Carbon nanotube-polymer nanocomposites The role of interfaces.pdf

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Composite Interfaces, Vol. 11, No. 8-9, pp. 567 – 586 (2005)  VSP 2005.

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Review Carbon nanotube-polymer nanocomposites: The role of interfaces C. VELASCO-SANTOS 1,2,3, A. L. MARTINEZ-HERNANDEZ 1,2,3 and V. M. CASTANO 1,∗ 1 Centro

de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autonoma de Mexico, A. P. 1-1010 Querétaro, Querétaro 76000, Mexico 2 Department of Materials Science, University of North Texas, Denton, TX 76203-5310, USA 3 Departamento de Materiales, Departamento de Mecatronica, Instituto Tecnológico de Querétaro, Av. Tecnológico, Col. Centro, Santiago de Querétaro, Querétaro 76000, Mexico Received 25 June 2004; accepted 14 September 2004 Abstract—Research aimed at producing new nanocomposites with improved properties has dramatically increased in the last decade, especially on materials tailored at a nanometric level, such as fullerenes and carbon nanotubes. The use of nanoforms as reinforcement of organic polymers has opened the possibility of developing novel ultra-strong and conductive nanocomposites. Nevertheless, the challenge of manufacturing multifunctional composite materials based on nanostructures is still open, in particular in the details of the corresponding interfacial properties, which are particularly relevant in these systems. This paper reviews the main technical activities in this field, focusing on the most important parameters that influence the behavior of their interface, discussing recent advances, as well as current and future trends in research. Keywords: Carbon nanotubes; polymer composites; interface; chemical functionalization.

1. INTRODUCTION

Carbon nanotubes (CNs) are among the most interesting materials discovered during the last twenty years; these materials have diverse arrangements at a nanometric level that lead to different properties depending on the specific kind of nanotubes. In fact, a number of different types of nanotubes, from singlewalled nanotubes (SWNTs), double-walled nanotubes (DWNTs), and multiwalled nanotubes (MWNTs) to their variants with helix and bamboo shapes are already known to this date. ∗ To

whom correspondence should be addressed. E-mail: [email protected]

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Also, a number of methods have been reported so far for producing carbon nanotubes: arc discharge [1, 2], pyrolysis of hydrocarbons over catalysts [3], laser vaporization of graphite [4], electrolysis of metal salts with graphite electrodes [5], and hydrothermal methods [6]; also, nanotubes have been found recently to be produced under relatively mild natural environments [7]. CNs display different properties that suggest potential for use in various advanced technological applications, such as tips for Atomic Force Microscopy (AFM) [8], cells for hydrogen storage [9– 11], nanotransistors [12, 13], electrodes for electrochemical applications [14, 15], sensors of biological molecules [16], catalysts [17], etc. In particular, one possible application that has attracted a great deal of attention in the materials engineering community is the incorporation of CNs as reinforcement of composite materials, mainly within an polymeric matrix. These materials are expected to exhibit outstanding mechanical properties, such as extremely high Young’s modulus, stiffness and flexibility, as has been demonstrated by both experimental data and theoretical models [18– 21]. In addition, the unique electronic properties of nanotubes suggest possibililities for use as either semiconductor or metallic conductive nanomaterials [22]. Also, these structures possess high thermal stability [23], which could be advantageous for aerospace applications. Additionally, the nearly perfect structure of CNs, their small diameter, and their high surface area and high aspect ratio, provide an amazing inorganic structure with unique properties extremely attractive to reinforcing organic polymers. However, in spite of the fact that CNs were originally discovered in 1991 and that the first report on their amazing mechanical and electronic properties appeared in 1996 [18, 22], the idea of the using CNs to produce new composite materials has only just begun to be explored in the past five years, when different papers have reported the incorporation of CNs in various polymer matrices, aiming to synergetically increase diverse properties of the material. Results so far have been contrasting, since several types of CNs have been used: some composites were made with SWNTs, others by using MWNTs, and the difference in the number of walls causes important variations on the diameter of the nanotubes, which in turn produces different characteristics in the composites. It has also been found that the specific route used to synthesize CNs yields different graphitization degrees and, therefore, diverse properties of the final material. Various polymers have been used along with different processing methods and, recently, some groups have used functionalized carbon nanotubes (f-CNs) to improve interfacial interaction between the inorganic filler and the organic matrix. In general, the field of developing strong, conductive and smart materials based on CNs is just beginning and promises to become a very important area of R&D. Accordingly, in this review we have compiled and discussed the leading results available in this emerging field of materials science; with special emphasis on the role of interfaces, aiming not only to provide an insight into the field, but also to try to foresee the main areas of activity in the coming years.

