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A novel approach to the chemical stabilization of gamma-irradiated ultrahigh molecular weight polyethylene using

arc-discharge multi-walled carbon nanotubes

P. Castell M. J. Martnez-Morlanes

P. J. Alonso M. T. Martinez J. A. Puertolas

Received: 18 March 2013/ Accepted: 16 May 2013 / Published online: 25 May 2013

Springer Science+Business Media New York 2013

Abstract A complete study was made of the stabilization

of gamma-irradiated ultrahigh molecular weight polyeth-ylene (UHMWPE) using arc-discharge multi-walled car-

bon nanotubes (MWCNTs) as inhibitors of the oxidative

process. MWCNTs were efficiently incorporated into the

polymer matrix by ball milling and thermo-compression

processes at concentrations up to 5 wt% and subsequently

gamma irradiated at 90 kGy. Raman spectroscopy dem-

onstrated the generation of radicals on the walls of the

MWCNTs and that the G/D ratio was altered by their

generation. The same spectra showed interactions between

the polymer chains as a series of shifts are observed in the

UHMWPE bands. The effect of the MWCNTs as inhibitors

for the oxidative process of the UHMWPE was evaluated

by means of Electron Spin Resonance (ESR) and Fourier

Transformed Infrared Spectroscopy (FTIR). ESR detection

of the radiation-induced radicals proved the radical scav-

enger behaviour of MWCNTs. FTIR measurements were

performed to ascertain the influence of the irradiation and

of the accelerated ageing protocol in the oxidation index of

the polymer and the composites. The results pointed to thepositive contribution of the MWCNTs in increasing the

oxidative stability of the composite when compared to pure

UHMWPE. A comparison is made between composites

obtained using MWCNTs produced by the carbon vapour

deposition and arc-discharge methods.

Introduction

Ultrahigh molecular weight polyethylene (UHMWPE) is a

polymer with high chemical inertness and excellent tribo-

logical and mechanical properties, such as low friction

coefficient, high wear resistance, suitable stiffness, tough-

ness and fatigue resistance, which have been exploited by

different industries. Moreover, owing to its biocompati-

bility, UHMWPE has been used as bearing material in total

joint replacements since 1962. Different gamma and elec-

tron beam irradiation processes have been applied in order

to provide good sterilization and to enhance the wear

resistance performance [1, 2]. Additionally, UHMWPE is a

highly hydrogenous material that offers effective radiation

shielding performance and therefore has potential appli-

cations in the aerospace industry or as radiation shielding

materials for personal protective equipment and electronic

devices [3].

To enhance these current or potential applications,

UHMWPE-based composites have been developed using

different reinforcing materials. An attempt was made in the

1970s to modify UHMWPE for commercial use by

blending powder with carbon fibres [4]. This composite,

commonly known as Poly II (Zimmer Inc., Indiana, USA),

was developed commercially and used in clinical applica-

tions until the 1980s. However, catastrophic short-term

P. Castell (&) M. T. Martinez

Department of Chemical Processes and Nanotechnology,

Instituto de Carboquimica ICB-CSIC, C/Miguel Luesma

Castan 4, 50018 Saragossa, Spain

e-mail: pere.castell@aitiip.com

P. Castell

AITIIP Technological Center, Polgono Industrial Empresarium

C/Romero 12, 50720 Saragossa, Spain

M. J. Martnez-Morlanes J. A. Puertolas

Department of Materials Science and Technology, Instituto de

Investigaciones en Ingeniera de Aragon-I3A, Universidad de

Zaragoza, 50018 Saragossa, Spain

P. J. Alonso J. A. Puertolas

Instituto de Ciencia de Materiales de Aragon, ICMA,

Universidad de Zaragoza-CSIC, 50009 Saragossa, Spain

123

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clinical failures associated with defects in consolidation led

to this composite being abandoned [5, 6]. On the other

hand, several radiation-resistant UHMWPE matrices for

cosmic, neutron, X-rays, electron or gamma radiation have

been used to develop composite materials filled with dif-

ferent reinforcing materials for aerospace applications. By

way of example, B4C and W nanoparticles have been used

as fillers in gamma radiation-protective UHMWPE com-posites [7], UHMWPE/Sm2O3 as an absorbing material to

shield against proton radiation [8], and UHMWPE fibre/

nano-epoxy for shielding against cosmic radiation [9].

