The influence of the matrix polarity on the morphology and properties of ethylene vinyl acetate copolymers–carbon nanotube nanocomposites

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 1659–1665 www.elsevier.com/locate/compscitech

The influence of the matrix polarity on the morphology and properties of ethylene vinyl acetate copolymers–carbon nanotube nanocomposites Sophie Peeterbroeck a, Laetitia Breugelmans a, Michae¨l Alexandre b, Janos BNagy c, Pascal Viville b, Roberto Lazzaroni b,d, Philippe Dubois a,b,* a

c

Service des Mate´riaux Polyme`res et Composites, Universite´ de Mons-Hainaut, 20, Place du Parc, B-7000 Mons, Belgium b Materia Nova ASBL, Avenue N. Copernic 1, B-7000 Mons, Belgium Laboratoire de Re´sonance Magne´tique Nucle´aire, Faculte´s Universitaires Notre-Dame de la Paix. 61, rue de Bruxelles, B-5000 Namur, Belgium d Service de Chimie des Mate´riaux Nouveaux, Universite´ de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium Received 9 January 2006; received in revised form 22 June 2006; accepted 2 July 2006 Available online 6 September 2006

Abstract In this study, nanocomposites based on three commercial ethylene vinyl acetate copolymers (EVA) and purified multi-walled carbon nanotubes (MWNTs) have been prepared via direct melt blending. The influence of the matrix polarity, related here to the relative content in vinyl acetate units, on the mechanical properties has been studied, by comparing the tensile properties of the nanocomposites to those of the unfilled matrices. The relative Young’s modulus of the nanocomposites is shown to slightly increase with the polarity of the matrix. Tapping mode atomic force microscopy has been used to study the state of dispersion of the MWNTs in the EVA matrices. The morphological results do not show a clear dependence between the polarity of the matrix and the dispersion state of the nanotubes: increasing the vinyl acetate content in the EVA matrix does not generate a much finer dispersion of the MWNTs. This was confirmed by the experimental determination of the relative Young’s modulus compared with the theoretical predictions. The increase, with the VA content, of the relative stiffness of the resulting materials is further explained by the determination of the mean aspect ratio of the dispersed particles, as evaluated using the Halpin–Tsai model.  2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanoparticle-reinforced composites; B. Mechanical properties; Atomic force microscopy

1. Introduction There is a high level of interest in using filler particles having at least one dimension in the nanometer range for producing polymeric nanocomposite materials with remarkably improved properties [1,2]. Since 1991, carbon nanotubes (CNTs) have been intensively studied for appli-

* Corresponding author. Address: Service des Mate´riaux Polyme`res et Composites, Universite´ de Mons-Hainaut, 20, Place du Parc, B-7000 MONS, Belgium. Tel.: +32 65 37 34 80; fax : +32 65 37 34 84. E-mail address: [email protected] (P. Dubois).

0266-3538/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.07.001

cations in polymer composites because of their superior mechanical properties [3]. There are two main types of CNTs that can have high structural perfection: single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). SWNTs consist of a single graphite sheet seamlessly wrapped into a cylindrical tube. MWNTs comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk. In terms of mechanical properties, CNTs are exceptionally strong (along their main longitudinal axis), meaning that they are characterized by a high Young’s modulus [4] and a high tensile strength while displaying high flexibility [5]. In the frame of the preparation

