Carbon Nanotubes Composites: Processing, Grafting and Mechanical ...

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Current Nanoscience, 2010, 6, 12-39

Carbon Nanotubes Composites: Processing, Grafting and Mechanical and Thermal Properties Ana L. Martínez-Hernández1,2, Carlos Velasco-Santos1,2,* and Víctor. M. Castaño1 1 Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Santiago de Querétaro, Querétaro, 76230, México 2

Departamento de Metal Mecánica, Ingeniería en Materiales, Instituto Tecnológico de Querétaro, Av. Tecnológico s/n, Col. Centro, Santiago de Querétaro, Querétaro 76000, México Abstract: Carbon nanotubes represent one of the most important materials in nanoscience and nanotechnology today. The exceptional properties that these materials possess open new fields in science and engineering. Additionally, the chemistryassociated to these materials starts to play an important role, inasmuch, new moieties insert by different chemical routes, inside and outside of carbon nanotubes surfaces, have shown able to modify their structure and properties. To this date, new properties have been found in chemically-modified nanotubes and diverse potential applications are suggested for these materials. One of the most frequent applications for these carbon materials is their inclusion as reinforcement in polymer matrices, due to the amazing structural, mechanical, electrical, chemical and thermal properties that carbon nanotubes possess, suggesting that these materials are ideal to produce new polymer nanocomposites. In this context, carbon nanotube nanocomposites have been developed by numerous research groups around the world aiming to produce new novel strong and light composite materials. Also, electrical conductivity and thermal properties have been studied for this kind of nanocomposites. However, new challenges to create a new age of multifunctional composite materials with these nanometric forms arise and, therefore, the study of new properties in these nanocomposites has increased significantly in the last few years. In this review we discuss a range of methods to properly utilize nanotubes in poymer-based composites, from the solubility behavior of carbon nanotubes, the processing methods to develop carbon nanotube polymer composites, interactions produced between carbon nanotubes and polymer grafted, to the most recent results on the mechanical and thermal properties of carbon nanotubes polymer composites, synthesized with different types of carbon nanotubes. In addition, we discuss the effect of different chemical modifications on nanotubes, with special focus on those developed to improve the compatibility between these nanostructures and engineering polymers, as well as their effect on the final composites properties. The significance of understanding, enhancing and controlling the behavior at the interface between nanotubes and polymer matrices towards the development of novel multifunctional applications with these composites, is also discussed in detail.

Keywords: Carbon nanotubes, chemical functionalization, polymer nanocomposites, polymer grafting, mechanical properties, thermal properties. INTRODUCTION Carbon nanotubes are one of the most studied nanomaterials in the last fifteen years. Due to their extraordinary physical and chemical properties that this carbon allotrope posseses has emerged as novel nanometric material, promising in most areas of science and engineering. These materials have their own features and properties related to structural arrangement and therefore the carbon nanometric materials find new specific research fields that are raised constantly. Nowadays, the carbon nanotubes research has been focused on diverse fields, inasmuch as no previous material has displayed the combination of outstanding mechanical, thermal, and electronic properties. The combination of these properties makes nanotubes appropriate for an extensive range of applications. One of these possible applications is certain their incorporation as reinforcement of polymers matrices in order to develop a novel strong, light, conductive, smart and multifunctional new age of nanocomposite materials. The properties of carbon nanotubes collectively with their size provide a unique material competent to form more interactions in various polymer matrices at molecular level. In this context different researches have been focus to synthesize polymer composites with as-obtained carbon nanotubes and modified nanotubes. Several experimental researches are directed to find the better compatibility between carbon nanotubes and polymer matrices, inasmuch as, there are many factors that play an important role in carbon nanotubes dispersion in polymers. However, chemical functionalization in carbon nanotubes has improved notably the compatibility of these materials with diverse solvents and polymer matrices. This later has allowed diversify the applications of carbon nanotubes and take advantage of their properties in vari-

*Address correspondence to this author at the Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Santiago de Querétaro, Querétaro, 76230, México; Tel: +52 442 2 38 11 50; Fax: +52 442 2 38 11 65; E-mail: [email protected] 1573-4137/10 $55.00+.00

ous polymeric matrices. Thus, in this paper are reviewed the dispersion of carbon nanotubes in different solvents, oxidation in nanotubes is also introduced, inasmuch as this approach is the base of several functionalizations with other chemical moieties, in addition is reviewed the functionalization with other chemical groups inserted on carbon nanotubes surface using other methods. The techniques used in the literature to produce polymer composites reinforced with carbon nanotubes are reviewed before to analysis the mechanical and thermo-mechanical properties of these kinds of nanocomposites reinforced with as-synthesized nanotubes. In addition are presented the polymer grafting nanotubes researches with the grafting to and grafting from techniques, follows of the review of mechanical properties reported to polymer composites developed with modified carbon nanotubes. Finally comments of the reviewed researches and advances in the field of carbon nanotubes polymer nanocomposites are also discussed. 1. DISPERSION OF CARBON NANOTUBES IN ORGANIC SOLVENTS AND WATER Recently, research to synthesize Carbon Nanotube Polymer Composites (CNPN) has focused on different routes aiming to adequately disperse these nanomaterials in a number of polymer matrices. Carbon Nanotubes (CNs) dispersion is influenced by the surface nature of these materials, which normally agglomerate due to the interactions between carbon nanotubes with each other. To get good dispersion during polymer composites processing is fundamental in order to aspire to all mentioned multifunctional properties desire in polymer composites, adequate dispersion improve conductive, mechanical, thermal properties and allows to access to other features in these kind of novel polymer nanocomposites. Thus, to disperse CNs in organic solvents solutions or polymer is an open area of R&D, with fundamental questions, unsolved yet. However, important advances have been achieved when CNs are modified by chemical or physical approaches, in order to change their surface nature, thus improve their solubility properties and therefore, their © 2010 Bentham Science Publishers Ltd.

Carbon Nanotubes Composites

dispersion. In this context, CNs are nowadays modified and their solubility in organic solvents analyzed. Functionalization allows carbon nanotubes to diversify their properties and potential uses. Different chemical routes that change CNs surface behavior and solubility properties in different organics solvent will be analyzed in this section. The modification of these materials with organic groups has constituted the basis to improve compatibility of these nanostructures. In this perspective, two important routes to insert different organic groups in CN’s surface have emerged: a) through a previous oxidation, by introducing organic carboxyl groups that, in turn, allow other chemical reactions on the CNs´ surfaces, and b) through direct reactions on CNs´ surface, aiming of putting new organic chemical groups to tailor and diversify properties and applications. Recently, the chemical functionalization of carbon nanotubes and other structures has been the objective of important theoretical and experimental research, since it represents one of the most promising routes to expand the applications of these and other nanomaterials. In the following sections recent studies, where organic groups (mainly carboxyl and carboxylate moieties) on carbon nanotube surfaces were generated by chemical functionalization and are taken as base of the insertion of other organic groups, will be reported. The review of this section begins with oxidation, that is generally recognized as the most typical approach to modified CNs and carbonaceous materials. Then, other chemical approaches and dispersion behaviors of CNs in organic solvents or water are discussed and their effect compared with other modified nanotubes.

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with other elements in order to form new materials, such as B-C, CN and B-C-N, among others. These nanotubes have been produced by different techniques; among which we can mention hydrocarbon pyrolisis, arc-discharge and laser ablation [14-16]. b) Opening and filling CNs with metals through chemical oxidation. c) Addition of new chains on the tips and surface of functionalized CNs, using as links the groups generated during the oxidation. Despite the fact that the first oxidations were developed to open and filling CNs, the carboxyl, hydroxyl and carboxylate groups generated all through the oxidation on the surface of these materials have demonstrated useful moieties to bond new reactive chains that improve solubility, processing and compatibility with other materials and, therefore, allow to take advantage of CNs properties. Due to the relevance of this type of functionalization, next sections shall dealwith the different studies related to chemical functionalization, after oxidation, by using the OH pertained to COOH groups generated during the oxidation process. The organic carboxyl groups formed on a nanotube surface are localized at the defects in functionalized single-walled and multiwalled nanotubes and suitable reactive organic groups from other chemical chains are prone to react in this zone. A representative scheme of functionalized CN’s with these organic moieties is shown in Fig. (1).

1.1. Oxidation CNs produced by various techniques are mixed with other carbon forms known as impurities, which difficult their manipulation and limit their processing and possible uses. Consequently, researchers have been developed a variety of purifications methods, most of which are based on oxidation. Oxidation techniques were developed with different targets, short after the discovery of CNs. The different approached reported are generally based on the difference on their resistance to be oxidized, between CNs and other nanoparticles formed during the production. It was observed that nanotubes, when oxidized, are consumed from the tips inward [1]. The first reports in this field proposed the oxidation as a route to open and filling CNs with others elements and then allow the adsorption of other molecules [2, 3]. These techniques were developed in gas phase using CO2 [4] and oxygen [5], slight variants of the latter approach were published as CNs gas phase purifications, although the yields reached were around only 1 weight percent [6]. The low yields in gas phase led to new efforts to oxidize the sample with improved results. Liquid phase oxidation methods have been proposed with different targets, from just purification to the opening and filling CNs. These approaches used strong acids, such as HNO3 [3] a mix of H2SO4- HNO3 [7], strong oxidants, such as KMnO4 in an acidic solution [8] and others, as H2O2/H2SO4 mix and HClO4 [9, 10]. Nowadays, different techniques have been reported as purification procedures by using chromatography [11], intercalation [12] and polymer engineering techniques [13]. However, the liquid phase purification has demonstrated to lead to important findings, for instance, it allows higher yields than the gas phase and thus gives the opportunity to have enough CNs free of impurities, but the most attractive point of this type of purification is the additional functionalization on CNs tips and sidewalls, which has opened a novel route in the chemical modification of these nanomaterials, inasmuch as the amazing properties and novel structure of CNs are ideal to form new arrangement at the nanometric level. Chemical modification has developed along different ways: a) Doped CNs, this approach consists in substituting C atoms

Fig. (1). A schematic representation of organic groups on functionalized carbon nanotube.

1.2. Functionalized Soluble Carbon Nanotubes One of the routes used as a complementary procedure to insert organic long chains is by using SOCl2 (thionylchloride) in dimethyl formamide. Thus, long chain alkylamine (Octadecylamine) has been coupled to activated f-SWNTs. The addition of this chain allows soluble SWNTs (s-SWNTs) in organic solvents, such as chloroform, benzene, toluene, 2-dichlorobenzene and CS2 [17]. Other organic chains that have been inserted successfully through the same route are alkyl-aryl amine and 4-dodecyl-aniline [18]. All these organic chains improve the solubility of SWNTs, forming also s-SWNTs. The method using organic carboxyl-terminated nanotubes with SOCl2 subsequent reaction has been used to attach different organic moieties. For instance this approach, followed by a reaction with monoamine-terminated poly(ethylene oxide) (PEO), was useful to facilitate the linkage of this chain in a grafted PEO-SWNTs system [19]. Also, a comparison of solubility of oxidized SWNTs, by using different solvents, was presented. Fig. (2) shows the temporary changes of the dispersed amount in a 10mM NaOH solution as well as the behavior after dispersion in organic solvents at room temperature. SWNTs are dispersed relatively well in DMF and water, but coagulate rapidly in tetrahydrofuran and CHCl3. Also, this system (f-SWNT + SOCl2) has been used in MWNTs to functionalize these structures, via addition of an aminopolymer, such as poly(propionyletthylemine-co-ethylenemine) (PPEI-EI) with the

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cently by Lin et al. [21] A scheme of the reaction using the SWNTs in [1-ethyl-3(dimethyl-amino)propyl] carbodiimide hydrochloride (EDAC) in aqueous KH2PO4 buffer solution is presented in Fig. (3). The functionalization is based on the carbodiimide-activated amidation reaction by using the organic groups on the nanotube surface. Fig. (4) shows the TEM image of PPEI-EI nanotubes.