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2. PROPERTIES OF CARBON NANOTUBES

Carbon nanotubes possess many features that make them unique materials: for example, although standard carbon fibers have long been utilized in industrial composites as reinforcements for polymers that produce excellent properties, such as low density with high specific strength and specific modulus, CNs have superior properties as compared to any carbon fiber, in spite of being chemically equivalent. The very first paper reporting the Young’s modulus of CNs-based composites was published by Treacy et al. [18], where a figure of 1.8 TPa was found as an average, and these authors mentioned there that Young’s modulus in SWNTs can reach values as high as 5 TPa. Recently, Yu et al. [21] found smaller Young’s modulus for MWNTs. However, the results varied depending on the specific approach used to perform the mechanical probes and also on the particular type of NT’s. For instance, Wong et al. [24] calculated the Young’s modulus through atomic force microscopy (AFM) and found a value of ≈1.28 TPa for MWNTs. The method was applied to silicon carbide nanorods which showed a Young’s modulus of around half of that of MWNTs. There, the modulus was calculated from bending force vs. displacement curves. Other studies have evaluated the mechanical properties of carbon nanotubes produced by arc-discharge and catalytic methods, since these two kinds of CNs present different advantages over each other. For example, catalytic CNs can be produced in aligned patterns, which produces useful results like well-defined arrangements and uniform electrical properties in the composites. In addition, the principle used promises to become an important tool to produce CNs in large quantities. However, the degree of graphitization in these structures is poor and this is detrimental to the mechanical and electronic properties. On the other hand, CNs produced by arc-discharge methods show a high degree of graphitization and excellent mechanical and electrical properties, which are ideal when these nanomaterials are used in polymeric matrix composites. The disadvantage is that they have a high content of impurities, which limits their possible uses. The arc-discharge approach to produce SWNTs requires catalyzed reactions, which induce defects in some regions where the bonds between the carbon material and the metallic element of the catalytic particle are formed, in the same way as those produced by catalytic methods. However, SWNTs produced by this method are the strongest fibers known to date, though the SWNT bundles formed by this process could cause some problems in the dispersion of nanotubes in polymer composites. In this context, Salvetat et al. [25] evaluated the mechanical properties of arc-discharge MWNTs (arc-MWNTs) and grown catalytic MWNTs (cat-MWNTs) using AFM. The results show an important range in the elastic modulus between two forms of CNs: the average elastic modulus (E) for arc-MWNTs has a value of 0.87 TPa while E for cat-MWNTs is around 0.027 TPa [25]. However, another recent report indicates that the tensile strengths of these CNs might be relatively high, inasmuch as the defects could assist in stress transfer between layers [26].

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Another team has measured the elastic modulus by nanoindentation in verticallyaligned multiwalled carbon nanotubes produced by catalytic method with Plasma Enhanced Chemical Vapor Deposition (PECVD) [27]. The parameters evaluated in these CNs are: effective bending modulus (E b ), axial modulus (E a ) and nanotube wall modulus (E w ). The mechanical results for these catalytic CNs are more promising than others with the aim of producing polymer composites, since E b varies from 0.9 TPa to 1.24 TPa, E a is in the range of 0.9 to 1.23 TPa and E w has a value between 4.14 and 5.61 TPa. The latter figure is in agreement with those obtained by theoretical calculations with the local density approximation model [28] and with the finite element approach [29], which report effective moduli of 4.7 TPa and 4.84 TPa, respectively. Other important reports have obtained different results in simulations with respect to the dependence of the elastic properties of nanotubes; Lu found that these properties in CNs are insensible to helicity, number of walls and size, with an elastic modulus around 1 TPa for all nanotubes with a radius larger than 1 nm [30]. However, other research with similar calculations has found that elastic modulus is dependent on the structure and tube diameter with a value for E of 1.24 TPa [31]. As mentioned before, the vast majority of theoretical and experimental results agree that CNs produce the strongest fibers in the world. Table 1. Comparison of diameter, mechanical and electrical conductive properties in different carbon materials Carbon fibersa (precursor) Rayon Polyacrylonitrileb L.T.c pitch mesophase H.T.d pitch mesophase Fibras tipo carburo resina L.T.c Fibras tipo carburo resina H.T.d Graphitee Carbon nanotubes a Reference

Young’s modulus (GPa) 390 230 41 41 340 690 1000 De 800 a 5000f

Electrical resistivity (10−4  cm) 10 18 100 50 9

Diameter (nm) 7000–9000 5000–7000 11 000 11 000 5000

1.8 0.4 0.05 a 2g 0.05 a 1.2g

5000 h

0.4–90

[32]. the reference [33] are cited different carbon fibers that were developed using polyacrylonitrile as precursor. The elastic modulus differs from 220 to 390 GPa in different commercial fibers such as: AVCO, Celanese, Courtaulds, Great Lakes, Carbon, Hercules, Union Carbide, USA, Toray. c Low temperature. d High temperature. e The values were calculated in the sheet plane. f The majority of the elastic modulus values in different researches fluctuate between these ranges. g Values of different researches Ref. [22] and cited in the reference [34]. Note: The values fluctuate depending on the arrangement in CNs. h Does not apply because the graphite is a sheet. b In

