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Use of Dynamic Rheological Behavior to Estimate the Dispersion of Carbon Nanotubes in Carbon Nanotube/Polymer Composites Qinghua Zhang,* Fang Fang, Xin Zhao, Yingzhi Li, Meifang Zhu,* and Dajun Chen State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua UniVersity, Shanghai 201620, People’s Republic of China ReceiVed: March 28, 2008; ReVised Manuscript ReceiVed: June 28, 2008

Well-dispersed multiwalled carbon nanotube (MWNT)/polystyrene composites have been prepared. Transmission and scanning electron microscopy were employed to observe the distribution of the MWNTs in the composites in a microscopic scale, indicating a nanotube network formed in the matrix. The dispersion of the nanotubes in the polymer was monitored by oscillatory rheology. It was found that the addition of MWNTs in the polymer had a drastic influence on the rheological behavior of the composites. As the MWNT loading increased, Newtonian behavior disappeared at low frequency, suggesting a transition from liquid-like to solidlike viscoelastic behavior. A more homogeneous dispersion or a greater loading of the nanotubes in the matrix produced stronger solid-like and nonterminal behavior, and the composites exhibited less temperature dependence at elevated temperature, compared to the matrix melt. 1. Introduction Since their discovery in 1991, carbon nanotubes (CNTs) have generated huge amounts of activity in most areas of science and engineering due to their unprecedented physical and chemical properties. No previous material has displayed the combination of superlative mechanical, thermal, and electronic properties attributed to CNTs. These properties make nanotubes ideal for a wide range of applications.1,2 These days, the materials can be produced on a relatively large scale in an economical way, such as by carbon-arc discharge and chemical vapor deposition. Incorporation of carbon nanotubes into polymer matrices has been shown to remarkably improve the mechanical properties of the resulting composites, and to provide sufficient conductivity for electrostatic charge dissipation. For example, CNT-based composites exhibited significant mechanical improvement,3,4 high electrical conductivity,5,6 and superior thermal properties.7 One of the key points to the successful development of CNTbased composites and to the improvement of their performance is the dispersion of the nanofillers in a polymer matrix. The surface functionalization of the nanotubes is an essential step to overcome the inherent incompatibility between the nanofillers and polymer matrices, thus ensuring significant energy and load transfer across the nanofiller-matrix interface. Covalent functionalization and the surface chemistry of CNTs have been envisaged as very important factors for nanotube processing and application.8,9 The surface functionalization of nanotubes improves the interfacial adhesion and mechanical properties and reduces the CNT loadings based on some polymers, such as epoxy,10,11 nylon,12-14 and polyolefin 15,16 and others.17 Monitoring the quality of dispersion within a composite system gives rise to additional problems. While the clustering of spherical particles had been well-studied for both spherical and highly asymmetrical (platelets, rods, and fibers),18-20 there are reliable direct techniques of observing the nanofillers in the bulk of a composite suspension. All optical methods cut off below a length scale of ∼0.2 - 0.5 µm, while the nanofillers * To whom correspondence should be addressed. Fax: +86 21 67792854. E-mail: [email protected]; [email protected].

