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Self-assembled natural rubber/multi-walled carbon nanotubecomposites using latex compounding techniques

Zheng Peng a,*, Chunfang Feng a,b, Yongyue Luo a, Yongzhen Li a, L.X. Kong c,**

a Chinese Agricultural Ministry Key Laboratory of Tropical Crop Product Processing, Agricultural Product Processing Research Institute,

Chinese Academy of Tropical Agricultural Sciences, Zhanjiang 524001, PR Chinab School of Materials, Hainan University, Haikou 571737, PR Chinac Centre for Material and Fiber Innovation, Institute for Technology Research Innovation, Deakin University, Geelong Vic 3217, Australia

A R T I C L E I N F O

Article history:

Received 9 December 2009

Accepted 10 August 2010

Available online 15 August 2010

A B S T R A C T

Functionalization of multi-walled carbon nanotubes (MWCNTs) plays an important role in

eliminating nanotube aggregation for reinforcing polymeric materials. We prepared a new

class of natural rubber (NR)/MWCNT composites by using latex compounding and self-

assembly technique. The MWCNTs were functionalized with mixed acids (H2SO4/

HNO3 = 3:1, volume ratio) and then assembled with poly (diallyldimethylammonium chlo-

ride) and latex particles. The Fourier transform infrared spectroscopy, transmission elec-

tron microscopy, and scanning electron microscopy were used to investigate the

assembling mechanism between latex particles and MWCNTs. It is found that MWCNTs

are homogenously dispersed in the natural rubber (NR) latex as individual nanotubes since

strong self-aggregation of MWCNTs has been greatly depressed with their surface function-

alization. The well-dispersed MWCNTs produce a remarkable increase in the tensile

strength of NR even when the amount of MWCNTs is only 1 wt.%. Dynamic mechanical

analysis shows that the glass transition temperature of composites is higher and the

inner-thermogenesis and thermal stability of NR/MWCNT composites are better, when

compared to those of the pure NR. The marked improvement in these properties is largely

due to the strong interfacial adhesion between the NR phase and MWCNTs. Functionaliza-

tion of MWCNTs represents a potentially powerful technology for significant reinforcement

of natural rubber materials.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Natural rubber (NR), one of the most important biosynthe-

sized polymers, has excellent chemical and physical proper-

ties, such as outstanding elasticity, flexibility, antivirus

permeation, and good formability and biodegradability [1]

and is widely used in various areas such as tyres, sport elas-

tomers, sealing materials and dairy rubber item [2,3]. The

raw NR latex is generally reinforced with carbon black [4],

ultra-fine calcium carbonate [5], modified montmorillonite

[6], silica [7] and starch [8] before being manufactured to prod-

ucts as the mechanical properties of raw NR including tensile

strength and tear resistance in most cases cannot meet the

requirements of applications. However, the reinforcement is

not so effective for natural rubber latex due to large dimen-

sion and agglomeration of these traditional reinforcing mate-

rials. Therefore, it is essential to exploit a new way to enhance

the mechanical properties of latex products. Several inorganic

nano-materials have already been studied and applied exten-

sively, such as nanosilica [9], nano-clay [10], nano-calcium

0008-6223/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2010.08.025

* Corresponding author: Fax: +86 759 222 1586.** Corresponding author.

E-mail addresses: zhengpeng8@yahoo.com (Z. Peng), lingxue.kong@deakin.edu.au (L.X. Kong).

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ava i lab le a t www.sc iencedi rec t . com

journal homepage: www.elsevier .com/ locate /carbon

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carbonate [11] and they are more effective to the reinforce-

ment of NR than those traditional fillers.

In recent years, carbon nanotubes (CNTs), as quasi

one-dimensional nano-materials, have become a potential

candidate for a wide range of application such as electronics,

composite fabrication and gas storage [12–14] since the publi-

cation of the landmark paper by Iijima [15]. Compared with

other nano-fillers, CNTs as an ideal reinforcing filler are ex-

pected to provide a better enhancement effect in polymer

composites, due to their inherent superior properties [16].

Consequently, CNTs have been widely exploited in different

polymers. Sekitani et al. [17] dispersed single-walled carbon

nanotubes as chemically stable dopants in a vinylidene

fluoride-hexafluoropropylene copolymer matrix to form a

composite film, by using an ionic liquid of 1-butyl-3-methyl-

imidazolium bis(trifluoromethanesulfonyl)imide. Yu et al.

