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Composites Science and Technology 72 (2012) 482–488
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Effect of block-copolymer dispersants on properties of carbon nanotube/epoxy systems M.R. Loos a, J. Yang a, D.L. Feke b, I. Manas-Zloczower a,⇑ a b
Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
a r t i c l e
i n f o
Article history: Received 23 April 2011 Received in revised form 23 November 2011 Accepted 30 November 2011 Available online 8 December 2011 Keywords: A. Carbon nanotubes A. Nano composites A. Polymer–matrix composites (PMCs) B. Mechanical properties Wind turbine blades
a b s t r a c t The effects of the addition of eight different block copolymers on the dispersion stability of multi-walled carbon nanotubes (MWCNTs) are reported. Suspensions of CNTs in different components of an epoxy system have been prepared using a tip sonicator and different amounts of block copolymers. The resistance to sedimentation of MWCNTs in various media was systematically investigated by using a centrifugation technique. Block copolymers that result in dispersions of MWCNTs in epoxy and hardener stable for more than 1 week have been obtained. Dispersions using a single or a combination of two different dispersing agents have been used for the fabrication of MWCNT nanocomposites. The effect of different preparation routes and use of block copolymers on the tensile properties and surface resistivity of the composites have been evaluated. The results obtained have been related with the dispersion stability of the MWCNTs in the epoxy components. Published by Elsevier Ltd.
1. Introduction Carbon nanotubes (CNTs) are considered to be the ultimate reinforcement agent for high performance polymer composites [1]. However, processing difficulties associated with obtaining homogeneous dispersions of CNTs or the potentially weak interfacial interaction between the CNTs and the polymer matrix often result in composite properties that fall short of the expectations associated with the promise of CNTs. As a result, advances in the development of materials for strategic applications may be hampered by the lack of processing approaches that overcome these difficulties. Consider, for example, the case of wind energy. Since its early development in the 1980s, the global market for wind energy has expanded exponentially. In the period 1990–2007 the total world wind electricity capacity has grown 50 times and is predicted to increase over the 2008 level by ten-fold by 2030, and twenty-fold by 2050, [2]. In order to achieve the expansion expected in this area, there is a need for the development of stronger and lighter materials which will enable manufacturing of blades for larger rotors. Carbon nanotube based composites could enable larger area rotors to be cost-effective. While a great deal of work has been done toward using CNTs as a reinforcing agent in polymer composites, their homogeneous ⇑ Corresponding author. Tel.: +1 216 368 3596; fax: +1 216 368 4202. E-mail addresses:
[email protected] (M.R. Loos),
[email protected] (J. Yang),
[email protected] (D.L. Feke),
[email protected] (I. Manas-Zloczower). 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2011.11.034
dispersion within polymer matrices remains a challenge. Due to the strong attractive long-ranged van der Waals interactions, CNTs tend to aggregate [3]. Thus, the high specific surface area of carbon nanotubes that potentially can act as the desirable interface for stress transfer, has also the downside of inducing strong attractive forces between the CNTs themselves, leading to an excessive agglomeration behavior. The critical challenges in the carbon nanotube composites technology are controlling the interfacial interaction between filler and matrix and the homogeneous dispersion of the filler in the composite [3]. Of main importance is also the stability of CNT dispersions. The principal methods for achieving dispersion of nanotubes are: mechanical methods, covalent functionalization, and non-covalent modification by using small molecules and polymer dispersants [4]. Each method has its advantages and disadvantages. The mechanical methods do not permanently stabilize the dispersion. The covalent methods are efficient in stabilizing dispersions but inevitably lead to the disruption of the electronic conjugation in CNTs during the formation of new covalent bonds in the graphene sheet [4]. On the other hand, non-covalent functionalization, such as by using block copolymers, is particularly desirable, as it leaves the electronic structure and mechanical properties of CNTs intact [4]. To the best of our knowledge, the first work proposing the use of block copolymers as dispersing agents in CNT based composites was presented by Li and collaborators [5]. In that work CNTs were dispersed in a solvent with the aid of a single block copolymer, prior to the addition of the epoxy resin. The amount of block copolymer used was the same as the amount of CNTs. The authors claimed an improved
