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multi-walled carbon nanotube composites
Composites Science and Technology 68 (2008) 2498–2502
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Melt rheological properties of nylon 6/multi-walled carbon nanotube composites Min Wang a, Weizhi Wang a, Tianxi Liu a,*, Wei-De Zhang b a Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, People’s Republic of China b Nano Science Research Center, College of Chemistry, South China University of Technology, Guangzhou 510640, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 18 September 2007 Received in revised form 18 April 2008 Accepted 2 May 2008 Available online 8 May 2008 Keywords: A. Carbon nanotubes A. Nano composites A. Polymer matrix composites (PMCs) D. Rheology
a b s t r a c t Nylon 6 (PA6) composites with different loadings of multi-walled carbon nanotubes (MWNT) were prepared by melt compounding technique. Melt rheological properties of PA6/MWNT composites were studied in linear viscoelastic response regions. The incorporation of MWNT into PA6 matrix resulted in higher complex viscosities (|g*|), storage modulus (G0 ), loss modulus (G00 ), and lower loss factor (tan d) than those of neat PA6, especially in low frequency region. The orientation of rigid molecular chains in the composites introduced by the addition of MWNT induced a strong shear thinning behavior and an increasing activation energy for the flow process. With the increase of MWNT loading, the composites experienced a transition from liquid-like to solid-like viscoelasticity. The decrease in the slope of the plot of log G0 versus log G00 with increasing MWNT loading suggested the microstructural changes of the polymer matrix due to incorporation of MWNT. At low frequencies, nonterminal solid-like rheological behavior of PA6 composites were observed and attributed to the formation of the network-like structures of MWNT in the polymer composites. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Polymer nanocomposites made by incorporating a tiny quantity of nanomaterial into a polymeric matrix have attracted much attention worldwide for their multi-functional, outstanding thermal, electrical and mechanical properties [1–3]. From the discovery by Iijima in 1991 [4], high aspect ratio carbon nanotubes (CNT) including single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) have been considered as the ideal enhancement fillers in polymer composites due to their high mechanical strength, and high electrical and thermal conductivity [5–9]. To achieve good performance for CNT filled polymer composites, homogeneous dispersion of the nanotubes in the polymer matrix and strong interfacial interaction between nanotubes and polymer remain two major challenges. At present, three main processing techniques have been usually used to fabricate CNT reinforced polymer composites: solution mixing or coagulation, in situ polymerization, and melt compounding [10–16]. Among these processing techniques, melt compounding has been accepted as the simplest and most effective method from an industrial perspective, because this process makes it possible to fabricate high performance composites at low cost, and also facilitates commercial scale-up. In our previous work [16,17], nylon 6 (PA6)/MWNT composites prepared by melt compounding approach show great * Corresponding author. Tel.: +86 21 5566 4197; fax: +86 21 6564 0293. E-mail address: [email protected] (T.X. Liu). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.002
improvement in mechanical properties due to the homogeneous dispersion of MWNT in the matrix as well as the strong interfacial interaction between MWNT and polymeric matrix. The mechanical testing shows that, compared with neat PA6, the elastic modulus and the yield strength of the composite are greatly improved by about 214% and 162% with addition of only small amount (2 wt%) of MWNT into the PA6 matrix. The rheological behavior of polymer composites is of great importance in polymer processing, particularly for the analysis and design of processing operations as well as understanding the relationship between structures and properties of polymer composites. Much research has been done on the rheological behavior of various polymer nanocomposites, especially for the two-dimensional nanoclay-based polymer composites; however, the rheological properties of one-dimensional CNT reinforced polymer composites have rarely been investigated and still remain much less explored to date. Krishnamoorti et al. [18] observed nonterminal low frequency rheological behavior in the composites containing nanoclay; and other researchers [19] also observed this nonterminal solid-like rheological behavior in polymer composites containing carbon fibers. Kharchenko et al. [20] reported that the viscosity of polypropylene/MWNT composites decreased strongly with increasing shear rate, and the composites exhibited large and negative normal stress. Du et al. [21] prepared poly(methyl methacrylate)/SWNT composites by coagulation method and observed that the rheological percolation threshold was smaller than that of electrical conductivity; and they attributed this difference
M. Wang et al. / Composites Science and Technology 68 (2008) 2498–2502
to smaller nanotube–nanotube distance required for electrical conductivity as compared to that required to impede polymer chain mobility. Hu et al. [22] also applied coagulation method to prepare poly(ethylene terephthalate) (PET)/MWNT composites. A coating layer on MWNT by PET chains was observed and considered to be the evidence of interfacial interaction between MWNT and PET chains; and they reported the melt composites showed transition from liquid-like to solid-like viscoelasticity. Kim et al. [23] investigated rheological properties of poly(ethylene 2,6-naphthalate)/MWNT composites prepared by melt compounding process, and observed the nonterminal behavior at higher MWNT content and attributed it to the formation of the interconnected structures of MWNT in the composites. The rheological properties of CNT reinforced polymer composites are related to the materials’ microstructures, the state of nanotube dispersion and orientation in the matrix, as well as the interactions between nanotubes and polymer chains. Therefore, the rheological properties should be characterized to correlate these characters with the composite properties and thus to optimize the processing conditions for achieving high performance polymer composites. Nylon 6 is an important kind of commercial polymer which is widely used in engineering plastic products, however, few reports could be found in the literature regarding the effect of CNT on the rheological properties of nylon-based composites [24]. In our previous report, the preparation, mechanical properties, crystallization and melting behavior of the PA6/MWNT composites have been investigated [16,17,25]. In addition, the PA6 composites reinforced by the novel CNT-based hybrid fillers have been prepared and studied in our early report [26,27]. In this study, the rheological properties in linear viscoelasticity region for PA6 composites with different MWNT loadings have been systematically characterized to investigate the effect of MWNT on the rheological properties of PA6 matrix.