[18] prepared conductive multi-walled carbon nanotube/poly-

styrene composites based on latex technology. Yet, little work

has been done in the field of latex reinforcement with carbon

nanotubes, because of their poor compatibility with NR phase.

From the standpoint of surface chemistry, the improvement of

CNT dispersion in the polymeric matrix by forming oxygen

functional groups on CNTs surface via chemical modification

plays an important role in reinforcing polymers [19–21].

In our previous work, we developed a novel process that

incorporates the latex compounding and self-assembly tech-

niques to prepare polyvinyl alcohol/silica composites [22],

and rubber/silica composites [23]. It was found that the chem-

ical and physical properties of these nanocomposites, com-

pared with the polymer host, were significantly enhanced.

In the present work, the surface of carbon nanotubes is mod-

ified with mixed acids and polyelectrolyte to improve the sol-

ubility of multi-walled carbon nanotubes (MWCNTs), leading

to the successful introduction of a novel self-assembly pro-

cess for the preparation of NR/MWCNT composites. The

self-assembly mechanism between MWCNTs and latex parti-

cles is studied and the impacts of MWCNTs on the morphol-

ogy and mechanical properties of the composites including

dynamic mechanical properties are also investigated.

2. Experimental

2.1. Materials

Latex with a total solid content of 60% was sourced from

Qianjin State Rubber Farm (Zhanjiang, PR China) and was

then pre-vulcanized. MWCNTs (average diameter: 10–30 nm;

average length: 5–15 lm; purity: P95%) was obtained from

the Tannamigang Co., (Shenzhen, PR China). The surfactant

used for dispersing the MWCNTs was sodium dodecyl sulfate

(SDS) (90%) provided by Merck Chemical Co., and poly (diallyl-

dimethylammonium chloride) (PDDA) (mol wt.% ca. 100,000–

200,000; 20 wt.% in water) was brought from Sigma–Aldrich

(Sigma–Aldrich, Louis, MO). All inorganic acids were AR grade.

2.2. Treatment of MWCNTs with mixed acid

A typical procedure was as follows: 1.08 g of MWCNTs were

added into a 250 mL flask charged with 20 mL of mixed acid

(H2SO4/HNO3 = 3:1, volume ratio) solution [24]. The mixture

was heated for 1.5 h and then diluted in a 1000 mL beaker

with 800 mL deionized water. After cooling to room tempera-

ture, the mixture was filtered through 200 nm pore diameter

membrane and washed with deionized water until neutral.

The resulted black solid was then dried under vacuum

freeze-dryer for 48 h.

2.3. Preparation of NR/MWCNT composites

The composite (NR/MWCNTs = 99:1 w/w) was prepared

according to the following procedures. 1.08 g acid treated

MWCNTs and 0.54 g SDS were dispersed in 100 mL water

and treated with an ultrasonic vibrator for 0.5 h [25], and its

pH was adjusted to 10 with 0.2 M KOH to obtain a negatively

charged MWCNT dispersion. 50 mL positively charged PDDA

solution (0.5 wt.%, pH 10) was dropped into the MWCNT dis-

persion, which was magnetically stirred for 0.5 h. Afterwards,

MWCNT/PDDA aqueous dispersion was then dropped into

180 g negatively charged latex (pre-vulcanized, pH 10) accom-

panying with gentle magnetic stir at room temperature for

24 h. Finally, the mixture was cast on glass plates and dried

at 50 �C to obtain NR/MWCNT composite films. The same pro-

cess was also employed to prepare the reference samples of

NR composites reinforced with untreated MWCNTs.

2.4. Characterizations

Surface chemistry of MWCNTs after oxidation was investi-

gated with a Perkin–Elmer Spectra GX-I Fourier transform

infrared spectroscopy (FTIR) (Perkin–Elmer, Fremont, CA) with

a resolution of 4 cm�1 in the transmission mode. The MWCNT

dispersion was dropped on a copper network for transmission

electron microscopy (TEM) observation with an accelerating

voltage of 100 kV (JEOL, Peabody, MA). Scanning electron

micrographs (SEM) of the composites were taken with a Phi-

lips XL30-EDAX instrument (Philips, Eindhoven, Netherlands)

at an acceleration voltage of 10 kV. The fracture surface was

obtained after the brittle failure of bulk sample in liquid

nitrogen.

