Preparation and Characterization of Linear Low Density Polyethylene/Carbon Nanotube Nanocomposites

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1 Journal of Macromolecular Science w, Part B: Physics, 46: , 2007 Copyright # Taylor & Francis Group, LLC ISSN print/ x online DOI: / Preparation and Characterization of Linear Low Density Polyethylene/Carbon Nanotube Nanocomposites JAMAL AALAIE, ALI RAHMATPOUR, AND SOMAYEH MAGHAMI Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran Linear low-density polyethylene (LLDPE)/multiwalled carbon nanotube (MWNT) nanocomposites were prepared via melt blending. The morphology and degree of dispersion of nanotubes in the polyethylene matrix were investigated using scanning electron microscopy (SEM). Both individual and agglomerates of MWNTs were evident. The rheological behavior and mechanical and electrical properties of the nanocomposites were studied using a capillary rheometer, tensile tester, and Tera ohm-meter, respectively. Both polyethylene and its nanocomposites showed non- Newtonian behavior in almost the whole range of shear rate. Addition of carbon nanotubes increased shear stress and shear viscosity. It was also found that the materials experience a fluid-solid transition below 1 wt% MWNT. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. With increasing nanotube content, the activation energy of flow increases. A decrease of about 7 orders of magnitude was obtained in surface and volume resistivity upon addition of 5 wt% MWNT. In addition, a difference between electrical and rheological percolation thresholds was observed. The results confirm the expected nucleant effect of nanotubes on the crystallization process of polyethylene. A slight increase in Young s modulus was also observed with increasing MWNT content. Keywords polyethylene, nanocomposite, rheology, electrical properties, mechanical properties, DSC, SEM Introduction Carbon nanotubes (CNT) are a new class of materials that are known to possess excellent mechanical, electrical and thermal properties. Carbon nanotubes comprise a single sheet or sheets of graphite rolled into a cylinder several microns in length and a few nanometers in diameter. [1] According to the number of graphite layers forming the tubes, carbon nanotubes are termed as single-walled (SWNT) or multiwalled (MWNT). MWNT are electrically conductive due to the graphite lattice whereas SWNT behave as conductors or semiconductors depending on the chirality of the graphite sheets. [2] CNTs possess a very high aspect ratio up to and higher. It has been reported that CNTs are Received 27 February 2007; Accepted 16 March Address correspondence to Jamal Aalaie, Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI), P.O. Box , Tehran, Iran. aalaiej@ripi.ir 877

2 878 J. Aalaie et al. extremely strong with the strength of tens of GPa and exceptionally stiff with Young s modulus in TPa range, yet remarkably flexible with the breaking strain larger than 5%. [3] These properties makes them promising as a reinforcement for composites. In addition, CNTs have excellent conductivity that makes them an ideal material for the production of conductive polymer composites, capable of dissipating the electrostatic charge [4 6] or as shielding devices from the electromagnetic radiation. As in most fiber or particle reinforced materials, the effective utilization of nanotubes in composite applications depends strongly on the ability to disperse the CNTs homogeneously throughout the matrix without destroying their integrity. Carbon nanotubes are strongly affected by Van der Waals forces due to their small size and large surface area. These forces give rise to the formation of aggregates, which in turn make dispersion of CNTs in polymers difficult. Some common methods for the preparation of polymer/cnt composites include: in-situ polymerization, [7,8] solution mixing [9 11] and melt blending. [2,12,13] The composite prepared by the first two methods may result in contamination because of the residual monomer or solvent. However, those by melt blending are essentially free of such contamination. In addition, the tendency of CNTs to from aggregates may be minimized by appropriate application of shear during melt mixing. [14,15] In recent years there has been an increasing interest in the investigation of the behaviors of polymer/cnt nanocomposites. [16 23] Some studies showed a significant improvement of the mechanical properties, for example, Ryan et al., [22] reported that SWNTs produce a 3-fold increase in Young s modulus and almost a 2-fold increase in tensile strength of polyvinyl alcohol matrix at a concentration of approximately 0.1 vol%. Also, a 100% and 58% increase, respectively, in bending strength and flexural modulus of an epoxy matrix with addition of 0.6 wt% MWNT-NH2 was obtained by Yaping et al. [21] Similar results have been reported by other researchers. [16 18] However, some results showed that when CNTs are incorporated into the polymer matrices they do not necessarily result in enhanced mechanical properties. Bhattacharya et al., [23] investigated melt blended single wall nanotube (SWNT)/polypropylene composites and observed a slight drop in tensile strength, elastic modulus and fracture strain with addition of 0.8 wt% CNT. In a polyethylene/mwnt composite system, Kuan et al., [24] found a decrease in tensile strength when untreated CNTs were used. Synthesis methods and mechanical and electrical properties of PE/CNT nanocomposites have gained much attention in the literature, [1,11 13,18,19,24,25] while the rheological behavior of the nanocomposites, especially with reference to those based on linear low density polyethylene, has not been investigated thoroughly. In this research we investigated the rheological behavior of LLDPE/MWNT nanocomposites. In addition mechanical, electrical, crystallization properties and morphology of these nanocomposites have been studied. Experimental Materials The LLDPE used in this study was extrusion grade LL209 (melt flow index (MFI) 0.8 g/ 10 min at 1908C and 2.16 Kg), obtained from Tabriz Petrochemical Co. (Iran). Polyethylene grafted maleic anhydride (PE-g-MA), as a compatibilizer, was obtained from Pluss Polymer Co. (India), under the commercial name of OPTIM E-118.