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of lightweight composite materials, CNTs further draw interest owing to their low density [6]. As a result of their reinforcing ability, CNTs proved efficient for fabricating nanocomposites with excellent tribological properties [7]. In order to achieve a significant improvement of the properties of the CNTs/composites, there are several key issues to be resolved, such as: (i) improving the dispersion of CNTs to optimize the surface of interaction between the filler and the matrix and (ii) tuning these interfacial interactions by modifying the chemical nature of the polymer matrix and its ability to wet the CNTs surface. In order to homogeneously disperse the CNTs throughout the polymer matrix, the CNTs entanglements, which occur during the synthesis, and the CNTs agglomerates, caused by the intermolecular van der Waals forces, must be disrupted. Indeed, MWNTs are generally entangled in the form of curved agglomerates and have therefore, a small effective specific surface area in interaction with the polymer matrix. Melt mixing in (very) high shear conditions, bulk polymerization, and sonication during the CNTs dispersion process represent the most cited techniques to overcome these agglomeration problems [8]. Therefore, an important parameter for producing nanocomposite materials with largely improved mechanical properties relies on the extent of dispersion of the individual CNTs. Unlike other families of nanocomposites such as clay or layered double hydroxide-based nanocomposites [9–12], for which the dispersion state is commonly detected by using X-ray diffraction (XRD) and transmission electron microscopy (TEM), the qualitative and quantitative characterization of CNTs dispersion is a difficult task since CNTs do not possess characteristic layerto-layer registry that could be observed by XRD. TEM analysis appears often inadequate to study the 3-dimensional distribution of fillers that are a few nanometers thick but several tens of microns long. Alternative techniques for morphological observation such as atomic force microscopy (AFM) may therefore be of interest. This study aims at evaluating the effect of the vinyl acetate (VA) content in ethylene vinyl acetate copolymers (EVA) on the dispersion state of carbon nanotubes and the mechanical (tensile) properties of the resulting nanocomposites. A direct microscopic approach by AFM has been chosen to observe cross-sections of the composite bulk and to assess the quality of the CNTs dispersion. The mechanical properties of the CNTs-based nanocomposites have been correlated to their morphology via the Halpin–Tsai model [13,14], which allows estimating the mean aspect ratio of the dispersed nanoparticles in the polymer matrices. 2. Experimental 2.1. Materials Three commercial ethylene vinyl acetate copolymers (Exxon), Escorene UL00112 (EVA 12), with 12 wt% in

VA and a melt flow index (190 C/2.16 kg) of 0.5 g/ 10 min, Escorene UL00119 (EVA 19), with 19 wt% in VA and a melt flow index (190 C/2.16 kg) of 0.65 g/10 min, and Escorene UL00328 (EVA 27), with 27 wt% in VA and a melt flow index (190 C/2.16 kg) of 3 g/10 min, were chosen as the matrices. High density polyethylene (HDPE, Dow) with a melt flow index (190 C/2.16 kg) of 1.1 g/10 min and polyvinylacetate (PVAc, Aldrich) with Mw = 12,800 were used for the sake of comparison. The multi-walled nanotubes (MWNTs) used in this work were produced at the nuclear magnetic resonance laboratory (FUNDP, Belgium) by catalytic decomposition of acetylene on transition metal particles (Co, Fe) supported on Al2O3 [15]. The catalyst contains 2.5 wt% cobalt and 2.5 wt% iron supported on alumina. The synthesis was carried out in a fixed-bed flow reactor at 700 C for a reaction time of 60 min. Purified MWNTs were obtained after dissolution of the support in boiling concentrated sodium hydroxide water solution and dissolution of the catalysts in concentrated hydrochloric acid water solution. The MWNTs are characterized by an average inner diameter of 5 nm and an average outer diameter of 15 nm, corresponding to ca. 14–15 concentric layers. Their average length is ca. 15 lm. 2.2. Preparation EVA–carbon nanotube composites and PVAc–carbon nanotube composites were prepared in a Brabender internal mixer at 140 C, for 12 min with a speed of 45 rpm to obtain materials filled with 3 wt% of MWNTs. For HDPE–carbon nanotube composites, the temperature of the mixer was set at 190 C. 2.3. Characterization Tensile properties were measured at 20 C on a Lloyd LR 10 K tensile tester following the ASTM D638 method with dumbdell-shaped specimens obtained from compression molded samples (type V). All tensile data are obtained from the average of five independent measurements; the relative errors are reported as well. Differential scanning calorimetry (DSC) measurements were performed on a TA instrument Q100 apparatus under N2 atmosphere at a heating rate of 20 K/min between 70 and 125 C. Test specimens for AFM studies were prepared with a LEICA cryomicrotome equipped with a diamond knife and maintained at 80 C. The AFM images were recorded in tapping mode (TM) in ambient atmosphere at room temperature with a nanoscope IIIa microscope (Veeco Inst., Santa Barbara, CA). The probes were commercially available silicon tips, with a spring constant of 24–52 N/m, a resonance frequency in the 264–339 kHz range, and a typical radius of curvature in the 10–15 nm range. Images obtained in phase detection were recorded