cannot be observed using optical methods. However, all electron microscopy methods can only provide the representative information for the selected fields of view, even though observation of individual nanotubes can be performed. Moreover, the above techniques to study the dispersion of nanofillers in composites are limited to merely a two-dimension (2D) observation; while the CNTs in 3D network form are distributed in polymer matrices. Each of these techniques suffers from unavoidable difficulty in the interpretation of results, which leaves reciprocal space techniques and global indirect techniques of characterizing a dispersed composite. Rheological methods have been widely used to study composites incorporated with fillers, since they can detect the presence of internal structures.21,22 The macroscopic connectivity with 3D network produced from physical interactions can be investigated using rheological methods. The long-range connectivity can be attributed to various forces, such as van de Waals, hydrogen bonds, and electrostatic interactions. At a critical loading of filler, the viscoelastic response of the composite system changes from liquidlike to solid-like behavior. The rheological behaviors of polymer composites filled with carbon black,23 carbon nanotubes,24,25 and nanoclays 26,27 have been reported. The rheological response of aqueous suspensions of CNTs has been used to model the percolation of rigidity induced by rod particles.28 Mitchell et al.29 described the rheological percolation in PS/SWNT nanocomposites prepared by blending SWNTs and PS in toluene; Kota et al.30 and Du et al.31 reported the correlation between electrical and rheological properties of MWNT/PS and SWNT/ PMMA nanocomposites, respectively. The viscoelasticity of polymer matrices filled with single- and multiwalled CNTs has also been investigated to some extent. In our previous reports,32,33 SWNT/UHMWPE (ultrahigh molecular weight polyethylene) and SWNT/HDPE (high density polyethylene) nanocomposites were prepared by melting and solution processing, respectively, and the rheology and electrical conductivity were investigated. In this report, we prepared composites containing MWNTs and polystyrene (PS) via an emulsion technique. MWNTs used to produce the composites were functionalized using strong acid, while some were not. The aim was to compare the different dispersion state of

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MWNTs in the matrix and to analyze the effect of dispersion on the rheological behaviors. The rheological data indicated that the addition of MWNTs in PS led to the changes of the materials from liquid-like to solid-like behavior. We found that for a homogeneous dispersion system, relatively low filler loading may cause the 3D network, and the composite system consequently, to exhibit solid-like behavior. The observation on the transmission electron microscope (TEM) and scanning electron microscope (SEM) confirmed the dispersion with individual MWNTs and 3D network in the polymer matrix. Meanwhile, the effect of temperature on the rheological properties of the composites was also discussed. 2. Experimental Section 2.1. Materials. MWNTs with a diameter of ∼20 nm and a purity of >95% were supplied from Shenzhen Nanotech Port Co. Ltd. Analytical grade styrene (Shanghai Xianfeng Chemicals, China) was used as the monomer, and was distilled under reduced pressure prior to use. HNO3 and H2SO4 were purchased from Pinghu Chemicals, China. Potassium persulfate (K2S2O8, KPS) and sodium dodecylsulfate (SDS) were purchased from Sinopharm Chemical Reagent Co., Ltd. The materials were directly used without further purification. 2.2. Preparation of MWNT/PS Composites. The composites were prepared by three simple steps: the modification of MWNTs, synthesis of PS, and preparation of the composites. (1) The crude MWNTs were first oxidized in a solution of HNO3 and H2SO4 (1:3) using reflux for 8 h, sonicated in a bath ultrasonicator. Carboxylic acid groups or hydroxyl groups were introduced onto the surface of the MWNTs after washing by deionized water. The MWNT suspension was prepared at the aid of SDS. (2) Synthesis of PS was carried out using a typical emulsion polymerization: 1.0 g SDS and 80 mL distilled water were put into a three-necked flask with a stirring rod. 0.15 g KPS and 30 mL styrene were sequentially added into the flask. The reaction was carried out at 80 °C for 4 h. (3) Aqueous MWNT suspension and PS emulsion were blended by ultrasonication for 1 h, and MWNT/PS composites were thus prepared after cold-drying at -20 °C for 24 h. 2.3. Characterization. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS-670 spectrometer with KBr pellets of MWNTs. Spectra in the range of 600-4000 cm-1 were obtained by the averaging of 32 scans at a resolution of 1 cm-1. To observe the functionalized MWNTs and their dispersion in PS, the aqueous MWNT solution or MWNT/PS emulsion was dripped onto a copper grid. After drying, the observation can be carried out on a Hitachi H-800 TEM at 250 kv. The fracture surface morphology of the composite was measured on a JSM-5600LV SEM at an accelerating voltage of 10 kv. The sample was broken in liquid nitrogen and goldsputtered prior to observation. To prepare the samples for rheological measurement, the above dried powders of MWNT/PS composites were extruded into a round chip with a diameter of 25 mm and a thickness of ∼2 mm on a DACA microcompounding instrument made in the United States of America. Dynamic rheological properties were investigated on a TA Orchestrator ARES-RFS at various temperatures under nitrogen atmosphere. The measurements were carried out in an oscillatory shear mode using parallel plate geometry with a diameter of 25 mm. Frequency sweeps between 0.01 and 100 rad/s were applied at a low strain of 1%. Specimens were placed between the preheated plates and were allowed to equilibrate for approximately 10 min prior to each frequency sweep run.