3 LLDPE/MWNT Nanocomposites 879 Multiwalled carbon nanotubes (MWNT) used in this study were prepared by chemical vapor deposition (provided by Jiang Youg Trade Co. (China)) with diameters ranging from 20 nm to 40 nm, lengths varying from 1 mm to10mm and purity of 85%. Preparation Method Nanocomposites were prepared with different MWNT loading (1 to 5 wt%) by melt mixing in a PL2000 Brabender internal mixer. A list of the prepared nanocomposites is given in Table 1. The mixing temperature and rotor speed were 1508C and 75 rpm, respectively. At first, LLDPE and PE-g-MA were fed to the mixer; after reaching a fixed torque, CNTs were added, and mixing was continued for 18 minutes. 2 mm sheets were prepared by compression molding under a pressure of about 10 MPa at 1808C for 6 minutes using a hydraulic press. Standard test specimens for mechanical and electrical tests were obtained by punching of the prepared sheets. Characterization The microstructure of samples was examined using Cambridge Instruments, S360, scanning electron microscopy (SEM). The samples for SEM examination were fractured in liquid nitrogen and coated with a thin layer of gold prior to SEM observation. The electrical conductivity of the composite sheets were measured with a Tera ohmmeter, CEAST Co. at 500 V, according to ASTM D257. The resistivity range in this experimental set up is limited to values above 10 8 Vm, which is the range of dissipating electrical charges. Measurement of the tensile properties was conducted on a Zwick/Roell Tensile/ Flexural tester according to ASTM D638. Tensile strength and elongation at break were measured at a crosshead speed of 50 mm/min and gauge length of 4.25 cm. Each result was averaged from five specimen tests. The melt flow behavior of polyethylene and its nanocomposites were measured using an Instron 3211 capillary rheometer (barrel diameter, cm). The entire barrel and the capillary assembly were electrically heated with a microprocessor-based temperature controller. The capillary used had a length to diameter ratio (L/D) of (length 51 mm, diameter 1.26 mm). The capillary studies were carried out at 150 and 1808C. The rate of shear variation was achieved by changing the speed of the plunger (0.2, 0.6, 2, 6, and 20 cm/min). The apparent shear stress (s app ) and apparent shear rate Table 1 Compositions of the LLDPE/MWNT nanocomposites Sample code LLDPE (wt%) PE-g-MA (wt%) MWNT (wt%) PN PN PN PN