The dynamic-mechanical thermal analysis (DMA) is taken

on rectangular specimens (10 · 4 · 0.1 mm) in tensile mode at

a frequency of 5 Hz using a Dynamic Mechanical Analyzer

(DMA242C, Netzsch). Tensile test experiments were con-

ducted on an Instron Series IX Automated Materials Testing

System (Instron, Acton, MA) at room temperature with a cross

head speed of 500 mm/min and the sample length between

the jaws was 25 mm, and the sample width was 4 mm.

3. Results and discussion

3.1. Surface functionalization and dispersion of MWCNTs

The water-solubility of MWCNTs dominates its dispersion in

latex. The pure MWCNTs (p-MWCNTs) are very stable and

their surface shows chemical-inert and hydrophobic proper-

ties unless being modified [26]. Therefore, we used mixing

acid system (H2SO4/HNO3 = 3:1, volume ratio) to modify

MWCNTs. An absorption band peaked at 1723 cm�1 is clearly

4498 C A R B O N 4 8 ( 2 0 1 0 ) 4 4 9 7 – 4 5 0 3

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present in the FTIR spectra (Fig. 1) for acid modified MWCNTs

(m-MWCNTs), a strong evidence suggesting the stretching

vibration of carbonyl group (–C@O) [27] while no band appears

in the spectra of p-MWCNTs in the region of 1700 cm�1. Com-

bining with a stronger band at about 3431 cm�1 assigned to

the stretching vibration of hydroxy group (–OH), we can draw

a conclusion that carboxylic acid group (–COOH) has been

successfully introduced onto the surface of MWCNTs

(Fig. 1). Such a modification on MWCNTs with acid and PDDA

leads to a very stable dispersion of the CNTs in the solution

after even more than 1 week (Fig. 2), while the p-MWCNTs

gradually precipitates within 1 h. Therefore, the MWCNTs

treated with mixed acid will lead to significant improvement

in solubility and chemical reactivity, which are essential for

MWCNTs to disperse in the latex as observed by Huang

et al. [28] on polymethyl methacrylate/acrylic acid.

The difference between p-MWCNTs and m-MWCNTs dis-

persion is also confirmed by TEM (Fig. 3). It is found that m-

MWCNTs are dispersed in water as a form of individual tubes,

no aggregation being observed (Fig. 3b), while the p-MWCNTs

aggregated badly (Fig. 3a). The improvement in water-solubil-

ity of m-MWCNTs is largely contributed to the introduction of

carboxyl groups onto the surface of MWCNTs and the interac-

tion between MWCNTs and PDDA. Zhang et al. [29] studied

the Poly(L-lactide)/multi-walled carbon nanotube composite

and observed that the modification of CNTs surface has im-

proved their water solubility and alleviated the aggregation

of CNTs.

In addition, surface modification of MWCNTs increases

the degree of their random orientation in the solution and

creates sections of irregular length (Fig. 3B). The shortening

in length of MWCNTs also can enhance the dispersion be-

cause shorter nanotubes may have less opportunity to twist

inter and intra MWCNTs.

3.2. The self-assembling mechanism between MWCNTsand latex particles

The self-assembly technique was first introduced by Decher

et al. in 1991 [30] and mainly used to fabricate layer-by-layer

composites. In the present study, self-assembly together with

latex compounding techniques is used to suppress the strong

self-aggregation generally encountered for p-MWCNTs and

enhance the interfacial adhesion between MWCNTs and latex

particles, to ultimately improve the properties and perfor-

mance of developed NR/MWCNT composites.

The process developed involves two assembly steps

(Fig. 4). Before the first assembly, the carboxylic group and hy-

droxy group were introduced onto the surface of MWCNTs

through acidic treatment. The introduction of hydrophilic

group greatly improves MWCNT water-solubility and makes

it possible for MWCNTs to be dispersed in water-based latex.

However, there is still a strong interaction between fillers and

matrix, and NR/MWCNT system could not form a stable

4000.0 3000 2000 1500 1000 400.0

Wavenumber (cm-1)

%T

A

B

1402.764 1723.502

3431.734

Fig. 1 – Infrared spectra of (A) p-MWCNTs and (B)

m-MWCNTs.