Figure 1. FTIR spectra of crude and surface-functionalized carbon nanotubes.

3. Results and Discussion 3.1. Morphology. To uniformly disperse the MWNTs in the PS matrix, the MWNTs were oxidized using HNO3/H2SO4 to introduce carboxylic acid groups or hydroxyl groups onto the surface of the MWNTs, according to the previous reports.34,35 Figure 1 gives the FTIR of the samples to identify the produced groups on the surface of the MWNTs by above reaction: For the crude MWNTs, the FTIR curve does not give a vibration. Vibrational modes of surface-functionalized MWNT at 3425, 1731, and 955 cm-1 are attributed to O-H stretching, carbonyl stretching, and out-of-plane O-H bending, respectively, similar to the other reported before.2,36 In our observations, for the dispersion of the crude MWNTs in water, MWNTs easily agglomerate and settle to the bottom of the container within several days; whereas, surface-functionalized MWNTs can be uniformly dispersed in water after a couple of weeks. Electron microscopy was employed to observe the morphologies of dispersion of MWNTs. Comparing the crude MWNTs to the surface-functionalized MWNTs, corresponding to Figure 2, parts a and b, respectively, it is apparent that the former gives the form of bundle and agglomeration, whereas the latter shows an individual form. The different morphologies of the crude and surface-functionalized MWNTs exhibit different dispersion in the PS emulsion. The TEM image shown in Figure 2c gives the agglomeration morphology of the crude MWNTs in the PS emulsion; whereas the surface-functionalized MWNTs exhibit the individual dispersion in the emulsion. The red arrows in Figure 2d point to the three individual nanotubes. The spherical particles with a diameter of 300-500 nm are PS particles synthesized by emulsion polymerization. It is apparent that individual nanotubes with a diameter of ∼20 nm embed around the PS particles and interpenetrate among the PS particles. The exfoliation of the surface-functionalized MWNTs indicates that the acidic oxidation on their surfaces improves the dispersion of the nanotubes in water and in PS emulsion. Latex technology and polymer emulsion were easy ways to uniformly disperse carbon nanotubes into a polymer and composites exhibited an excellent performance.37-39 As shown in Figure 3a, the SEM image of a fracture surface of the composites containing 5% MWNTs shows that nanotubes are uniformly distributed in the polymer matrix at a length scale of ∼500 nm. In addition, the nanotubes appear as interconnected structures at this MWNT loading, which is consistent with other nanofilled systems and suggests a nanotube network.31,32,40 However, for the composites containing 1% MWNTs, the fillers do not form the nanotube network in the composite, as shown in Figure 3b. In general, the surface-functionalized MWNTs in

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Figure 2. TEM images of MWNTs: (a) crude MWNTs in the bundle or agglomeration form, (b) surface-functionalized MWNTs in the individual form, (c) crude MWNTs dispersed in PS emulsion, and (d) individual surface-functionalized MWNTs dispersed in PS emulsion, the red arrows point to three individual nanotubes.