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shear rate range to obtain the apparent viscosities (g) for the neat PA6 and the PA6/MWNT composites. To maintain all the rheological experiments in the linear viscoelastic region, strain sweeps were performed in advance for each sample, and consequently a strain of 1% was selected for all frequency sweep data in this work. The rheological properties were reproducible after repeated temperature cycling and frequency sweep, indicating that there is almost no chain degradation during measurements. In order to further confirm whether molecular weight degradation occurs due to melt rheology measurements, the ideal experiment to perform would be to extract the carbon nanotubes out of the composites and measure the molecular weight of the remnant. However, such an experiment would be exceedingly difficult to perform given the fact that the carbon nanotubes are only nanometers in size. Furthermore, the possibility that some polymer molecules remain attached to the large surface area of carbon nanotubes cannot be ruled out, thus rendering complete extraction or separation improbable. The comparison was finally made between the composite (with 1 wt% MWNT, after repeated extraction using formic acid and centrifugation) and the neat PA6 after being experienced melt rheology tests, by measuring their respective intrinsic viscosities. The intrinsic viscosity, which is obtained from dilute solution measurements, is known to be particularly sensitive to the high molecular weight fractions of polymers with a molecular weight distribution and thus would be useful for signaling any significant degradation. Consequently, the intrinsic viscosities, [g], of neat PA6 and the composite containing 1 wt% carbon nanotubes after melt rheology measurements were determined to be 0.82 dL/ g and 0.81 dL/g, respectively, using formic acid as the solvent. It can be seen that the difference between the intrinsic viscosity values is within experimental scatter. Given the typical uncertainties in such measurements, it can be suggested that the melt rheology measurements do not significantly degrade the molecular weight of PA6 matrix.
2. Experimental 3. Results and discussion 2.1. Materials and preparation of PA6/MWNT composites PA6 pellets (Grade SF1080A) used here are the product of Ube Industries under license from Toyota. The MWNT were prepared by catalytic chemical vapor deposition of methane on Co–Mo/MgO catalysts [28]. The as-prepared MWNT were purified by dissolving the catalyst in hydrochloride acid followed by refluxing in nitric acid for increasing more carboxylic and hydroxyl groups [16,17]. PA6 composites containing different MWNT contents (from 0.2 to 2 wt%) were prepared via melt compounding method using a Brabender twin-screw extruder at 250 °C for 10 min with a screw speed of 100 rpm. The sample disks (with thickness of 1.7 mm and diameter of 25 mm) were prepared using a mixing molder (LMM, ATLAS Electric Devices Co.) with the extruded pellets at 245 °C for 5 min, and the mold temperature was 200 °C. 2.2. Rheological measurements Rheological properties of PA6/MWNT composites and neat PA6 were measured on a strain-controlled rheometer (ARES, TA Instruments) over a temperature range of 225–245 °C. Small-amplitude oscillatory shear measurements were performed using parallelplate geometry with the plate diameter of 25 mm and the plate gap setting of 1.0 mm to get the complex viscosities (|g*|), the storage moduli (G0 ), the loss moduli (G00 ) and the loss factor (tan d), by applying a time-dependent strain, c(t) = c0 sin(xt), and measuring resultant shear stress, r(t), which is interpreted as: r(t) = c0 [G0 sin(xt) + G00 cos(xt)], where x is the oscillation frequency of the rheometer. Steady rate sweep tests were also performed at low
The main melting region of neat PA6 and its MWNT composites begins at about 200 °C and ends at about 225 °C, as observed by differential scanning calorimetry results [16,17]. The decomposition temperature of PA6 and its MWNT composites is higher than 378 °C by thermogravimetric analysis. Hence, in this study the melt rheological measurements for PA6 and its MWNT composite samples were carried out between 225 and 245 °C in order to investigate the temperature effect on the rheological properties. The complex viscosities (|g*|) at 225 °C for PA6 and its MWNT composites as a function of frequency are presented in Fig. 1a, and the variation of the |g*| with the MWNT loading at different frequencies is shown in Fig. 1b. The complex viscosities decrease with the increase of frequency, indicating a non-newtonian behavior and pseudoplastic characteristics of PA6/MWNT composites. The shear thinning behavior may be attributed to the orientation of the rigid CNT molecular chains which disturbs the formation of PA6 chain entanglements in the composites during the applied shear force in the PA6/MWNT composites. Similar phenomenon was observed for other CNT reinforced polymer composites [20,23]. It can be seen from Fig. 1 that the incorporation of a small quantity (0.2 or 0.5 wt%) of MWNT into the PA6 matrix slightly decreases the complex viscosity compared to that of the neat PA6. This phenomenon may be attributed to the formation of the viscous surface layers around the dispersed MWNT thus leading to an increase in the free volume and making it easier to flow. When further increasing the MWNT loading, the physical interactions between the neighboring MWNT and/or between the MWNT and the matrix increase remarkably, which result in the increase of the
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M. Wang et al. / Composites Science and Technology 68 (2008) 2498–2502
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Fig. 1. (a) The complex viscosity versus frequency with different MWNT loadings, and (b) the complex viscosity versus MWNT loading at various frequencies for PA6/ MWNT composites at 225 °C.