Fig. 2 – Photographs of MWCNTs dispersed in water: (A)

p-MWCNTs; (B) m-MWCNTs.

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dispersion to avoid aggregation. As the interaction between

non-polar NR and polar m-MWCNTs is relatively weak, this

may still lead to heavy self-aggregation of MWCNTs. There-

fore, the first assembly step is to assemble negatively charged

template MWCNTs with positively charged PDDA molecular

chain, using the electrostatic adsorption as driving force. As

the surface of MWCNTs is covered with PDDA, the interaction

between carbon nanotubes is greatly obstructed, and the self-

aggregation of MWCNTs is further depressed.

Due to a large difference in rigidity between MWCNTs and

PDDA and the charge density of PDDA being significantly

higher than that on the surface of MWCNTs, all PDDA charges

cannot form short-distance ion pairs with the surface charges

of rigid MWCNTs.

Therefore, the positive charge on PDDA cannot be com-

pletely neutralized by the negative charge on MWCNTs during

the first step assembly; the m-MWCNTs will maintain posi-

tive and be ready for the second assembly with negatively

charged latex particles. This is demonstrated from TEM obser-

vation where short MWCNTs are attracted onto the latex par-

ticles (presented as dark circle pies) like little worms (Fig. 5a);

MWCNTs with middle size are assembled around the latex

particles surfaces as circles (Fig. 5b); and the long MWCNTs

are individually dispersed, having various latex particles

linked (Fig. 5c). After the second assembly, the latex particles

and MWCNTs are strongly interacted and the MWCNTs are

uniformly dispersed in NR matrix after drying.

3.3. Morphology of NR/MWCNT composites

Generally, it is hard to individually disperse MWCNTs in poly-

mer matrix, due to strong aggregation of p-MWCNTs [31] and

high viscosity of the polymer. PDDA, as discussed in the above

section, was used as a bridge to connect MWCNTs and latex

Fig. 4 – The schematic of the self-assembly process.

Fig. 3 – TEM micrographs of MWCNTs dispersed in water: (A)p-MWCNTs; (B) m-MWCNTs.

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particles, aiming to alleviate the aggregation of MWCNTs and

enhance interfacial adhesion between MWCNTs and NR ma-

trix. Therefore the modification of MWCNTs with PDDA plays

a critical role in preparing composites.

As demonstrated from the fracture SEM morphology

images of NR/MWCNT composites, the p-MWCNTs agglomer-

ate and are not well distributed in NR (Fig. 6a), while the

m-MWCNTs are not presented as congregated but loose

nanotubes (Fig. 6b). In addition, some outcrops of the

MWCNTs exist on the fracture surface, indicating that the

interaction between the MWCNTs and the rubber macromol-

ecules is enhanced. When the self-assembly process is used,

the MWCNTs disperse in NR matrix as individual tubes

(Fig. 6c) and the interface between MWCNTs and NR seems

very smooth which indicates the excellent compatibility be-

tween MWCNTs and NR matrix.

3.4. Mechanical property

Natural rubber, as a typical elastomer, shows an excellent

flexibility. However, its insufficient mechanical properties

cannot meet the requirement of applications unless it is rein-

forced with inorganic fillers. The traditional inorganic fillers

Fig. 5 – TEM micrographs of different length m-MWCNTs

assembling with latex particles: (A) small size m-MWCNTs;

(B) middle size m-MWCNTs; (C) large size m-MWCNTs.Fig. 6 – SEM micrographs of NR/MWCNT composites: (A) p-

MWCNTs dispersed in NR matrix; (B) MWCNTs treated with

acid dispersed in NR matrix; (C) m-MWCNTs dispersed in

NR matrix using self-assembly technique.

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need to be added into NR with a large amount to gain suffi-

cient mechanical properties due to the relatively weak inter-

facial adhesion of the fillers with NR. For example, more

than 50% carbon black, in some cases, is needed before NR

is manufactured to product [32].

However, incorporating even a very small amount of

MWCNTs can markedly reinforce natural rubber. The self-

assembled NR with 1 wt.% MWCNTs receives an increase of

58% in tensile strength (from 19.0 to 30.1 Mpa), and a similar

increase in tensile modulus at different elongations (Table 1).