Given their high strength and stiffness, unprecedented

length/diameter ratio and excellent thermal and electrical

conductivities, amongst other properties, carbon nanotubes

(CNTs) make successful candidates for reinforcing poly-

mers [10]. It is also worthy of note that CNTs have electron

affinities similar to those of fullerenes, which are often

called electronic sponges [11], showing great capacity to

accept and donate electrons. In fact, CNTs have a marked

ability to react with free radicals generated by radiation, asshown in several studies present in the literature which

support the antioxidant capacity of CNTs [12, 13]. In this

regard, Zeynalov et al. [14] performed a simulation of the

thermo-oxidative processes in polymer chains, which

showed an inhibition of oxidation when CNTs were

incorporated. Martinez-Morlanes et al. [15] also pointed

out that CVD multi-walled carbon nanotubes (MWCNTs)

behave as radical scavengers in UHMWPE, decreasing the

amount of radicals introduced by the c-irradiation to an

even greater extent than the action of other common radical

scavenger antioxidants introduced into UHMWPE. The

literature shows stabilization against thermal oxidation for

high density polyethylene composites containing CNTs

[16].

Nevertheless, since the CNT structure may vary

depending on the type or method of production, the prop-

erties of CNTs may differ significantly. Different produc-

tion methods are currently being used, such as arc

discharge [17], chemical vapour deposition (CVD) [18]

and laser ablation [19].

The arc-discharge method entails the striking of an arc

between graphite electrodes in a hydrogen or helium

atmosphere [20]. The MWCNTs produced in this way are

very straight and highly graphitized materials, whilst CVD

MWCNTs are more spaghetti-like and less graphitized.

Another significant difference between these two types of

MWCNTs is their metal content.

The aim of this work was to assess the influence of arc-

discharge MWCNTs on the properties of UHMWPE

composites when they are subjected to high doses of

gamma irradiation. The particular structure of these

MWCNTs is different from other carbonaceous materials.

First, we studied the effect of gamma irradiation that was

able to induce structural changes or defects on the surface

of the MWCNTs, in order to show their different capabil-

ities in acting as scavengers for the free radicals generated,

consequently inducing different oxidation resistance

behaviours to the composites. Second, we evaluated the

influence of arc-discharge MWCNTS on the thermal sta-

bility of these composites. Finally, a comparison was made

of the thermal and chemical stability of the new arc-dis-charge MWCNT composite with other composites based

on previously studied CVD MWCNTs.

The enhanced stability of the resulting composites filled

with arc-discharge MWCNTs opens a series of potential

applications for these additives as stabilizers for the oxi-

dative process of gamma-irradiated UHMWPE.

Experimental section

Materials

UHMWPE in powder form was supplied by Goodfellow

(Huntingdon, UK) with an average particle size of 150 lm,

a molecular weight between 3 9 106 and 6 9 106 g/mol

and no other additives.

MWCNTs were produced by the arc-discharge method.

These MWCNTs were straight, highly graphitized, of mi-

crometre length, between 20 and 30 nm in diameter and

were prepared in a purpose-built electric arc-discharge

apparatus under standard conditions as described elsewhere

[20]. Since the production of arc-discharge MWCNTs does

not require a catalyst, the resulting material has no metal

content, making it a suitable candidate to enhance the

thermal stability of composites.

Melt blending and irradiation

UHMWPE powder was blended with 0.5, 1, 3 and 5 wt%

of MWCNTs and the mixture was homogenized in a ball

mill for 2 h at 400 rpm. The powder was subsequently

compressed using a thermo-compression cell (Perkin

Elmer) for 15 min at 175 C under 10 MPa pressure, fol-

lowed by cooling in air down to 40 C under constant

pressure. The obtained composites were denoted as

UHMWPE (raw material) and MWCNT/UHMWPE (sam-

ples containing different amounts of MWCNTs). Several

UHMWPE and MWCNT/UHMWPE specimens (n = 3)

were gamma irradiated at 90 kGy and eventually at

150 kGy doses and denoted as UHMWPE-I and MWCNT/

UHMWPE-I. The gamma irradiation was performed using

a 60Co-irradiator (Aragogamma, Barcelona, Spain) and was

carried out in air atmosphere at room temperature, with the

average dose rate of 3 kGy/h. The samples were stored at

-20 C after irradiation until their characterization.