4 880 J. Aalaie et al. (ġ app ) were calculated using the following equations: [26] s app ¼ DP D ð1þ 4L _g app ¼ 32Q pd 3 ð2þ where DP is the pressure drop across the capillary length, D and L are diameter and length of the capillary, respectively. Q is the volumetric flow rate of the material. The apparent shear stress was taken as equal to the true shear stress (s w ). The true shear rate (ġ w ) was calculated using the Rabinowitch correction: [26] 3n 0 þ 1 _g w ¼ _g app 4n 0 ð3þ where n 0 is the slope of the log s app versus log ġ app. The shear viscosity was calculated by using the following equation: m ¼ s w _g w ð4þ The flow behavior index, n, and consistency index, k, were calculated by using the power law model: s w ¼ k _g n w h ¼ k _g w n 1 log h ¼ log k þðn 1Þlog _g w ð5þ ð6þ ð7þ In order to evaluate the influence of temperature on viscosity, the activation energy (E)of melt flow was calculated for all systems, as given by the Arrhenius-Frenkel-Eyring equation. On the basis of the Arrhenius assumption, the dependence of viscosity on temperature can be assessed by the following equation: [27] 1 1 ¼ R T 1 T 2 E ln h 1 ð8þ h 2 where h 1 and h 2 are the viscosities at temperatures T 1 and T 2. R is the gas constant (8.315 J/mol). In order to investigate polymer crystallinity within the samples, DSC measurements were conducted using a DSC7 Perkin-Elmer instrument. Samples were held for 1 minute at 508C and then were heated from 50 to 1808C at108c/min. Results and Discussion Morphology from SEM The fracture surface morphology of PE and its nanocomposites was investigated by SEM analysis (Fig. 1). Although there are some individual nanotubes (visible as white points in images) scattered in the matrix, most of them are clumped together in aggregates. With increasing nanotube content from 1 to 5 wt%, the number of aggregates decreased while the size of them increased from 0.25 to 0.5 mm. In Fig. 1d one can observe white

5 LLDPE/MWNT Nanocomposites 881 Figure 1. SEM images of fracture surface of (a) PN0, (b) PN1, (c) PN3, and (d) PN5. areas of the size of approximately 0.5 mm. These areas represent regions of relatively high concentration of carbon nanotubes in the composite. In addition to these clusters of nanotubes, the filler is dispersed uniformly in the polymer matrix. Rheological Behavior In view of possible technological applications of the prepared nanocomposites, we analyzed their viscoelastic response under the application of a steady shear flow field, which plays a critical role for the processabilty of polymer systems. In this regard, steady shear viscosity measurements were performed. Understanding the rheological behavior of nanocomposite melts is not only important in gaining a fundamental knowledge of the processability, but also is helpful in understanding the structure property relationships in these materials. Figures 2 5 show the shear rate dependency of the viscosity and shear stress for LLDPE/PE-g-MA and corresponding nanocomposites measured at 150 and 1808C. The shear rates examined, between 10 and 1900 S 1, covers the shear experienced during most polymer processing techniques. Both PE and its nanocomposites display non-newtonian behavior in almost the whole range of shear rate. The figures indicate that the viscosity continuously decreases with increasing shear rate; i.e., the shear thinning effect occurs for all the systems. This behavior is typical of polymer melts, coming from the disentanglement process and increasing the average end-to-end distance of polymeric chains due to shearing.

6 882 J. Aalaie et al. Figure 2. Shear viscosity versus shear rate of LLDPE nanocomposites at 1508C. The shear viscosity and shear stress increase with addition of the carbon nanotubes, indicating a transition from a liquid-like to a solid-like behavior due to the formation of a percolated nanotube network which impedes the motion of polymer chains. Figure 6 shows the viscosity versus MWNT content for these nanocomposites. Adding 1 wt% MWNT causes a drastic change in viscosity, indicating the rheological percolation threshold. The effect of nanotubes is most pronounced at low shear rates and the relative effect diminishes with increasing shear rate. At high shear rates, the viscosity of the nanocomposites is comparable to those of the pure LLDPE/LDPE-MA, thus they have similar processability. In other words, at high rate of deformation, the matrix viscosity was nearly recovered. The above observations suggest that the nanotubes are strongly oriented toward the flow direction at high shear rates, which causes the shear viscosity of the nanocomposites to decrease nearly to that of neat polyethylene. Our results are similar to those reported by some other researchers. [2,12,13,25,28] Abdel-Goad et al., [2] reported a fluid-solid transition at the composition of 0.5 wt% MWNT, beyond which a continuous network of CNTs was formed throughout the polycarbonate matrix and, in turn, caused an increment in viscosity. Zhang et al., [12] obtained a low rheological percolation of 1.5 wt% for Figure 3. Shear viscosity versus shear rate of LLDPE nanocomposites at 1808C.