Fig. (2). Dispersing properties of acid-treated SWNTs in different solvents. Filled circles are 10mM NaOH and solid curve is results of least-squares fitting by an exponential function with a time of 470 h. Copyright American Chemical Society 2001.

Fig. (3). A schematic representation of PPEI-EI attached to carbon nanotube surface.

Fig. (4). TEM image of PPEI-EI functionalized SWNTs. Copyright American Chemical Society 2001.

purpose of forming hydrophilic MWNTs. The behavior of these materials changes by the organic groups attached to the surface and could be very useful for biological systems [20], inasmuch as many nanostructured biological molecules have amphiphilic behavior, and sometimes hydrophilic activities are required. This is also promising in the field of assembling nanotubes through long organic chains. The characterization of individual nanotube bundles, with PPEI-EI organic chain on the surface, has been reported re-

Carbonyl groups and amine groups have much affinity, inasmuch as these organic molecules form the amide groups, responsible for the structure in peptides and proteins. Therefore, direct reactions in carboxyl-terminated SWNT have been realized with molten octadecylamine, in the fange 120 ºC to 130 ºC, the sequence reaction allows to first form a carboxylate group in SWNTs, which has interaction with octadecylamine functionalized free radical [22]. This produces larger soluble SWNTs (s-SWNTs) than those formed using SOCl2 [17]. Other researchers have developed an extensive variety of aliphatic amines coupled to carboxyl groups in the SWNTs surface. In these procedures, SWNTs are treated with amine vapors at temperature betwwen 160-170 ºC, under reduced pressure [10]. Following this affinity between organic groups and nanotubes surfaces, water solubilization of SWNTs has been reached by the insertion of glucosamine bonded through amine moieties, the molecule was inserted in hydroxyl groups corresponding to COOH in CN’s [23]. Recently, a new approach to modify MWNTs surface through the ester linkage with carboxylate groups in nanotubes was presented. The method shows the possibility to insert different alkyl chains, by using a variety of alkyl halides. The system employs the phase transfer reaction of carboxylate salts with alkyl halides. The sequence presents firstly the change of carboxyl groups to carboxylate salts, which react with alkyl halides in the phase-transfer reagent presence. This method has the advantage that the reaction takes place in water after several hours, constituting a simple and efficient method to insert different chains through similar reactions. In addition, alkyl-terminated nanotubes are obtained easily from the solution, inasmuch as these CN’s are insoluble in water [24]. So, the method represents an important route to join nanotubes with other alkyl materials to produce hybrid devices at the nanoscale. Other reactions that involve the insertion of alkyl groups are those produced with alkyllithium reagents and fluoro nanotubes. In this case, the study was realized with two types of CN’s, namely, those produced by laser oven method and those by high pressure of carbon monoxide (HiPCO). The functionalization degree in this case was measured by combining Thermo Gravimetric Analysis (TGA) and UV-Vis-NIR spectroscopy. The process to achieve the functionalization consists of different steps. First, one electron is transferred from the alkyllithium reagent to the fluoro nanotubes, as a result, a fluoride expulsion occurs, leaving a radical site on the SWNT. Recombination of the alkyl radical with the SWNT produce alkyl-terminated nanotubes. The results show clear evidence that the type of CN’s, diameter and isomer type of alkyllithium are important parameters to achieve the insertion of the alkyl chain. The SWNTs produced by HiPCO exhibit a higher degree of alkylation than those produced by the laser-oven method. In addition, smaller diameter fluoro tubes are alkylated more readily and the reaction with tert-butyllithium is less effective than those produced with nbutyllithium [25]. Fluorotubes are soluble in Tetrahydrofuren but are no soluble neither in ether nor n-hexane. In general, the method demonstrates interesting results and reveals the significant parameters that play an important role in the functionalization of fluoro nanotubes. Other flouro nanotubes have been synthesized at temperatures between 150 and 400 ° C [26]. At higher temperatures, the graphitic structure decays noticeably. The highest degree of functionalization has been estimated about C2F using elemental analysis. Nevertheless, when fluorination was applied to small diameter SWNT produced by HiPCO (High Pressure CO), CNTs were cut to an average



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the nanotubes to toluene solutions of dodecanethiol-stabilized Au nanoparticles, used for labeling the location of the chemical reaction sites on the tube wall. The reaction was followed by opticalabsorption measurements. Fig. (6) shows the absorption spectra of as-grown nanotubes suspended in toluene with dodecanethiol 1% by volume after sonication for 10, 40 and 100 min.

Fig. (5). a) SWNTs in SDBS, after sonication and centrigugation, b) SWNTs in water, sonicated but not centrigugated. c-f) SWNTs in peptide solution after sonication and centrifugation, c)K4, d)E4, e)E4C4 oxidised, f) E4C4 reduced. Copyright Royal Society of Chemistry 2007.

length of less than 50 nm [27]. Fluorinated nanotubes were reported to have a moderate solubility (1 mg/mL) in alcoholic solvents [28]. Electrophilic addition of chloroform to CNTs have been developed in the presence of a Lewis acid, followed by alkaline hydrolysis [29]. Further esterification of the hydroxy groups to the nanotubes led to increased solubility. In other approach, SWNTs with a solid-phase milling technique have been used to obtain hydroxylmodified CNTs using potassium hydroxide [30]. The nanotube surface was covered with hydroxyl groups, and the derivative displayed an increased solubility in water (up to 3 mg/mL). Ruthenium-based olefin metathesis catalysts have been attached at the defect sites of acid-treated nanotubes [31]. These catalystfunctionalized tubes result effective in the ring-opening metathesis polymerization of norbornene monomer. The polymer-modified tubes have improved solubility in organic solvents. A complete review of the chemistry of carbon nanotubes is presented by Tasis et al. [32], where modification in nanotubes is realized with different targets. Witus et al., [33] realized a multiple functionalization using a series of peptides that non-covalently solubilize carbon nanotubes in water, linked molecules without modifying the electronic structure in carbon nanotubes, as corroborate by using vis-NIR absorbance, fluorescence, and standard and cryo-TEM. Analysis of these materials reveals that the peptides exhibit some diameter selectivity. Additionally, one of the modifications addresses the poor stability of non-covalently solubilized SWNT suspensions by including cysteine residues for covalent crosslinking between adjacent peptides. The samples of soluble SWNTs in water were analyzed after sonication. SWNTs in water remain as black precipitates, while SWNTs sonicated in the peptide solutions yield homogeneous dark suspensions. The darkness of the solution is an indication of the nanotube concentration. In Fig. (5), the different sequence peptides: an anionic coating (E4 sequence), a cationic coating (K4 sequence), and a covalently linked network around the nanotubes (E4C4 sequence) in comparison with sodium dodecyl benzenesulfonic acid (SDBS), a standard surfactant, are shown. The K4 peptide yields the lightest suspension, while the darker E4 and E4C4 suspensions show that those peptide sequences are able to dissolve higher concentrations of SWNTs [33]. Cui et al., modified SWNTs using dodecanethiol as the reaction agent [34]. The thiolated nanotubes are soluble in toluene. In these nanotubes, the conductance decreases by three orders of magnitude and leads to a gate dependence with a memory effect, which is attributed to the chemical reaction between the tube wall and the thiol. The thiolation process was also observed on the exposure of

Fig. (6). UV-Vis spectra of SWNTs in toluene with dodecanethiol after different times of sonication, 10 min, (slid line), 40 min (dash line), 110 min (dot line). The open circles represent the reference spectrum of the solvent. Copyright American Institute of Physics 2005.

Tagmatarchis et al., report covalent functionalization of carbon nanohorns CNHs using a chemical methodology applied for the functionalization of fullerenes and carbon nanotubes, namely the 1,3-dipolar cycloaddition of azomethine ylides [35]. Contrary to CNTs, the reaction with CNHs proceeds well in either dimethylformamide (DMF) or toluene as a solvent. The authors explained this effect in terms of the nature of as-grown CNHs, where the rough surface structure of CNHs aggregates forbids the increase of the aggregate–aggregate contact area, leading to their weak interactions through van der Waals forces and, therefore, to a higher dispersion of CNHs in the examined solvents. Authors of this research mention that this dispersion is in contrast with CNTs, as they can only slightly disperse in DMF and not at all in toluene. Also, is mentioned that the surface curvature of CNHs plays a crucial role towards their chemical reactivity, due to the presence of unbalanced strain at one edge of the CNHs. Fig. (7) illustrates functionalized CNHs, where a typical dahlia-like morphology of CNHs aggregates is observed. Chen and Zhang, use polyaromatic molecules, such as rhodamine 6G and methylene blue in order to precipitate DNAsolubilized single-walled carbon nanotubes from solution, through a competitive binding mechanism, whereby DNA is displaced from the nanotube surface, allowing the nanotubes to rebundle. These polyaromatic molecules posses extended aromatic systems capable of forming - interactions with SWNTs [36]. Delamination of DNA also is achieved when complementary oligonucleotide (Poly A 30) is used to hybridize specifically to the DNA coating on the nanotubes. Fig. (8) shows soluble and precipitated SWNTs prepared in this research. A group of pyrene-containing poly(phenylacetylene)s (PPAs) of high molecular weight (Mw up to 170 000) were reported by Yuan et al. [37]. Materials are obtained by mixing the polymers and MWNTs in an appropriate solvent, producing the polymer/MWNT



Fig. (7). Images of High resolution transmission electron microscopy (HRTEM) of soluble functionalized Carbon Nanohorns (CNH) , a, c) low magnification, b, d) high magnification. Copyright Wiley Interscience 2006.