complex viscosities of the composites. Therefore, the |g*| of the PA6 composites was increased to a large degree as the MWNT loading reaches 1 wt% or higher. The frequency dependence of the complex viscosities of the neat PA6 and the PA6/MWNT (0.5 wt%) composites measured at various temperatures is shown in Fig. 2. It can be seen that the |g*| decreases with increasing temperature for both the neat PA6 and the PA6/MWNT composite, indicating that the free volume of the neat PA6 and its composites increases with increasing temperature and results in the decreased viscosity. It can also be observed that the temperature has much effect on the rheological properties of the PA6 composites, especially at high shear force region. For neat PA6 (Fig. 2a), the complex viscosity remains almost constant under frequency between 0.01 and 10 rad/s, and begins to slightly level off above 10 rad/s. However, with increasing the frequency the |g*| for the PA6 composites significantly but steadily decreases over the whole frequency range investigated (Fig. 2b). The polymer melt can be taken as a transient physically cross-linked network of entangled random polymer coils, and the physical cross-linking sites could be destroyed due to the orientation of polymer chains under shear force. In neat PA6 melt, the rebuilding of polymer chain entanglements can keep up with the destroying of the physical cross-linking sites at low shear force region, thus the viscosity remains almost constant showing a newtonian behavior; as the shear rate increases to a certain extent, the number of chain entanglements reduces, and the viscosity begins to decrease with
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Fig. 2. The frequency dependence of the complex viscosity for (a) neat PA6 and (b) PA6/MWNT (0.5 wt%) composites at different temperatures.
increasing shear rate showing a shear thinning behavior. However, the PA6 composite with 0.5 wt% MWNT shows a non-newtonian behavior even at a very low shear rate. It is obvious that the rigid MWNT bundles in PA6 matrix tend to orient under shear force and disturb the formation of the polymer chain entanglements, resulting in the strong shear thinning behavior for PA6 composites [20,23]. Similar results were observed for the composites with 0.2, 1 and 2 wt% MWNT (not shown here for abbreviation). In this study, the shear thinning exponents (n) and the activation energy (Ea) for flow process of the PA6/MWNT composites were used to quantitatively describe the influence of MWNT on the rheological properties of PA6 composites. The shear thinning exponents can be obtained from the power law relationship of |g*| = Axn (where A is a sample specific pre-exponential factor; and x is the oscillation frequency of the rheometer equivalent to shear rate) by fitting a straight line to the data at low frequency in the plot of log |g*| versus log(x) [29]. The n values for the neat PA6 and the composites are listed in Table 1 as a function of MWNT loading. It can be seen that the shear thinning exponents steadily decrease with increasing MWNT content, indicating that shear thinning behavior of the PA6/MWNT composites significantly depends on MWNT loading: the higher the MWNT loading, the stronger shear thinning behavior exists in the composites. The activation energy Ea of flow process for the neat PA6 and its composites was estimated by the plot slope of log g0 (T) versus 1/T, according to the Arrhenius equation, g0 ðTÞ ¼ KeEa =RT , where g0 is the zero shear rate viscosity, R is the molar gas constant (8.314 J mol1 K1), and T is the temperature (K). The zero shear
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M. Wang et al. / Composites Science and Technology 68 (2008) 2498–2502 Table 1 The shear thinning exponent n, the activation energy Ea, and the low frequency slopes of G0 and G00 versus x for PA6/MWNT composites with different MWNT loadings in 225 °C Materials Neat PA6 PA6/MWNT PA6/MWNT PA6/MWNT PA6/MWNT
0.2 wt% 0.5 wt% 1 wt% 2 wt%
n
Ea (kJ mol1)
Slope of G0
Slope of G00
0.02 0.09 0.13 0.14 0.14
36.15 42.58 46.71 77.92 90.83
1.15 0.79 0.68 0.52 0.43
0.98 0.81 0.77 0.73 0.71