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Table 1 shows a comparison of the mechanical properties and other features between carbon fibers, graphite and carbon nanotubes of different sources. There is no doubt that another important reason to incorporate CNs in nanocomposites is the electronic arrangement of these materials, inasmuch as different works have shown that CNs can be metallic or semiconducting depending on their diameter and structure [35– 37] and another report has shown that the properties change in each tube depending on its structure [22]. Thus, the electrical conduction range is rather broad, which diversifies potential uses of CNs. Table 1 also shows a comparison of electrical properties of CNs with other carbon materials. These properties could be very useful in order to use CNs in multifunctional composites. However, other features need to be taken into account if CNs are to be effectively incorporated in polymer composites, since CNs must take part in the load transfer through a good interfacial interaction, to truly take advantage of their excellent mechanical properties. It is expected that a deformation in the CNs could modify their electrical properties in some parts of the nanotube and change the behavior of the composite. Accordingly, the effect of the deformation in SWNTs has been calculated, where the electronic structure, and therefore electrical conduction, is affected when the nanotubes are deformed [38]. The authors found that the conducting SWNTs may become semiconducting once they were mechanically deformed, but semiconducting SWNTs will never become conducting when they are mechanically deformed. This could be useful to build novel smart polymeric composite materials, since a different electrical response could be expected depending on the mechanical deformation. Other studies of SWNTs, produced by approaches such as laser ablation and arc-discharge, were aimed to form ropes and bundles [39], and some theoretical researches have studied the mechanical properties of these bundles in different conditions, with the object of predicting mechanical and electrical properties of these aggregates of CN groups. These models could be useful in predicting properties and employing CNs in composites and other applications [40, 41]. In addition to the relevant CN properties discussed above, there exist other important features that position CNs well above any other existing reinforcement fibers for polymer composites. Indeed, in nanotubes, the nanometric scale diameter is a key factor in improving the interface, since their very small size produces close interaction at the molecular scale, useful in minimizing the zones without contact at the interface, enhancing the interactions and thus providing compatibility between two quite different materials: inorganic CNs and organic polymer matrix. CN diameter plays an important role in providing a near-molecular interaction, and the corresponding high aspect ratio is key to reinforce composites. Another important characteristic of CNs becomes evident as they are first deformed and then the stress released: they recover their original shape. This flexibility gives nanotubes unique opportunities as reinforcements, for neither carbon fibers nor other known reinforcements exhibit this property [26]. It is known that the syn-

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thetic fibers used nowadays are more sensitive to fracture when they are deformed. In addition, the mechanical deflections in CNs can be electrically induced [42]. All the properties described above make CNs exciting materials to develop completely new polymer composite materials. However, the development of those novel composites still has important barriers to overcome, mainly related to the understanding of the phenomena at the interface between CNs and organic polymers. The path to develop these new materials began only a few years ago, and the results available at this time are scattered and partial and sometimes misleading, for some studies promise that these materials are ready for technological applications, while other reports reveal still a poor adhesion and poorly performing composites. Thus, it is extremely important to analyze and discuss the results reported so far in the light of the underlying fundamental scientific problem: the interfaces.

3. CARBON NANOTUBE POLYMER NANOCOMPOSITE

Different approaches have been used to produce carbon nanotube polymer nanocomposites (CNPN) from melt blend [43], dissolving and casting [44], in situ polymerization [45] and extrusion [46] among others. Nevertheless, the first studies that incorporated nanotubes in polymers were developed to align and to observe the possible interaction of CNs with the polymer. The first report where CNs were incorporated in polymer dates back ten years. In this study Ajayan et al. [47] aligned CNs in an epoxy matrix, then cut small slides to obtain a partially aligned segment. Aligned carbon nanotubes in polymer matrices can be considered as a designed arrangement, inasmuch as this would produce possibilities of control of the properties along the direction of the carbon nanotubes in the nanocomposites. This first evidence of aligned CNs in a polymer matrix applies only to small areas of these nanocomposites. Jin et al. [48] report alignment of CNs in polymer matrix by mechanical stretching. They used MWNTs synthesized by the arc-discharge method and the thermoplastic polymer polyhydroxyaminoether that was dissolved in chloroform and later molded in Teflon. The results show a partial alignment that depends on the stretching ratios and on the carbon nanotube fraction. In another report, SWNTs are aligned by a combination of solvent casting and melt processing methods: there the CNs were dispersed in polymethylmethacrylate (PMMA) using dimethylformamide as solvent. Alignment in composites provides higher conductivity along the flow direction. Elastic modulus (E) and yield strength (Y ) were measured in melt-spun fibers: both E and Y increase with nanotube content and draw ratio. Although these increases are not uniform, in both parameters higher values are reached with 5 wt% and 8 wt% of SWNTs [49]. Wagner et al. developed an interesting study where stress transfer ability was shown in carbon nanotube polymer nanocomposite. In this report, the fragmentation of MWNTs under tensile stresses in thin films was observed. The results show that the efficiency to transfer stress in this composite is at least one order of

