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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: s.kumar@eng.oxon.org

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%.

J Mater Sci

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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.

J Mater Sci

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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.

References

[1] Kurtz SM (2009) UHMWPE biomaterials handbook. 2nd ed.

Academic Press, Boston, pp 1–6. https://doi.org/10.1016/

B978-0-12-374721-1.00001-8

[2] Bi S, Liu M, Shen J, Hu XM, Zhang L (2017) Ultrahigh self-

sensing performance of geopolymer nanocomposites via

unique interface engineering. ACS Appl Mater Interfaces

9(14):12851–12858. https://doi.org/10.1021/acsami.

7b00419

[3] Chung DDL (2007) Damage detection using self-sensing

concepts. Proc Inst Mech Eng Part G: J Aerosp Eng

221(4):509–520

[4] Chung D (2012) Carbon materials for structural self-sensing,

electromagnetic shielding and thermal interfacing. Carbon

50(9):3342–3353

[5] Gupta TK, Kumar S, Khan AZ, Varadarajan KM, Cantwell

WJ (2018) Self-sensing performance of MWCNT-low den-

sity polyethylene nanocomposites. Mater Res Express

5:015703. https://doi.org/10.1088/2053-1591/aa9f9e

[6] Liu H, Gao J, Huang W, Dai K, Zheng G, Liu C, Shen C,

Yan X, Guo J, Guo Z (2016) Electrically conductive strain

sensing polyurethane nanocomposites with synergistic car-

bon nanotubes and graphene bifillers. Nanoscale

8(26):12977–12989. https://doi.org/10.1039/c6nr02216b

[7] Dai H, Wong EW, Lieber CM (1996) Probing electrical

transport in nanomaterials: conductivity of individual carbon

nanotubes. Science 272(5261):523–526

[8] Treacy MJ, Ebbesen T, Gibson J (1996) Exceptionally high

Young’s modulus observed for individual carbon nanotubes.

Nature 381(6584):678–680

[9] Arif MF, Kumar S, Shah T (2016) Tunable morphology and

its influence on electrical, thermal and mechanical properties

of carbon nanostructure-buckypaper. Mater Des

101:236–244. https://doi.org/10.1016/j.matdes.2016.03.122

[10] Berber S, Kwon Y-K, Tomanek D (2000) Unusually high

thermal conductivity of carbon nanotubes. Phys Rev Lett

84(20):4613–4616

[11] Yu A, Ramesh P, Sun X, Bekyarova E, Itkis ME, Haddon RC

(2008) Enhanced thermal conductivity in a hybrid graphite

nanoplatelet—carbon nanotube filler for epoxy composites.

Adv Mater 20(24):4740–4744. https://doi.org/10.1002/adma.

200800401

[12] Danafar F, Fakhru’l-Razi A, Salleh MAM, Biak DRA (2009)

Fluidized bed catalytic chemical vapor deposition synthesis

of carbon nanotubes. Chem Eng J 155(1–2):37–48. https://

doi.org/10.1016/j.cej.2009.07.052

[13] Ranjith Kumar VSN, Kumar S, Pal G, Shah T (2016) High-

ampacity overhead power lines with carbon nanostructure-

epoxy composites. J Eng Mater Technol

138:041018–041026. https://doi.org/10.1115/1.4034095

[14] Kumar S, Pal G, Shah T (2017) High performance overhead

power lines with carbon nanostructures for transmission and

distribution of electricity from renewable sources. J Clean

Prod 145:180–187. https://doi.org/10.1016/j.jclepro.2017.

01.053

[15] Samad MA, Sinha SK (2011) Mechanical, thermal and tri-

bological characterization of a UHMWPE film reinforced

with carbon nanotubes coated on steel. Tribol Int

44(12):1932–1941. https://doi.org/10.1016/j.triboint.2011.

08.001

[16] Bakshi SR, Tercero JE, Agarwal A (2007) Synthesis and

characterization of multiwalled carbon nanotube reinforced

ultra high molecular weight polyethylene composite by

electrostatic spraying technique. Compos A Appl Sci Manuf

38(12):2493–2499. https://doi.org/10.1016/j.compositesa.

2007.08.004

[17] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y,

Dubonos SV, Grigorieva IV, Firsov AA (2004) Electric field

effect in atomically thin carbon films. Science

306(5696):666–669. https://doi.org/10.1126/science.

1102896

[18] Cui Y, Kundalwal SI, Kumar S (2016) Gas barrier perfor-

mance of graphene/polymer nanocomposites. Carbon

98:313–333. https://doi.org/10.1016/j.carbon.2015.11.018

[19] Gupta TK, Kumar S (2018) Fabrication of carbon nanotube/

polymer nanocomposites. Carbon nanotube-reinforced

polymers: from nanoscale to macroscale. Elsevier, Amster-

dam, pp 61–81

[20] Ruan SL, Gao P, Yang XG, Yu TX (2003) Toughening high

performance ultrahigh molecular weight polyethylene using

multiwalled carbon nanotubes. Polymer 44(19):5643–5654.

https://doi.org/10.1016/S0032-3861(03)00628-1

J Mater Sci

Author's personal copy

[21] Liu T, Eyler A, Zhong W-H (2016) Simultaneous improve-

ments in wear resistance and mechanical properties of

UHMWPE nanocomposite fabricated via a facile approach.

Mater Lett 177:17–20. https://doi.org/10.1016/j.matlet.2016.

04.072

[22] Gao J-F, Li Z-M, Q-j Meng, Yang Q (2008) CNTs/

UHMWPE composites with a two-dimensional conductive

network. Mater Lett 62(20):3530–3532. https://doi.org/10.