rate viscosities g0 were determined by the shear rate independent viscosity plateau at low shear rates from the flow curves (i.e. apparent viscosity g versus shear rate x) obtained in the steady shear rate sweep experiments. The activation energy values (shown in Table 1) of the PA6/MWNT composites increase with increasing MWNT loading, suggesting that the incorporation of MWNT into the PA6 matrix leads to more rigid and stiffer polymer chains in the composite systems and results in higher activation energy for the flow process. The storage modulus (G0 ) and loss modulus (G00 ) of the PA6/ MWNT composites at 225 °C as a function of frequency are shown in Fig. 3. It is apparent that MWNT has a distinct effect on the rheological behavior of PA6 matrix, even at loading level as low as 0.2 wt%. The storage modulus and loss modulus of the PA6/MWNT composites were significantly improved relative to the PA6 matrix, particularly at low frequency. It is known that the polymer chains
a
are fully relaxed and exhibit characteristic terminal flow behavior with the power law relation of approximately G0 x2 and G00 x for linearly homo-dispersed polymer melts [30]. The neat PA6 exhibits homopolymer-like terminal behavior at low frequencies as shown in Fig. 3 with the scaling properties of G0 x1.15 and G00 x0.98. The low frequency power law indexes of G0 (x) and G00 (x) for neat PA6 are smaller than 2 and 1, respectively, probably due to the polydispersity of polymer chains [18]. With the incorporation of MWNT into PA6 matrix, the dependence of G0 and G00 on x becomes comparatively weak at low frequency, and nonterminal solid-like rheological behavior were observed. The slopes of the terminal zone of G0 and G00 are also listed in Table 1. The flow curves for the PA6/MWNT composites can be expressed by a power law relation of G0 x0.790.43 and G00 x0.810.71. Compared to the case for neat PA6, the decrease in the slopes of G0 and G00 for the composites can be explained by the microstructural changes of the polymer matrix due to incorporation of MWNT. The nanotube– nanotube and strong polymer–nanotube (interfacial) interactions increase with increasing MWNT content [16,17], and lead to the formation of the interconnected or network-like structures of MWNT in the polymer composites which restrains the long-range motion of polymer chains and results in the pseudo-solidlike behavior [12,31,32]. As reported in the literature [33], the rheological percolation threshold for PA6/MWNT composites is about 1 wt% of carbon nanotubes. Therefore, the PA6 composites undergo a transition from liquid-like to solid-like viscoelasticity with the increase of MWNT loading at low frequency region. At high frequencies, the effect of the nanotubes on the rheological behavior
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Fig. 3. The frequency dependence of (a) storage modulus and (b) loss modulus of PA6/MWNT composites with different MWNT loadings at 225 °C.
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is relatively weak as shown in Fig. 3, suggesting that the MWNT does not significantly influence the short-range dynamics of the PA6 chains. Therefore, the presence of nanotubes has a substantial influence on large-scale polymer chain relaxations but has little effect on short-range motion of polymer chains. The variation of loss factor (tan d, G00 /G0 ) with frequency for PA6 composites is shown in Fig. 4a. It can be seen that the tan d decreases with increasing the MWNT loading, indicating the improved elastic properties by introducing MWNT into PA6 matrix. At high frequency region, the decrease of tan d with increasing frequency was attributed to the partial orientation of polymer chains caused by shear deformation. The tan d maximum for PA6 composites shifted to higher frequency compared with that of the neat PA6, probably implying the changes in the microstructures and the formation of network structures. The property enhancement of PA6 due to the addition of MWNT can be further improved by the plot of the phase angle (d) versus the complex modulus (|G*|), which is known as the van Gurp-Palmen plot in the literature [34], as shown in Fig. 4b. The PA6/MWNT composites exhibited smaller d values with increasing MWNT loading, implying that the incorporation of MWNT enhanced the elastic property of the PA6/MWNT composites.
