leon3

Preparation, Characterization, and Physical Propertiesof Multiwall Carbon Nanotube/Elastomer Composites

Leon D. Perez,1,2 Manuel A. Zuluaga,1 Thein Kyu,3 James E. Mark,2 Betty L. Lopez11 Instituto de quımica Universidad de Antioquia, Apartado 12-26, Medellın, Colombia

2 Polymer Research Center, Department of Chemistry, The University of Cincinnati, Cincinnati,Ohio 45221-0172

3 Department of Polymer Engineering, University of Akron, Akron, Ohio 44325-0301

The present article describes preparation, characteri-zation, and physical properties of nanoparticles-filledcomposites consisting of multiwall carbon nanotubes(MWCNT) and styrene-butadiene rubber and nitrile-butadiene rubber. The reinforcing MWCNT fillers weresynthesized by chemical vapor deposition on iron andcobalt catalysts supported by calcium carbonate sub-strates. These MWCNT were further treated with nitricacid to produce hydroxyl and carbonyl functionalgroups on the carbon nanotubes (CNT) surfaces. Ofparticular importance is that these functionalized CNTswere found to exert profound influence on the elasto-meric matrices, particularly the vulcanization activationenergy, resistance to solvent swelling, enhanced glasstransition temperature, and improved storage and lossmoduli. POLYM. ENG. SCI., 49:866–874, 2009. ª 2009 Societyof Plastics Engineers

INTRODUCTION

In practical applications, elastomers have been invaria-

bly reinforced with mineral fillers such as silica and/or car-

bon black to improve their physical and mechanical proper-

ties. The extent of property improvement depends on sev-

eral factors, notably the size of particles, their aspect ratio,

and the strength of interactions between the filler and the

matrix polymer. The filler-matrix interactions are especially

crucial to improve the filler dispersion as well as the adhe-

sion of the matrix polymer to the filler surface, which in

turn increases the effective filler volume and also promotes

the stress transfer from the matrix to the filler when the

material is subjected to mechanical deformation [1].

Recent advances in nanotechnology have shown the

effectiveness of novel nano-sized particles in reinforcing

the polymeric materials, and thus nanoparticle-reinforced

composites have gained considerable interest, especially

when this goal can be achieved at unusually low filler

loadings. The typical reinforcing fillers include layered

silicate (clays), expanded graphite, polyhedral oligomeric

silsesquioxane (POSS), and single-wall and multiwall car-

bon nanotubes (MWCNT) [2–6]. Recently, the carbon

nanotubes (CNTs) have become increasingly attractive

because of the possible large scale production of these

materials. Moreover, MWCNT have several advantages in

industrial applications over other fillers because their pro-

duction is less complex and more cost effective due to

the requirement of the low loading to achieve comparable

composite properties [7]. In addition, syntheses routes

based on the chemical vapor deposition (CVD) processes

are currently well established that can produce high yields

of acceptable CNT products [8].

One of the challenges in dealing with the neat CNT is

its inherently weak interactions with the matrix polymers,

because the pristine CNTs are made-up of cylindrical

graphite layers. These graphites do not have the func-

tional groups required to promote filler-polymer interac-

tions to enhance the reinforcement effects of the nano-

particles in the CNT/rubber composites. However, CNTs

can be functionalized during the purification process by

exposing them to strong oxidizing acids to introduce

hydroxyls and carboxyl groups on the CNT surface. These

functional groups of the CNT promote the strong interac-

tions with the elastomer matrix such as electron donor–

acceptor interactions, particularly those containing oxy-

genated and nitrogenated functional groups interacting

with unsaturated carbon bonds of the elastomers.

Although it is possible to incorporate MWCNT into an

elastomer via solution blending [9], this approach is not

suitable for industrial practice due to its higher environ-

Correspondence to: Leon Perez; e-mail: [email protected]

Contract grant sponsor: National Science Foundation (Polymers Program,

Division of Materials Research); contract grant number: DMR-

0314760; contract grant sponsor: The University of Antioquia.

DOI 10.1002/pen.21247

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

mental and economic costs. Melt blending is the most

convenient and efficient technique for the preparation of

CNT reinforced elastomers, but there are only limited

studies reporting on the physical and mechanical pro-

perties of these composite materials produced by melt

processing.