Figure 3. SEM images of cross section of (a) 5% MWNT/PS composite indicating formation of nanotube networks within the composite and (b) 1% MWNT/PS composite exhibiting the individual nanotubes in the matrix.

the form of the individual nanotubes appear as homogeneous dispersions in the polymer matrix, but a few of them aggregate, showing a large diameter of 50-80 nm. Most of MWNTs embedded into the matrix show some degree of waviness or entanglements along the axial direction. Such curvature probably reduces the efficient structural reinforcements in comparison to the theoretical promises.41 3.2. Effect of Nanotubes on Rheological Behaviors. In general, the shear viscosity of the neat polymer is characterized by two distinct regions, called the Newtonian region and the shear thinning region. At low shear rate or oscillatory frequency, the Newtonian region is observed with independence of shear rate, followed by the shear thinning region, where the viscosity linearly decreases with an increase in the shear rate. The dependence of complex viscosities (η*) on the oscillatory frequency (ω) of the MWNT/PS composites and neat PS is shown in Figure 4. Rheological behavior of neat PS exhibits an obvious two regionssa Newtonian region at low frequency (1 rad/s). It is apparent from the Figure 4 that nanotubes have a dramatic effect on the rheological behaviors. As the MWNT

Figure 4. Dependence of complex viscosity of the composites containing crude MWNTs on applied frequency at 180 °C.

loadings of 1.25% or 5% are added into the polymer matrix, the Newtonian region of the composites disappears and only the shear thinning region remains at the frequency from 0.01 to 100 rad/s. As shown in Figure 4, the complex viscosity increases with increasing MWNT content. The effect of the

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Figure 5. Dependence of storage modulus of the composites containing crude MWNTs on applied frequency at 180 °C.

TABLE 1: Terminal Slopes of G′ and G′′ for MWNT/PS Composites MWNT loadings (%)

low-freq. slope of log G′ vs log ω

low-freq. slope of log G′′ vs log ω

0 1.25 5

1.17 0.62 0.26

0.84 0.46 0.10

nanotubes is most pronounced at low frequencies and the relative effect diminishes with increasing frequency due to shear thinning, and the viscosities are orders of magnitude higher than that of neat PS at low frequency. For the sample with higher loadings of 5% MWNTs, the plot of η* vs ω exhibits a linear relationship throughout the studied range of frequency. This is in accordance with theoretical expectations and experimental observations for fiber reinforced composites.42,43 At high frequencies, the fraction of SWNTs has a relatively minor influence on the complex viscosities of the composites. It should be noted that according to our previous report, the addition of SDS in a matrix polymer has a very slight influence on the complex viscosity, so that the effect of SDS on the rheological properties will be not taken in account here.33 Remarkable shear thinning behaviors have been reported in some composites embedded with nanofillers such as nanofibers,32 nanoclays,44 and silica.45 In these composites, it is believed that filler-polymer and filler-filler interactions increase with increasing the filler loadings. The filler-filler interactions play a dominant role in the rheological behavior for the composites, resulting in an increase in the shear viscosity without the Newtonian plateau region. The physical network of the nanotubes caused by the filler-filler interaction can be understood as the so-called rheological percolation. Figure 5 shows the function of the storage modulus of the composites as the addition of MWNTs into the polymer matrix. At low frequencies, PS chains are fully relaxed and exhibit typical homopolymer-like terminal behavior with the scaling properties of approximately G′ ≈ ω2 and G′′ ≈ ω. This power law relation may vary because of the polydispersity of polymer chains. However, with the addition of MWNTs, this terminal behavior disappears and the dependence of G′ on ω at low frequency is weak. Thus, large-scale polymer relaxations in the composites are effectively restrained by the presence of MWNTs. The low frequency power-law dependence of G′ decreases monotonically with increasing nanotube loading, from ω1.17 for PS to ω0.26 for composite with 5% MWNT loadings, as listed in Table 1. Meanwhile, the frequency dependence of G′′ decreases from ω0.84 for PS to ω0.10 for one with 5% MWNT

Figure 6. Dependence of tanδ of the composites containing surfacefunctionalized MWNTs on applied frequency at 180 °C.