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magnitude larger than in conventional fiber composites [50]. A later report by Lourie and Wagner indicates that the polymer nanotube interface is not inert and load is transferred to the nanotubes via the surrounding matrix. In this paper, it was suggested that, in spite of the huge difference in scale between traditional fiber composites and carbon nanotube nanocomposites, the fundamental concepts of the mechanics of conventional composites could hold at the nanometric level [51]. Deformation studies by transmission electron microscopy (TEM) of polymer CN composites were developed with a nanocomposite containing MWNTs in a polystyrene matrix. The studies were realized using a TEM strain holder and a film with a notch. The results show that nanotubes tend to align and bridge the crack when a tensile stress was applied to the composite, showing load transfer across the nanotube–polystyrene interface [52]. Other PMMA-based studies have explored other parameters, such as CN distribution and the possible modification of CNs as they are incorporated in a polymer matrix. The studies were realized with SWNTs obtained from arc-discharge method and not purified. The investigation shows that low concentrations of CNs in polymer give a uniform distribution with this kind of CN [53, 54]. Although some papers claim a good distribution and load transfer at the interface, this should be reflected in the properties of nanocomposites. However, contradictory results have been obtained in this kind of composite, mainly with respect to the mechanical properties, showing that many factors affect the production, the interface and, therefore, the successful development of carbon nanotube polymer nanocomposites. Schadler et al. developed MWNTs-epoxy composite cured with triethylene tetraamine. They evaluated both tensile and compression strength. The results show that compression modulus is higher than tensile modulus in this composite. The load transfer was evaluated through Raman spectroscopy, and it was found that strain in carbon bonds only shifts significantly under compression. The authors speculate that all the walls in MWNTs only participate in compression, unlike the case when the composite is subjected to tensile stress, where the outer walls participate [55]. Shaffer and Windle formed CN-polyvinyl-alcohol nanocomposite with different loads of catalytically grown nanotubes: the content of CN was varied from 10 wt% to 60 wt% and dynamical mechanical parameters were measured. The values of storage modulus (E  ) increased over two-fold, but only by using a huge quantity of CN. At the same time, the onset of thermal degradation was retarded in this composite. Therefore, these authors suggest the application of CNs as modifiers. Also, the conductive properties in composites were measured, and a percolation threshold was found at a concentration between 5 and 10% of CN [56]. In other study, MWNT-PMMA composites were developed and tested mechanically. Two types of MWNTs were incorporated in the polymer matrix: as-synthesized and treated MWNTs (t-MWNTs: CNs ground in a high rate ball mill for 20 min and then boiled for 0.5 h in nitric acid); the method used to form the composites was polymerization using azoiso-butyronitrile (AIBN) as initiator. Satisfactory

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increases in the tensile strength and hardness were reached with 3 wt% and 5 wt% of t-MWNTs; however, with higher loads the tensile strength decreased. The results are attributed by the authors to the good interaction between CNs and polymer matrix, due to the fact that the initiator could break the π bonds of CNs and, in this way, it is possible to form a chemically bonded interface. However, many other factors could also be responsible for this success: for example, the fact that t-MWNTs were treated with nitric acid solution could cause a slight oxidation in some zones of the nanotubes, where the defects might be increased by the initiator action, producing functionalized nanotubes that would interact with the polymer chains. More research is needed to confirm this and to understand the role played by the polymeric initiator in CN bonds and on the development of nanocomposites [57]. PMMA is one of the most studied polymers for these composites. Cooper et al. evaluated mechanical properties of nanocomposites formed with MWNTs, SWNTs and nanofibrils. The results of these analyses present a tensile modulus almost insensitive to the presence of CNs; however, the impact properties are significantly improved by both MWNTs and SNWTs [58]. Tatro et al., in another recent study with PMMA and MWNTs, have evaluated thermal and electrical properties, Vickers microhardness and dynamical mechanical analysis in nanocomposites irradiated with a Cesium-137 source, with the aim of determining the effect of carbon nanotubes when the sample is irradiated. The results show that MWNTs may improve radiation hardness of mechanical properties and glass transition temperature when a composite is formed. The effect is attributed to conjugated bonds in CNs, which absorb some part of the radiation energy, limiting the damage of PMMA molecules. More aging studies with radiation and evaluation of the interface are proposed and currently being developed by these authors [59]. Another recent report evaluates the dynamical mechanical properties of pure PMMA and nanocomposites formed with SWNTs and PMMA matrix at two different frequencies. The study was developed with the aim of understanding the effect produced by the incorporation of the nanotubes on the ability of PMMA to store and dissipate the mechanical energy. SWNTs used in this research were produced by laser ablation method and the nanocomposites were prepared by in situ polymerization using a solution of dimethylformamide (DMF) and 2,2-azobisisobutyronitrile (AIBN) as initiator. α-Relaxation, related with the onset of the glass transition temperature (Tg ) and β-transition associated with hindered rotation of the side chain, were evaluated in PMMA and SWNT-PMMA nanocomposites. Transitions were detected in both tan δ curves and elastic modulus curves (E  ) at low and high temperatures. Results at two different frequencies show that PMMA and nanocomposites formed with slight quantity of SWNTs exhibit similar β-transition temperature and identical Tg ’s. However, E  at low temperature (−150◦ C) in the nanocomposite with a tiny quantity (0.014 wt%) of SWNTs exhibits an important increase in comparison with E  of the pure PMMA, from ≈5.1 GPa in PMMA to 7.2 GPa for the nanocomposite, both evaluated with a