In the present article, we begin with the synthesis of

CNTs by CVD using iron and cobalt catalysts supported

by calcium carbonate substrates, followed by the prepara-

tion, characterization, and properties of the nanoparticles-

filled composites using styrene-butadiene rubber (SBR)

and nitrile-butadiene rubber (NBR) as elastomeric matri-

ces. The emphasis is placed on the determination of vul-

canization behavior, morphology, thermal, mechanical,

and electrical properties.

EXPERIMENTAL

Carbon Nanotube Synthesis

The MWCNT were synthesized by CVD according to

the procedure described by Giraldo et al. [10] using the

cobalt (12 wt%) and iron (6 wt%) catalyst supported by

calcium carbonate. The catalyst was prepared by impreg-

nation of carbonate from an aqueous solution of the

metallic salts, viz., Co(NO3)2�6H2O and Fe(NO3)3�9H2O.

The chemical deposition reaction was carried out at

7508C for 30 min. Ethylene, the source of carbon, was

mixed with nitrogen at a 3:1 ratio of ethylene/nitrogen

under a constant nitrogen flow rate of 400 mL/min into

the reactor. The material thus obtained from the CVD

reaction was treated with the 30 wt% nitric acid solution

for 72 hr to remove the support and the metal, and also to

reduce the amorphous carbon by-products. This procedure

presumably caused some oxidation to the CNTs. The

resulting product was then thoroughly washed using dis-

tilled water and finally dried at 1258C for 24 hr.

Appropriate amounts of CNTs corresponding to 2, 5,

10, and 15 phr were compounded into the elastomers by

melt–mixing using a two-roll open mill. The elastomer

compounds were vulcanized with the aid of sulphur

according to the conventional vulcanization method; their

compositional recipes are shown in Table 1. The vulcaniza-

tion times for each composition were determined by differ-

ential scanning calorimetry (DSC) using a Q100 TA instru-

ments calorimeter under isothermal conditions at 1508Cduring 1 hr to ensure that the vulcanization was completed.

The degree of vulcanization a was calculated based on the

heat of vulcanization reaction in what follows,

a ¼ DHt

DH1(1)

where DHt and DH1 are the reaction enthalpy at time, tand the total enthalpy change for the completed reaction,

respectively. The samples were vulcanized at 1508C and

at 3000 psi using a compression molding to obtain thin

sheets (�0.8 mm thick).

The morphological features of the carbonaceous mate-

rials and their dispersion in the elastomeric composites

were characterized by scanning electron microscopy

(SEM) (Model XL30 FEI, Phillips), and the composites

were frozen in liquid nitrogen, fractured, and shadowed

with gold in vacuum. The cross sections were analyzed.

The carbon nanotube sample was dispersed in propanol, a

small droplet was put in the holder, and the solvent was

evaporated under vacuum.

Structural characteristics of the CNTs were determined

by TEM, the instrument utilized was Model CM20, Phil-

lips, and the CNTs samples were prepared in the same

way mentioned before for the SEM analysis.

The values of equilibrium swelling ratios, Qr were

determined using small rubber pieces (weighed 0.5 g),

which were placed in a steel mesh cage and immersed in

toluene for 72 hr. The swollen pieces were then blotted

and reweighed, and the values of Qr were calculated in

accordance with the following equation,

Qr ¼m=dr þ ðms � mÞ=dt

8:

9;

m=dr(2)

where m and ms are the rubber weights before and after

the swelling, respectively, and dr and dt are the densities

of the rubber and toluene, respectively. The activation

energies for the vulcanization were calculated based on

the Ozawa equations [11], using the temperature of the

vulcanization peak (Tmax) obtained at four heating rates,

viz., 5, 10, 15, and 208C/min.

In the first thermogravimetric analyses (TGA), thermal

stabilities of the CNTs were determined via weight loss

by ramping from room temperature to 8008C at 108C/min

in air. For the purpose of comparison, another sample was

annealed at 4808C for 1 hr and then analyzed by TGA

scanned at 108C/min in air. Similarly, the TGA scans of

the CNTs-filled rubbers were acquired by increasing tem-

perature from ambient to 7008C at a heating rate of 108C/

TABLE 1. Sample compositions in phr.

NX SX

SBR 0 100

NBR 100 0

CNT 0–15 0–15

Zinc oxide 5 5

Stearic acid 1 1

Agerite Resin Da 2 2

Sulphur 1.5 1.5

CBSb 2 2

ZDECc 0.8 0.8

a 2,2,4-trimethyl-1,2-hydroquinoline.b N-cyclohexylbenzothiazole-2-sulphenamide.c Zinc diethyl dithiocarbamate.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 867

min under N2 circulation. For all the analysis, a thermo-

gravimetric analyzer (Q500, TA) instruments was used.