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Characterization techniques

To evaluate the possible effect of gamma irradiation on the

polymer structure, Raman spectra were obtained using a

HORIBA JobinYvon Raman spectrometer HR 800UV,

with a 532-nm laser equipped with an air-cooled CCD

detector.

Calorimetric studies were carried out in a TA InstrumentQ2000 thermal analyser under N2 at 10 C/min. Samples

weighing *7.0 mg were used in all the experiments. All

the dynamic scans (n = 3 for each material) were regis-

tered from 20 to 200 C. The melting point, Tm, was

measured as the maximum temperature of the endothermic

peak in a dynamic scan. The degree of crystallinity of the

composites was also calculated as the area under the

endothermic peak divided by the total enthalpy of melting

of a 100 % crystalline UHMWPE, which was taken to be

290 J/g [21]. An estimation of the lamellar thickness, Lc,

was obtained using the ThomsonGibbs equation [22].

To estimate the thermal stability of the composites,thermogravimetric experiments were conducted under air

atmosphere and, in the case of UHMWPE, some experi-

ments were performed under nitrogen atmosphere on a TA

Instruments thermobalance (accuracy: 10-4 mg) from

room temperature to 700 C at a heating rate of 10 C/min.

The samples weighed *7.0 mg; three specimens per

sample were tested.

To investigate the oxidative stability of UHMWPE,

sections were microtomed from all the materials (dimen-

sion of samples 10 9 2 9 0.15 mm3) and then subjected to

a severe oxidative ageing protocol. In the present study, we

used a non-standard method consisting of ageing under air

at 120 C for 36 h [23]. This accelerated ageing process

succeeded in promoting subsurface areas of high oxidation,

as reflected in the presence of white banding on fracture

surfaces of artificially aged UHMWPE tensile specimens.

The Fourier transformed infrared (FTIR) spectra were

recorded on a PerkinElmer model 1600 spectrometer (32

scans, 4 cm-1 resolution). The FTIR spectra of the powder

materials were obtained using pressed discs of the solid

powders combined with KBr. The oxidation index (OI) was

calculated as the area ratio of the carbonyl bands centred at

1718 cm-1 and the reference band at 1360 cm-1 following

ASTM F2102 guidelines.

ESR spectra were measured at room temperature in a

Bruker Elexsys E580 spectrometer working at X-band. The

microwave power was 0.2 mW and the modulation ampli-

tude 0.1 mT. Prism-shaped samples (2 9 2 9 10 mm3)

were fixed with vacuum grease to a methacrylate sample

holder. It was checked to ensure that the sample holder and

the vacuum grease do not give any detectable signal. At

least two different samples from the same batch were

measured. As MWCNT/UHMWPE samples could show

dielectric losses, which depend on the CNT content, the

concomitant modification in the cavity Q-factor was mon-

itored by the intensity change in the cubic Cr3? signal

observed in a small (2 9 2 9 0.5 mm3) nominally pure

MgO single crystal [24]. The intensity of the radiation-

induced radical spectra was corrected for Q-factor changes

and normalized to the sample weight. However, the actual

samples showed lower dielectric losses than MWCNTsmeasured in a previous study [13] and even pristine

MWCNT powdered samples could be measured. In this

case, powdered samples were introduced into a quartz tube

which had previously been checked to ensure it was free of

any radical signal.

Results and discussion

Effect of irradiation on UHMWPE structure. Raman

spectroscopy

Before the preparation of the composites, a complete

Raman spectroscopy characterization of both components,

the MWCNTs and the UHMWPE, used was conducted in

order to identify the bands related to both materials.

Figure 1a details the Raman spectra of the MWCNTs used.

The spectra shows the two characteristic lines, the tan-

gential band (G band) in the region of 1580 cm-1 and the

so-called disorder-induced band or D band at 1340 cm-1

[25] with a ratio of intensities between G and D band of

G/D = 3.92, which is a relatively high value and a clear

indication of the high graphitic structure of the MWCNTs

used.