1016/j.matlet.2008.03.053

[23] Ren P-G, Di Y-Y, Zhang Q, Li L, Pang H, Li Z-M (2012)

Composites of ultrahigh-molecular-weight polyethylene with

graphene sheets and/or MWCNTs with segregated network

structure: preparation and properties. Macromol Mater Eng

297(5):437–443. https://doi.org/10.1002/mame.201100229

[24] Hu N, Masuda Z, Yan C, Yamamoto G, Fukunaga H,

Hashida T (2008) The electrical properties of polymer

nanocomposites with carbon nanotube fillers. Nanotechnol-

ogy 19:215701. https://doi.org/10.1088/0957-4484/19/21/

215701

[25] Eslami H, Behrouz M (2014) Molecular dynamics simula-

tion of a polyamide-66/carbon nanotube nanocomposite.

J Phys Chem C 118:9841–9851. https://doi.org/10.1021/

jp501672t

[26] Pal G, Kumar S (2016) Modeling of carbon nanotubes and

carbon nanotube–polymer composites. Prog Aerosp Sci

80:33–58. https://doi.org/10.1016/j.paerosci.2015.12.001

[27] Pal G, Kumar S (2016) Multiscale modeling of effective

electrical conductivity of short carbon fiber-carbon nan-

otube-polymer matrix hybrid composites. Mater Des

89:129–136. https://doi.org/10.1016/j.matdes.2015.09.105

[28] Pal G, Kumar S (2018) Mechanical properties of isolated

carbon nanotube. Carbon nanotube-reinforced polymers:

from nanoscale to macroscale. Elsevier, Amsterdam,

pp 173–199

[29] Kim K, Regan W, Geng B, Alemn B, Kessler BM, Wang F,

Crommie MF, Zettl A (2010) High-temperature stability of

suspended single-layer graphene. Phys Status Solidi (RRL)

Rapid Res Lett 4(11):302–304

[30] Dresselhaus MS, Dresselhaus G, Saito R, Jorio A (2005)

Raman spectroscopy of carbon nanotubes. Phys Rep

409(2):47–99. https://doi.org/10.1016/j.physrep.2004.10.006

[31] Briscoe BJ, Pelillo E, Sinha SK (1996) Scratch hardness and

deformation maps for polycarbonate and polyethylene.

Polym Eng Sci 36(24):2996–3005

[32] Pak SY, Kim HM, Kim SY, Youn JR (2012) Synergistic

improvement of thermal conductivity of thermoplastic

composites with mixed boron nitride and multi-walled car-

bon nanotube fillers. Carbon 50(13):4830–4838. https://doi.

org/10.1016/j.carbon.2012.06.009

[33] Zabihi Z, Araghi H (2016) Effect of functional groups on

thermal conductivity of graphene/paraffin nanocomposite.

Phys Lett A 380(45):3828–3831. https://doi.org/10.1016/j.

physleta.2016.09.028

[34] Zabihi Z, Araghi H (2017) Effective thermal conductivity of

carbon nanostructure based polyethylene nanocomposite:

influence of defected, doped, and hybrid filler. Int J Therm

Sci 120:185–189. https://doi.org/10.1016/j.ijthermalsci.

2017.06.010

[35] Yu A, Ramesh P, Sun X, Bekyarova E, Itkis ME, Haddon

RC (2008) Enhanced thermal conductivity in a hybrid gra-

phite nanoplatelet–carbon nanotube filler for epoxy com-

posites. Adv Mater 20:4740–4744. https://doi.org/10.1002/

adma.200800401

[36] Wunderlich B (1973) Thermal analysis of polymers. J Therm

Anal 5(1):117–136

[37] Lahiri D, Dua R, Zhang C, de Socarraz-Novoa I, Bhat A,

Ramaswamy S, Agarwal A (2012) Graphene nanoplatelet-

induced strengthening of ultrahigh molecular weight poly-

ethylene and biocompatibility in vitro. ACS Appl Mater

Interfaces 4(4):2234–2241. https://doi.org/10.1021/

am300244s

[38] Mironi-Harpaz I, Narkis M (2001) Thermoelectric behavior

(PTC) of carbon black-containing TPX/UHMWPE and TPX/

XL-UHMWPE blends. J Polym Sci Part B: Polym Phys

39(12):1415–1428. https://doi.org/10.1002/polb.1113

[39] Khanam PN, Ponnamma D, Al-Madeed MA (2015) Elec-

trical properties of graphene polymer nanocomposites. In:

Sadasivuni KK, Ponnamma D, Kim J, Thomas S (eds)

Graphene-based polymer nanocomposites in electronics.

Springer, pp 25–47. https://doi.org/10.1007/978-3-319-

13875-6_2

[40] Oskouyi ABSU, Mertiny P (2014) Tunneling conductivity

and piezoresistivity of composites containing randomly

dispersed conductive nano-platelets. Mater Chem Phys

7(4):2501–2521

[41] Zabihi Z, Araghi H (2016) Monte Carlo simulations of

effective electrical conductivity of graphene/poly(methyl

methacrylate) nanocomposite: landauer-Buttiker approach.

Synth Met 217:87–93. https://doi.org/10.1016/j.synthmet.

2016.03.024

[42] Grossiord N, Loos J, Regev O, Koning CE (2006) Toolbox

for dispersing carbon nanotubes into polymers to get con-

ductive nanocomposites. Chem Mater 18(5):1089–1099.

https://doi.org/10.1021/cm051881h

[43] Bauhofer WKJ (2009) A review and analysis of electrical

percolation in carbon nanotube polymer composites. Com-

pos Sci Technol 69(10):1486–1498

J Mater Sci

Author's personal copy