Electrical conductivities were measured on the flat vul-

canized rubber sheets using two parallel flat electrodes

operated at various voltages ranging from 5 to 15 volts.

The sample thickness was �0.7 mm.

The dynamic mechanical experiments were conducted

from 2100 to 508C at a frequency of 1 Hz in tensile

mode using a DMA (Diamond, PerkinElmer). Heating

rate was 38C/min.

For the stress–strain measurements, the dumbbell-

shaped specimens were cut from the vulcanized rubber

sheets. Tensile tests were carried out using a tensile tester

(MTS TEST/5) at a strain rate of 50 mm/min; each

reported value corresponds to an average of at least three

measurements.

RESULTS AND DISCUSSION

Structural and Physical Characterization ofNeat MWCNT

Electron Microscopy. Figure 1A shows the SEM image

for the as-synthesized carbon nanotube materials, showing

the appearance of loose curly CNTs together with some

amorphous-like particles. These nanotubes seemingly have

a broad length distribution. The same CNT appearance

can be identified in the corresponding TEM image of

Figure 1B with some residual metal particles in the dark

background, which may be a consequence of low metal/

support interactions. The nanotube diameters range from

20 to 60 nm.

Thermogravimetric Analysis. The TGA and the deriv-

ative of thermogravimetric (DTG) thermograms of the

unannealed and annealed CNTs are depicted in Fig. 2.

The unannealed CNT manifested dual weight losses in

the DTG curve. The first DTG peak, located at 4428Ccorresponding to a loss of 22.3 wt%, may be attributed to

the degradation of the less stable amorphous carbon. The

second weight loss peak appearing at 5548C corresponds

to a loss of 64.2 wt% which is attributable to the CNTs

themselves. In contrast, the annealed nanotubes showed

only a single degradation peak (at 572.58C) in the DTG

curve, the increasing in the degradation temperature is

due to the elimination of amorphous carbon and func-

tional groups on the nanotubes.

Infrared Spectroscopic Characterization. Figure 3

exhibits the infrared spectrum for the CNTs in which the

characteristic band at 3432 cm21 corresponding to

hydroxyl groups and the bands at 1650 and 1730 cm21

corresponding to carbonyl groups suggest functionalized

CNT surface. These functional groups are presumably

FIG. 1. Electron micrographs of the carbon nanotubes. A: SEM showing curly CNT and some particles of

amorphous carbon, and B: TEM showing CNT with diameter from 20 to 60 nm.

FIG. 2. TGA and DTG for the decomposition of the nanotubes in air

as prepared and annealed in a previous stage.

868 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

formed due to the chemical treatment with the strongly

oxidizing nitric acid during the purification of CNTs.

Characterization of MWCNT/Elastomeric Composites

SEM Analysis. The SEM photographs for the NBR and

SBR composites containing 15 phr of CNTs are presented

in Fig. 4. The CNTs in both elastomeric composites are

seen to be randomly dispersed and well wetted by the

elastomers. However, some large CNT bundles are visible

in the zoomed part of the CNT/SBR composite (Fig. 3B),

protruding out from the rubbery matrix. It appears that

the CNT dispersion in the NBR composite is finer than

that in the CNT/SBR.

Determination of the Vulcanization Activation

Energy. In this study, the activation energy was calcu-

lated from the DSC experiments under dynamic condi-

tions, that is, vulcanization reactions were performed at

four different heating rates (b), 5, 10, 15, and 208C, and

the maximum temperature of the DSC thermogram at

each rate was collected and hereafter designated as Tmax.

The Kissinger model was utilized for the calculation of

the activation energy in what follows:

Kissinger’s equation reads

Ea ¼ �Rd lnðb=T2

maxÞdð1=TmaxÞ

(3)

Figure 5 shows the relative activation energies for the

NBR and SBR matrices reinforced with various amounts

of CNTs. The observed increasing trend in the activation

energy of vulcanization may be attributed to the adsorp-

tion of the accelerator compound utilized. Also, the

adsorption of polymer chains at the CNT filler surface

may reduce the mobilities of the rubbery chains, making

them less reactive and thus requiring more energy for the

vulcanization to trigger, therefore a complete vulcaniza-

tion is hard to come by.