4. Conclusions Melt rheological properties of PA6 and its composites with various MWNT loadings have been systematically studied in this work. Melt rheological results show that at low frequency the MWNT has a substantial influence on the linear viscoelastic properties of the PA6/MWNT composites, but only has a modest effect at high frequency. It has been found that the incorporation of MWNT in PA6 matrix results in higher complex viscosities, storage modulus, loss modulus, and lower loss factor than those of neat PA6, especially in low frequency region. Strong shear thinning behavior is observed in PA6/MWNT composites, probably due to the orientation of rigid polymer chains caused by the addition of MWNT. A gradual decrease in the terminal zone slope of G0 illustrates that the composites undergo a transition from liquid-like to solid-like viscoelasticity with the increase of MWNT loading, and the network-like structures of MWNT are formed in the polymer composites. Acknowledgements The authors thank financial support from the National Natural Science Foundation of China (20774019), Shanghai Leading Aca-
demic Discipline Project (Project Number: B113), and the Opening Project of Key Laboratory of Biomedical Polymers of Ministry of Education, Wuhan University (20070503). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Melt rheological properties of nylon 6/multi-walled carbon nanotube composites Min Wang a, Weizhi Wang a, Tianxi Liu a,*, Wei-De Zhang b a Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, People’s Republic of China b Nano Science Research Center, College of Chemistry, South China University of Technology, Guangzhou 510640, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 18 September 2007 Received in revised form 18 April 2008 Accepted 2 May 2008 Available online 8 May 2008 Keywords: A. Carbon nanotubes A. Nano composites A. Polymer matrix composites (PMCs) D. Rheology
a b s t r a c t Nylon 6 (PA6) composites with different loadings of multi-walled carbon nanotubes (MWNT) were prepared by melt compounding technique. Melt rheological properties of PA6/MWNT composites were studied in linear viscoelastic response regions. The incorporation of MWNT into PA6 matrix resulted in higher complex viscosities (|g*|), storage modulus (G0 ), loss modulus (G00 ), and lower loss factor (tan d) than those of neat PA6, especially in low frequency region. The orientation of rigid molecular chains in the composites introduced by the addition of MWNT induced a strong shear thinning behavior and an increasing activation energy for the flow process. With the increase of MWNT loading, the composites experienced a transition from liquid-like to solid-like viscoelasticity. The decrease in the slope of the plot of log G0 versus log G00 with increasing MWNT loading suggested the microstructural changes of the polymer matrix due to incorporation of MWNT. At low frequencies, nonterminal solid-like rheological behavior of PA6 composites were observed and attributed to the formation of the network-like structures of MWNT in the polymer composites. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Polymer nanocomposites made by incorporating a tiny quantity of nanomaterial into a polymeric matrix have attracted much attention worldwide for their multi-functional, outstanding thermal, electrical and mechanical properties [1–3]. From the discovery by Iijima in 1991 [4], high aspect ratio carbon nanotubes (CNT) including single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) have been considered as the ideal enhancement fillers in polymer composites due to their high mechanical strength, and high electrical and thermal conductivity [5–9]. To achieve good performance for CNT filled polymer composites, homogeneous dispersion of the nanotubes in the polymer matrix and strong interfacial interaction between nanotubes and polymer remain two major challenges. At present, three main processing techniques have been usually used to fabricate CNT reinforced polymer composites: solution mixing or coagulation, in situ polymerization, and melt compounding [10–16]. Among these processing techniques, melt compounding has been accepted as the simplest and most effective method from an industrial perspective, because this process makes it possible to fabricate high performance composites at low cost, and also facilitates commercial scale-up. In our previous work [16,17], nylon 6 (PA6)/MWNT composites prepared by melt compounding approach show great * Corresponding author. Tel.: +86 21 5566 4197; fax: +86 21 6564 0293. E-mail address: [email protected] (T.X. Liu). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.002
improvement in mechanical properties due to the homogeneous dispersion of MWNT in the matrix as well as the strong interfacial interaction between MWNT and polymeric matrix. The mechanical testing shows that, compared with neat PA6, the elastic modulus and the yield strength of the composite are greatly improved by about 214% and 162% with addition of only small amount (2 wt%) of MWNT into the PA6 matrix. The rheological behavior of polymer composites is of great importance in polymer processing, particularly for the analysis and design of processing operations as well as understanding the relationship between structures and properties of polymer composites. Much research has been done on the rheological behavior of various polymer nanocomposites, especially for the two-dimensional nanoclay-based polymer composites; however, the rheological properties of one-dimensional CNT reinforced polymer composites have rarely been investigated and still remain much less explored to date. Krishnamoorti et al. [18] observed nonterminal low frequency rheological behavior in the composites containing nanoclay; and other researchers [19] also observed this nonterminal solid-like rheological behavior in polymer composites containing carbon fibers. Kharchenko et al. [20] reported that the viscosity of polypropylene/MWNT composites decreased strongly with increasing shear rate, and the composites exhibited large and negative normal stress. Du et al. [21] prepared poly(methyl methacrylate)/SWNT composites by coagulation method and observed that the rheological percolation threshold was smaller than that of electrical conductivity; and they attributed this difference
M. Wang et al. / Composites Science and Technology 68 (2008) 2498–2502
to smaller nanotube–nanotube distance required for electrical conductivity as compared to that required to impede polymer chain mobility. Hu et al. [22] also applied coagulation method to prepare poly(ethylene terephthalate) (PET)/MWNT composites. A coating layer on MWNT by PET chains was observed and considered to be the evidence of interfacial interaction between MWNT and PET chains; and they reported the melt composites showed transition from liquid-like to solid-like viscoelasticity. Kim et al. [23] investigated rheological properties of poly(ethylene 2,6-naphthalate)/MWNT composites prepared by melt compounding process, and observed the nonterminal behavior at higher MWNT content and attributed it to the formation of the interconnected structures of MWNT in the composites. The rheological properties of CNT reinforced polymer composites are related to the materials’ microstructures, the state of nanotube dispersion and orientation in the matrix, as well as the interactions between nanotubes and polymer chains. Therefore, the rheological properties should be characterized to correlate these characters with the composite properties and thus to optimize the processing conditions for achieving high performance polymer composites. Nylon 6 is an important kind of commercial polymer which is widely used in engineering plastic products, however, few reports could be found in the literature regarding the effect of CNT on the rheological properties of nylon-based composites [24]. In our previous report, the preparation, mechanical properties, crystallization and melting behavior of the PA6/MWNT composites have been investigated [16,17,25]. In addition, the PA6 composites reinforced by the novel CNT-based hybrid fillers have been prepared and studied in our early report [26,27]. In this study, the rheological properties in linear viscoelasticity region for PA6 composites with different MWNT loadings have been systematically characterized to investigate the effect of MWNT on the rheological properties of PA6 matrix.