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frequency of 100 rad/s. This represents a significant increase, considering the small quantity of SWNTs used. Thermogravimetric analysis (TGA) and nuclear magnetic resonance (NMR) studies have been used to evaluate possible changes in the structure of the polymer nanocomposite caused by the addition of SWNTs. The authors cite that neither microstructure nor thermal degradation changes were localized in nanocomposites, as compared to pure PMMA. This indicates that these parameters could not be liable for the change in the mechanical properties of the nanocomposites. However, the interface in these nanocomposites plays an important role and the properties of the polymer changed in the vicinity of the SWNTs, probably by cohesive interactions produced by the large surface area of CNs [60]. This research also involved discussion and possible adaptation of the traditional theory of CN nanocomposites; the authors found that nanotubes have a greater effect on the properties of composites than those expected from traditional composite theory [60]. Therefore, new models will be needed to adjust and predict CN nanocomposite properties because the interface produced with polymers and nanomaterials is generating unexpected properties. Dufresne et al. processed and characterized nanocomposites that were developed with catalyzed MWNTs and poly(styrene-co-butyl acrylate) [61]. The composites were prepared by casting, after stirring with different loads of CNs from 0 to 15 wt%. Mechanical properties of these composites were evaluated by dynamical mechanical analysis (DMA) and tensile test: increases in the tensile modulus with the content of CN were found in almost all samples. However, an unexpected decrease in the modulus was observed in the sample with 7 wt% of CNs. Although good distribution was achieved in most samples, some areas with nanotubes appeared like clusters causing unpredictable behavior in those samples. This has been observed in other composites prepared with CNs when the content is higher than 5 or 7 wt% and explains the unexpected behavior in this particular study. Wong et al. have analyzed two kinds of nanocomposite that show agglomerates that cause similar effects to those just mentioned. Although it was shown by field emission scanning electron microscopy (FESEM) and TEM that CNs have good interaction and adhesion with the polymers, other regions show agglomerates rather than even distribution and cause a deterioration in the nanocomposite properties. In this research, CNs reinforcing polystyrene rod and epoxy thin film were analyzed. In the case of CN–polystyrene composites, tensile strength increases from 22.1 MPa (for polystyrene) to 24.4 MPa (for the sample with 0.1 wt% of CN); however, when the content of CNs was increased at 5 wt% the strength was diminished to 17.9 MPa. Furthermore, the samples with 1 wt% and 2 wt% CN showed lower values in tensile strength than the sample of pure polystyrene [62]. Also in this study, epoxy thin films were evaluated mechanically. Microscopy results of these samples revealed the two mentioned regions. One corresponds to the failure of the matrix but not at the CN-epoxy interface. Nevertheless, in other regions, CN agglomerates were observed showing poor dispersion locally.

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Finally, the paper published by Wong et al. gives an interesting analysis involving molecular simulations of CN pullout. Molecular models of SWNTs in polystyrene matrix and SWNTs in epoxy matrix were generated, and then total work required for fiber pullout was calculated. The results obtained for CN-epoxy are very similar to those obtained in experimental pullout for the same system. The authors conclude that the results of this simulation and the previous experimental studies suggest that the interfacial shear stress of CN–polymer systems is at least one order of magnitude higher than micro fiber reinforced composites and it is assumed that the surface of the CN is important to achieve intimate contact at nanometric level between CNs and polymer matrix [62]. Thus, these results show that a strong interface can be achieved between inorganic CNs and organic matrix when the polymer interacts with nanotubes. However, once more, large quantities cause problems by forming agglomerates. An opposite effect to those discussed above has been reported in CN nanocomposites when the hardness has been measured; here, the increase in the quantity of nanotubes incorporated in the polymer has a favorable effect. The addition of 2 wt% of CNs improves the hardness of an epoxy matrix by some 20%; however, when quantities higher than 1.5 wt% were used, the hardness diminished as compared to the hardness of pure epoxy. The authors have speculated that this effect is caused by a weak interface between CNs and the polymer. On the other hand, a favorable effect on the hardness with 2 wt% of CNs is attributed to the meshlike structures formed when the concentration of CNs is increased. The authors mentioned that the formation of these structures is caused by the high aspect ratio of the nanotubes, which produces a tangle arrangement that enhances the scratch resistance. In this report, different temperatures were used to treat CN nanocomposites and to test their failure after flexural strength in diverse conditions; since CNs have been proposed for new composites for the space industry these variabletemperature states are needed to simulate this environment. Results indicate that the mesh-like structures mentioned do not improve the flexure strength under the different conditions studied. The authors conclude that, in this case, the cause of the reduction could be due to structural non-homogeneity or the presence of a weak bonding interface [63]. Epoxy matrix is another polymer used in many studies in CN nanocomposites, showing excellent electrical properties with small amounts of carbon nanotubes. Allaoui et al. report a percolation threshold between 0.5 wt% and 1 wt% of carbon nanotubes whereas the tensile strength improved significantly with 1 wt% of CNs in yield point and in 10% in strain [64]. However, only a small increase was observed when 4 wt% of CNs was added. As we mentioned, this effect has been observed by other researchers, with the addition of unpurified CNs. In fact, the majority of the results demonstrate that the spatial distribution is an open question and that it is possible that the inhomogeneity of the samples has a strong influence, as important as the type of polymer used [44].