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M.R. Loos et al. / Composites Science and Technology 72 (2012) 482–488
dispersion of the CNTs in the system as evidenced through an enhancement in tensile properties. However, the stability of the CNT suspensions was not directly addressed. In another study, Cho and collaborators modified carbon fiber/epoxy composites with MWCNTs [6]. The CNTs were dispersed with the aid of a block copolymer and a solvent. The authors reported an enhancement of the thermomechanical properties of the composites due to the improved dispersion of the CNTs. The same group extended this work and investigated the dispersion of CNTs with different aspect ratios [7]. Again improved dispersion was obtained with the aid of a block copolymer and solvent. Similarly, with the aid of block copolymers, Zhao et al. improved the dispersion of CNTs in a glass fiber/epoxy composite [8]. The addition of dispersed CNTs enhanced the flexural strength of these composites. Due to the inherent processing difficulties, the use of CNTs for industrial applications has been limited. In the case of CNT reinforced thermoplastics, master batches comprising pellets with high concentrations of CNTs are a viable alternative. Due to the solid state of this material, the long-term stability of the dispersion during storage is not an issue. As a result, master batches of many thermoplastics containing CNTs are already commercially available [9]. The broadening of the same idea of master batches to thermoset resin precursors, i.e. liquid systems, is difficult and the necessity of maintaining CNTs stably dispersed for long-term periods becomes evident. At the present time, the number of thermoset resins containing CNTs commercially available in the form of master batches is very limited [9]. The long-term stability of CNTs dispersions has not often been addressed in the literature [10]. In addition, as thermosets are usually two-part systems, e.g. resin and curing agent, this brings about the possibility of dispersing the CNTs in either or both components with the aid of appropriate block copolymers. In light of the above, to increase the attractiveness of CNTs to industrial applications, there is a need for the development of strategies that result in homogeneously dispersed and long-term stable CNT-resin dispersions. This is especially the case for systems with a broad range of applications such as epoxy resins. This work presents an approach for preparing long-term stable suspensions of MWCNTs in different components of an epoxy system with the goal of developing a strategy for preparing master batches and composites with high strength to mass ratio. 2. Experimental 2.1. Materials In this study the epoxy matrix used is a modified diglycidyl ether of bisphenol-A (DGEBA, L135i, Hexion) with an amine hardener (RIMH 1366, Hexion). The MWCNTs (Baytubes C150P) were supplied by Bayer Material Science and exhibit an average diameter of 13 nm and a length larger than 1 lm. Spectroscopic analyses of these MWCNTs are available in the literature [11,12]. A number of block copolymers have been tested as dispersing agents for the epoxy system: BYK-2150, BYK-2155 and BYK-9077 are block-copolymers with pigment affinic groups (BYK-Chemie); BYK-9076 is an alkylammonium salt of a high molecular weight copolymer; L-7500 and L-7602 are polyalkyleneoxide modified polydimethylsiloxanes (Momentive); L-7607 is a siloxane polyalkyleneoxide copolymer and B60H consists of a polyvinyl butyral (Kuraray). All the materials were used as received. 2.2. Experimental methods Suspensions of MWCNTs in the epoxy resin or hardener have been prepared with different amounts of block copolymers relative
483