The large increases in activation energy in N15 may

be explained by the very strong adsorption of vulcanizing

polar reagents because NBR is a polar polymer. The non-

polar additives such as the accelerators are less soluble in

polar matrix and thus easily absorbed by the oxidized

nanotubes. Among the experimental methods for deter-

mining the activation energy, torque vulcanization rheom-

etry and isothermal or/and nonisothermal DSC are the

most widely used techniques. The values obtained from

those techniques showed consistent trends, and the same

behavior was observed for nanoparticles-filled rubbers

[12–14].

Swelling Behavior. Table 2 shows the comparison of

the swelling behavior of the unfilled elastomers and the

CNT-reinforced elastomers. The reduction in the equilib-

rium swelling ratio Qr is a measure of the degree of adhe-

sion between the polymer chains and the filler particles.

The swelling is usually reduced in the case of adsorbing

filler surfaces when compared with the case of nonadsorb-

FIG. 3. Infrared spectra of the nanotubes treated with nitric acid, indi-

cating the presence of carbonyl and hydroxyl groups.

FIG. 4. SEM photographs for samples (A) N15 and (B) S15, showing randomly dispersed CNT and wetted

by the polymer.


ing fillers [15]. Table 2 shows the values of Qr and Qr/

Qr0, where Qr0 is the swelling ratio for the unfilled elasto-

mer. The swelling ratio for the unfilled SBR is larger than

for the corresponding NBR sample because the toluene is

a better solvent for the SBR as evidenced from the mag-

nitude of the respective Flory-Huggins interaction para-

meters [16, 17]. For both the elastomers, the ratio Qr/Qr0

decline with increasing CNT loading, suggestive of the

strong filler-elastomer interactions. However, a larger

reduction in Qr is seen in the CNT/SBR composites rela-

tive to the polar NBR. In view of the nitrile electron

acceptor groups in the NBR which can interact strongly

with the electron donating hydroxyl groups on the CNT

surface, a stronger interaction is anticipated between the

nanotubes and the NBR matrix. On the other hand, when

the elastomer chains interact more strongly with the filler,

a single macromolecular chain can cover sizable numbers

of active sites on the filler surface, and therefore, only a

smaller number of chains may be anchored at the surfaces

[18]. Presumably, unanchored NBR chains may be con-

tributing to the swelling of the elastomeric composite

network.

It should be pointed out that the swelling ratio of the

CNT/NBR is slightly reduced when compared with those

of the CNT/SBR composites in all the compositions

tested. This finding is consistent with the notion that the

NBR contains polar nitrile groups which strongly interact

with the hydroxyl functionalized CNT surfaces via the

electron donor–acceptor interaction. One cannot rule out

the fact that when the elastomer chains interact more

strongly with the filler, a single macromolecular chain can

cover sizable numbers of active sites on the filler surface

and therefore only a smaller number of chains can be

anchored at the surfaces [18]. It is likely unanchored

NBR chains may be contributing to the swelling of the

elastomeric composite network, but to a lesser extent with

increasing the CNT loading level.

Electrical Conductivity. Figure 6 shows the electrical

conductivity for the SBR and NBR nanocomposites as a

function of the nanotube concentration. The conductivity

of the CNT/NBR is orders of magnitude higher than that

of the CNT/SBR composites. With increasing the CNT

loading, the conductivity value increase for approximately

two orders of magnitude and levels off at around 10 phr

CNT, which will be regarded as the NBR the percolation

threshold. In the case of the SBR, there is no discernible

percolation threshold, except for a minor increase in the

conductivity with CNT loading.

Thermal Stability. The CNT/SBR composite showed

only a single drop in the weight loss curve with the

increasing temperature. The addition of nanotubes to the

matrix polymer raises the onset of degradation as well as

its maximum rate. In the case of the CNT/NBR compo-

sites, the unfilled elastomer as well as the composite con-

taining 2 and 5 phr of nanotubes revealed dual drops in

the weight loss curve, and the corresponding DTG ther-

mograms showed two degradation peaks. However, when

the CNT content is increased to 10 and 15 phr, only one

degradation peak can be discerned in these CNT/NBR

composites, as shown by the electrical conductivity test;

when the content of CNT is 10 phr, the percolation

threshold was reached. Probably, the established filler net-

FIG. 5. Relative activation energies for the SBR and NBR reinforced

with nanotubes, measured by dynamic DSC and obtained using Ozawa

equation.