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shear rate range to obtain the apparent viscosities (g) for the neat PA6 and the PA6/MWNT composites. To maintain all the rheological experiments in the linear viscoelastic region, strain sweeps were performed in advance for each sample, and consequently a strain of 1% was selected for all frequency sweep data in this work. The rheological properties were reproducible after repeated temperature cycling and frequency sweep, indicating that there is almost no chain degradation during measurements. In order to further confirm whether molecular weight degradation occurs due to melt rheology measurements, the ideal experiment to perform would be to extract the carbon nanotubes out of the composites and measure the molecular weight of the remnant. However, such an experiment would be exceedingly difficult to perform given the fact that the carbon nanotubes are only nanometers in size. Furthermore, the possibility that some polymer molecules remain attached to the large surface area of carbon nanotubes cannot be ruled out, thus rendering complete extraction or separation improbable. The comparison was finally made between the composite (with 1 wt% MWNT, after repeated extraction using formic acid and centrifugation) and the neat PA6 after being experienced melt rheology tests, by measuring their respective intrinsic viscosities. The intrinsic viscosity, which is obtained from dilute solution measurements, is known to be particularly sensitive to the high molecular weight fractions of polymers with a molecular weight distribution and thus would be useful for signaling any significant degradation. Consequently, the intrinsic viscosities, [g], of neat PA6 and the composite containing 1 wt% carbon nanotubes after melt rheology measurements were determined to be 0.82 dL/ g and 0.81 dL/g, respectively, using formic acid as the solvent. It can be seen that the difference between the intrinsic viscosity values is within experimental scatter. Given the typical uncertainties in such measurements, it can be suggested that the melt rheology measurements do not significantly degrade the molecular weight of PA6 matrix.
2. Experimental 3. Results and discussion 2.1. Materials and preparation of PA6/MWNT composites PA6 pellets (Grade SF1080A) used here are the product of Ube Industries under license from Toyota. The MWNT were prepared by catalytic chemical vapor deposition of methane on Co–Mo/MgO catalysts [28]. The as-prepared MWNT were purified by dissolving the catalyst in hydrochloride acid followed by refluxing in nitric acid for increasing more carboxylic and hydroxyl groups [16,17]. PA6 composites containing different MWNT contents (from 0.2 to 2 wt%) were prepared via melt compounding method using a Brabender twin-screw extruder at 250 °C for 10 min with a screw speed of 100 rpm. The sample disks (with thickness of 1.7 mm and diameter of 25 mm) were prepared using a mixing molder (LMM, ATLAS Electric Devices Co.) with the extruded pellets at 245 °C for 5 min, and the mold temperature was 200 °C. 2.2. Rheological measurements Rheological properties of PA6/MWNT composites and neat PA6 were measured on a strain-controlled rheometer (ARES, TA Instruments) over a temperature range of 225–245 °C. Small-amplitude oscillatory shear measurements were performed using parallelplate geometry with the plate diameter of 25 mm and the plate gap setting of 1.0 mm to get the complex viscosities (|g*|), the storage moduli (G0 ), the loss moduli (G00 ) and the loss factor (tan d), by applying a time-dependent strain, c(t) = c0 sin(xt), and measuring resultant shear stress, r(t), which is interpreted as: r(t) = c0 [G0 sin(xt) + G00 cos(xt)], where x is the oscillation frequency of the rheometer. Steady rate sweep tests were also performed at low
The main melting region of neat PA6 and its MWNT composites begins at about 200 °C and ends at about 225 °C, as observed by differential scanning calorimetry results [16,17]. The decomposition temperature of PA6 and its MWNT composites is higher than 378 °C by thermogravimetric analysis. Hence, in this study the melt rheological measurements for PA6 and its MWNT composite samples were carried out between 225 and 245 °C in order to investigate the temperature effect on the rheological properties. The complex viscosities (|g*|) at 225 °C for PA6 and its MWNT composites as a function of frequency are presented in Fig. 1a, and the variation of the |g*| with the MWNT loading at different frequencies is shown in Fig. 1b. The complex viscosities decrease with the increase of frequency, indicating a non-newtonian behavior and pseudoplastic characteristics of PA6/MWNT composites. The shear thinning behavior may be attributed to the orientation of the rigid CNT molecular chains which disturbs the formation of PA6 chain entanglements in the composites during the applied shear force in the PA6/MWNT composites. Similar phenomenon was observed for other CNT reinforced polymer composites [20,23]. It can be seen from Fig. 1 that the incorporation of a small quantity (0.2 or 0.5 wt%) of MWNT into the PA6 matrix slightly decreases the complex viscosity compared to that of the neat PA6. This phenomenon may be attributed to the formation of the viscous surface layers around the dispersed MWNT thus leading to an increase in the free volume and making it easier to flow. When further increasing the MWNT loading, the physical interactions between the neighboring MWNT and/or between the MWNT and the matrix increase remarkably, which result in the increase of the
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Fig. 1. (a) The complex viscosity versus frequency with different MWNT loadings, and (b) the complex viscosity versus MWNT loading at various frequencies for PA6/ MWNT composites at 225 °C.