7 LLDPE/MWNT Nanocomposites 883 Figure 4. Shear stress versus shear rate of LLDPE nanocomposites at 1508C. Figure 5. Shear stress versus shear rate of LLDPE nanocomposites at 1808C. Figure 6. Viscosity versus MWNT content of LLDPE nanocomposites at 1508C.

8 884 J. Aalaie et al. Table 2 n and k values of various LLDPE nanocomposites Sample code n (1808C) k (1808C) n (1508C) k (1508C) PN PN PN PN SWNT/high density polyethylene composites. At this concentration, the viscosity exhibits a large difference from the pure PE matrix, indicating a solid like behavior. The consistency index (k) and the flow behavior index (n) of the LLDPE/PE-g-MA and its nanocomposites at 150 and 1808C are reported in Table 2. The values indicate that LLDPE/PE-g-MA and its nanocomposites behave as pseudoplastics, obeying the power law equation. As can be seen, the shear thinning exponent, n, decreases with increasing MWNT content. The activation energy of flow for polyethylene nanocomposites is plotted against nanotube content in Fig. 7. The activation energy values increases with increasing MWNT content up to 1 wt%. From Equation (8), it can be inferred that a system with higher activation energy of melt flow requires a smaller change in temperature for the same degree of viscosity reduction. On the other hand, the higher the E, the more temperature sensitive is the melt. Adding nanotubes, generally, causes a large energetic barrier for segmental motions of polymer chains in the confined space [29] and thus increasing of the flow activation energy. In addition, Gu et al., [29] reported that strong interactions between polymer matrix and fillers may also cause greater activation energies. For MWNT loadings greater than 1 wt%, there is a slight decrease in activation energy values. We attribute this behavior to the formation of aggregates at higher concentration of MWNTs. Figure 7. Activation energy of flow versus MWNT content for LLDPE nanocomposites.

9 LLDPE/MWNT Nanocomposites 885 Table 3 Electrical and mechanical properties of LLDPE/MWNT nanocomposites Sample code Tensile modulus (MPa) Tensile strength (MPa) Elongation at break (%) Surface resistivity (ohm. cm) Volume resistivity (ohm) PN PN PN PN ,10 8,10 8 Electrical Properties In order to examine the electrical conductivity and the state of percolation in these nanocomposites, electrical volume and surface resistivity tests were carried out; the results are presented in Table 3. The electrical conductivity of polyethylene was improved by increasing MWNT content. At very low concentration of MWNT (,3 wt%), electrical resistivity gradually decreases with increasing nanotube content. However, surface and volume resistivities of the nanocomposite containing 5 wt% MWNT decreases to,10 8 ohm. cm from that of 3 wt% of ohm. cm. This stepwise change in electrical resistivity is a result of the formation of an interconnected structure of MWNT and can be regarded as an electrical percolation threshold. At the percolation threshold, a very high percentage of electrons are permitted to flow through the sample due to the formation of the interconnecting conductive channels. The electrical percolation threshold for our nanocomposites occurs at 5 wt% MWNT. At this concentration resistivities are in the range that is suitable for charge dissipation. As can be seen, there is a difference between the electrical and rheological percolation thresholds. Zhang et al. [12] explained that this phenomenon is a result of different tube-tube distances required for electrical and rheological percolations. Assuming that the electron hopping mechanism applies to the electrical conductivity of nanotube/polymer composites, the required tube-tube distance has to be less than 5 nm for the composites to be electrically conductive. However, as long as the tube-tube distance is comparable to the diameter of random coils of the polymer chains, which is essentially more than 10 nm, the nanotube network can effectively restrain polymer motion, which is the main factor of determining rheological percolation threshold. Therefore, the electrical percolation threshold for a given nanotube/polymer system needs a higher loading level of nanotubes than the rheological percolation threshold. This phenomenon had also been reported by some other researchers. [30,31] DSC Analysis DSC measurements were used to investigate polymer crystallinity within the samples. DSC curves are presented in Fig. 8. Melting temperature and enthalpy values are reported in Table 4. The crystalline fraction of the composites, X, was calculated from: X ¼ DH ð9þ 276F c