loadings. The storage modulus G′ is almost independent of frequency at low frequencies, especially at the nanotube loading of 5%, which is indicative of a transition from liquid-like to solid-like viscoelastic behavior. This nonterminal low frequency behavior can be attributed to the formed nanotube network, which restrains the long-range motion of polymer chains. As the nanotube content increases in this composite system, nanotube-nanotube interactions begin to dominate, and eventually lead to percolation and the formation of an interconnected structure of nanotubes. This critical composition is regarded as a rheological percolation composition. Once the nanotubes form a network structure, the rheological properties exhibit a similar behavior, even though we further increase the nanotube loadings. Moreover, the plot of loss modulus G′′ vs ω exhibits a similar trend to G′, which is not shown here. Figure 6 gives the relationship between the applied frequency and the tangent loss angle (tanδ) for the composites with different MWNT loadings with respect to the oscillatory frequency. In contrast to purt PS (no MWNT), the composites exhibit lower tanδ, especially for the composites containing 5% MWNTs. Since tanδ ) G′′/G′, it is used to characterize the viscoelasticity of a material, and the less tanδ means that the material is exhibiting relatively more solid-like behavior. As shown in Figure 6, the addition of MWNTs into the composites increases the solid-like viscoelastic behavior of the composites and decreases the liquid-like behavior at a given oscillatory frequency, which is attributed to the formed nanotube network in the composites. 3.3. Effect of Nanotubes Dispersive State on Rheological Behavior. According to the analysis above, the addition of the MWNTs in PS results in sudden changes of the dynamic rheological behavior of the composites. The nonterminal effect at low frequency is caused by the formation of 3D nanotube networks in the composites, which impedes the motion of macromolecular chains. Therefore, a composite system with more homogeneous dispersion of the nanofillers exhibits a more obvious nonterminal effect, compared to a system with poor dispersion of the nanofillers. In other words, at a given MWNT loading, homogeneous dispersion of MWNTs results in a higher complex viscosity and storage modulus, compared to a poor dispersion. As mentioned above, in our observations, for the dispersion of the crude MWNTs in water, MWNTs easily agglomerate and settle to the bottom of the container within several days; whereas, surface-functionalized MWNTs can be uniformly dispersed in water after a couple of weeks. The previous reports by researchers have shown the obvious improvement of nano-

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Figure 7. Dependences of η* (a) and G′ (b) on ω for the composites containing different MWNTs of 5% at 180 °C

Figure 8. Changes of complex viscosities with applied frequencies at various temperatures for the PS matrix (a), composites containing 1.25% crude MWNTs (b), 5% surface-functionalized MWNTs (c), and 5% crude MWNTs (d).

tube dispersion in a polymer matrix by their surface functionalization, leading to the improvement of the electrical properties and mechanical performance.31,33,46 Figure 7a shows the effect of crude MWNTs and surface-functionalized MWNTs added into the polymer matrix on the complex viscosity of the composites. Apparently, for the composites containing 5% crude MWNTs or 5% surface-functionalized MWNTs, the two plots of η* vs ω exhibit parallel linear relationships throughout the studied range of frequency; whereas the neat PS shows a Newtonian fluid at the low frequency. However, by contrasting the surface-functionalized MWNT/PS with the crude MWNT/ PS composites, it is found that the former shows the higher complex viscosity than the latter. This fact indicates that the MWNTs functionalized using strong acid produce more homogeneous networks in the polymer matrix than the crude MWNTs. Figure 7b gives the changes of the storage modulus of PS, crude MWNT/PS, and surface-functionalized MWNT/PS with the applied frequency. As discussion in Figure 4, the G’s for the composites are almost independent of frequency and exhibit the nonterminal behavior at low frequencies, which is attributed to the formed nanotube network and restrains the longrange motion of polymer chains. However, the surface-func-