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Table 2. Comparative results (E  , % increase in E  ) of Carbon nanotubes composites obtained in different polymers matrices at 40◦ C 1 Hz of frequency [44]a Polymer

E  (40◦ C, 1 Hz) (matrix) MPab

PMMA PS PSBA PVA MEMA

∼ =800 ∼ =2400 ∼ =0.681 ∼ =5000 708

E  (40◦ C) CN composite MPac ∼ =1600 ∼ =3500 ∼ =1.584 ∼ =11 200 2340

CNs wt%d

Increase % E e

Reference

26 5 7 60 1

100 44 132 124 230

[43] [65] [61] [56] [44]

a Modified

table of the reference [44]. modulus at 40◦ C of the polymer matrix. c Storage modulus at 40◦ C of the carbon nanotube composite. d Weight percent of carbon nanotubes in the composite. e Increase percent in storage modulus of composite with respect to polymer matrix. b Storage

The alignment of carbon nanotubes in nanocomposites has an important influence on the electrical properties. Mechanical properties also change when CNs are aligned. Thostenson and Chou found relevant improvements in tensile stress properties and toughness when 5 wt% of CNs were incorporated into a polystyrene matrix, up to five times with respect to the randomly oriented composite, when the same quantities of CNs were aligned by extruder [65]. Other important topic that helps to enhance the compatibility at the interface level between CNs and an organic polymer matrix is the incorporation of additives and other polymers that could assist in the dispersion and compatibility. In this context, different behaviors have been observed when an additive is incorporated into nanocomposites. Gong et al. [66] incorporated nonionic surfactants in thermosetting polymer to improve the interfacial interaction between nanotubes and the polymer. The results show higher increases in storage modulus (E  ) and in glass transition temperature when both surfactant and CNs were used, with respect to the composite where only carbon nanotubes were incorporated. In both cases only 1 wt% of CNs was employed; however, E  with surfactant is increased by more than 30% and glass transition increases by 25◦ C. In contrast, the addition of CNs without surfactant produced only a moderate increase in the temperature and mechanical properties. In other reports, better results have been reached without additives, providing great care is taken during mixing [44]. A non-ionic surfactant was also used but with thermoplastic copolymer, methylethylmethacrylate (MEMA). This study is one of the first that compares their own results with other researchers where both only carbon nanotubes and additives are used. An important increase in E  by more than 200% at 40◦ C is reached with only 1 wt% of CNs, without additives. The comparative results presented involving different groups are shown in Table 2, for CN nanocomposites where additives were used, and in Table 3 for studies where only CNs were used. The authors suppose that, although the non-ionic surfactant utilized is a good dispersant for carbon, the wetting could occur with the impurities

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Table 3. Comparative results of DMA on carbon nanotubes composites using a surfactant in two different polymersa Sample

Polymer Polymer + Surf Polymer 1 wt% CN Polymer 1 wt% CN, 1 wt% surf

Epoxy (66) E  40◦ C (MPa)b 1390 1150 1550 1750

Tg ◦ C (tan δ)c 63 62 72 88

MEMA (44) E  40◦ C (MPa)b 708 774 2340 1983

Tg ◦ C (tan δ)c 92 89 99 102

a Modified

table of the reference [44]. modulus at 40◦ C. c Glass transition temperature (tan δ). b Storage

in the CN’s surroundings and an interface is created with only with some part of the CNs. This would produce interfaces that are not so effective, since they form a little barrier that produces different conductivity behavior [67]. The results of the conductive behavior in these films are currently in preparation. Reports where other substances take part in the interfacial and dispersion behavior of CN nanocomposites were published by Jin et al. [68]. In this study, MWNTs were sonicated in a dimethylformamide solution of poly(vinylidene fluoride) (PVDF). The mix was melt blended with PMMA to form a nanocomposite. The results show that there is a threshold in the addition of PVDF, since slight amounts of the PVDF improve the storage modulus, whereas higher PVDF contents lower the storage modulus.

4. CHEMICAL FUNCTIONALIZATION OF CARBON NANOTUBES

The results analyzed in the previous section show that CNs obtained from methods that include impurities together with the CNs lead to unpredictable properties, even though the composites in some cases show an important increase in mechanical properties below a content of 5% of CNs. Other papers report a poor interface due to poor wettability [69], which could be influenced by the impurities. Accordingly, different methods have been proposed to eliminate impurities in CNs. The most important impact has been produced by oxidation methods which, in addition to reducing impurities, cause chemical modifications of CNs. This has proven useful for composites and other applications of these materials. This section describes some important studies in the field of chemical modification. The oxidation technique has opened a new gate in this field, by producing carboxyl and carboxylate groups on the tips and surfaces of CNs. These moieties have been used to attach different chains in CNs that allow different possible uses for these materials [70]. Oxidation approaches were developed few years after CNs were discovered. The theoretical basis is the different resistance to oxidation

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(a)

(b) Figure 1. A schematic representation (a) carbon nanotube (b) oxidized and functionalized carbon nanotube.