Fig. (8). Vials of centrifugated SWNTs after precipitation by (A) rhodamine R6G, (B) methylene blue MeB (C) oligonucleotide PolyA30, (D) DNAsolubilized SWNTs without desolubilizing agent. (R6G, MeB, PolyA30). American Chemical Society 2006.

hybrids with MWNT contents up to 25 wt %. This material has high solubility in common organic solvents, such as chloroform and THF. Solubility reachs 637.5 mg/L in THF, due to the additive effect of the PPA skeleton and the pyrene pendants in solubilizing the MWNTs. The solubilization is realized through the spontaneous wrapping of the polymer chains around the MWNT shells, produced by - interactions. The pyrene -containing PPAs were synthesized by using the organorhodium complexes as catalysts. In this research, several new homo and copolymers of PAs bearing pyrene pendats, P1, P(1-co-PA) and P2(m) are prepared. Fig. (9) shows the chemical structure of the monomers used. Fig. (10) presents the solubility results of wrapped nanotubes in THF. 2. PROCESSING OF CARBON NANOTUBE COMPOSITES Research focused to synthesize and evaluate carbon nanotubes composites has shown that different parameters play an important role on the final properties of polymer-based carbon nanotube composites. In this section, only the methods employed in the majority of these studies are discussed. Mechanical and thermal properties

obtained with these materials are analyzed in other sections, along with other important factors, such as chemical functionalization. Thus, the three most used synthesis methods in Carbon Nanotube Polymer Nanocomposites (CNPN) are mentioned in what follows. 2.1. Solution Processing To dissolve the polymer matrix and incorporate the CNTs in the solution is one of the most used methods. The polymer solution allows dispersing nanotubes relatively easy, but some challenges are faced, depending on the polymer matrix used, carbon nanotube type and processing conditions. This section describes some experimental details using this approach. Results and discussion of these studies are found in section four. Shaffter and Windle developed composite films with a ample range of nanotube concentrations, by mixing of aqueous poly(vinyl alcohol) solutions, with CNTs, dispersions, followed by casting, and controlled water evaporation [38]. The concentrations for the two solutions were equal and guaranteed that the overall volumes of each sample as-mixed were identical, in spite of the CNTs loadings.


Fig. (9). Schematic representation of poly (phenylacetylene) homo and copolymers used by Zhang et al. 2006. Copyright American Chemical Society 2006.


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In other research, a copolymer of 96% wt of Methyl Methacrylate (MMA) and 4% wt of Ethyl Methacrylate (EMA) was developed by Velasco-Santos et al. [40]. In this case, MWNT obtained by the arc discharge approach were used with 10-20 nm of outer diameter, 2-6 nm of inner diameter, characterized by HRTEM (High Resolution Transmission Electron Microscopy) and lengths of 1-10 μm. The composites were produced by solution mixing and then cast in Teflon molds. The surfactant triton X-100 (toctyphenoxypoly-ethoxyethanol) and the Plasticizer trytolyl phosphate were used to improve the dispersion of CNTs in the polymer solution. The preparation of CNTs-composite is as follows: the MEMA copolymer was dissolved in TetraHydrofuren (TH) and MWNT were sonicated in TH by 5 minutes (the samples with surfactant or plasticizer were added with CNTs and solvent at this stage) and added to the polymer solution. The whole mix was sonicated at three intervals of 10 minutes each during the first hour in the teflon molds and sonicated by 5 minutes during the next five hours in order to ensure better distribution, The solvent was evaporated at room temperature; the films have an average thickness of 0.35 mm. The composites were prepared with 1wt % CNTs (1), 1 wt % CNTs and 1 wt % surfactant (1S), 1 wt % CNTs and 1 wt % plasticizer (1P) . The films were analyzed by optical microscopy and the sample that presented apparently better distribution (1S) was prepared at different concentrations, explicitly 5 wt % CNTs and surfactant (5S), 7 wt % CNTs and surfactant (7S) and 10 wt % CNTs and surfactant (10S). In this research, the following samples were prepared as reference: 0 wt % CNTs(0), 0 wt % CNTs and 1 wt% surfactant (0S), 0 wt % CNTs and 1 wt % plasticizer (0P). Mechanical properties of these nanocomposites are analyzed in other section of this article. Other research that involves solution processing for obtains polymer reinforced with CNs was developed by Ruan S.L., et al, [41]. The composites were synthesized introducing purify MWNTs in Ultrahigh molecular weight poly(ethylene) (UHMWPE). CNs were dispersed in xylene during 2 hours by magnetic stirring and after sonicated 2 hours more at room temperature. This mixture was poured into the UHMWPE xylene solution refluxed previously at 140 °C during 3 hours. Once that CNs solution is added the mixture is refluxed again by 30 minutes in order to get good dispersion of CNs in polymer solution. Films were prepared by solution casting with 1 wt% of CNs. Polymer films were also prepared in this research.

Fig. (10). Solubility in THF of MWNTs wrapped in the pyrene containing PPA chains. The contents of MWNTs in the nanohybrids is given in parentheses calculated theorically. Copyright American Chemical Society 2006.

Composite films present similar degree of CNTs alignment to the plane of the film. This conclusion was confirmed by the authors using X-ray diffraction patterns. The thickness of the films varied from 53 mm down to 44 mm, increasing with CNTs concentration, as a result of the density of nanotubes ~1.75 g/cm3 and PVOH ~1.3 g/cm3. Densities of these materials were measured by a liquid immersion technique and found to obey a linear law of mixtures. On the other hand, Dufresne et al., used CVD MWNTs with mean diameters around 30–50 nm and a polymer matrix of styrene 34 wt% and butyl acrylate 64 wt%, which contained 1 wt% acrylic acid and 1 wt% acrylamide [39]. In this research, aqueous polymer suspension with spherical particles with an average diameter around 150 nm and 30 wt% solid fraction was used. The CNTs dispersion was mixed with the suspension of polymer to composite films using weight fractions of CNTs from 0 to 15 wt%. The mix was stirred and cast in a Teflon mold at 35 oC to allow both water evaporation and polymer particle coalescence.

Safadi et al. [42], used MWNTs produced by CVD to form Polystyrene (PS) films composites, first MWNTs were dispersed in toluene using ultrasonic wand dismembrator. The mixture was poured in toluene solution of 30% PS. Films produced contain 1, 2 and 5 wt% of CNs. These composites were developed by two methods, film casting and spin casting. In the first case approximately 20 ml of each solution was decanted into glass culture dish in order to evaporate the toluene. The obtained films were dried to vacuum for 7 days at 80°C and 25 in Hg with an average thickness of 0.4mm. In the other hand films were produced by spin casting, solutions containing PS and MWNTs were spun on a Spin Coater. 3 gr. of the solution were distributed onto each substrate. The obtained films were formed at 30 s with speeds between 1000-2500 rpm and dried at the same conditions of the films obtained by the first technique. To dissolve the polymer matrix, mix solution with nanotubes and then cast is an approach employed also for using, as reinforcement, functionalized nanotubes. Next are presented some methods used to produce polymer nanocomposites reinforced with CNs using solution processing and functionalized nanotubes. Bhattacharyya et al. used a protein, ferritin, covalently attached on to the side walls of MWNTs. The CVD CNTs are used as the reinforcement agent in the thermoplastic polymer PVA. MWNTs were purified via reflux in 2 M HNO3 solution for 24 h, using centrifugation, washing with deionized water, and drying under vacuum. Then



CNTs were treated with HCl to create acid functionality, mainly through carboxyl groups on the side walls of the nanotubes, which can produce an amidation reaction with the amino NH2 groups in the protein molecules. For protein immobilization, acid-treated MWNTs were dispersed in 5 mL phosphate buffer solution with pH=6.7 by sonication using a bath sonicator for l h. 2 ml of l0 mg/mL solution of 1-ethyl-3-3-dimethylaminopropyl carbodiimide hydrochloride (EDAC) was added to the dispersion and then stirred for 30 min as commercial ferritin solution (0.15 mL) was added and then stirred for 24 h at room temperatures. The next sample was centrifuged and washed with deionized water. For the synthesis of 1.5 wt % composite films, 0.4 gm of PVA were dissolved in 20 mL of water at 90 °C and cooled to room temperature. 6 mg of functionalized MWNTs were dispersed in 10 mL of deionized water and added and stirred for 24 h at room temperature. The solution was centrifuged at 5000 rpm to remove any undissolved part and then decanted into a plastic Petri dish and dried under vacuum to obtain the composite film. Also, in this research, separate composite films using nonfunctionalized MWNTs are synthesized by the same process [43].

MWNTs (MWNT–Cl). After centrifugation, the residual solid was washed with THF several times and dried for 2 h under vacuum at room temperature. MWNT–Cl are converted to hydroxylfunctionalized MWNTs (MWNT–OH) with glycol at 120 °C for 48 h. For this reaction, macromolecular MWNT initiator (MWNT–Br) was prepared by reacting MWNT–OH with 2-bromoisobutyl bromide using triethylamine [46].

Geng et al. use fluorinated CNTs with semi-crystalline polymer poly(ethylene oxide) PEO. Authors prepared by roll-casting the composites with fluorinated single walled nanotubes (F-SWNTs) using 1,4,6 and 10 wt% of load and control sample of pure PEO. SWNTs used were produced by laser ablation. PEO powder with a molecular weight of 30,000g/mol was used as matrix. In the roll cast system F-SWNTs were first suspended in isopropanol by sonication and then mixed with a clear PEO/methanol solution. The cast was achieved in a teflon roller using a pipette dropwise. A solid film is obtained after evaporation of the solvent. The film obtained with F-SNWTs is considerably more uniform than those formed using pristine CNTs [44].

Composites using SAN and PMMA-grafted MWNT composites were prepared by solution casting using THF as solvent. The SAN and MWNTs-grafted THF solution was stirred overnight at room temperature to obtain homogeneous solutions. Then, samples were decanted into Petri dishes and evaporate the solvent. The composite films were then dried in a vacuum oven at 60 °C for two weeks. Three composites were prepared with PMMA-grafted MWNT/SAN at weight ratios of 5/95, 10/90 and 20/80 [46].

Paiva et al. functionalize CNTs with Poy(vinyl alcohol) using as matrix also PVA with a MW of 70,000–100,000 (99% hydrolyzed). The PVA used for nanotube functionalization had a MW of 22,000 (87–89% hydrolyzed). SWNT and MWNT were produced by the arc-discharge method and the chemical vapor deposition (CVD) method, respectively. The CNTs were purified using an aqueous nitric acid solution. Then, nanotubes were functionalized with low molecular weight PVA by the N,N-dicyclohexyl carbodiimide-activated esterification reaction. Nanocomposite films were fabricated from water-solutions of PVA–SWNT and PVA–MWNT using the wet-casting method. PVA polymer sample (MW 70,000– 100,000) was dissolved in water by heating at 80 °C and stirring for 4 h to obtain a 20% (wt/wt) PVA solution. Functionalized nanotube solution was prepared with specific quantity of CNTs and added to the PVA solution; the sample was sonicated and stirred overnight for effective mixing. The resulting solution was cast onto a glass slide. The film was dried at room temperature for 24 h and dried at approximately 100 °C. A neat PVA film and a composite film with SWNTs without functionalization were prepared for comparison purposes, using the same procedure. The dimensions of the films were 15 mm X 30 mm with a thickness of 50–100 m. The carbon nanotube content in nanocomposite films varied from 2.5% and 5% (wt/wt) [45]. Wang et al., use CNTs grafted with Poly (methyl-methacrylate) PMMA and poly Styrene-coAcryloNitrile (SAN) with an acrylonitrile content of 25 wt% The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of SAN are 66,000 and 156,000, respectively. MWNTs (purity 95%, diameter within 10–20 nm) used in this research are produced by CVD. For the synthesis of CNTs-grafted. MWNTs were refluxed in concentrated nitric acid 67% for 24 h. The excess nitric acid was removed by centrifugation. Then, the sample was washed with deionized water and dry THF. The treated MWNTs were dried at 50 °C under vacuum. Functionalized MWNT was then stirred with 100 mL of thionyl chloride at 70 °C for 24 h to form acid chloride-functionalized

Grafting of PMMA onto MWNT–Br was achieved by atomtransfer radical polymerization. Appropriate amounts of monomer were mixed with MWNT–Br, CuBr, and 2,20-bipyridine. The mixture was degassed and then heated at 70 °C in an oil bath under nitrogen for 72 h. The solid reaction mixture was dissolved in dry THF. This solution was filtered and washed with dry THF several times to completely remove monomer and non-grafted PMMA. The PMMA content of PMMA-grafted MWNTs is 90 wt% as determined by thermogravimetry. The molecular weight of PMMA grafted onto MWNTs was determined by gel permeation chromatography. In this procedure, PMMA is detached from the MWNTs in a THF/NaOH solution. The Mn and Mw values are 1800 and 2000, respectively.