The effect of the gamma irradiation on the structure of

the MWCNTs was measured at two different dosages, 90

and 150 kGy. Figure 1a shows the presence of the same

bands in the Raman spectra, although with a shift to higher

frequencies. The G/D relation was increased up to 4.84 at

the 90 kGy dose, indicating that the irradiation had a

graphitization effect, decreasing the number of defects in

the structure, whilst this index decreases down to 2.05 at

150 kGy due to the generation of more defects. Chen and

coauthors [26] reported an improvement in the graphiti-

zation of MWCNTs that were gamma irradiated up to

200 kGy. They observed a decrease in the interspacing of

the MWCNT layers owing to the formation of chemical

bonding between the different layers. In this study, we

measured a decrease in the G/D ratio at the highest dose,

which was probably due to the generation of defects that

damaged the CNT structure. The different nature of the

MWCNTs could be the explanation for this differential

behaviour. In order to avoid the generation of more defects

on the surface of the MWCNTs, we have only irradiated

the composites at 90 kGy.

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The non-irradiated and the gamma-irradiated composites

containing different loadings of MWCNTs were also stud-

ied by Raman spectroscopy and no significant differences

were noted between them. Figure 1b shows the comparison

of non-irradiated and gamma-irradiated MWCNT/

UHMWPE composites containing two different loadings of

MWCNTs. However, significant differences were observed

at the different MWCNT loadings. Figure 1c shows the

spectra of the MWCNT/UHMWPE composites containing

different loadings of MWCNTs. Owing to the resonantly

enhanced signal from the MWCNTs [27], the Raman

vibrational modes of the CNTs are visible even at the low

MWCNT loading. In all the composites, the D and G bands

from the MWCNTs increased their intensity as the CNT

content increased. On the other hand, the reduction in the

intensity of the UHMWPE bands is also noticeable and

more pronounced when the MWCNT content increased.

Moreover, there were slight differences on the tangential

bands of the MWCNTs in both the irradiated and non-

irradiated composites. A shift was observable in the G band

region in the composites containing different MWCNT

loadings. The G band shifts from 1580 cm-1 in pristine

MWCNTs towards 1583 cm-1 in the composites contain-

ing 5 wt% of MWCNTs. This shift is probably related to the

stiffening of the tangential vibrations of the G band because

of the polymer wrapping as it was previously reported for

other types of CNTs [15].

Thermal properties

Evaluation of crystallinity. DSC measurements

Different DSC scans were carried out in order to study the

influence of the MWCNTs on the thermal properties of the

UHMWPE composites. The values for melting point,

crystallinity and lamellar size of all composites are listed in

Table 1. With the incorporation of arc-discharge

MWCNTs, the melting temperatures remained constant

even up to 5 wt% MWCNT content and only a slight

Fig. 1 a Raman spectra of raw MWCNTs (solid line), 90 KGy

irradiated MWCNTs (dashed line) and 150 KGy gamma-irradiated

MWCNTs (dotted line), b Raman spectra of the composites

containing 1 and 3 wt% of MWCNTs (non-irradiated and 90 KGy

irradiated) and c Raman spectra of the 90 KGy gamma-irradiated

composites at different MWCNT loading (0, 0.5, 1.0, 3.0 and

5.0 wt%)

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increase in lamellar size was noticeable at the highest

MWCNT loading.

DSC data also showed that gamma irradiation increased

the melting temperature of the crystals in the neat

UHMWPE and the resulting crystallinity. This effect was

not observed when CVD MWCNTs were incorporated into

UHMWPE by Martinez-Morlanes et al. [15] in which casea slight decrease was shown in the melting temperatures of

the studied composites. In this study, surprisingly, the

crystallinity of the composites increased with MWCNT

content, from an initial crystallinity of 55 % up to 58 % in

the 5 wt% composite. This observation was in agreement

with previous studies by Kanagaraj et al. [28], where an

increase in the crystallinity of the UHMWPE was observed

due to incorporation of MWCNTs. The MWCNTs may act

as new nucleation sites, resulting in higher crystallinity

values. However, there are other studies that are not in

agreement with these findings and which observed a

decrease in crystallinity when UHMWPE was reinforced

with MWCNTs, such as those by Bakshi et al. [29] and

Martnez-Morlanes et al. [15]. The explanation of the

observed decrease was that this was mainly caused by the

friction between MWCNTs and the polymer chains that

hindered the mobility of the polymeric chains, resulting in

lower crystallinity. So, we can conclude that MWCNTs

produced by arc discharge may induce higher crystallinity

probably due to their more linear shape acting as nucleation

sites and resulting in higher crystallinity.