Interestingly, the flexibility of host NR is not affected by

the introduction of MWCNTs as its elongation at break even

receives a small increase even though the composite shows

a dramatic enhancement in rigidity. This is different from

other polymeric/inorganic composites where their valuable

flexibility is compromised although their rigidity usually in-

creases [33].

Without modifying the MWCNTs, natural rubber compos-

ite reinforced with p-MWCNTs exhibits less improvement in

its mechanical properties due to the aggregation of MWCNTs

(Table 1). Similar to our previous work [22,23], the improved

properties show a strong correlation with the morphology of

the composites. The better the MWCNTs disperse, the better

the properties of the composites. The theoretical modeling

studies on the mechanical properties of NR/MWCNT compos-

ites will be the subject of our future work to understand the

fundamental interactions between the carbon nanotubes

and the NR.

3.5. Dynamic thermal mechanical analysis

To further understand the reinforcement mechanism by

MWCNTs, DMA is conducted for the NR and NR/MWCNT

composites. NR possesses viscoelasticity, so hysteresis may

occur. When alternating stress is imposed, strain will lag a

phase angle. The lag is not only relevant to their chemical

structure, but also external conditions [34].

A typical glass transition turn is observed in the DMA

curves for NR and NR composites (Fig. 7). For composites,

the movement of NR molecular chains is restricted due to

strong interfacial adhesion between NR phase and MWCNTs.

Compared to that of the NR, the glass transition temperature

(Tg) of composites, corresponding to the peak temperature of

the curves, moves to a higher temperature. Particularly, the Tg

of composites which are prepared through self-assembly pro-

cess moves to �55 �C from �69 �C of the NR, while the Tg of

composites with p-MWCNTs remains almost the same as that

of the NR, because of the low interaction between NR and

aggregated MWCNTs. This has been observed for polyvinylal-

cohol (PVA)/silica (SiO2) nanocomposites where Tg shifted

from 65 �C (PVA) to 76 �C (PVA composite with 0.5 wt.% SiO2)

when strong interaction between PVA and SiO2 was present

[35].

Another interesting phenomenon is that the maximum

tan d of the composites is lower than that of the NR (Fig. 7).

In other words, the loss angle of NR is bigger than that of

the NR/MWCNT composites, suggesting that NR is able to ab-

sorb more energy than NR/MWCNT composites do under an

acute strain environment. The accumulation of absorbed en-

ergy will cause thermal degradation and further reduce the

mechanical property of materials. Therefore, the inner-ther-

mogenesis or thermal stability of prepared NR/MWCNTs with

smaller tan d is better than that of NR [35]. This is crucial for

the NR products to be used in distortional and rolling

conditions.

4. Conclusions

A self-assembly process combining latex compounding tech-

nique has been successfully used to develop NR/MWCNT

composites. After two steps of assembly, the strong self-

aggregation of MWCNTs has been greatly depressed and

MWCNTs are homogenously distributed throughout the NR

matrix as individual nanotubes, due to an excellent interfa-

cial adhesion of MWCNTs with NR phase. The introduction

of well dispersed acid modified MWCNTs in the NR matrix

contributes to a significant improvement in tensile strength,

tensile modulus at different elongations, and inner-thermo-

genesis or thermal stability for NR host while the flexibility

of NR matrix is still maintained. Compared to that of the pure

NR, the Tg of NR/MWCNT composites move to a higher

temperature.

Table 1 – Mechanical properties of NR and NR/MWCNTcomposites.

MWCNT loading(wt.%)

0 1(p-MWCNTs)

1(m-MWCNTs)

Tensile strength(Mpa)

19.0 (±1.2) 24.9 (±1.2) 30.0 (±1.2)

Tensile modulus(Mpa)100% Elongation 0.6 0.7 0.8300% Elongation 1.0 1.2 1.3500% Elongation 1.5 1.9 2.3Elongation atbreak (%)

930 932 962

Fig. 7 – The temperature dependent loss factor for NR and

NR/MWCNT composites: (A) NR; (B) p-MWCNT composite;

(C) m-MWCNT composite.

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Acknowledgements

The financial support from the Natural Science Foundation of

China (contract Grant number: 50763006) and Ministry of Sci-

ence and Technology R & D research institutes special fund

(contract Grant number: 2008EG134285) is gratefully

acknowledged.

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C A R B O N 4 8 ( 2 0 1 0 ) 4 4 9 7 – 4 5 0 3 4503