Other research that involved solution and casting approach, in order to develop nanocomposites, was achieved by Owens; In this study, the PAN/SWNT composite was prepare, using 0.076 g of PAN and 0.0053 g of SWNTs, both dissolved in acetone, allowing to slowly evaporate while in an ultrasonic bath. The evaporated PAN containing the SWNTs was dried for two hours at 70 oC. A similar process was used to incorporate functionalized fluorinate nanotubes witin the PAN. Both functionalized and nonfunctionalized carbon nanotubes were obtained from Carbon Nanotechnologies Inc., grown by the HiPco process. In general, solution and casting have been used to incorporate different types of CNTs, both functionalized and non-functionalized. Nanocomposites developed with this approach are relatively easy to process; even though the method is very useful at the research level, it is difficult to scale-up in order to develop nanocomposites for specific applications or industrial uses [47]. 2.2. Melt Processing Even thought the solution and casting approach is very popular, it presents the difficulty of the size and quantity of composites produced. Other promising methods involve polymer melt and processing. The composites produced by this technique present the advantages of uniform mixing, are easier to scale than other process and it is possible to produce composites with specific spatial arrangements. In addition melting is used frequently to produce composites with thermoplastic polymer matrix in industrial scale. Thus, the technique is very useful to find different alternatives to produce CNPN. In what follows, we present some approaches that use melt processing to produce CNPN, by using extrusion or hot pressing. Coppet et al., synthesized composites with CNTs and nanofibrils. The matrix used was PMMA as dry spherical particles, 100– 500 mm in diameter. The carbon nanofibrils used in this research were pyrolitically stripped and, therefore, had no surface organic contamination. The diameter of nanofibril was 200 nm, and the length range was 200–500 m. The MWNTs have diameters ranging 10–15 nm and length range 2–3 m.


In the composites preparation, the components were mixed using a high intensity ultrasonic processor, with a small amount of ethanol separately, at 10% amplitude for 90 s, first mechanically then by sonication at 50% for 90 s. This allowed the distribution of carbon materials over the surface of the PMMA spherical particles. After sonication, the mix was spread out finely on a foil surface and dried at 50 °C for 1–2 h, then mechanically mixed using a Molinex Attritor. The attritor propeller had three de-phased blades, and was operated at 2000 rpm for 30 min. The mixing was heated at a temperature of 170 °C in a Brabender Plasti-Corder, speed range of 30– 50 rpm for 10–30 min. These conditions allowed good distribution of nanofibrils and nanotubes. The reference sample of pure PMMA was not sonicated with ethanol or mixed in the Brabender prior to extrusion. The final step of the composite preparation was to extrude the ‘‘pre-mixes’’ in a extruder to orient the reinforcement in the flow direction. The apparatus was a Brabender DSK 25 singlescrew extruder with 25 mm screw diameter and a L/D ratio of 22. The melt temperature was 232 °C, and the pressure was 16 bar [48]. Wong et al., used MWNT synthesized by pyrolysis of hydrocarbon to prepare CNPN using, as matrix, Polystyrene (PS). First, the raw carbon powders were purified in diluted nitric acid for several hours to dissolve the catalyst particles. Then, the samples were washed with distilled water and dried at an elevated temperature in an argon atmosphere. MWNTs were obtained of this process with diameter ranging from 20 to 60 nm and length ranging from 1 mm to several tens of microns, with 96 wt% purity. To fabricate CNT/PS composite, 6 g of PS were dissolved in 40 g of toluene, and the mixture was heated in a plate with stirring. CNTs were weighed according to the required percentage and added to the solution. The mixture was stirred for 1 h to form a homogeneous suspension. The suspension was sonicated for 1 h and then cast into an aluminum mould as a film. Then, the suspension in the mould was dried in an oven at 100 °C for 3 h. The samples were cut and extruded in a mini extruder at 180 °C under an extrusion load of 30 N. The reservoir of CNT/polymer melt was extruded through a die of 1 mm (diameter) by 10 mm (depth) with a pressure of 7.4 x 104 Pa. Composite rods with approximately 0.1, 0.5, 1.0, and 2.0 wt% of CNT were obtained. In this research, other composites are obtained using a different method and epoxy as polymer matrix [49]. Wei et al., prepared CNPN with Ultrahigh Molecular Weigth Poly(ethylene) and MWNTs. Precursory UHMWPE fibers without/with 1 wt% and 5 wt% of MWNTs prepared by the gel extrusion process were preheated for 30 min at 120 °C, pressed with a load of 20 tons for 30 min to get precursor films with a thickness 10 m and width 2 mm, and then cooled to room temperature. The highly oriented fibers present a DR (length ratio) of 5, 11, 15, 20, 25, and 30 and were synthesized in two steps, in the first the precursor films were drawn to five times the length, DR = 5, at 120 °C and, in the second step, the above fibers were drawn to a predefined DR at 130 °C. After, a hot press was applied with a load of 25 tons for 30 min at 120 °C, and then cooled to room temperature [50]. Tatro et al., fabricated a CNPN by hot pressing. They use MWNTs which were first sonicated in DMF for 2 h. The DMF with the dispersed MWNTs was then mixed with a 10% (w/v) PMMA/DMF solution. The mixture was sonicated by 2 h. The PMMA was precipitated using methanol. The 1 wt% MWNT/ PMMA composite produced was dried under vacuum at 110 °C for two days and then at 140 °C for an extra two days. The residual neat PMMA was also processed using this technique. The 1 wt% MWNT/PMMA composite was mixed with neat PMMA utilized a C.W. Brabender Plasticorder with a mixer accessory to synthesize 0.10, 0.26, and 0.50 wt% MWNT/PMMA composites. Samples were compression molded at 135 °C and 3000 lbs of pressure for 15 min in a Carver Press [51].


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In other research Zoo et al., used CNTs obtained by catalytic decomposition in order to produce hot press composites. CNTs was mixed with UHMWPE, the specimens were prepared with 0.1, 0.2, and 0.5 wt%. Also UHMWPE without CNTs was synthesized. Toluene was used in this process to provide active blending with UHMWPE and CNTs. The samples were mixed in the ultrasonic bath for about 1 h and then placed into the hood for 2 days to remove toluene, before being formed into 50 x 50 x 2 mm shapes by hot pressing at 180 °C, 25MPa and 1 h. Also, for the wear tests, specimens were cut to 15 x 15 mm pieces and cleaned in ethanol in an ultrasonic cleaner [52]. Details of the mechanical results of these composites are analyzed in other section of this manuscript. Functionalized Carbon Nanotubes have been used in different process in combination with melt and mix approaches in order to fabricate CNPN. In spite of the polymer chains have been formed previously to this processing and therefore, less probability of interactions could be achieved, different groups use this type of CNTs, inasmuch as using CNTs with different groups in the surface could contribute to improve dispersion. Xue et al., fabricated a material consisted of 80% UHMWPE and 20% High Density Polyethylene (HDPE) reinforced with MWNTs in order to improve the tribological properties. The resulting compound is possible to process in standard processing equipment, like a win-screw extruder. Due to the viscosity, the mix of these polymers allows easy dispersion of the CNTs in the matrix. As a reference material, specimens of neat UHMWPE and a HMWPE/HDPE composite without MWNTs were synthesized [53]. Samples were produced with untreated CNTs and with pretreated CNTs. For the latter, the dried CNTs were mixed into 4.0 mol/l HNO3 with a weight ratio of 1:3 (CNTs:HNO3). The mixture was continuously boiled in a reflux at 100 °C for 2 h and stirred at 300 rpm. Afterwards, the mixture was washed with distilled water. The product was dried at 100 °C in an oven. Both CNT samples (pre-treated and untreated) were dried in an oven at 100 °C for 2 h and then compressed. The UHMWPE/HDPE blends were produced by mixing 20 wt% HDPE granules and 80 wt% UHMWPE powder in a Hakke laboratory kneader at a temperature of 210 °C. The rotational speed was 10 rpm for 5 min, and then increased to 45 rpm for 10 min. After, the blend was compression-moulded as the UHMWPE samples at a temperature of 180 °C and a pressure of 10 MPa. The composites were produced in the same way as the UHMWPE/HDPE samples. Different weight contents of CNTs untreated or pre-treated respectively were mixed with the UHMWPE powder and the 20 wt% HDPE. The mixture was processed in the kneader and compression moulded in the hot press to plates. Wang et al., employed modified MWNTs to prepare composites using PEO-terminated NH2 as compatibilizer and PMMA as polymer matrix. Materials were manufactured by melt mix. The CNTs were oxidized using acid and then dried in vacuum at 100 °C for 3 days. The presence of carboxylic acid groups in acidified MWNTs was corroborated by spectroscopy. Then, PEO-NH2 and PMMA composites were prepared in a laboratory mixing molder, the melt blending of PEO-NH2, MWNTs and PMMA was prepared at 200 °C for 30 min. The resulting materials were then pressed at 200 °C in a hydraulic press for 30 min at atmospheric pressure, and then at 200 °C for 15 min under 1 metric ton pressure, three quick press/ release cycles were performed [54]. 2.3. “In situ” Polymerization In previous section we have discussed how the carboxyl groups on the tip of the nanotubes change their surface behavior, which makes them useful for tailoring the interaction between CNs and other compounds. Thus, the use of these moieties to improve the interfacial bonding in CN-polymer composites has been proposed,