With regard to the effect of the irradiation, the irradiated

MWCNT/UHMWPE composites showed higher melting

points and higher lamellar sizes and crystallinity than the

non-irradiated ones. The molecular chain scission caused

by the irradiation and the higher mobility of the shortened

chains generated more perfect crystals and overall higher

crystallinity, an effect that has already been observed

previously by other authors [3032]. Although a similar

effect was observed [15] in previous MWCNT composites,

the increase noted in the melting temperature and the

crystallinity in this study was lower than in previously

described composites. Martnez-Morlanes et al. observed

an increase of up to 20 % in crystallinity in irradiated

MWCNT/UHMWPE composites, whilst only a slight

increase of 2 % was caused by the irradiation in the present

study. These differences are caused by the different

structures of the MWCNTs which have a high impact on

the properties of the composite. There are no previous

examples of UHMWPE reinforced with arc-dischargeMWCNTs.

Evaluation of thermal stability of the composites.

Thermogravimetric analysis

TGA analysis was used to monitor the thermal stability of

the prepared composites and to provide a comparison with

the thermal stability of the CVD MWCNT/UHMWPE

composites.

Figure 2a shows the TGA curves of the neat UHMWPE

under N2 and air atmospheres. It can be observed that the

highest thermal stability corresponds to the neat UHMWPE

curve registered under N2 atmosphere, as expected,

showing only a single degradation process approximately

at 460 C corresponding to the thermal degradation of the

polymer. The curve registered under air atmosphere

showed two main degradations for the UHMWPE, the first

corresponding to the oxidative degradation that occurs at

lower temperatures between 300 and 400 C and the sec-

ond corresponding to the thermal degradation at 440 C.

The TGA curve of the irradiated polymer, also plotted in

Fig. 2a, showed slightly higher thermal stability owing to

the cross-links generated during the gamma irradiation

process. Figure 2b shows the curves corresponding to the

non-irradiated and gamma-irradiated composites obtained

with the arc-discharge MWCNTs measured under air

atmosphere. In both materials, the oxidative stability is

similar to that observed in the neat polymer and thermal

degradation occurs at a higher temperature, even higher

than for UHMWPE-I. It should be mentioned that the

gamma-irradiated composite showed a slightly lower value

than the non-irradiated composite, but in both cases it was

Table 1 Melting temperature, crystallinity and lamellar size of the studied composites

Sample UHMWPE MWCNT/PE 0.5 wt% MWCNT/PE 1.0 wt% MWCNT/PE 3.0 wt% MWCNT/PE 5.0 wt%

Tm (C) 135.3 0.4 135.0 0.2 135.3 0.4 136.0 0.1 136.0 0.3

Cryst (%) 54.8 0.4 59.2 0.9 55.8 0.3 57.8 0.4 58.0 0.4

Lc (nm) 25.7 0.9 24.9 0.4 25.8 1.0 27.5 0.4 27.6 0.8

Sample UHMWPE-I MWCNT/PE 0.5 wt%-I MWCNT/PE 1.0 wt%-I MWCNT/PE 3.0 wt%-I MWCNT/PE 5.0 wt%-I

Tm (C) 140.1 0.4 137.2 0.2 137.3 0.4 137.7 1.2 137.9 0.6

Cryst (%) 57.9 0.9 60.4 0.2 59.3 0.6 60.5 0.9 60.4 0.6

Lc (nm) 47.8 3.6 31.4 0.7 32.0 1.5 33.7 5.3 34.2 2.8

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higher that of the neat irradiated polymer, a clear indication

of the effective influence of the MWCNTs. The values of

maximum decomposition rate and the residue at 600 C are

summarized in Table 2.

The study of the TGA curves of different arc-discharge

MWCNT/UHMWPE composites under air (see Fig. 2b)

denotes the stabilization effect of both MWCNTs and

gamma irradiation as mentioned previously. The presence

of only 0.5 wt% of MWCNTs increased the thermal sta-

bility of the composite from 398 to 435 C, and at the

highest MWCNT content, thermal stability was increased

up to 445 C.