to the concentration of MWCNTs using a tip sonicator. In a typical experiment, 10 mg of MWCNTs were added to 10 g of epoxy/hardener and dispersed by using simultaneous sonication (Sonics CP750, 165 W) and magnetic stirring for 10 min. Suspensions without the addition of dispersing agents were prepared as a control. The concentration of MWCNTs was fixed at 0.1 wt.% in relation to epoxy resin, which renders composites reinforced with 0.075 wt.% MWCNTs. The amount of dispersing agent has been 1 (one times), 5 or 10 the amount of MWCNTs. Aiming to observe the resistance to sedimentation of the dispersions obtained, photographs of the undisturbed suspensions have been taken over a time frame of 0–168 h. In order to confirm their stability, the suspensions have been subjected to centrifugation at 150 g for 2 h. Based on the preliminary dispersion results, epoxy composites have been prepared via different routes for mechanical characterization. For the preparation of the samples, MWCNTs (0.10 wt.% in relation to epoxy) and block copolymers were dispersed in epoxy or hardener using simultaneous magnetic stirring (400 rpm) and sonication during 30 min. The suspension of MWCNTs was then degassed by applying vacuum for 30 min to remove trapped bubbles. Further, the suspension was mixed with the second component of the system for 3 min by stirring and the mixture was allowed to cure at room temperature overnight and finally post-cured at 90 °C for 6 h. Reference samples of neat resin and resin containing block copolymers were also prepared following the same route. Composites have been also prepared using a combination of two different dispersing agents. Specifically, MWCNTs have been dispersed in epoxy resin using L-7602 while BYK-9077 has been added to the hardener. The influence of the processing route above on the tensile properties of the composites has also been analyzed. 2.3. Characterization The tensile properties of the composites were investigated according to ASTM 638–03 using an Instron 1011 universal tensile tester at a crosshead speed of 1 mm/min. Surface resistivity measurements were performed using a resistivity meter model 291 made by Monroe Electronics. Specimens with dimensions 70 70 3 mm have been characterized. The reported values correspond to an average of four specimens. Fracture surfaces of the composite samples were observed using a scanning electron microscope (SEM) (JEOL JSM-6510LV) with an operating voltage of 30 kV. 3. Results and discussion 3.1. Screening of dispersing agents The resistance to sedimentation of MWCNTs dispersed in epoxy and hardener is summarized in Fig. 1. Dispersions stable for at least 168 h were obtained for the neat epoxy and by using 1 of any of the eight block copolymers. The finding that all block copolymers are suitable for the stabilization of the suspensions of the CNTs may result from the elevated viscosity of the system which hinders the reagglomeration of the fillers. If the block copolymers actually do not aid stability, then by increasing the amount of surfactant (which decreases suspension viscosity), gravity-driven sedimentation of MWCNTs clusters might be induced. Moreover, even with the aid of block copolymers, the sedimentation of large aggregates of CNTs can be expected due to the density differences. Considering the dispersions in hardener, only BYK-9076 was not effective for the stabilization of the suspensions. As shown in Fig. 1, the CNTs are stably dispersed in the neat epoxy and hardener, thus the result obtained by the addition of various block copolymers via this method are not sufficient to confirm a real
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Neat
BYK-2150
L-7500
L-7607
L-7602
B-60H
BYK-9077
BYK-2155
BYK-9076
Epoxy
Epoxy
Epoxy
Epoxy
Epoxy
Epoxy
Epoxy
Epoxy
Epoxy
Neat
BYK-2150
L-7500
L-7607
L-7602
B-60H
BYK-9077
BYK-2155
BYK-9076
Hardener
Hardener
Hardener
Hardener
Hardener
Hardener
Hardener
Hardener
Hardener
Fig. 1. Dispersion stability of MWCNTs in epoxy and hardener after 168 h. The amount of dispersing agent used is the same as the CNTs (1). Differences in the color of some images are due to lighting variations only. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