complex viscosities of the composites. Therefore, the |g*| of the PA6 composites was increased to a large degree as the MWNT loading reaches 1 wt% or higher. The frequency dependence of the complex viscosities of the neat PA6 and the PA6/MWNT (0.5 wt%) composites measured at various temperatures is shown in Fig. 2. It can be seen that the |g*| decreases with increasing temperature for both the neat PA6 and the PA6/MWNT composite, indicating that the free volume of the neat PA6 and its composites increases with increasing temperature and results in the decreased viscosity. It can also be observed that the temperature has much effect on the rheological properties of the PA6 composites, especially at high shear force region. For neat PA6 (Fig. 2a), the complex viscosity remains almost constant under frequency between 0.01 and 10 rad/s, and begins to slightly level off above 10 rad/s. However, with increasing the frequency the |g*| for the PA6 composites significantly but steadily decreases over the whole frequency range investigated (Fig. 2b). The polymer melt can be taken as a transient physically cross-linked network of entangled random polymer coils, and the physical cross-linking sites could be destroyed due to the orientation of polymer chains under shear force. In neat PA6 melt, the rebuilding of polymer chain entanglements can keep up with the destroying of the physical cross-linking sites at low shear force region, thus the viscosity remains almost constant showing a newtonian behavior; as the shear rate increases to a certain extent, the number of chain entanglements reduces, and the viscosity begins to decrease with
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Fig. 2. The frequency dependence of the complex viscosity for (a) neat PA6 and (b) PA6/MWNT (0.5 wt%) composites at different temperatures.
increasing shear rate showing a shear thinning behavior. However, the PA6 composite with 0.5 wt% MWNT shows a non-newtonian behavior even at a very low shear rate. It is obvious that the rigid MWNT bundles in PA6 matrix tend to orient under shear force and disturb the formation of the polymer chain entanglements, resulting in the strong shear thinning behavior for PA6 composites [20,23]. Similar results were observed for the composites with 0.2, 1 and 2 wt% MWNT (not shown here for abbreviation). In this study, the shear thinning exponents (n) and the activation energy (Ea) for flow process of the PA6/MWNT composites were used to quantitatively describe the influence of MWNT on the rheological properties of PA6 composites. The shear thinning exponents can be obtained from the power law relationship of |g*| = Axn (where A is a sample specific pre-exponential factor; and x is the oscillation frequency of the rheometer equivalent to shear rate) by fitting a straight line to the data at low frequency in the plot of log |g*| versus log(x) [29]. The n values for the neat PA6 and the composites are listed in Table 1 as a function of MWNT loading. It can be seen that the shear thinning exponents steadily decrease with increasing MWNT content, indicating that shear thinning behavior of the PA6/MWNT composites significantly depends on MWNT loading: the higher the MWNT loading, the stronger shear thinning behavior exists in the composites. The activation energy Ea of flow process for the neat PA6 and its composites was estimated by the plot slope of log g0 (T) versus 1/T, according to the Arrhenius equation, g0 ðTÞ ¼ KeEa =RT , where g0 is the zero shear rate viscosity, R is the molar gas constant (8.314 J mol1 K1), and T is the temperature (K). The zero shear
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M. Wang et al. / Composites Science and Technology 68 (2008) 2498–2502 Table 1 The shear thinning exponent n, the activation energy Ea, and the low frequency slopes of G0 and G00 versus x for PA6/MWNT composites with different MWNT loadings in 225 °C Materials Neat PA6 PA6/MWNT PA6/MWNT PA6/MWNT PA6/MWNT
0.2 wt% 0.5 wt% 1 wt% 2 wt%
n
Ea (kJ mol1)
Slope of G0
Slope of G00
0.02 0.09 0.13 0.14 0.14
36.15 42.58 46.71 77.92 90.83
1.15 0.79 0.68 0.52 0.43
0.98 0.81 0.77 0.73 0.71
rate viscosities g0 were determined by the shear rate independent viscosity plateau at low shear rates from the flow curves (i.e. apparent viscosity g versus shear rate x) obtained in the steady shear rate sweep experiments. The activation energy values (shown in Table 1) of the PA6/MWNT composites increase with increasing MWNT loading, suggesting that the incorporation of MWNT into the PA6 matrix leads to more rigid and stiffer polymer chains in the composite systems and results in higher activation energy for the flow process. The storage modulus (G0 ) and loss modulus (G00 ) of the PA6/ MWNT composites at 225 °C as a function of frequency are shown in Fig. 3. It is apparent that MWNT has a distinct effect on the rheological behavior of PA6 matrix, even at loading level as low as 0.2 wt%. The storage modulus and loss modulus of the PA6/MWNT composites were significantly improved relative to the PA6 matrix, particularly at low frequency. It is known that the polymer chains