The effect of irradiation on UHMWPE produces a cross-

linking which causes degradation to occur at higher tem-

peratures, from 396 to 444 C [33]. The effect of irradia-

tion produced an increase in the thermal stability of neat

UHMWPE, which is somehow comparable to the

enhancement achieved with the MWCNTs. The irradiation

of the composites had only a slight effect on their thermal

stabilities, since their maximum temperatures only

increased to 452 C at the highest loading. The negligible

effect in the present composites could be attributed to the

different interaction of the arc-discharge MWCNTs with

gamma irradiation, resulting in less cross-linking that

produces comparable thermal stability. However, it should

be mentioned that the overall thermal stability is enhanced

even at the lowest MWCNT loading, but the irradiation

produced no effect on their thermal stability.

Finally, the TGA curves of the non-irradiated and

gamma-irradiated composites containing 3 wt% of CVD

MWCNTs were studied and are plotted in Fig. 2b. The

thermogram showed lower temperatures for oxidative

degradation, and thermal degradation occurred in both

cases at lower temperatures than for the arc-discharge

MWNCT/UHMWPE composites, which pointed out the

worse thermal stability of the CVD MWCNT/UHMWPE

composites. The main difference between the CVD

MWCNT/UHMWPE composites and the arc-discharge

MWCNT/UHMWPE composites studied in the present

manuscript is the oxidative stability which was dramati-

cally higher in the latter case and can be attributed to the

absence of metals in the reinforcing material used. Thus,

the arc-discharge MWCNT/UHMWPE composites pre-

sented improved thermal stability performance in com-

parison with previous MWCNT/UHMWPE composites

[15]. The presence of small amounts of metals as impuri-

ties observed in some types of MWCNTs may induce some

thermal oxidation, thus resulting in lower thermal stability

in the final composites. The use of arc-discharge MWCNTs

has been proved very efficient in increasing the thermal

Fig. 2 a TGA curves of UHMWPE in N2 (solid line), UHMWPE in

air (dashed line), UHMWPE-I (dotted line), and b TGA curves in air

of 3 wt% arc-discharged MWCNT/UHMWPE (solid line), 3 wt%

arc-discharged MWCNT/UHMWPE-I (dashed line), 3 wt% CVD

MWCNT/UHMWPE (dotted line) and 3 wt% CVD MWCNT/

UHMWPE-I (dashed-dotted line)

Table 2 Temperature of maximum decomposition rate and final residue for UHMWPE and MWCNTs composites

Sample UHMWPE MWCNT/PE 0.5 wt% MWCNT/PE 1.0 wt% MWCNT/PE 3.0 wt% MWCNT/PE 5.0 wt%

Tmax (C) 396 8 435 19 446 13 446 13 445 6

Residue (%) 0.00 0.01 0.60 0.55 0.23 0.2 2.03 0.54 3.28 1.4

Sample UHMWPE-I MWCNT/PE 0.5 wt%-I MWCNT/PE 1.0 wt%-I MWCNT/PE 3.0 wt%-I MWCNT/PE 5.0 wt%-I

Tmax (C) 444 4 445 11 440 6 448 2 452 13

Residue (%) 0.01 0.01 0.62 0.62 1.01 0.64 2.60 0.98 3.99 0.52

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stability of UHMWPE, even at the lowest MWCNT load-

ing. The absence of metals is not only of interest in

increasing the thermal stability, given that oxidation reac-

tions are not catalysed, but also increases the biocompati-

bility of the composites. Thus, the use of arc-discharge

MWCNTs is very beneficial in comparison with the CVD

MWCNTs that were previously used to reinforce

UHMWPE with regard to biocompatibility and thermalstability.

Electron spin resonance measurements

The electron spin resonance (ESR) spectra of raw

MWCNT/UHMWPE samples with different MWCNT

loadings are shown in Fig. 3a. These results contrast with

those previously reported in MWCNT/UHMWPE com-

posites prepared with CVD MWCNTs where no ESR sig-

nal was observed before irradiation [13]. For comparison

purposes, the spectrum of a raw UHMWPE sample

(labelled as 0.0 %) has been also included. No signal wasdetected in MWCNT-free samples prior to irradiation.

However, a complex spectrum is observed in MWCNT/

UHMWPE samples, the intensity of which increases with a

higher MWCNT content.