10 886 J. Aalaie et al. Figure 8. DSC curves for LLDPE/PE-g-MA nanocomposites. where DH is the enthalpy of fusion (J/g), 276 J/g is the theoretical enthalpy of fusion for a 100% crystalline polyethylene [32] and F c is the weight fraction of polymer. A slight increase in melting temperature of nanocomposites was found. In addition, the crystalline fraction increases from 22.1% for neat polymer matrix to 30.32% for 5 wt% MWNT composite. This might be caused by the effect of nanotubes on the nucleation process of crystallization. Carbon nanotubes act as nucleating agents to initiate crystallization; resulting in a crystalline layer around the nanotubes. It can be seen from the DSC curves that some broadening of the melting peak occurred with increasing nanotube content. This most likely can be attributed to a broad distribution of crystal sizes. A broader melting peak was also observed in a polypropylene/swnt composite as compared to pure polypropylene. [33] Mechanical Properties The maximum tensile strength, s f, Young s modulus E and failure strain 1 f of the neat LLDPE/PE-g-MA and its nanocomposites are summarized in Table 3. Adding MWNTs decreased tensile strength of nanocomposite from to as the MWNT content increased from 0 to 5 wt%. This phenomenon is attributed to the aggregation of CNTs in the polymer matrix. Similar results have been reported by some other researchers. [23,24] Table 4 Enthalpy of fusion, melting temperature and crystalline fraction of nanocomposites Sample code T m (8C) DH (J/g) X (%) PN PN PN PN

11 LLDPE/MWNT Nanocomposites 887 The elongation at break, as an indicator for the toughness of the materials, decreases dramatically with addition of carbon nanotubes to the polymer matrix. This rapid decease of 1 f is believed to be caused by the premature failure starting at the CNT aggregates. The embrittlement of polymers on addition of CNTs is not unusual and has been reported previously for polar polymers also. [34] Young s modulus increases from MPa to 333 MPa by adding 5 wt% nanotubes. So MWNTs have only a small reinforcement effect. The small increment of Young s modulus may be because of the slight increase in crystalline fraction of PE due to incorporation of MWNTs. There are similar results reported in the literature. [22,35] Conclusion LLDPE/PE-g-MA/MWNT nanocomposites with different loading levels of MWNT were prepared via melt compounding. The morphology, rheological, electrical and mechanical properties of the nanocomposites were investigated. There are individual and aggregate forms of MWNTs in the LLDPE matrix, from SEM images. The rheological studies indicated that, for all compositions, the shear viscosity decreases with increasing shear rate, which is shear-thinning behavior, following the power law equation. Addition of nanotubes increases shear stress and shear viscosity. The activation energy of flow increases with increasing nanotube content up to 1 wt%. Adding greater quantities of nanotubes causes a decrease in activation energy. A difference between rheological and electrical percolation thresholds was found. Electrical percolation occurs between 3 and 5 wt% MWNT, while the rheological one occurs below 1 wt% MWNT. This difference was attributed to the different mechanisms of reaching electrical and rheological percolation thresholds. Adding nanotube may cause a slight increase of Young s modulus, in addition of embrittlement. Nanotubes were observed to increase polymer crystallinity, through their nucleation mechanism of crystallization. References 1. Tang, W.; Santare, M.H.; Advani, S.G. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite film. Carbon 2003, 41, Abdel-Goad, M.; Potschke, P. Rheological characterization of melt processed polycarbonatemultiwalled carbon nanotube composites. J. Non-New. Fl. Mech. 2005, 128, Baughman, R.H.; Zakhidov, A.A.; Heer, W.A. Carbon nanotubes. The route toward applications. Science 2002, 297, Kim, Y.J.; Shin, T.S.; Choi, H.D.; et al. Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites. Carbon 2005, 43, Smith, J.G., Jr.; Delozier, D.M.; Connell, J.W.; Watson, K.A. Carbon nanotube-conductive additive-space durable polymer nanocomposite films for electrostatic charge dissipation. Polymer 2004, 45, Sandler, J.; Shaffer, M.S.P.; Prasse, T.; Bauhofer, W.; Schult, K.; Windle, A.H. Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 1999, 40, Wang, Z.; Lu, M.; Li, H.L.; Guo, X.Y. SWNTs-polystyrene composites preparations and electrical properties research. Mat. Chem. Phys. 2006, 100, Xiong, J.; Zhang, Z.; Qin, X.; Li, M.; Wang, X. The thermal and mechanical properties of a polyurethane/multi-walled carbon nanotubes composite. Carbon 2006, 44, 2701.

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