TABLE 2. Equilibrium swelling ratios Qr for the NBR and SBR

elastomers reinforced with multiwall carbon nanotubes.

Composition

SBR NBR

Qr Qr/Qr0a Qr Qr/Qr0

0 3.21 1 2.2 1

2 2.8 0.87 2.05 0.93

5 2.59 0.81 2.12 0.96

10 2.4 0.75 2.04 0.93

15 2.11 0.66 1.61 0.73

a Qr0 is the swelling ratio for the unfilled elastomers.

FIG. 6. Electrical conductivities for the NBR and SBR elastomers rein-

forced with the nanotubes.


work can act as a barrier to the diffusion of gaseous sub-

products generated during the thermal degradation, which

in turn can shift the first degradation of matrix to higher

temperatures, such that both the degradations are over-

lapped. Table 3 summarizes the TGA results for the

CNT/SBR and CNT/NBR elastomeric composites at

different levels of the CNT loading. In both the SBR

and NBR elastomers, the percentage of residual weight

increased with increasing amount of the CNT.

The CNT filler’s ability to suppress thermal degrada-

tion of the elastomers may be attributed to barrier effects,

as is the case for clay fillers, or a reduction of the pyroly-

sis rate, due to the decrease of the polymer global mobil-

ity. In fact, it has been shown that polymer chains con-

fined in the mesoporous structures show greater thermal

stabilities [5].

Dynamic Mechanical Analysis and Differential Scan-

ning Calorimetry. Table 4 shows the Tg values as

determined by DSC in comparison with the E00 and tan dpeaks of the DMA measurement. The Tg value of DSC is

closer to that of E00 relative to tan d. The DSC results of

the CNT/NBR composites show only a slight increase in

the Tg, but there is no discernible movement in the Tg of

CNT/SBR composites. In contrast, the DMA results

reveal definite increase in Tg with increasing filler content

in the both composite, but this change was more pro-

nounced for the CNT/NBR composite.

This discrepancy in the Tg behavior between the two

techniques may be attributed to the different sensitivity of

each technique utilized. In principle, the glass transition

temperature Tg of a polymer is a manifestation of the

chain mobilities reflecting the glassy-rubbery transition

in the bulk. The addition of the filler particles affects the

polymer two ways. First, the filler particles tend to

restrict the polymer chain mobility through the filler-ma-

trix interaction. In view of the fact that the polar NBR

interacts more strongly with the functionalized CNT sur-

face relative to that of the nonpolar SBR/CNT, the Tg of

the CNT/NBR is expected to increase more drastically

with increasing filler content. Second, as discussed ear-

lier in the TGA studies, the thermal stability of the ma-

trix polymer can be enhanced by the addition of the min-

eral filler. Because the DSC technique is an indirect

measure of the Tg through the enthalpy change, the fill-

ers can suppress the heat flow and thus the DSC thermal

signal may be affected showing poor sensitivity to the

Tg change in the CNT/elastomeric composites with the

filler loading level.

An alternative account of the different Tg behavior in

the two composites is that the chain mobility depends on

the polymer cohesive forces, as advocated by Lipatov

[18]. In nonpolar elastomers, the cohesive forces are low

and therefore only minor changes in Tg are expected for

the CNT/SBR composite.

In addition, Table 4 shows the relative areas of the tan

d peaks, which are normalized by the elastomer content

in the composite. According to Arrighi et al. [19], the

normalized intensity of the Tg relaxation peak is found to

be inversely proportional to the bound rubber fraction.

The suppressed Tg peak is indicative the restrictive chain

mobility of the bound rubber fraction. It has been estab-

TABLE 3. TGA results for the SBR and NBR reinforced with multiwall carbon nanotubes under N2 atmosphere.

CNT (phr)

SBR NBR

First loss First loss Second loss

Maximum (8C) Residual (wt%) Maximum (8C) Residual (wt%) Maximum (8C) Residual (wt%)

0 447.4 5.0 423.9 64.9 445.7 9.2

2 450.0 6.4 424.1 64.1 446.5 12.0

5 451.3 8.8 425.8 61.7 425.8 11.8

10 454.8 12.9 443.5 17.7

15 455.1 16.3 452.7 20.6

TABLE 4. DMA and DSC results.