This observation is similar to the results obtained for

pristine arc-discharge MWCNTs (Fig. 3b). This spectrum

could be simulated as the superposition of two signals

(labelled as C1 and C2) which are plotted by dotted lines in

Fig. 3b; the calculated spectrum as a superposition of both

contributions is given by the broken line. It is worth noting

that the C2 contribution (centred at g = 2. 011 0.002)

has a Lorentzian shape, whilst the contribution of C1

(centred at g = 2. 020 0.002) shows a dysonian char-

acter (A/B & 1.42 where as usual A and B denote,

respectively, the positive and negative lobe intensity)

which suggests that C1 is due to conductor particles of sizecomparable to the skin depth [34].

Similar spectra have previously been reported in differ-

ent MWCNT samples produced by arc discharge [3538].

In all those cases, the relative intensity of the two compo-

nents could be modified by either the production conditions

or further treatment. That clearly proves that the C1 and C2

signals are due to different paramagnetic entities. However,

there is some controversy about assignation to a specific

one. In the work by Yocomichi [38], the intensity ratio of

both signals is changed by thermal annealing and the C2

signal becomes undetectable when the annealing tempera-

ture is 700 C. This author then concluded that the C1signal is associated with MWCNTs, whereas C2 is due to

graphitic particles. The dysonian shape for the C1 signal

favours that assignation. As a similar spectrum is also

observed in MWCNT/UHMWPE prior to irradiation, we

conclude that the same distribution of carbon species

appears in the composite material as in pristine MWCNT.

Whilst no modification of the ESR spectrum of pristine

arc-discharge MWCNTs is detected after irradiation, some

Fig. 3 a X-band EPR spectra measured a RT of different MWCNT/

UHMWPE samples prior to irradiation, b continuous line gives the

X-band ESR spectra measuring a RT of a powder sample of

MWCNT. Dashed line corresponds to the calculated one as super-

position of C1 and C2 signals represented by dotted lines (see text),

and c X-band ESR spectra measured a RT of different MWCNT/

UHMWPE samples after 90 kGy gamma irradiation. The vertical

scale of all the traces is the same and the intensity of the spectra has

been corrected of the dielectric losses (see text) and normalized to the

sample mass. Microwave frequency 9.88 GHz, microwave power

0.2 mW, modulation amplitude 0.1 mT

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new signals are observed in the ESR spectrum of MWCNT/

UHMWPE-I (see Fig. 3c). In spite of the overlapping with

the C1 and C2 signals, particularly for the highest MWCNT

contents, the high field wing of this radiation-induced signal

can be clearly resolved. For comparison purposes, the

spectrum of UHMWPE-I is shown in the top of Fig. 3c. This

spectrum consists of a complex signal that could be tenta-

tively associated with the more stable polyallyl radical (stickmarked in Fig. 3c) and with oxygen-centred radicals

(labelled as S in Fig. 3c) [39, 40]. The similarity of the

radiation-induced signal in MWCNT/UHMWPE-I allows us

to conclude that the same defects are created during the

radiation process in the present case. This becomes clear in

the 0.5 % MWCNT/UHMWPE-I as long as the C1 and C2

signals are kept low enough. In that case, a comparison

between the UHMWPE-I and 0.5 % MWCNT/UHMWPE-I

spectra allows us to conclude that the presence of a low

concentration of MWCNTs noticeably reduced allyl radical

production (at least by a factor three). An increase in the

MWCNT content did not induce a further significantreduction in the allyl radical production. These last results

contrast with the observations found when CVD MWCNTs

are used for preparing MWCNT/UHMWPE composites [13]

where the radiation-induced allyl radical is more severely

reduced than in the present stage. However, a monotonous

decreasing of the radicals responsible for the S signal is

observed as the MWCNT concentration increases. Figure 4

shows the comparison of CVD MWCNTs vs arc MWCNTs

in UHMWPE composites. For this MWCNT concentration,

allyl production is more efficiently inhibited when arc-

discharge MWCNTs are incorporated to UHMWPE,

whereas the contrary occurs with the radical responsible for

the S signal. On the other hand, the reduction in radical

production as the MWCNT concentration increases is less

efficient when arc-discharge MWCNTs are used compared

to that observed if CVD MWCNTs are incorporated to

UHMWPE.