either by inserting other chemical groups or by using the carboxyl groups produced during the oxidation [55]. Jia et al. [56] suggested that the initiator opens the bonds of CNs, allowing them to take part in the polymerization. However, oxidation offers more possibilities for bonding the nanotubes to the matrix, due to reactive chemical groups such as COOH, COO- and C=O, found on the tip and on the wall surfaces. Other groups [55] have proposed that the best approach for effective interactions between the functional groups located on the CNs surface and polymer chains, is by “in situ” processing, since the free radical formed in monomers by the initiator could either react with the CNs moieties easier than when the polymer is already made and then dissolving or melting to produce the final composites. In situ polymerization has allowed better interactions at the molecular level, provided the CNs are previously functionalized. The disadvantages of this technique are the difficult control of polymerization parameters once the CNs are included as well as the control of the viscosity and nucleation produced by the nanotubes in some semicrystalline polymers. Here, we shall review the processing details of some relevant results. We include epoxy composites when a curing agent is used, inasmuch as the process allows having some dangling bonds in the polymer when functionalized nanotubes are incorporated. Velasco-Santos et al., manufactured CNPN using Methyl Methacrylate monomer (MMA) and 2-2 azoisobutironitrile (AIBN) to process PMMA polymer matrix and carboxyl-terminated MWNTs. Functionalized arc discharge MWNTs were developed using KMnO4. Composites were produced by “in situ” polymerization using AIBN as initiator. The reaction to produce each composite was carried out in a flask with MMA monomer and AIBN. the CNTs were added after 30 minutes of reaction and constant stirring, the temperature was maintained at 70°C for 2 hours, then the mix was casting in a glass mold with a latex frame in order to control the shrinkage. The composites produced have a thickness about 1.8 mm. The manufactured samples are identified as follows: sample A (only PMMA), sample B (1 wt % of u-MWNT), sample C (1 wt % of f-MWNT), extra composite Sample D (1.5 wt% of f-MWNT) [55]. Details of mechanical properties are reviewed in section four. Other work that involves PMMA as polymer matrix and “in situ” polymerization is that by Putz et al. [57]. In this study, SWNTs produced by laser ablation were used. Nanocomposites were prepared by in situ polymerization in solution. Equal amounts of MMA and dimethylformamide (DMF) were added to the reactor, along with 0.08 wt % of the free-radical initiator 2,2azobisisobutyronitrile (AIBN), the reaction temperature was maintained at 80 °C. For the preparation of the nanocomposites, the SWNTs were sonicated in DMF at approximately 0.01% (w/w), and the resulting solution was centrifuged and decanted prior to its incorporation into the reaction flask. Reaction time was 2 hr and then samples were precipitated in methanol, washed with hexane, and dried at room temperature in vacuum. Reaction conditions were controlled during the synthesis of these composites and the molecular weights are maintained, so no polydispersity is presented. The control of processing conditions allows the SWNTs to preserve tacticity and that the stereochemical structure of the polymerized samples was not affected by adding the SWNTs. Gojny et al. 2003 use MWNTs arc discharge functionalized using a mix of H2SO4 and HNO3. Afterwards, oxidised CNTs were modified with triethylenetetramine in order to form CNPN with epoxy matrix. For the preparation of the nanocomposites, suspensions of oxidised MWNTs and aminofunctionalised nanotubes were prepared in acetone and sonicated for 30 min and slowly dispersed in the heated epoxy resin. After cooling to room temperature the hardener Ruetadur Teta was added to each of the two samples and finally cured for 24 h at room temperature [58].


Liu and Wagner, 2005 used chemically-functionalized multiwalled carbon nanotubes (MWNTs) in order to prepare two kinds of CNPN with epoxy resin matrix, using two aliphatic amines curing agents: the first is synthesized using Jeffamine T-403 glycolitic polypropyleneoxidetriamine as a curing agent, which resulted into a glassy epoxy resin, the second nanocomposite was prepared using Jeffamine D-2000, w-polypropyleneoxide diamine as a curing agent, which resulted into a rubbery epoxy resin. CNTs used have a diameter range around 30–50 nm and the length range was 1–5 μm, and purity was >95%. The epoxy resin used in this work was diglycidyl ether of bisphenol A (Epon 828 weight ratio: 25/75), the resin was mixed and stirred for 1 h at 75 °C, then degassed in vacuum for 1 h, and cast into a mould. The curing schedule was then 75 °C for 3 h and 125 °C for an additional 3 h. For the Epon 828/T-403 system (weight ratio: 100/42) mixing was accompanied by stirring for 15 min at room temperature, followed by degassing in vacuum for 2 h, and by curing at 80 °C for 2 h and 125°C for 3 h. The composites were prepared using 1 wt. % nanotube in epoxy resin (Epon 828/D2000), first f-MWNTs sample was dispersed in chloroform. D-2000 curing agent was added to the solution for 1 h using a 50–60 Hz bath sonicator. Then, the chloroform is evaporated with continuous stirring, a stoichiometric amount of Epon 828 was added into the mixture to produce 1 wt.% nanotube–epoxy composites. A similar procedure was used to prepare nanotube- based Epon 828/T-403 composites [59]. In other research Yaping et al., 2006 use MWNTs synthesized by CVD. The method produces CNTs and carbon nanofibers of different diameters and structure. The MWNTs used in this study were purified. The average diameter of the MWNTs was about 30– 100 nm, and the purity was >95 wt. %. As polymer matrix epoxy resin TDE-85 (epoxy number of 0.85 eq/100 g) was employed. Amine 593 was utilized as curing agent. MWNTs were heated with an excess 2.5% and 20% weight of triethylenetetramine ethanol solution for 30 min. Amino-functionalized carbon nanotubes (MWNTs- NH2) and MWNTs were added at 120 °C and dispersed in a high-speed homogenizer of 20,000 rpm for 20 min. The mixtures were degassed at 90 oC for 30 min. Then amine curing agent was incorporated to the epoxy resin, this decreased the temperature until 40–50 oC. Afterwards, the mixture was decanted into a preheated steel mould. The mould is maintained at 60 oC for 1 h. The epoxy resin was mixed with 0.2%, 0.4%, 0.6%, 1.0% of MWNTs and MWNTs-NH2 separately [60]. Zhao et al., 2005 incorporated oxidized MWNTs treated with acid. The –COOH content in MWNT was about 1 carboxyl group per 50 carbon atoms as determined by chemical titration. Polymerization grade 3-caprolactam which chemical formula is C6H11NO was used to the form polymer matrix. For the preparation of these composites 3.0 g MWNTs or MWNT-COOH are combined with 60 g 3-caprolactam and 60 g H2O, then the mix is ultrasonicated for 0.5 h. the solution was mixed with additional 3-caprolactam (540 g) in a 2 l closed autoclave for the hydrolytic polymerization. The temperature during polymerization was maintained at 230 °C for 2 h, 245 °C for 2 h, and then increased to 265 °C for 2.5 h. Nitrogen atmosphere was maintained to exhaust water during the last 1.5 h. The products removed from the autoclave were pelletized, extracted five times with boiling water and dried in vacuum at 110 °C for 8 h. The composites Polyamide 6 CNTs contain ca. 0.5 wt% CNTs (non modified nor functionalized) and are indicated as PA6/MWNT and PA6/MWNTCOOH, respectively. Those formulations were chosen because no significant improvements were obtained at higher CNTs loading by simple melt compounding. Pure PA6 was synthesized under the same conditions as a comparison [61]. Polyamide composites by using CNs were developed by Kang et al. [62]. In this report, Poly(hexamethylenesebacamide) (nylon 610) nanocomposites, reinforced with MWNTs, were successfully


produced via in situ interfacial polymerization of two liquid phases. The carbon nanotubes used in this study were CVD MWNT, purified with 3 M HNO3 at 60 °C for 12 h, and refluxed in 5 M HCl at 120 °C during 6 h. The purified MWNTs were dispersed in pure water containing Triton X-100 and then sonicated for 7 h at 25 °C. Then, two solutions were prepared to synthesize the composites. The first mixture consisting of 5.4 ml (6.048 g, 25.2 mM) of sebacoyl chloride in 180 ml carbon tetrachloride. To this solution, 90 ml of an aqueous MWNT dispersion containing 7.95 g of hexamethylenediamine and 2.16 g of sodium hydroxide, was added. The polymerization reaction starts with the addition of MWNT and a film of nylon 610/MWNT is formed at the liquid interface and. The nylon 610/MWNT composite was washed with distilled water. The sample was first air-dried and then dried in a vacuum oven at 80– 100 °C. Yan and Yang have used functionalized MWNTs to synthesize polyamide-matrix composites. Oxidized MWNTs were previously modified with isocyanate groups to prepare polymer composites by in situ anionic ring-opening polymerization (AROP). First, 100 g of oxidized nanotubes were dispersed into 50 ml of toluene 2, 4diisocyanate (TDI) under stirring. Functionalization was achieved in a dry nitrogen atmosphere at 80 °C for 72 h. The resulting nanotubes, terminated in isocyanate moieties (MWNTs-NCO), were filtrated and washed with toluene and then with N,Ndimethylformamide (DMF) in a Soxhlet extraction by 48 h, to remove completely the unreacted TDI. Samples were dried in vacuum at 40 °C for 24 h. To synthesize the polymer nanocomposites through in situ polymerization, the MWNTs-NCO prepared as described, were added to a solution of 80 g of e-caprolactam (CL) with 20 g DMF, and then sonicated by 1 h at room temperature. Afterwards, DMF was removed via vacuum distillation at 60 °C, and the mixture heated at 170 °C under vacuum for 20 min. TDI was added to ensure that the isocyanate groups contained in all the preparations was identical. Then, 4 wt % of e-Caprolactam sodium salt (CLNa), was added. Finally, the mixture was poured into a preheated mold into an air-circulating oven at 160 °C and polymerized for 10 min. polyamide 6 -MWNTs nanocomposites (MCPA6MWNTs), with different loads of MWNTs-NCO (0.5, 1.0, and 1.5 wt %), were obtained. In this case, MCPA6 and MCPA6, with oxidized MWNTs, nanocomposites were also prepared by the same procedure [63]. Poly(p-phenylene benzobisoxazole) (PBO) is other polymer used as matrix in CNPN. Li et al. produced composite fibres prepared by in situ polymerization and dry-jet wet spinning, by employing CVD MWNTs. Pre-treated CNs were prepared using H2SO4 and HNO3 3:1 (v/v) and refluxed for 30 min under partial vacuum. The CNs were poured into water at room temperature and neutralized with NaOH and thus washed with water. For preparing the composites, liquid crystalline solutions of CNs/PBO were produced by in situ polycondensation of 4,6-diaminoresorcinol dihydrochloride (DADHB) and terephthalic acid (TA) in the presence of CNs. In situ polymerizations were carried out in poly(phosphoric acid) PPA at a CNs/polymer concentration of 10 wt%, which formed an anisotropic mixture. The CNs concentrations were 8 and 2 wt%, respectively, with respect to the polymer concentration utilized. The CNs/PBO mix was dry-jet wet spun using a piston-driven system. The mix was transferred under a dry nitrogen atmosphere to the spinning case at 50 °C heated at 160 °C for 4–6 h prior to spinning. The processing temperature was kept at 200 °C; while coagulation and washing with water was at room temperature. The fibres so produced were washed in running water for five days and subsequently dried overnight under vacuum at 100 °C. For comparison, pristine PBO polymerization and fibre spinning were prepared under the same conditions with no CNs added. The PBO and CNs/PBO fibres in this report were not heat treated under tensile strength to avoid the influence of the heat-treatment process on the initial fibre structure [64].