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with the highest sampling resolution available, i.e., 512 · 512 data points.

a

30 EVA 12 EVA 19 EVA 28

25

3. Results and discussion Stress (MPa)

20 15 10 5 0 0

b

Stress (MPa)

In order to compare the effect of the polarity of the EVA matrix on the extent of dispersion and distribution of CNTs and the mechanical properties of the prepared nanocomposites, 3 wt% of MWNTs have been melt blended with three matrices containing 12, 19, and 27 wt% VA units, respectively, within an internal mixer at 140 C for 12 min. The torque values obtained during melt blending are compared in Fig. 1, for both the unfilled matrices and the MWNTs-based composites. These values correspond to the stabilized torques, reflecting the intrinsic viscosity of the material reached after ca. 10 min of mixing. It can be observed that the three composites are all characterized by an increase in melt viscosity as compared to their respective unfilled matrices. Regardless of the presence of the nanotubes, the torque values measured for the EVA 12- and EVA 19-based materials are always higher than for the EVA 27 matrix, which can be explained by the lower inherent melt indices shown by EVA 12 and EVA 19, i.e., the polymers containing the lowest contents in VA units. The dispersion of purified MWNTs within the three EVA matrices leads to the formation of uniformly black samples, even at this low filler content. The MWNTs dispersion and distribution throughout the EVA matrices can thus be considered as macroscopically homogeneous.

24 22 20 18 16 14 12 10 8 6 4 2 0

200

400

600 800 1000 1200 1400 Strain (%)

EVA 12 + 3wt% MWNTs EVA 19 + 3wt% MWNTs EVA 28 + 3wt% MWNTs

0

200

400

600

800

1000

Strain (%)

3.1. Tensile properties The mechanical properties of the nanocomposites and the unfilled matrices have been evaluated by tensile testing. Stress–strain curves are similar in shape before and after adding fillers (Fig. 2a and b), indicating that the presence

20 Unfilled Matrix Composite

18 16 14 Torque (Nm)

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12 10

Fig. 2. Stress–strain curves of (a) unfilled EVA matrices and (b) EVA/ MWNTs composites.

of the nanotubes does not modify the overall mechanical behavior of the matrices. However, all the composites show an increase in stiffness, characterized by the Young’s modulus, which even reaches a twofold increase in the case of EVA 27 (Table 1). When considering the relative Young’s modulus of the three composites, i.e., the ratio between the composite modulus and the modulus recorded for the respective unfilled matrix (Fig. 3), one can observe that increasing the VA content in the matrix triggers a slight Table 1 Tensile properties of EVA and EVA-based composites filled with 3 wt% MWNTs

8 6

Sample

Stress at break (MPa)

Strain at break (%)

Young’s modulus (MPa)

EVA EVA EVA EVA EVA EVA

21.9 ± 1.6 13.6 ± 1.6 27.8 ± 1.2 17.9 ± 0.8 28.1 ± 0.8 19.4 ± 0.7

760 ± 120 350 ± 90 1150 ± 40 590 ± 30 1330 ± 30 890 ± 30

80.6 ± 5.0 117.2 ± 13.1 28.3 ± 1.5 44.9 ± 2.5 11.7 ± 0.5 24.3 ± 0.9

4 2 0 EVA 12

EVA 19

EVA 28

Fig. 1. Stabilized torque values measured during melt processing, for the EVA matrices either unfilled or added with 3 wt% MWNTs: effect of vinyl acetate content in EVA.