positive effect of the aid of dispersing agents on the stability of the suspensions. Therefore, to further check the stability of the suspensions of nanotubes, a more extreme approach has been undertaken by having the dispersions centrifuged at 150 g during 2 h. Again suspensions of MWCNTs in different components of epoxy have been prepared with the aid of eight different dispersing agents. The resistance to sedimentation of the MWCNTs in different media obtained using the centrifuge method is summarized in Table 1. According to this qualitative methodology, we consider that if a suspension does not exhibit visible phase separation after centrifugation, the block copolymer used during dispersion is suitable for the stabilization of CNTs. The neat epoxy results in unstable CNT dispersions; however all of the different dispersing agents studied are suitable for the stabilization of MWCNTs suspensions. Note that the concentration of dispersing agents is relatively low: 1 the amount of CNTs. Using the screening method via centrifugation, only four of the dispersing agents, namely BYK-2155, L7500, L-7602 and B-60H, were found to be suitable for stabilizing CNTs into the hardener. Therefore the use of centrifuge has proved to be a reliable and faster method for the analysis compared to leaving the samples undisturbed for several days. 3.2. Tensile properties of CNT/epoxy composites Fig. 2 shows the effect of the various surfactants on the tensile properties of the epoxy composites containing 0.075 wt.% Table 1 Stability state observed using the centrifuge method for dispersions of CNTs in epoxy and hardener using various dispersing agents at 1 the CNT concentration. Key: (U) = stable; () = unstable. Dispersing Agent
Epoxy
Hardener
None BYK-2150 BYK-2155 BYK-9076 BYK-9077 L-7500 L-7602 L-7607 B-60H
U U U U U U U U
U U U U
MWCNTs. Control samples of neat epoxy and epoxy with block copolymers have also been also analyzed. Interestingly, dispersion of CNTs in the hardener rather than in the epoxy, leads to an improvement in the Young’s modulus of the composites (Fig. 2a) which can be attributed to an improved interaction between filler and matrix. The non-covalent interactions of the nanotubes with the aromatic groups in the hardener are based on the ability of the extended p-system of the carbon nanotubes sidewall to bind guest molecules via p–p-stacking interactions [4]. There are only few reports in literature on the dispersion stability of carbon nanotubes in the hardener [10,13–16]. Our screening of the suspensions via centrifugation revealed that the suspensions of CNTs in the hardener do not exhibit long-term stability, unless an appropriate block copolymer is added. The use of L-7602 results in a stiffening of the matrix, increasing the Young’s modulus from 2.64 to 3.01 GPa (Fig. 2a). When the CNTs are added in the hardener with the aid of the same block copolymer, the modulus further increases to 3.09 GPa. Composites prepared by the dispersion of CNTs in epoxy with the aid of BYK9077 have a Young’s modulus of 3.06 GPa. The effect of CNTs and dispersing agent on the elongation at break of the composites is shown in Fig. 2b. The best enhancement is achieved by dispersing the CNTs in the hardener with the aid of BYK-9077, when the elongation at break increases from 8.62% (neat epoxy) to 10.60%. A careful analysis of Fig. 2b confirms that the improvement in the composite ductility is mainly due to the reinforcing effect of the CNTs. Note that the addition of BYK-9077 in the neat epoxy does not increase the elongation at break of the polymer. The overall enhancement in the tensile behavior of the samples prepared using L-7602 and BYK-9077 can be better observed in the stress–strain curves (Fig. 3a and b). The Young’s modulus of epoxy/CNTs composites can be estimated using well-known micromechanical models. The HalpinTsai model has been used successfully to calculate the modulus of CNT reinforced polymer composites [4]. For randomly oriented CNTs in a polymer matrix, the modulus of the composite is given by the following equations [17]
EC 3 1 þ 2ðl=dÞgL V NT 5 1 þ 2gT V NT þ ¼ 8 1 gT V NT Em 8 1 gL V NT
ð1Þ
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3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Epoxy + Disp. Agent (No CNTs) Epoxy + CNTs Hard. + CNTs
12
(b)
10 8 6 4 2
15 0 L7 50 0 L7 60 7 L7 60 2 B6 0 BY H K9 0 BY 77 K2 15 5
BY
N
on e
N o BY ne K2 15 0 L7 50 0 L7 60 7 L7 60 2 B6 0 BY H K9 0 BY 77 K2 15 5
0
K2
Young’s Modulus (GPa)
(a)
Epoxy + Disp. Agent (No CNTs) Epoxy + CNTs Hard. + CNTs
Elongation at break (%)
14
4.0
Fig. 2. (a) Effect of MWCNTs and block copolymers on the Young’s modulus and (b) elongation at break of epoxy based composites.