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In other research Li et al. [65] produced PBO-based CNPN, by using CVD MWNTs. Functionalization of CNs was achieved by mixture solution of concentrated H2SO4 and HNO3 (1:3,v/v) at 100°C for 3 h. After oxidation, MWNTs were washed with distilled water until no residual acid was detected. Then, the samples were dried at 80°C under vacuum. Finally, the O-MWNTs were dispersed by ultrasonication in H3PO4 at room temperature at 38 kHz for 40 min. The dispersed O-MWNTs were identified as UMWNTs. Composites were prepared by in situ polymerization by employing 25 mmol of 4,6-diamionophenol dihydrochloride(DADHB) and 25 mmol of Terephthalic acid (TA) mixed together with 83 wt% polyphosphric acid (PPA) (25.30 g) in a 100 ml glass flask, equipped with a mechanical stirrer and gas ports (inlet/outlet). In this reaction system, a 10.487 g mixture, containing H3PO4 and U-MWNTs was first added, then a total of 21.11 g fresh P2O5 was added to the mixtures until a final P2O5 concentration to 83 wt% is achieved. The mixture was heated at 160 °C for 24 h under constant stirring. Opalescence was observed in the mix, indicating the presence of the nematic phase. The mixture was finally heated to 200 °C for 6 h. From this mix, a threadlike sample was withdrawn by using a glass stick and cast into a film by using a blade with a spacer, then coagulated and washed in distilled water to remove the PPA completely and dried under vacuum at 100°C. Under the same conditions O-MWNTs/PBO composites and pure PBO polymer were prepared. The two kinds of composites concentrations and the PBO polymer are finally mixed at 9 wt% in PPA. The CNs concentration is 5 wt% with respect to the polymer content. 3. CARBON NANOTUBE POLYMER-GRAFTING In the synthesis of CNTs – polymer composites, dispersion is doubtless one of the most important challenges to overcome. In this sense, chemical functionalization has represented an outstanding route to improve compatibility with many polymeric matrices. Thus, a number of strategies have been applied for the functionalization of CNTs through covalent and non-covalent reactions with organic molecules, including synthetic polymers, biomolecules such as enzymes, proteins, polysaccharides or DNA, in order to improve the hydrophilicity of CNTs and therefore overcome the dispersion problem. With this purpose, grafting of macromolecules has opened a new area of opportunity to the introduction of hyperbranched polymers or biomolecules onto the surface of CNTs. Herein; we focus on recent developments in the modifications of CNTs by grafting techniques. Attachment of organic chains to the surface of CNTs can be accomplished by either “grafting to” or “grafting from” techniques. “Grafting to” involves the bonding of a preformed endfunctionalized polymer to reactive surface groups on the substrate. The “grafting from” involves the immobilization of initiators onto the substrate followed by in situ surface polymerization to generate the tethered polymer chains [66]. Both approaches have many useful applications and advantages depending on the polymer to be attached, their efficiency vary widely according to the reaction conditions. Grafting from, for example, has the advantage of preparing polymer brushes with high grafting density (up to 85 mg/m2) as compared to the “grafting to” method (about 1 mg/m2). Also, “grafting from” methods have been used for better control of the molecular weight and the molecular weight distribution of the polymer chains [66]. On the other hand “grafting to” approach allows the successful introduction of complex macromolecules such as copolymers: poly(propionylethylenimine-co-ethylenimine), poly(styrene-co-aminomethylstyrene), poly(styrene-co-p-(4-(40vinylphenyl)-3-oxabutanol)) and poly(styrene-cohydroxymethylstyrene) among many others [67]; or biomolecules (DNA, polysaccharides, proteins), thus producing multifunctional nano-structures useful for biocompatible materials, sensors, electronic devices and



groups on the CNTs surfaces. The links could be achieved by physical adsorption or covalent attachment mechanisms, however, in agreement with Baskaran et al. [68], the quantification of covalently-attached polymer and non-covalently adsorbed polymer during grafting reaction is an outstanding issue that needs to be addressed thoroughly. CNTs can be modified successfully as they were produced with the “grafting to” method. This allows maintaining their structural integrity without losing mechanical properties while increasing their solubility and dispersion. When chemical modification of CNTs is made by “grafting to”, “polymer wrapping” or “polymer absorption”, but without oxidation, the basic principle of interaction is by non-covalent bond. In spite of Liu P. distinguishing clearly between these methods [67], we prefer describing them as “grafting to related methods”, considering the next facts: a) the links between CNTs and polymers are achieved presumably by stacking, van der Waals interactions and by less grade mechanical anchoring, b) non-covalent functionalization can be accomplished without disrupting the primary structure of CNTs, c) these techniques can be considered as non-invasive for CNTs, d) dispersion and solubility are increased and e) high stability in any commonly used organic or inorganic solvents is finally reached. Thus, here we describe briefly some results with the application of “grafting to” and “grafting to related methods”, these research results are representative examples, but, of course, they are not the only ones available in the literature. An interesting example of “grafting to” without oxidation is given by Lou et al., they propose a simple reaction that involves three kinds of alkoxyamine end-capped polymer: PolyStyrene (PS), Poly(3-CaproLactone) (PCL) and their copolymer (PCL-b-PS). In this experiment commercially available CVD MWNTs without further purification are added into a glass reactor together with PS, PCL or PCL-b-PS and toluene, the mixture is degassed and heated at 130ºC under stirring for 24 h. Their results show a better dispersion of grafted MWNTs that can be appreciated in Fig. (11). In this study, authors report the grafting ratio, defined as the weight ratio of the grafted polymer to the nanotubes and determined by TGA. The highest grafting ratio, 30%, corresponds to PCL-b-PS and also is observed that grafting ratio increases with the PS molecular weight [69].

Fig. (11). Dispersion of MWNTs in toluene 7 days after ultrasonication. (A) Pristine MWNTs, (B) PS-Grafted MWNTs, (C) PCL-Grafted MWNTs, (D) PCL-Grafted MWNTs after acid treatment . Transmission Electrón Microscopy of (II) PS-Grafted MWNTs and (III) MWNTs. Copyright Elsevier 2004.

preparation of novel composite materials. Thus “grafting to” allow to insert complex structural natural polymers, which could be grafting in CNs surface previous activation. Then, the advantages or disadvantages that show each method are depending on which the polymer to be grafting is, or which the grafting target is. 3.1. “Grafting To” Technique This approach can be described as a true interaction between readilymade polymers with reactive end groups and functional

Olek and co-workers use polymer wrapping to functionalize MWNTs produced by CVD, with high purity. In this case, the chosen polymer was PolyAllylamine Hydrochloride (PAH), that later is useful as platform to join nanocrystalls over PAH-wrapped MWNTs. As in other non-covalent functionalization, the procedure is strightforward: nanotubes were dispersed in PAH salt solution (0.5M NaCl, 500 ml) and sonicated for 4 h, stirred overnight at 80ºC, and again sonicated for 3 h more. Excess PAH was removed with water and by centrifugation until a stable, homogenous PAHMWNTs suspension was obtained. PAH confers to MWNTs amine functionality, this ensures good separation and stability due to electrostatic interactions in water or organic solvents, in addition to the covalent attachment of different kinds of nanocrystalls: CdSe-ZnS in CH-Cl3, ZnO in ethanol and Fe2O3 in aqueous solution. Fig. (12) shows a clear TEM image with the nanocrystalls attached covalently to PAH-MWNTs, these new nanohybrid materials open up new properties for a wide range of possible purposes, such as electronic and optic devices, sensors, solar cells and catalytic materials [70]. Besides countless synthetic polymers, natural molecules have been also attached to CNTs; this kind of functionalization shows an endless number of potential applications in biomaterials, bioengineering and an emerging nano-biotechnology. Due to the complexity and delicate characteristics of biomolecules, “grafting to” technique has been amply used.



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ties, which make the biological and medicinal applications of CNTs possible [73]. Yan et al. investigated dispersion of SWNTs with CS and its various neutral pH water-soluble derivatives: O-Carboxymethylchitosan (OC) and OC modified by poly(ethylene glycol) at the -COOH position (OPEG). It is important to mention that these dispersants have amine pendant groups. In this research, SWNTs without any treatment are dispersed in CS, OC and OPEG solutions by sonication. From their results, authors conclude that links between the CS dispersants and SWNTs are due to polymer structure, since the degree of interaction between SWNTs and amines is associated to the basic character of the amines. SWNT-amine interaction strength decreases in the order of primary, secondary and tertiary amines. This suggests that the interaction between SWNT and amines is sensitive to steric hindrance around the nitrogen atom. Besides, SWNTs are considered as good electron acceptors and amine group present in CS solutions is a reasonably good electron donor [72]. In addition, nitrogen-containing functional groups (such as amines and amides) posses an important affinity for physi- or chemisorption with attendant weak charge transfer on the SWNT sidewalls due to high nucleophilicity of the N-based groups [74]. Fig. (12). Transmission Electron Microscopy of MWNTs-NP Hybrid materials, (A) MWNTs-CdSe-ZnS in CHCl3, (B) MWNT-ZnO in EtOH (C) MWNT-Fe2O 3 in water. Copyright Elsevier 2007.

Between natural molecules, polysaccharides are one of the most abundant and interesting materials. Chitosan (CS), a copolymer of 2-acetamido-2-deoxy--Dglucopyranose and 2-amino-2-deoxy-D-glucopyranose through a -(14) linkage, is a naturally abundant polysaccharide, generally obtained by extensive deacetylation of chitin. This natural polymer has been used in many industrial and pharmaceutical applications, but recently it has been investigated for uses as a biopolymer matrix for electrochemical sensing applications by incorporating CNTs [71], also CS has been shown to be a good polymeric dispersant for SWNTs in acetic acid, showing diameter selectivity [72]. CS-CNTs system is especially interesting because it presents biocompatibility, electronic and optical proper-

In other work by Liu et al., CS was deposited over MWNTs surface; their method can preserve the pristine CNTs and is made simply by dispersion of MWNTs in CS-acetic acid solution by ultrasonic for 10 min, after CS-MWNTs blend was stirred for 1 h. During this step, according with authors, CS macromolecules were adsorbed on the surface of the MWNTs, acting as polymer cationic surfactants to stabilize the CNTs. Diluted ammonia solution was added for increasing the pH value of MWNTs-CS dispersion, when it happens, the ionized CS would be deionized and become nondissolvable in aqueous media. Adsorbed and soluble CS covers the surface of MWNTs forming a layer of CS coating, which later undergoes a crosslinking promoted by heating the blend to 60ºC and adding glutaraldehyde. As it is observed in Fig. (13), CS covers completely the surface of MWNTs giving them a potential use in biosensing, gene and drug delivering as well as other chemical,

Fig. (13). Scanning Electron Microscopy images of a- b) MWNT , c-d) Chitosan surface decorated MWNTs. Copyright Elsevier 2005.