12 12 + 3 wt% MWNTs 19 19 + 3 wt% MWNTs 27 27 + 3 wt% MWNTs

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Fig. 3. Evolution of the relative tensile Young’s modulus (Erelative) with the VA content.

beneficial effect on the relative Young’s modulus. Since the DSC analyses do not reveal any significant modification in the thermograms of the EVA matrices upon MWNTs addition (Fig. 4), such increase in relative modulus with the VA content is not due to factors dealing with the degree of interaction between the matrix and the filler or to variations in the crystallinity of the matrix. The stress and strain at break decrease in all cases after addition of 3 wt% CNTs (Table 1). The strain at break of the composite based on EVA 12 is decreased by more than 50% compared to the value of the neat EVA 12 while the

EVA 27

EVA 19

Heat Flow

EVA 12

1 W/g

-50

0

50

100

decrease recorded for the composite based on EVA 27 is only around 30%. This is typically what is observed for composites filled with (nano)fillers characterized by a variable quality of dispersion. The literature reports [16] that, when CNTs are added in a polymer matrix, the elongation at break tends to substantially decrease with the nanotube content and the extent of dispersion within the polymer matrix. Clearly, a much larger reduction is observed for the composites with poorly dispersed CNTs [8]. This variation in the quality of CNTs dispersion might originate from the relatively weak bonding/interaction between the CNTs and the polymer matrix for copolymers with a low VA content. Indeed, for other polymer matrices, it has been reported [5,8,17,18] that when the interfacial bonding between the nanotubes and the polymer matrix is weak, the load transfer from the polymer to the CNTs is not large enough for the CNTs agglomerates to be broken during melt blending and processing. In order to verify that the evolution of the mechanical properties, and more particularly the increase in relative Young’s moduli with the VA content, may be linked to some extent with the improvement in CNTs dispersion, the results have been analyzed with the Halpin–Tsai model for randomly-oriented (nano)particles [13,14]. This model has been specifically developed to account for the Young’s modulus of composite materials based on polymer matrices filled with particles of high aspect ratio. This model assumes favorable interactions between the matrix and the filler as well as a good filler distribution. The chosen Halpin–Tsai equation, which links the relative Young’s modulus of a composite to the morphological and mechanical properties of its constituents and taking into account the specific geometrical characteristics of MWNTs is represented as      3 l ðEf =Em Þ  ðd=4tÞ En ¼ 1þ2 / 8 d ðEf =Em Þ þ ðl=2tÞ f   1 ! ðEf =Em Þ  ðd=4tÞ Em  1 / ðEf =Em Þ þ ðl=2tÞ f     5 ðEf =Em Þ  ðd=4tÞ 1þ2 þ / 8 ðEf =Em Þ þ ðd=2tÞ f   1 ! ðEf =Em Þ  ðd=4tÞ Em  1 ð1Þ / ðEf =Em Þ þ ðd=2tÞ f where En = Young’s modulus of the nanocomposite (measured), Em = Young’s modulus of the neat matrix (measured), Ef = Young’s modulus of the nanofiller (1.106 MPa) [18,19], (l/d)mean = mean aspect ratio of dispersed (nano) particles (to be calculated), d = diameter of the carbon nanotube, l = length of the carbon nanotube, t = thickness of graphite layer (0.34 nm) and /f = volume fraction in nanofiller.

Temperature (˚C)

Fig. 4. DSC thermogram of unfilled EVA matrices (plain line) and the related 3 wt% MWNTs-filled composites (dotted line). Thermograms have been shifted on the ordinate scale for sake of clarity.