CNT (phr)

NBR SBR

DSC

DMA

DSC

DMA

E00 Tan d E00 Tan d

Tg (8C) Max. (MPa) Tg (8C) Max Rel. area Tg (8C) Tg (8C) Max. (MPa) Tg (8C) Max Rel. area Tg (8C)

0 220.8 16.1 223.9 1.58 1 214.2 247.4 54.7 250.6 1.64 1 241.6

2 220.5 17.1 220.0 1.43 0.90 212.2 247.4 51.7 249.5 1.63 0.92 240.5

5 220.3 24.7 220.8 1.39 0.89 213.7 246.9 47.7 249.9 1.53 0.89 241.6

10 219.7 28.1 218.2 1.11 0.83 212.7 246.1 60.2 248.4 1.24 0.80 240.0

15 219.1 31.9 218.5 0.98 0.78 211.8 247.0 66.2 247.6 1.12 0.76 239.2


lished that the conformational change is more restricted

for the chains in close contact with the filler surface. As a

result, a higher activation energy is required for those

chains to undergo over all motions and consequently,

these chains may not contribute significantly to the tan dpeak intensity. The areas under the Tg peaks are similar

for both elastomer matrices, but this does not necessarily

mean that the interactions of the two elastomers with the

nanotubes are the same.

Figures 7A and 8A compare the trends of tan d for

the CNT/SBR and CNT/NBR composites, as a function

of amount of CNT. A significant reduction of the tan dpeak intensity is clearly seen in the case of the nano-

composites. Figures 7B and 8B compare the values of

the storage modulus E0 for the SBR and NBR when

unfilled and when filled with the nanotubes. The filled

samples show higher storage and loss modulus at each

temperature. This could be due to the hydrodynamic

effect and the adsorption of polymer chains on the filler

surfaces that concurrently increases the filler effective

volume [20] (with reductions of the bulk mobility, as al-

ready mentioned).

To interpret the variation of the elastic moduli of poly-

meric composites with the CNT amount, the values of

elastic modulus are fitted to the Guth model [1] because

this model is the most relevant for this kind of composite,

specially for anisotropic filler. More detailed description

of this model can be found elsewhere [21, 22]. The Guth

equation reads,

E0

E00

¼ ð1þ 2:5ffþ 14:1f 2f2Þ (4)

where E0 and E00 are the elastic modulus of the unfilled

and filled polymer, and f and f are the filler volume frac-

tion and the aspect ratio, respectively. In the present

study, the Guth model (Eq. 4) was compared with the rel-

ative increase in the modulus as determined by DMA at

208C (see Fig. 9). The effect of the filler aspect ratio and

the volume fraction on the elastic modulus as well as the

FIG. 7. DMA curves for the composites SBR/carbon nanotubes, (A) tan d and (B) storage modulus.

FIG. 8. DMA curves for the composites NBR/carbon nanotubes, (A) tan d and (B) storage modulus.


interaction between the dispersed particles are considered

by the inclusion of the quadratic term into the origin

expression of the Einstein’s equation [23].

Values of the aspect ratio obtained by fitting the exper-

imental results to the Guth model are lower than

expected. This could be due to agglomeration that reduces

the effective anisotropy of the filler. A higher aspect ratio

was needed in the case of the CNT/NBR nanocomposites,

which probably is a result from the better filler dispersion

mentioned earlier. Moreover, the nitrile groups of the

NBR are good electron acceptors and therefore can inter-

act more strongly with the electron-donating hydroxyl

groups of the CNTs.

Tensile Tests. The effects of the nanotubes on the me-

chanical properties of the SBR and NBR are illustrated in

Fig. 10. The tensile strengths of the elastomers increase

gradually with increase in the amount of CNTs. When

compared with the unfilled elastomers, the stress at 100%

strain, the stress at rupture, and the tensile modulus of the

composite filled with 15 phr CNTs were increased by

262, 293, and 230%, respectively, for the CNT/SBR com-

posites. The corresponding values for the CNT/NBR ones

were 170, 192, and 229%, respectively. Such improve-

ments should attract greater interest in the application of

nanocomposite technology of the present kind.

FIG. 9. Elastic modulus measured from DMA results for the NBR and

SBR elastomers reinforced with the nanotubes, and fitted to the Guth

model.

FIG. 10. Tensile test results for SBR and NBR as a function of the nanotube concentration. A: Stress at

100% strain; B: Stress at rupture; and C: Tensile modulus.