a
are fully relaxed and exhibit characteristic terminal flow behavior with the power law relation of approximately G0 x2 and G00 x for linearly homo-dispersed polymer melts [30]. The neat PA6 exhibits homopolymer-like terminal behavior at low frequencies as shown in Fig. 3 with the scaling properties of G0 x1.15 and G00 x0.98. The low frequency power law indexes of G0 (x) and G00 (x) for neat PA6 are smaller than 2 and 1, respectively, probably due to the polydispersity of polymer chains [18]. With the incorporation of MWNT into PA6 matrix, the dependence of G0 and G00 on x becomes comparatively weak at low frequency, and nonterminal solid-like rheological behavior were observed. The slopes of the terminal zone of G0 and G00 are also listed in Table 1. The flow curves for the PA6/MWNT composites can be expressed by a power law relation of G0 x0.790.43 and G00 x0.810.71. Compared to the case for neat PA6, the decrease in the slopes of G0 and G00 for the composites can be explained by the microstructural changes of the polymer matrix due to incorporation of MWNT. The nanotube– nanotube and strong polymer–nanotube (interfacial) interactions increase with increasing MWNT content [16,17], and lead to the formation of the interconnected or network-like structures of MWNT in the polymer composites which restrains the long-range motion of polymer chains and results in the pseudo-solidlike behavior [12,31,32]. As reported in the literature [33], the rheological percolation threshold for PA6/MWNT composites is about 1 wt% of carbon nanotubes. Therefore, the PA6 composites undergo a transition from liquid-like to solid-like viscoelasticity with the increase of MWNT loading at low frequency region. At high frequencies, the effect of the nanotubes on the rheological behavior
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is relatively weak as shown in Fig. 3, suggesting that the MWNT does not significantly influence the short-range dynamics of the PA6 chains. Therefore, the presence of nanotubes has a substantial influence on large-scale polymer chain relaxations but has little effect on short-range motion of polymer chains. The variation of loss factor (tan d, G00 /G0 ) with frequency for PA6 composites is shown in Fig. 4a. It can be seen that the tan d decreases with increasing the MWNT loading, indicating the improved elastic properties by introducing MWNT into PA6 matrix. At high frequency region, the decrease of tan d with increasing frequency was attributed to the partial orientation of polymer chains caused by shear deformation. The tan d maximum for PA6 composites shifted to higher frequency compared with that of the neat PA6, probably implying the changes in the microstructures and the formation of network structures. The property enhancement of PA6 due to the addition of MWNT can be further improved by the plot of the phase angle (d) versus the complex modulus (|G*|), which is known as the van Gurp-Palmen plot in the literature [34], as shown in Fig. 4b. The PA6/MWNT composites exhibited smaller d values with increasing MWNT loading, implying that the incorporation of MWNT enhanced the elastic property of the PA6/MWNT composites.
4. Conclusions Melt rheological properties of PA6 and its composites with various MWNT loadings have been systematically studied in this work. Melt rheological results show that at low frequency the MWNT has a substantial influence on the linear viscoelastic properties of the PA6/MWNT composites, but only has a modest effect at high frequency. It has been found that the incorporation of MWNT in PA6 matrix results in higher complex viscosities, storage modulus, loss modulus, and lower loss factor than those of neat PA6, especially in low frequency region. Strong shear thinning behavior is observed in PA6/MWNT composites, probably due to the orientation of rigid polymer chains caused by the addition of MWNT. A gradual decrease in the terminal zone slope of G0 illustrates that the composites undergo a transition from liquid-like to solid-like viscoelasticity with the increase of MWNT loading, and the network-like structures of MWNT are formed in the polymer composites. Acknowledgements The authors thank financial support from the National Natural Science Foundation of China (20774019), Shanghai Leading Aca-
demic Discipline Project (Project Number: B113), and the Opening Project of Key Laboratory of Biomedical Polymers of Ministry of Education, Wuhan University (20070503). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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