Fig. (14). Schematic showing reversible dispersion of CNTs using Chitosan derivates. Copyright Elsevier 2007.

Fig. (15). Variation of transmittance with pH in the systems Chitosan derivates-MWNTs. Copyright Elsevier 2007.

medical and biological applications [75]. Similar procedure was developed by Zhang et al. [73]. In this work, SWNTs and MWNTs were used without purification; CNTs were added to solutions of CS, CarboxyMethyl Chitosan (CMCS), N-Succinyl Chitosan (NSCS) and 2-HydroxypropyltrimethylAmmonium Chloride Chitosan (HACCS), and then sonicated for 20 min. After, the mixture was centrifuged at 5000 rpm for 1 h and the above suspension was carefully collected. Derivatives of CS could be used to disperse CNTs homogeneously by manipulating pH, while endowing them with biocompatibility and maintaining their intact electronic structure, as shown in Fig. (14). Fig. (15) presents the reversible behavior of CNTs between aqueous solution and solid state as precipitate in function of pH variation. Authors conclude that chemical and conformational changes of CS and its derivatives, while changing pH, may be responsible for this reversible behavior. CS adsorbed onto surface of CNTs performs as a polymeric cationic surfactant in acidic solutions. The CS chains drive away one another and are extended in acidic solution due to the protonated –NH2 groups, while the –NH3+ are deprotonated progressively with increasing pH, causing CS-CNTs precipitate because of extensive intramolecular hydrogen bonding. On the other hand, CMCS, NSCS and HACCS act as polymeric amphoteric surfactants due to the coexistence of – NH2 and introduced functional groups, thus as CS, -NH2 groups of CMCS are protonated in acidic solution, forming the homogeneous

dispersion of CNTs. Whereas, in basic solution –NH3+ and –COOH groups are deprotonated, then negatively charged –COO could also form a homogeneous dispersion of CNTs. Around the isoelectric point of CMCS (pH=7), the -NH2 and –COOH groups can form intramolecular and intermolecular complexes and thus precipitate CNTs. In the case of NSC, in spite of this derivative containing also –NH2 and –COOH, the presence of the succinyl group as hydrophobic section causes the extreme irreversible solubility. The last substance, HACCS, posses the group CH2CHOHCH2CN+(CH3)3Cl-, which could not be deprotonated by changing pH, therefore, it maintains the CNTs solubility over all the range of pH studied by these authors. These interesting results can be applied in drug delivery or other medical applications due to the compatibility given to the CNTs and reversible solubility behavior. Besides these non-covalent funcionalizations, chemical modification of CNTs can start with purification treatments; these remove residual catalyst, eliminate amorphous carbon and at the same time open end caps and external walls in MWNTs. The defect sites on the surface of carbon nanotubes, as open-ended nanotubes with terminal carboxylic acid groups, allow covalent linkages of oligomers or polymers with the nanotubes. As we mentioned before, purification treatments generally involve different acids as well other oxidants. Once purified and oxidized, CNTs are ready to grafting.


Different biomolecules have been grafted beginning the procedure with purification of CNTs, which causes the presence of carboxylic groups on their surface. Wu et al. [71], Shiehand and Yang [76], Ke et al. [77], among other authors, have attached successfully CS through different routes using covalent bonding. DNA is another interesting biomolecule that has been amply studied in order to attach it to CNTs. Daniel et al. mentioned in their review several important advances in this sense. According to these authors, DNA chains have been used to create various functional structures and devices through the sequence-specific pairing interactions [78]. Generally, the reactive sites on the CNTs were created by the acid treatment to introduce the carboxyl groups on their tips. DNA molecules with functional links are then coupled to the carboxyl groups on the CNTs. On the other hand, Yang et al. propose a novel route for attaching DNA to oxidized SWNTs in organic solvent and aqueous media [79]. In this paper, single strands of pre-synthesized DNA containing a 3’ linker with a terminal, primary amine group were covalently attached to the oxidized SWNTs via an amide bond, after activating carboxyl groups on the SWNTs. A specific and ample discussion on biomolecules interactions with CNTs is given by Yang in other report [80]. 3.2. “Grafting From” Technique Grafting of polymers is a method wherein monomers are covalently bonded (modified) onto the polymer chain, in case of CNTs, this means the same, but the attachment is on their surface. Similar to polymers, covalent attachment of polymers on CNTs can be achieved by different techniques, which include chemical, radiation, photochemical, plasma-induced or enzymatic grafting. Chemical grafting involves that attachment can proceed by free radical, ionic or atom transfer radical polymerization (ATRP). An interesting application of this is the study developed by Matrab et al. [66]. They reported successful ATRP grafting of Poly(n-Methyl MethAcrylate) (PMMA) and PolyStyrene (PS) brushes over aligned MWNTs grown on Si substrate by aerosol-assisted catalytic CVD. The surface functionalization is developed through electrochemical reduction of phenyl ethyl bromide groups, the resulting MWNTs-Br served as a platform for the growth of PMMA and PS chains by ATRP. Results obtained by these authors permit to observe the tethered polymer chains forming dense organic adlayers over the surface of MWNTs. On the other hand, novel techniques have been used for grafting, for example SWNTs have been functionalized through argon plasma-assisted UV grafting. Yan et al. used purified SWNTs that were exposed to Ar plasma; this treatment generates defect sites at the tube ends and side walls, which act as the active sites for the subsequent UV grafting of VZ monomer. Once treated, Ar-SWNTs were sonicated in 1-vinylimidazole. After, the mixture was irradiated by 24 h using an UV lamp. Authors demonstrate that UV radical grafting of polymer takes place on the defect sites of the SWNTs. The UV irradiation is useful in other vinylic reactions; therefore, this novel technique probably could be useful with different vinyl monomers such as vinylpyridine, acrylic acid, acrylate, acrylonitrile and even styrene, permitting a potentially wide range of functional chemistries that can be developed [81]. Not only vinylic polymers have been grafted, also other kind have been attached successfully, as hyperbranched poly(etherketone). This polymer specifically is interesting due to its many applications in aerospace and electrical industry, coating and as insulating materials. Thus, carboxylic acid-terminated Hyperbranched Poly(Ether-Ketone)s (HPEKs) were successfully grafted onto the surfaces of SWNTs and MWNTs. They were prepared via in situ polymerization of 5-phenoxyisophthalic acid as an AB 2 monomer for the HPEK in the presence of SWNT or MWNT in polyphosphoric acid (PPA)/phosphorous pentoxide (P2O5) medium


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[82]. In addition to these articles, Liu describes several of these “grafting from” techniques, followed by different authors [67]. Certainly, one of the most important advances in carbon nanotube field is Chemical Functionalization, since allows developing innumerable applications in composite materials, sensors and biocompatibility, among others. The results obtained in this sense has been wide and important, however there are even many areas worthy to be explored. 4. MECHANICAL AND THERMAL PROPERTIES OF POLYMER –CARBON NANOTUBE COMPOSITES 4.1. Non-Functionalized Carbon Nanotube Polymer Composites Carbon Nanotube-Polymer Nanocomposites (CNPN) are a very active area of R&D in the whole world. Diverse results have been obtained in this kind of composites, principally with respect to the mechanical and electrical properties, although, recently, other properties have been evaluated. These researches have shown that many factors play an important role and influence the interface affecting therefore properties in carbon nanotube composites. Thus, interesting properties are obtained, depending on the conditions used in the synthesis, dispersion quality and raw material features, showing that, in some cases, it is possible to obtain some interesting properties using carbon nanotubes as-synthesized. In addition, the first studies performed with this kind of nanotubes realized that it is needed to known which is the response of chemical unprocessed nanotubes in polymer composites. In this section, are analyzed different researches in CNPN synthesized with different polymer matrix and carbon nanotubes without chemical modification. Some of the processes used are: melt blend [83], dissolving and casting [40], “in situ “polymerization [55], extrusion [84] layer by layer [70] among others. A number of these synthesis methods and others have been used in different works using carbon nanotubes as-synthesized. Results show diverse mechanical and thermal properties which allow establishing some relevant conclusion in CNPN. 4.1.1. Mechanical and Thermal Properties of Carbon Nanotube Polymer Nanocomposites Shaffer and Windle, produced carbon nanotube-polyvinylalcohol nanocomposite with different loads of catalytically grown nanotubes, from 10 wt% to 60 wt%. Dynamic mechanical parameters were measured in these CNPN. The values of storage modulus (E’) were increased more than two-fold but by using a huge quantity of CNs. Simultaneously, the onset thermal degradation was retarded with these composites, suggesting the use of CNs as modifiers [38]. Nowadays, in the field of CNPN, PMMA is one of the most studied matrices. Cooper et al., characterized mechanical properties of PMMA nanocomposites formed with MWNTs, SWNTs and nanofibrils. In this research, tensile modulus is nearly insensitive to CNs loads; nevertheless, impact properties are considerably improved by both MWNTs and SWNTs [48]. In other recent study with PMMA using MWNTs Tatro et al., [51] have estimated thermal and electrical properties, Vickers microhardness and dynamical mechanical analysis in nanocomposites irradiated with a Cesium137 source. The effect that might have carbon nanotubes reinforcing polymer matrix when the sample is irradiated is shown. Results show that MWNTs may improve radiation hardness of mechanical properties and glass transition temperature when a composite is formed. The effect is assigned to conjugated bonds in CNs which may absorb part of the radiation energy, restraining in this way the damage of PMMA molecules. More aging studies with radiation and evaluation of the interface are planned and currently developed by these authors. Other research by Putz et al., reports the dynamical mechanical properties of PMMA and nanocomposites produced with SWNTs