70
70
(a)
50 40 30 20
Epoxy L7602 (No CNTs) L7602 / Epoxy L7602 / Hardener
10 0 0
1
(b)
60
Stress (MPa)
Stress (MPa)
60
2
3
4
5
6
7
8
9
50 40 30 20
Epoxy BYK 9077 (No CNTs) BYK 9077 / Epoxy BYK 9077 / Hardener
10 0
10
Strain (%)
0
1
2
3
4
5
6
7
8
9
10 11
Strain (%)
Fig. 3. Representative stress–strain curves for the epoxy based composites prepared using (a) L-7602 and (b) BYK-9077.
where
gL ¼
ðENT =Em Þ 1 ðENT =Em Þ þ 2ðl=dÞ
gT ¼
ðENT =Em Þ 1 ðENT =Em Þ þ 2
ð2Þ
Here EC, Em, ENT are the elastic moduli of the composite, matrix and CNT respectively, VNT is the CNT volume fraction and, l and d are the length and average outer diameter of the nanotubes. Thostenson and Chou [17] modified the Halpin-Tsai theory towards its applicability to nanotube reinforced composites. Considering that the outer wall of the nanotubes effectively acts as a solid fiber, with the same deformation behavior, diameter and length of the nanotube, the parameters gL and gT can be expressed as
gL ¼
ðENT =Em Þ ðd=4tÞ ðENT =Em Þ þ ðl=2tÞ
gT ¼
ðENT =Em Þ ðd=4tÞ ðENT =Em Þ þ ðd=2tÞ
ð3Þ
where t is the thickness of graphite layer (0.34 nm). Also, the volume fraction of the nanotubes can be calculated according to
qNT 1 mNT 1 V NT ¼ 1 þ qm mNT
ð4Þ
where mNT is the weight fraction of the CNTs, and qNT and qm are the density of the CNTs and the polymer matrix respectively. The moduli of the composites was calculated considering Em = 2.71 GPa, ENT = 900 GPa [17], l = 10 lm, d = 16 nm, qNT = 1.5 g/ cm3, qm = 1.19 g/cm3 and VNT = 0.056 vol.%. The calculated Young’s modulus obtained using the Halpin-Tsai (Eqs. (1) and (2)) and modified Halpin-Tsai model (Eqs. (1) and (3)) have values of EC = 2.87 GPa and EC = 2.73 GPa, respectively. The difference in the values obtained using the two models arise from the assumption in Eq. (3) that the outmost wall of the CNT, rather than all walls, carries the load. The experimentally determined Young’s moduli for the
samples prepared by dispersing CNTs in the hardener without block copolymers and with the aid of L-7602 as well as the samples prepared by dispersing CNTs in the epoxy with the aid of BYK-9077 were above the theoretical predictions. Noteworthy is that the Halpin-Tsai equations assume perfect wetting of the CNTs by the epoxy matrix and a uniform distribution of the CNTs. Therefore these results suggest that the BYK-9077 and L-7602 improve the interfacial interaction between filler and matrix and also the dispersion. Carbon nanotubes possess kinetic (thermal) energy and are subjected to Brownian movement. As a consequence, after the dispersion steps the CNTs persistently encounter one another and can collide such that van der Waals forces could reagglomerate them, thereby destroying the dispersion. Including an additional source for inter-particle repulsion such as with block copolymers which are capable of preventing the CNTs from close contacts, the dispersion can persist longer despite the presence of Brownian motion. The enhancements observed in dispersion stability and in tensile properties obtained by using block copolymers can be explained in terms of specific interactions as outlined below. BYK-9077 is a high molecular weight copolymer with pigment
Fig. 4. Schematics for CNTs interactions with block copolymers [6].
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Table 2 Tensile properties of epoxy composites prepared using a combination of two dispersing agents. E: Epoxy; H: Hardener. Et: Young’s modulus. rM: Tensile strength. eB: Elongation at break.