tionalized MWNT/PS composite gives a higher modulus than the crude MWNT/PS by ∼5× at a fixed frequency. This difference of G’s further proves that the former system possesses more homogeneous dispersion of the functionalized nanotubes. 3.4. Effect of Temperature on Rheological Behavior. The influences of temperature on the complex viscosities of the composites are demonstrated in Figure 8. In general, for the neat PS or the composites, the complex viscosities decrease with increasing temperature. The dependence of viscosity on the temperature can be interpreted by a free volume concept: an increase in temperature allows more thermal motion of macromolecules and greater free volume in the polymer, which leads to a decrease in intermolecular or intramolecular resistances associated with viscosity. Two models, including the WLF equation and the Arrehenius-type equation, have been employed to explain the influence of temperature on the viscosity. However, the sensitivity of the viscosity to temperature is profoundly affected by the addition of nanotubes. Compared to Newtonian fluid of the polymer matrix at the low frequency in Figure 8a, the plots of η* vs ω for the composites containing various MWNTs exhibit an obvious linear relationship. For the composite with 1.25% crude MWNTs, the η* decreases by more

Rheology of CNT/Polymer Composites than 1 order of magnitude at low frequency (0.01 rad/s) with a temperature increase from 170 to 190 °C, as shown in Figure 8b; whereas for the sample with 5% crude MWNTs in Figure 8d, the η* only decreases by a half-order of magnitude at the same reduction of temperature. This difference indicates that more MWNT loadings result in more obvious solid-like behavior of the composite. Alternatively, comparing the composite with 5% surfacefunctionalized MWNTs to one with 5% crude MWNTs, as shown in Figure 8, parts c and d, respectively, the dependence of η* on the temperature of the surface-functionalized MWNT system is less sensitive than that of the crude MWNT system. For the former composite, the η* reduces by only 3× at low frequency (0.01 rad/s) as the temperature increases from 170 to 190 °C, which means that the three lines in Figure 8c are very close. This difference further demonstrates that the more homogeneous dispersion of MWNTs in the polymer matrix results in more obvious solid-like behavior and higher complex viscosity of the composites. Therefore, we can monitor the dispersion state in large scale through measuring the dynamic rheological behaviors of the composites: at a given nanotube loading, the higher the viscosity of a composite system becomes, the weaker sensitivity of η* on temperature becomes, and the more homogeneous the dispersion of the nanofillers becomes. 4. Conclusions The objective of the present work is to examine the effect of nanotube loadings and the dispersive state on the rheological behavior of MWNT/PS composites and to thus monitor the dispersion of the nanotubes in a polymer matrix using this convenient method. The addition of MWNTs in PS had a dramatic influence on the rheological properties of the composites, especially at low frequency. The plots of complex viscosity vs the applied frequency for the composites exhibited an obvious linear relationship, indicating the disappearance of Newtonian fluid at low frequency. The formation of nanotube networks in the polymer matrix and the transition from liquid-like to solid-like resulted in the dramatic increase of complex viscosity and the storage modulus of the composites. More homogeneous dispersion or more loading of the nanotubes in the matrix led to stronger solid-like and nonterminal behaviors, and the composites exhibited less temperature-dependence at elevated temperatures, compared to the matrix melt. Meanwhile, morphological observation by SEM and TEM confirmed that most surface-functionalized MWNTs in an individual form produced a nanotube network in the polymer matrix, which restrains the long-range motion of polymer chains. Therefore, we can monitor and analyze the dispersion state in large scale using the rheological behaviors of the composites: at a given nanofiller loading, the higher η* and G′ of a composite system become, the weaker the sensitivity of the rheological behavior on temperature becomes, and the more homogeneous the dispersion of the fillers becomes. Acknowledgment. This work is supported by the Program for New Century Excellent Talents in University (NCET-060421), Shanghai Rising-Star Program (06QH14001), 863 Plan (2005AA302H30), Shanghai Leading Academic Discipline Project (B603), and 111 Program (111-2-04 & B07024). References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (2) Zhang, Q.; Chang, Z.; Zhu, M.; Mo, X.; Chen, D. Nanotechnology 2007, 18, 115611. (3) Qian, D.; Dickey, E. A.; Andrews, R.; Rantell, T. Appl. Phys. Lett. 2000, 76, 2868–2870.

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