/f ¼ 

df dm

1 

Mm Mf



ð2Þ þ1

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where df = density of the nanofiller (2.1 g/cm3) [6], dm = density of the matrix (from technical sheets : EVA 12 = 0.935 g/cm3, EVA 19 = 0.942 g/cm3, EVA 3 27 = 0.951 g/cm ), Mm = mass fraction in matrix (0.97) and Mf = mass fraction in nanofiller (0.03). Since MWNTs are characterized by a distribution in both lengths and diameters and since complete CNTs dispersion is probably not reached, the aspect ratio of the filler (l/d) that can be extracted from Eq. (1) will correspond to a mean value that takes into account both the size distribution and incomplete dispersion of the CNTs, as well as the random character of the individual CNTs orientation. To determine the mean aspect ratio of CNTs particles in the composites and to observe any evolution of its value with the VA content, (l/d)mean has been extracted from Eq. (1) . When the values are included in Eqs. (1) and (2), the following theoretical Ec/Em values can be calculated for a good quality of dispersion (l/d = 1000) (complete deagglomeration) Ec =Em ðEVA 12Þ ¼ 4:9 Ec =Em ðEVA 19Þ ¼ 7:6 Ec =Em ðEVA 28Þ ¼ 9:5 The predicted values are also increasing with the VA content. This is clearly a pure mathematical effect easily checked with the Halpin–Tsai equations and can be easily explained. Adding the same amount of the same reinforcement into two different matrix systems, one exhibiting a low Young’s modulus, and the other one exhibiting a high Young’s modulus, leads to a much higher increase in relative Young’s modulus in the composite with the softer matrix. It is important to note that the experimental values are largely inferior to the theoretical values, which means that the dispersion is not homogeneous. This will explain the variation of the ultimate properties of the composites. The mean aspect ratios can be extracted for the MWNTs dispersed in the three EVA matrices ðl=dÞmean ðEVA 12Þ ¼ 110 ðl=dÞmean ðEVA 19Þ ¼ 118 ðl=dÞmean ðEVA 27Þ ¼ 120 Since the MWNTs used for preparing the nanocomposites belong to the same batch, the slight evolution in the observed mean aspect ratio cannot originate from a variation of the CNTs dimensions. Therefore, it should originate from the small variation of the agglomeration state of the CNTs within the three matrices. Indeed, assuming that CNTs agglomerates have a relatively low aspect ratio (globular agglomerates), one can state that if a larger number of CNTs are separated from the agglomerates, their contribution to the overall mean aspect ratio (CNTs bundles/agglomerates and isolated CNTs) will tend to increase its value. In the EVA/MWNTs nanocomposites investigated here, the mean aspect ratio does not largely increase

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with the polarity of the EVA matrix used, indicating no really significant increase in the destructuration of CNTs bundles with the VA content of the polymer matrix. Therefore, the observed increase in the relative Young’s modulus is essentially a mathematical effect as verified by the Halpin–Tsai model. 3.2. Morphology To confirm the proposed hypothesis of the effect of the EVA matrix polarity (i.e., the relative content in VA units) on the extent of MWNTs dispersion and thus on the mechanical properties of the composite materials, the microscopic morphology of MWNTs-filled EVA samples has been observed by atomic force microscopy in tapping mode (TM-AFM). The large difference in stiffness between the ‘‘soft’’ EVA matrices and the ’’hard’’ MWNTs surface leads to a sizable phase contrast between the two components. The surface of the EVA 12-based composite material displays very large zones totally free of MWNTs (the continuous dark matrix in the image), whereas one can detect sporadically micron-size areas containing large CNTs agglomerates. Fig. 5a exhibits such a typical agglomerate of ca. 1 lm2, for instance the one pointed by the white arrow, where short segments of CNTs appear highly and densely entangled. The inset details the organization of the CNTs within the agglomerate. Clearly, MWNTs exhibit a poor dispersibility in the EVA 12 matrix. It is worth noting here that this result only reflects the poor dispersion of the CNTs in the matrix. The tensile data shown before however, attest for a good distribution of these micronscale aggregates within the matrix, resulting in a homogeneous black color of the sample. In the EVA 19-based composite, agglomerates are again observed (Fig. 5b) but a few isolated CNTs are however, also detected (inset in Fig. 5b). Finally, the morphology of the EVA 27-based composite is shown in Fig. 5c. The AFM phase image shows a mixture of some isolated MWNTs spread throughout the EVA matrix as well as remaining agglomerates. These morphological observations thus confirm that the increased polarity of the EVA matrix is not sufficient to induce a large destructuration/deaggregation of the CNTs bundles. Some favorable interactions (H-bonding, dipolar interactions) between the acetate groups of the EVA matrix and some polar groups known to be available on the outer surface of the MWNTs produced by the catalytic decomposition technique [19] are responsible of the slight increased dispersion state. This small increase is unfortunately not sufficient to induce significant changes in Young’s modulus excepting those arising from the mathematical effect of matrices. This hypothesis was further confirmed by the observation of the dispersion in composite materials based on HDPE and PVAc, which thus contain 0 and 100 wt% VA units, respectively. These two nanocomposites display a totally different morphology. The HDPE-based composite