S0 S1 S2 S3 S4 S5 S6 S7 S8
Preparation route
Et (GPa)
rM (MPa)
eB (%)
– CNT+L-7602+E CNT+L-7602+H CNT+BYK-9077+E CNT+BYK-9077+H CNT+L-7602+H BYK-9077+E CNT+BYK-9077+E L-7602+H CNT+L-7602+E BYK-9077+H CNT+BYK-9077+H L-7602+E
2.64 ± 0.21 2.78 ± 0.21 3.09 ± 0.13 3.06 ± 0.18 2.75 ± 0.27 3.09 ± 0.11
65.46 ± 1.57 61.40 ± 1.50 63.81 ± 0.92 61.48 ± 1.49 65.00 ± 1.81 60.71 ± 0.77
8.24 ± 0.83 7.26 ± 1.00 7.72 ± 0.43 8.97 ± 0.68 10.60 ± 0.25 5.93 ± 0.92
2.79 ± 0.19
61.41 ± 1.43
7.58 ± 1.20
2.88 ± 0.14
61.69 ± 2.15
7.93 ± 1.06
2.88 ± 0.13
62.11 ± 2.47
8.08 ± 0.46
affinic groups, consisting of a lyophobic and a lyophilic block. The lyophobic part adsorbs onto the surface of MWCNTs, while the lyophilic part is swollen by the solution, preventing agglomeration (Fig. 4). In addition, block copolymers may also act as interfacial binding agents improving stress transfer between the filler and the polymer matrix [6]. The block copolymer L-7602 is a polyalkyleneoxide modified polydimethylsiloxanes (PDMS). It consists of a flexible –Si–O–Si– hydrophobic main chain and hydrophilic pendant parts of PDMS [18]. The literature shows that the Si–O–Si chain interacts strongly with CNTs [19]. The flexible Si–O–Si chain can easily wrap onto the surface of CNTs via hydrophobic and other intermolecular interactions [19], while the hydrophilic part of
PDMS provides good dispersion in the aqueous solution and prevents CNTs aggregation through steric stabilization. The addition of BYK-9077 in the epoxy and the addition of L-7602 in the hardener lead to an improvement in the tensile properties of the composites. Therefore one could speculate that the combination of these two dispersing agents, in different parts of the epoxy system, could lead to an even greater enhancement in the tensile properties. Following this idea, composites have been prepared with also a combination of BYK-9077 and L-7602. Various routes of preparing the composite systems and the ensuing tensile properties are shown on Table 2. Comparison between the composites prepared using a single dispersing agent (S1–S4) and samples prepared using two dispersing agents (S5–S8) shows that the combination of dispersing agents does not offer any advantages. On the contrary, in most of the cases the combination of dispersing agents leads to a decrease in Young’s modulus, tensile strength and elongation at break. The possible interaction between these dispersing agents cannot be totally described since their detailed chemical structure is unknown. Nevertheless we believe that a detrimental complexing effect could take place due to the combination of the different block copolymers. 3.3. Morphology of CNT/epoxy composites The fracture surfaces of the composites prepared with and without dispersing agents have been analyzed by SEM. The surface of the composites without dispersing agent is presented in Fig. 5a while the fracture of the composites prepared with the aid of L-7602 and BYK-9077 are presented in Fig. 5b–d. In Fig. 5a
(a)
(b)
(c)
(d)
Fig. 5. SEM images of the fracture surface of composites prepared: (a) without block copolymers, (b) with the aid of L-7602 and (c, d) with the aid of BYK-9077.
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10 13 10
Dispersion in Epoxy
Dispersion in hardener
Surface resistivity (Ω /sq)
12
10 11 10 10 10 9 10 8
10 7 10 6 10 5
10 4 10 3 10 2 10 1
could also be a result of a more uniform distribution of CNTs, which would decrease the likelihood of bundles touching to form a conductive network. Static dissipative materials display surface resistivity values in the range of 105–1011 X. These materials are not too conductive but will slowly conduct static charges away, rendering them effective in preventing electrostatic discharge (ESD). Our results show that the dispersion of CNTs in both hardener and epoxy with the aid of L-7500, L7607 and L-7602 leads to composites with suitable ESD properties. The dispersion of CNTs in the hardener using BYK2150, BYK-9077 and BYK-2155 also result in materials useful for ESD.
