.
The concentration of nanofillers was found to have a
critical impact on mechanical, thermal and electrical
properties of the nanocomposites. Compared with
neat UHMWPE, a remarkable increase in the elastic
modulus of about 37% was observed for the hybrid
nanocomposites with 0.1 wt% CNT and 0.3 wt%
GNP. Similar improvement was observed for yield
strength with a maximum improvement of 33% at the
same concentration. A very low percolation threshold
of 0.1 wt% CNT ? 0.1 wt% GNP was achieved with a
significant increase in the electrical conductivity of
about 9 orders of magnitude. These nanocomposites
also showed good piezoresistive response with a
gauge factor of 6.2 in linear elastic regime, for
0.1 wt% CNT ? 0.3 wt% GNP composite. Therefore,
these nanocomposites in linear elastic regime can be
useful for strain sensing applications. In inelastic
regimes, these nanocomposites provide high sensi-
tivity (gauge factor up to 557) for strains up to 400%.
However, the piezoelectric response was highly
nonlinear in the inelastic regime. Therefore, piezore-
sistance in this regime may be more useful for self-
sensing of damage. The findings of this study suggest
that the CNT/GNP/UHMWPE composite with GNP
content lower than 0.5 wt% coupled with 0.1 wt%
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CNT is suitable for strain and/or damage sensing
applications in artificial implants.
Acknowledgements
The authors would like to thank to Abu Dhabi Edu-
cation Council (ADEC) for providing the research
Grant (EX2016-000006) through ‘‘the ADEC Award
for Research Excellence (A2RE) 2015.’’
Electronic supplementary material: The online
version of this article (https://doi.org/10.1007/
s10853-018-2072-3) contains supplementary material,
which is available to authorized users.
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