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Fig. 5. TM-AFM phase images of the nanocomposites: (a) (3 · 3 lm2) image of the composite based on EVA 12 + 3% MWNTs; the inset shows a (750 · 750 nm2) zoom on the aggregate, (b) (3 · 3 lm2) image of the nanocomposite based on EVA 19 + 3% MWNTs with micron-scale bundles of nanotubes; the inset is a (2.5 · 2.5 lm2) area where isolated MWNTs are observed and (c) (5 · 5 lm2) image of the composite based on EVA 27 + 3% MWNT.

Fig. 6. Left: TM-AFM phase image (5 · 5 lm2) of the composite based on HDPE + 3% MWNTs; right: TM-AFM phase image (5 · 5 lm2) of the composite based on PVAc + 3% MWNTs; the inset shows a (500 · 500 nm2) height image of a 20-nm thick isolated carbon nanotube.

presents very large zones of crystalline polyolefin matrix totally free of MWNTs and some 2 lm-wide agglomerates containing closely entangled MWNTs (encircled in Fig. 6, left). In contrast, in the PVAc-based composite, large number of MWNTs isolated throughout the PVAc matrix can be observed (Fig. 6, right). Clearly, importantly increasing the VA content in the system allows for destructuring, at least partially, the CNTs bundles and improving the dispersion of the nanotubes throughout the polymer matrix.

composites. Increase in the matrix polarity tends to slightly favour dispersion of the CNTs. Larger extent of CNTs dispersion is only observed for PVAc. DSC analysis has shown that, in a first approximation, the presence of the MWNTs in an EVA matrix does not disturb to a significant extent the crystalline structure of EVA. The influence of CNTs on the EVA crystallization kinetics is under investigation and will be reported in a forthcoming paper.

4. Conclusions

Acknowledgements

Nanocomposites based on EVA matrices with various VA units contents and MWNTs (3 wt%) have been prepared by conventional melt blending. The evolution of the mechanical properties (stiffness, tensile strain at break) and the morphology (as observed with AFM) strongly suggests only a slight effect of the VA content in the EVA matrix (and hence, its overall polarity) on the dispersibility of the MWNTs in the resulting nano-

The authors thank ‘‘Re´gion Wallonne’’ for financial support in the frame of the Nanotechnology project: BINANOCO. SMPC and SCMN thank the Belgian Federal Government Office of Science Policy (SSTC-PAI 5/3) for general support. M.A. Ph.D., P.V and R.L. are much indebted to both ‘‘ Re´gion Wallonne ’’ and the European Commission ‘‘FSE and FEDER’’ for financial support in the frame of Phasing-out Hainaut: Materia Nova.

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