0H K9 07 7 BY K2 15 5 BY K9 07 6
B6
L7 60 2
L7 60 7
4. Conclusions
BY
N
ea te po xy BY K2 15 0 L7 50 0
10 0 10
487
Fig. 6. Surface resistivity of neat epoxy and composites reinforced with 0.075 wt.% CNTs. The reported values correspond to an average of four values. The error bars are too small to be displayed.
individual nanotubes are found as white short threads inside the matrix and the fracture surface is relatively smooth. Fig. 5b and c indicate that the dispersion of CNTs treated by both L-7602 and BYK-9077 in the composites has enhanced. Voids are observed in the composite prepared with the aid of L-7602. The formation of voids in the specimens is expected to reduce the ductility of the composites. In fact this can be observed as a decrease in elongation at break for these composites (Fig. 2b). The improved reinforcement effect of CNTs obtained by using BYK-9077 is reflected by a significant larger roughness of the fracture surface (Fig. 5c) in relation to the other composites. In Fig. 5c the CNTs are not easily identified. This is most likely due to the wetting of the nanotubes by the block copolymer which decreased the contrast between the CNTs and the epoxy making it difficult to discern one from another. This wrapping of the block copolymers around the CNTs is expected to influence the charge migration between nanotubes and therefore the electrical conductivity of the composites. Fig. 5d shows another sign of reinforcement: the pull-out of CNTs indicated by arrows. Note that the dispersion states of the CNTs observed in the composites are in agreement with the tensile properties measured. 3.4. Surface resistivity of CNT/epoxy composites The surface resistivity of the neat epoxy and composites containing 0.075 wt.% CNTs, as a function of the dispersing agent used during preparation, is shown in Fig. 6. Given that the structure of the CNTs is that of ‘‘spaghetti-like’’ agglomerates, the concentration of 0.075 wt.% is most likely too low for reaching the percolation threshold. Therefore a transition between insulating and conducting behavior is not expected. Fig. 6 shows that the neat epoxy presents an insulative behavior, with a surface resistivity of 1012 X/sq. The same is true for the composites prepared using B-60H and BYK-9076. The dispersion of CNTs in epoxy, using BYK-2150, BYK-9077 and BYK-2155 also leads to insulative materials. It is possible that the affinity of the dispersing agents or the hardener for the MWCNT surface allows the chains to completely coat and insulate the CNT surface [20]. Indeed the SEM images of the fracture surface of composites prepared by dispersing CNTs in the epoxy with the aid of BYK-9077 suggest the coating of the CNTs with the block copolymer molecules. Thus, the MWCNTs or agglomerated CNTs are coated with the insulative material in such a way as to prevent the charge migration from one nanotube to another. In addition, the elevated resistivity for some of the samples
In this work, different strategies for the development of homogeneously dispersed and long-term stable suspensions of CNTs in different components of epoxy have been analyzed. The use of a centrifugation technique for the investigation of the stability of MWCNTs in various media has proved to be effective. Various block copolymers suitable for the preparation of stable dispersions of CNTs have been identified. The combination of dispersing agents did not show any advantage over the use of a single dispersing agent. Enhancements of the tensile properties for the epoxy based composites have been achieved by using two different block copolymers (L-7602 and BYK-9077) which also provided dispersions resistant to sedimentation for more than 168 h. The use of dispersing agents for the CNTs in either hardener or epoxy leads to composites with electrostatic discharge properties. Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgements This material is based upon work supported by the Department of Energy and Bayer MaterialScience LLC. under Award Number DE-EE0001361. We thank BYK Chemie Company, Momentive performance materials, Kuraray America Inc., Bayer MaterialScience and Molded Fiber Glass Company for generous offers of the materials used in this study. We would like to thank Dr. Usama Younes and Dr. Serkan Unal for their advice and constructive discussions. We also thank Bayer MaterialScience for performing the surface resistivity measurements. References [1] Gomes D, Loos MR, Wichmann MHG, de la Vega A, Schulte K. Sulfonated polyoxadiazole composites containing carbon nanotubes prepared via in situ polymerization. Compos Sci Technol 2009;69:220–7. [2] EWEA. Wind energy – The facts, a guide to the technology, economics and future of wind power. Belgium: Earthscan Publications Ltd.; 2009.
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