CONCLUSIONS

Nitric acid treatment of the CNTs introduced the func-

tional groups on the CNT surfaces such as carboxyl and

hydroxyl groups. The nitrile groups of the NBR elasto-

meric chains interact strongly with these functional groups

of the CNT filler surface through the electron donor–

acceptor interaction. Such filler-matrix interaction showed

the improved properties such as resistance to solvent

swelling, enhanced glass transition temperature, and

improved storage and loss moduli of the CNT/NBR rela-

tive to those of the CNT/SBR composites. Although the

nanocomposites showed some improvements in the me-

chanical properties, the electrical conductivities showed

minor improvement in the CNT/NBR composite (i.e., two

orders of magnitude), but virtually no improvement in the

CNT/SBR composite, presumably because of structural

defects of the CNT fillers introduced, viz., oxidations dur-

ing the nitric acid treatment.

ACKNOWLEDGMENTS

It is a pleasure to acknowledge Colciencias (Colombia)

to the program ‘‘Apoyo a la comunidad cientıfica Nacio-

nal a traves de las becas para estudios doctorales.’’

REFERENCES

1. L. Bokobza, Polymer, 48, 4907 (2007).

2. B. Lopez, L. Perez, M. Mesa, L. Sierra, E. Devaux, M.

Camargo, C. Campagneand, and G. Giraud, ‘‘Effect of a

Mesoporous Silica and Silane Coupling Agents on the Rein-

forcing of Styrene Butadiene Rubber,’’ in Annual TechnicalConference—ANTEC, Conference Proceedings (2005).

3. J.E. Mark, Acc. Chem. Res., 39, 881 (2006).

4. X.W. Zhou, Y.F. Zhu, and J. Liang, Mater. Res. Bull., 42,456 (2007).

5. L.D. Perez, L.F. Giraldo, W. Brostow, and B. Lopez,

e-Polymers, 29 (2007).

6. L.D. Perez, L.F. Giraldo, B.L. Lopez, and M. Hess, Macro-mol. Symp., 245–246, 628 (2006).

7. M. Moniruzzaman and K.I. Winey, Macromolecules, 39,

5194 (2006).

8. C.E. Baddour and C. Briens, Int. J. Chem. React. Eng., 3, 1(2005).

9. D. Yue, Y. Liu, Z. Shen, and L. Zhang, J. Mater. Sci., 41,2541 (2006).

10. L. Giraldo, B. Lopez, and G. Barrera, ‘‘Thermal Stability of

Carbon Nanotubes Produced by Catalytic Chemical Vapor

Deposition of Ethylene on Different Supports,’’ in 2007NSTI Nanotechnology Conference and Trade Show—NSTINanotech 2007, Technical Proceedings (2007).

11. T. Ozawa, Polymer, 12, 150 (1971).

12. M.A. Kader and C. Nah, Polymer, 45, 2237 (2004).

13. G. Sui, W.H. Zhong, X.P. Yang, and Y.H. Yu, Mater. Sci.Eng. A, 485, 524 (2008).

14. W.H. Zhong, J. Li, C.M. Lukehart, and L.R. Xu, Polym.Compos., 26, 128 (2005).

15. L. Bokobza and M. Kolodziej, Polym. Int., 55, 1090 (2006).

16. K.N.A.-E.-N.S.L. Abd-El-Messieh, J. Appl. Polym. Sci., 88,1613 (2003).

17. J. Yang, M. Tian, Q.-X. Jia, J.-H. Shi, L.-Q. Zhang, S.-H. Lim,

Z.-Z. Yu, and Y.-W. Mai, Acta Mater., 55, 6372 (2007).

18. Y.S. Lipatov, Polymer Reinforcement, ChemTec Publishing,

Canada (1995).

19. V. Arrighi, I.J. McEwen, H. Qian, and M.B. Serrano Prieto,

Polymer, 44, 6259 (2003).

20. A.V. Shenoy, Rheology of Filled Polymer Systems, KluwerAcademic Publishers, London (1999).

21. M.D. Frogley, D. Ravich, and H.D. Wagner, Compos. Sci.Technol., 63, 1647 (2003).

22. Y.P. Wu, Q.-X. Jia, D.-S. Yu, and L.-Q. Zhang, Polym.Test., 23, 903 (2004).

23. H.S. Hsich, J. Mater. Sci., 17, 438 (1982).


leon3

Documents