FTIR measurements. Measurement of oxidation

stability

Infrared spectroscopy was used to evaluate the possible

effect of gamma irradiation on CNT structure. The FTIR

spectra of MWCNT/UHMWPE-I showed no significant

differences with respect to raw MWCNTs (spectra not

shown). The spectra of both the as-received MWCNTs and

MWCNT/UHMWPE-I showed a band at 3444 cm-1

attributed to the presence of hydroxyl groups (OH) on the

surface of the MWCNTs, which are believed to result from

polar groups bonded to the MWCNTs or chemisorbedwater. The peak at around 1635 cm-1 can be attributed to

the stretching of the carbon nanotube backbone (C=C) [41].

The oxidative index (OI) of the neat polymer and the

different composites was calculated before and after the

ageing treatment, as detailed in the experimental section, in

order to study the effect of the incorporation of MWCNTs

on the oxidative stability of the UHMWPE. Thus, the OI of

the UHMWPE before and after the ageing protocol was

0.06 0.02 and 1.49 0.08, respectively. In other words,

an important increase in the OI was produced as a result of

the ageing due to the oxidative degradation. The irradiated

UHMWPE showed higher OI values (0.23 0.07) due to

the presence of radicals generated during the irradiation

process and the aged material showed an OI of

2.30 0.53. The OI of the composites was slightly higher

than that of raw UHMWPE, for example, the (MWCNT05/

UHMWPE) showed a value of 0.15 0.01, whilst the

irradiated composite (MWCNT05/UHMWPE)-I showed a

value of 0.64 0.10. This effect is probably caused by the

presence of the MWCNTs and their intrinsic contribution

to the OI measurements, since part of the C=C peak

appears in the frequency range associated with the carbonyl

band range at around 1718 cm-1 as was discussed in a

previous work [13]. However, the relevant finding is that

after the ageing protocol, the OI values of the (MWCNT05/

UHMWPE) increased up to 0.70 0.20 in the non-irra-

diated composite and up to 1.35 0.22 in the irradiated

one. Both values are significantly lower than the OI found

for the aged UHMWPE (1.49 and 2.30 for the non-irradi-

ated and irradiated material, respectively). The inhibitor

effect of the arc-discharge MWCNTs is similar to the effect

observed in CVD MWCNT composites [13], which

showed a lower OI for both irradiated and aged composites.

Fig. 4 X-band ESR spectra measured a RT of 0.5 % MWCNT/

UHMWPE-I. Upper trace arc-discharged MWCNT, lower trace CVD

MWCNT. Microwave frequency 9.88 GHz, microwave power

0.2 mW, modulation amplitude 0.1 mT

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This fact, together with the results obtained by ESR,

confirms the antioxidant activity of the MWCNTs in pre-

venting the oxidation of the polyethylene, which is bene-

ficial for its final properties.

Conclusions

We have successfully prepared UHMWPE composites con-

taining different loadings of arc-discharge MWCNTs by a

simple mixing process.The obtainedresultsdemonstrated that

a small loading of MWCNTs (0.5 wt%) can be efficiently

dispersed and integrated into UHMWPE, resulting in com-

posites with enhancedthermalstability. The Raman spectra of

the irradiated composites confirmed the modification of the

structure of the MWCNTs as the G/D ratio changes with the

irradiation dosage due to the reorganization of the defects on

thesurface of theMWCNTs.The generation of radicals on the

surface of theMWCNTs is beneficial for the final properties of

the irradiated composites as they react with the polymericchains enhancing their thermal stability and decreasing the

oxidation of the chains. Composites containing a low con-

centration of MWCNTs showed a noticeably reduction of the

allyl radical production in comparison with the neat

UHMWPE (at least by a factor three) demonstrating that they

can act as inhibitors of the oxidative process. Further incre-

ments in the MWCNT content did not induce significant

reduction in allyl radical production confirming that only

small amounts of MWCNTs are required to benefit from their

stabilization. The reduction of radicals in the irradiated com-

posites resulted in lower oxidation indexes than for the neat

irradiated polymer. This observation proves the radical

scavenger effect of the MWCNTs in the final composites.

Thus, we have demonstrated that low concentrations of arc-

discharge MWCNTs not only enhance the chemical stability

of gamma-irradiated UHMWPE but also improve other

properties. The obtained results open new technological

applications for these materials.

Acknowledgement The research was funded by the Comision

Interministerial de Ciencia y Tecnologa (CICYT), Spain, Project:

MAT 2010-16175.

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