between CNs and other nanoparticles formed during the production. Thus, it was observed that the nanotubes when oxidized are consumed from the tips inward at the same time [71] and the surface wall is modified if the oxidation is severe. Although the first oxidations were developed with the aim of opening and filling CNs, the carboxyl, hydroxyl and carboxylate groups generated through the oxidation on the surfaces of these materials are useful moieties in order to bond new reactive chains that improve solubility, processability and compatibility with other materials and, therefore, improve the interfacial interactions of CNs with other substances. This route has become one of the most important: it is the focus of many current researches because of the outstanding results it has produced [72]. The defects caused by oxidation have been studied and it has been found that nanotubes tolerate a limited quantity of them, before a macroscopic sample loses its special mechanical and electronic properties [73– 76]. This kind of functionalization is currently being researched in order to understand and to improve the utilization of these chemical modifications. The organic carboxyl groups formed in the nanotube surface are localized on the defects on functionalized single-walled and multi-walled nanotubes (f-SWNTs and f-MWNTs, respectively); suitable reactive organic groups for other chemical chains tend to react in this zone. An idealize scheme of unfunctionalized and functionalized CNs with these organic moieties is shown in Fig. 1. Next, different reports have focused on the chemical functionalization after oxidation. In these papers, the COOH groups generated in the oxidation process are used to attach different molecules useful to improve surface compatibility of CNs with other materials. The relevance of these groups to join CNs with organic polymer matrices is also mentioned.

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One of the routes used as complementary procedure to insert long organic chain is the use of SOCl2 (thionyl chloride) in dimethyl formamide. In this way, a long chain of alkylamine (octadecylamine) has been coupled to activated f-SWNTs [77]. This method is very useful to insert different chains. The approach also has been used to enhance the solubility properties of SWNTs with the mentioned chains and other moieties as alkyl-aryl amine and 4-dodecyl-aniline [78]. The same method with a subsequent reaction has been used to add other organic moieties. For instance, it has been followed by a reaction with monoamine terminated poly(ethylene oxide) (PEO), which was useful to facilitate the linkage of this chain in a grafted PEOSWNTs system [79]. Also, the system (f-SWNTS + SOCl2 ) has been used in MWNTs to functionalize these structures via addition of an aminopolymer, such as poly(propionylethylemine-co-ethylenemine) (PPEI-EI) with the purpose of forming hydrophilic MWNTs. Direct reactions have been carried out with molten octadecylamine in carboxylterminated SWNTs, in the range of 120–130◦ C, forming a carboxylate group in the SWNT, which interact with an octadecylamine functionalized free radical [73]. This produces greater solubility in SWNT (s-SWNTs) than those formed using SOCl2 [77]. Other researchers have developed a wide variety of aliphatic amines coupled to carboxyl groups in the SWNT surface [80], showing that amines in general can be attached to CNs successfully. The insertion of these groups in the CN surface could be useful in improving compatibility between these nanomaterials and aminopolymer matrices. A comprehensive study of SWNTs and MWNTs using carboxyl organic groups treated with SOCl2 , with further functionalization through esterification reactions with lipophilic and hydrophilic dendra, has been presented recently [81]. Defunctionalization reactions of these nanostructures under alkaline and acid catalyzed hydrolysis have been carried out to provide evidence of the ester linkages between the carboxyl-terminated carbon nanotubes and the mentioned organic moieties [82]. Dodecylamine groups have been bonded to CNs through carboxyl moieties via the SOCl2 reaction. The functionalized carbon nanotubes have shown an improvement in the solubility with the polymer matrix and organic solvents and have been used to develop carbon nanotube-oxotitanium phthalocyanine composite. The material showed an important improvement in photosensitivity with that content of functionalized CNs and an important five-fold increase over that with oxotitanium phthlocyanine alone [83]. The carboxylic terminated organic groups on SWNTs have been used by Quin et al. [84] in order to attach n-butylmethacrylate via atom transfer radical polymerization. In this reaction, the polymerization is controlled and might be useful to graft copolymer blocks and other polymers, thus improving the compatibility between nanotubes and synthetic macromolecules. A recent research study promotes the insertion of many functional groups on the CN surface. Using the same reaction, different organosilanes were added to MWNT via carboxyl groups on modified CNs. The method allows diversifica-

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tion of CN properties depending on the terminal chain of the coupled organosilane. The oxidation and silanization achieved in this research yield functionalized moieties, which contain terminated organosilane groups designated as R groups that are bonded to the nanotubes by silanol groups. This has allowed modification of organic-terminated CNs with many different chains, chemically-bonded to MWNTs, with the silanization reaction, where the R group (methacryl, glycidoxy, etc.) in the organofunctional silanes can be changed depending on the specific polymeric matrix employed. The behavior of these MWNTs changes as the R terminal changes, so it has been found that 3-methacryl-trimethoxysilane (3-MAT) produces soluble MWNTs in organic solvent such as acetone or ethanol [67, 85], since the methacryl chain attached as R group on the MWNT’s surface changes the chemical character of this material, increasing their solubility. This does not occur when 3-methacryl-mercaptosilane (3-MPT) is attached in MWNT, for the terminal thiol group in the chain attached to CNs produces different properties on the MWNT surface. Figure 2 shows a scheme of the reaction proposed in these studies. Chemical functionalization has reached an important position in the CN field, as different chemical processes have been developed to diversify CN properties. The modifications described above represent only an intermediate stage in the route to improve the interactions between nanotubes and organic polymer matrices. Indeed, other chemical reactions have already been explored to couple these materials to proteins [86] and other substances. Also, the carboxyl groups in the CN surface have been shown to improve the compatibility of CNs with biomolecules. This has been employed for nanotechnology applications [87] and might be an important tool to develop good compatibility between organic biopolymers and CNs, which could find applications in biomaterials and biomedicine.

Figure 2. Sequence of silanization reactions on nanotubes surfaces [72].

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5. FUNCTIONALIZED CARBON NANOTUBE-POLYMER NANOCOMPOSITES

Chemical modification is not a new tool to enhance the compatibility between a reinforcement and a matrix, since chemical attack and grafting have long been used in natural fibers, synthetic fibers and particles [88– 90]. However, in the field of the nanocomposites, chemical functionalization offers an important tool in order to improve the interface and achieve the amazing properties that carbon nanotubes can provide. Although different groups are very active in this field, interfacial interactions still need to be fully understood to improve the interface between carbon nanotubes and polymer matrix. The recent results in chemical modification of carbon nanotubes are promising and could be the way to form strong and functional new advanced materials. Some results obtained in the area of functionalized carbon nanotube nanocomposites will be reviewed in this section. It was described above how the carboxyl groups found in the tip and surface of CNs are useful for the interaction between CNs and other compounds. Thus, the use of these moieties to improve the ‘link’ in CN–polymer composites have been proposed, either by inserting other chemical groups or by using the carboxyl groups produced during the oxidation [85, 91]. As mentioned before, Jia et al. [57] suggested that the initiator opens the π bonds found in CNs and, in this way, CNs take part in the polymerization. However, although opening the π bonds in CNs would allow them to join to other chemical groups, oxidation provides more possibilities to bond the nanotubes to the matrix, due to reactive chemical groups such as COOH, COO− and C O that are found on the tip and on the wall surface. In another report Geng et al. [92] use fluorinated single-walled nanotubes (flSWNTs) to enhance the uniformity and the nanotube dispersion using poly(ethylene oxide) (PEO) as matrix, and dissolving the nanotubes in methanol. Nevertheless, a recent report [45] proposes that the best moment to achieve interactions between the functional groups (found in tips and walls in CNs) and polymer chains is definitely when the polymer is created by in situ processing, inasmuch as the free radical formed in monomer molecules by the initiator could either interact or react with the CN moieties more easily than when the polymer is made and after it is dissolved or melted to produce the composites. In that research, methyl methacrylate monomer (MMA), 2-2 azobisisobutyronitrile (AIBN) and oxidized f-MWNT were used, and CN composites were produced by in situ polymerization using AIBN as initiator. The AIBN quantity, reaction time and temperature were controlled to yield uniform molecular weights in all samples. Three samples in this research were manufactured with only polymer (poly(methyl methacrylate PMMA)): polymer with 1 wt% of unfunctionalized MWNTs (u-MWNTs), and polymer with 1 wt% of functionalized MWNTs (f-MWNTs), additional composite with polymer and 1.5 wt% of f-MWNTs was also produced to observe the behavior when the addition of f-MWNTs was increased slightly. Thee interaction of PMMA with f-MWNTs was analyzed by infrared and Raman spectroscopy. In addition, mechanical results show an important increase in E  at 40◦ C by 66% and 88%, with 1 wt% and 1.5 wt% of f-MWNTs, respectively, and both samples increase the glass transition

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Figure 3. SEM image after tensile test of f-MNWT composite where it is possible to observe the dispersion of the f-MNWTs and the wetting of f-MNWTs with the polymer. Copyright American Chemical Society [45].

temperature Tg by some 40◦ C. This is different from the case with unfunctionalized nanotubes, where E  at 40◦ C increased by 50% and the Tg by only 6◦ C. Another important fact is that the performance of f-MWNT composites increases more than 11-fold, at relatively high temperature. A comparison of different properties of this and other f-CN composites is presented in Ref. [72]. A scanning electron micrograph (SEM) of functionalized carbon nanotube nanocomposites is presented in Fig. 3. Other recent papers have shown that carboxyl-terminated carbon nanotubes treated with amines and incorporated into epoxy matrix have better interaction than those that were not treated chemically, showing an important improvement in the interface links between these two materials. In this study, TEM pullout tests were carried out, showing a better interaction due to chemical groups on the CN surface [93]. Other authors have develop in situ functionalized MWNTs in phenoxy composites by melt mixing with 1-(aminopropyl)imidazole [94]. In this research, the composites with more than 4.8 wt% show higher storage modulus than the polymer matrix. In this research the modulus at high temperature is diminished with the incorporation of in situ functionalized nanotubes.

6. FUTURE PROSPECTS IN CARBON NANOTUBE-POLYMER NANOCOMPOSITES

CNs promise to open up a new age of advanced multifunctional materials; however, the wide variety of results available today demonstrate that the addition of large quantities of CN in polymer-based composites is not favorable in all cases, and the reason for this behavior is not completely understood yet. Several of the

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analyzed reports agree that CNs in large amounts form clusters. This diminishes the interaction and, therefore, the interface is not optimized. Nevertheless, when small quantities of nanotubes are incorporated into the polymer, the electrical, optical and mechanical properties improve significantly, reaching important values unseen in other materials. The remarkable properties obtained when f-CNs are incorporated into polymeric composites represent a promising route to design ideal materials for aerospacerelated structural applications. However, the field requires much deeper fundamental research.

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