and PMMA matrix at two different frequencies. The study was developed with the target to know the effect generated by the inclusion of the nanotubes on the ability of PMMA to store and disperse 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. relaxation, related with the onset of the glass transition temperature (Tg) and -transition associated with hindered rotation of the side chain, were evaluated in PMMA and SWNTs–PMMA nanocomposites. Transitions were detected in both tangent delta curves and elastic modulus curves (E’) at low and high temperatures. Results in two different frequencies show that PMMA and nanocomposites formed with slight quantity of SWNTs exhibit similar -transition temperature and identical Tg’s. However E’ at low temperature (150°C) in the nanocomposite with a little quantity of 0.014 wt % of SWNTs, exhibits an important increase, as compared to E’ of the pure PMMA, from 5.1 GPa in PMMA to 7.2 GPa for the nanocomposite, both evaluated with a frequency of 100 rad/s. This represents an important significant rise, by considering the slight quantity of SWNTs used [57]. Also, research involving different studies of Termogravimetric Analysis (TGA) and Nuclear Magnetic Resonance (NMR) in order to evaluate possible changes in the structure of the polymer nanocomposite caused by the incorporation of SWNTs, has been reported. Neither microstructure nor thermal degradation changes were localized in these nanocomposites as compared to pure PMMA. This discards that these parameters could be susceptible in mechanical properties changes of the nanocomposites. Therefore, it is suggested that the interface in these nanocomposites plays an important role and the properties of the polymer are changed in the neighborhood of the SWNTs by the cohesive contacts, caused by the large surface area of CNs . Additionally, this study involves the discussion and possible adaptation of the traditional composite theory in the case of CNs nanocomposites; however the authors agree with other researchers, mentioning that nanotubes have a great deal of effect on the properties of the composites, larger than those expected for the traditional composite theory. Therefore, it is possible that new models will be required to adjust and predict CNPN properties, inasmuch as these nanomaterials and the interface produced with polymers are producing unexpected properties. Dufresne et al., processed and evaluated nanocomposites developed with catalyzed MWNTs and poly(styrene-co-butyl acrylate). The composites were synthesized 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 carbon nanotube were reported in almost all samples [39]. However, an unexpected decrease in the modulus was observed in the sample with 7 wt% of CNs. Thus, even though a good distribution is obtained in the majority of the samples, nanotubes appear like clusters in a number of areas, causing unpredictable behavior in some samples. This has been observed in other nanocomposites obtained by Velasco-Santos et al. [40], reinforced with carbon nanotubes which involve weight percents higher than 5 or 7 wt % and explains the unexpected behavior in this study. Nevertheless, it is possible than not only this factor affects the development of CNPN with relative high concentrations. Consequently it is needed to continue with the research within a similar context. Also, Wong et al., have analyzed two kinds of nanocomposites that show clusters, these bundles originate similar effects than those mentioned before, where the mechanical behavior is similar in several cases when carbon nanotube content is increased. Although it was probed that CNs have good interaction and adhesion with the polymers by techniques as Field Emission Scanning Electron Mi-


croscopy (FESEM) and Transmission Electron Microscopy (TEM), other regions shown agglomerates that restrict the adequate dispersion and cause the decrease in the nanocomposite properties. Polystyrene rod and epoxy thin films evaluated in this research as CNPN, present the following results: in the case of CNs polystyrene composites, tensile strength enhance from 22.1 MPa (for polystyrene) to 24.4 MPa (for the sample with 0.1 wt% of CNs), however, when the content of CNs was increased at 5 wt%, the strength is diminished until 17.9 MPa. Besides, the samples with 1 wt% and 2 wt% show lower values in tensile strength than the sample of pure polystyrene. In the case of epoxy thin films evaluated mechanically in this study, microscopy of these samples exposed the two mentioned regions. One corresponds to the failure of the matrix but not on the CN-epoxy interface; nevertheless, in other regions CNs agglomerates were observed, showing a local poor dispersion [49]. Authors suggest that this later behavior is the possible cause of the failure initiation. Similar results where some agglomerates affect mechanical properties mainly at relatively high concentration have been found in poly (methyl-ethyl methacrylate) by Velasco-Santos et al., [40]. In this report, CNPN which incorporate arc-discharge MWNTs as reinforcement, are analyzed. There, samples were produced by dissolution of copolymer and additives, such as surfactant and plasticizers, were included in some cases in order to improve dispersion. Fig. (16) shows the storage modulus (E’) evaluated by DMA (Dynamical Mechanical Analysis) in this study, the most outstanding modulus is obtained using only 1 wt % of CNs without additives (sample 1), increasing the modulus by more than 200% at 40°C. Fig. (17) illustrate the results of storage modulus (E’) obtained in DMA for the samples where a surfactant was used (0S, 1S, 5S, 7S, 10S) in order to improve dispersion. In this research, E’ is increased in the samples with 1 wt % (1S) and 5 wt % of CNs (5S) as compared to the sample with only 1 wt % of surfactant, nevertheless, in the samples with 7 wt % (7S) and 10 wt % (10S), E’ decreases with respect to the sample 5S and 1S. As remarked before, this behavior is present in different researches where 5 wt % of nanotubes were used, inasmuch as cluster zones produced by CNs in this concentration difficult the interaction at interface level, avoiding that numerous nanotubes to maintain contact with the polymer matrix. Fig. (18) shows a SEM image of agglomerates typically found in these nanocomposites. Fig. (19) shows similar bundle found in the polystyrene nanocomposite developed by Wong et al. [49]. Schadler et al. [85] developed MWNTs-epoxy composite cured with triethylene tetraamine. They evaluated both modulii: tensile and compressive. 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 shift significantly under compression. The authors point on that all the walls in MWNTs only participate in compression and do not when the composite is subjected to tensile stress, inasmuch as it is proposed that in this case only the outer walls of carbon nanotubes participate. Qian et al., have measured the elastic modulus in polystyrene (PS) – carbon nanotubes composites [86]. In this research, MWNTs were incorporated in the polymer by a simple solution-evaporation method, using toluene as solvent and short sonication time to disperse nanotubes in PS maintaining the integrity of the nanotubes. Tensile test in this study show an increase in elastic modulus of 36% and 42% with respect to the polymer matrix, with 1 wt% of MWNTs as reinforcement. The homogeneity of these nanocomposites as well as the deformation mechanism are analyzed by in situ Transmission Electron Microscopy (TEM), this provided information concerning interfacial bonding between the multiwall nanotubes and polymer matrix. Fig. (20) shows crack propagation in



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Fig. (16). Storage Modulus of Carbon nanotubes films: a) samples: 0 poly (methyl-ethyl methacrylate) (MEMA), 0S (MEMA - 1wt% surfactant), 0P (MEMA - 1wt% plasticizer), 1(MEMA – 1wt% CNTs), 1S (MEMA – 1wt% surfactant- 1wt% CNTs), 1P (MEMA – 1wt& plasticizer- 1wt% CNTs). Copyright 2003. Institute of Physics Publishing.

Fig. (17). Storage Modulus of Carbon nanotubes films: 0S (MEMA - 1wt% surfactant), 1S (MEMA -1wt% surfactant- 1wt% CNTs), 5S (MEMA -5wt% surfactant- 5 wt% CNTs), 7S (MEMA -7wt% surfactant- 7wt% CNTs), 10S (MEMA -10wt& surfactant- 10wt% CNTs). Copyright 2003. Institute of Physics Publishing.

MWNT-PS composite films. In these pictures, it is observed that the external load can be effectively transferred to the nanotubes. Inasmuch as nanotubes lie to align and link crack borders and next break and/or pull out of the polymer matrix. 4.1.2. Tribological Properties of Carbon Nanotube Polymer Nanocomposites Carbon nanotube polymer nanocomposites (CNPN) are thought to be used as multifunctional materials due to the properties that these composites have presented in many researches. Thus, it is important to evaluate different properties in order to known the response when the materials are exposed to different conditions. One of the less studied properties in CNPN are the tribological ones. However, even though preliminary results have been presented in this field, in comparison with other mechanical properties reported, tribological properties have been evaluated have shown important contribution of carbon nanotube loads. Next, we present novel tribological properties reported in CNPN, where carbon nanotubes play an important role, by improving and diversifying hardness and wear resistance with respect to the polymer matrix used.

Fig. (18). Scanning Electron Microscopyn (SEM) image of carbon nanotube polymer composite showing bundles of CNTs in matrix. Copyright 2003. Institute of Physics Publishing.



Fig. (19). Field Emission SEM image of fracture surface of CNT-PS composite: a CN agglomerate being pulled half way out from the Polystyrene matrix. Copyright 2003 Elsevier.

Fig. (21a). Carbon nanotubes in composite a) before wear test, b) after wear test. Copyright 2006 American Institute of Physics.

Fig. (20). In situ Transmission Electron Microscopy (TEM) images of a) crack nucleation and b) crack propagation in MWNT–PS thin composite induced by thermal stresses. Copyright 2000 American Institute of Physics.

The majority of reports about tribological properties of CNPN have used carbon nanotubes produced by catalytic decomposition of hydrocarbons. Yeong-Seok Zoo et al. utilized MWNTs produced by thermal CVD in order to synthesize nanocomposites with Ultra High Molecular Weight Polyethylene (UHMWPE) polymer matrix [52]. In this research, different loads of nanotubes were probed as reinforce in order to evaluate the influence of CNs in wear resistance. The results revealed that CNs in low quantities increase hardness and decrease wear loss. Other papers report also the use of UHMWPE as matrix reinforced with MWNTs. Wei et al., detail the tribological behavior

under nanoscratch and nanowear tests observed in MWNTs/ UHMWPE composite films [50]. The authors conclude that nanocomposite films exhibit higher wear resistance and lower friction coefficient than the un-reinforced films. In that sense, the friction coefficient decreases with increasing the content of MWNTs incorporated in the films. Also, it is important to emphasize the observation of a new microstructure on the surface of the MWNTs/ UHMWPE composite films; authors suggest that this microstructure is the interface between the MWNTs and polymer, and therefore the responsible for the change in the properties of the pure UHMWPE film. As a consequence of the interface achieved, the fracture cracks in composite films are propagated more difficultly, which is reflected in the nanocomposites exhibiting higher wear resistance than pure polymer film. Doubtless, in spite of their resistance and excellent properties, carbon nanotubes can undergo some damages during the wear tests, which could be the main reason why several composites do not reach the extraordinary properties that could be expected would be similar to other mechanical or conductive parameter. Damage in MWNTs was studied by Zarudi and Zhang, where the deformation mechanism is critical to the tribological behavior of epoxy composites reinforced with CVD MWNTs [87]. In this article, the composites were synthesized with MWNTs whose initial length was between 2 and 5 μm and with a bamboo conical structure, as shown in Fig. (21a). After a wear test, most of the MWNTs were fragmented to segments from 100 to 400 nm, Fig. (21b), further MWNTs were opened during wear test, which can be observed in Fig. (21c). The



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Fig. (21c). High Resolution Transmission Electron Microscopy.(HRTEM) images, showing CNs with an open end. Rupture of left wall and imperfection in the structure of right wall are observed. Copyright 2006 American Institute of Physics.

Fig. (21b). High Resolution Transmission Electron Microscopy.(HRTEM) images, showing CNs after wear test; a) Regions A-C. Indentation is distinguished of the outer wall of the outer walls that leads to distortion walls deeply, b)Region A. in more detail . sinusoidal shape of outer shells is observed. Copyright 2006 American Institute of Physics.

walls are no longer smooth, instead they have atomic distortion and dislocations, as shown in Fig. (21b). The explanation given by the authors consists in that the deformation is caused by the interaction with the roughness of the steel disk used for the wear tests, these roughness can act as sharp indenters that deform the walls in MWNTs, and also can cause tensile deformations, since nanotubes are well fixed in epoxy matrix, because it seems that the adhesion and stress transfer between nanotubes and matrix are strong. As we discussed in previous sections, only few kinds of CNPN have presented significant increase in tensile modulus or storage modulus, when carbon nanotubes are added in relatively big quantities. The majority reports indicate important improvements in mechanical properties of polymer nanocomposites when very low quantities of nanotubes are included, however the researches considered in this section show that the converse has been reported in

CNPN when hardness has been measured. Other report presented by Lau K.T. and Shi S., evaluating wear resistance, found that the addition of 2 wt % of CNs, improve the hardness of an epoxy matrix in 20% [88]. However, when less than 1.5 wt% were employed, the hardness diminishes in comparison with the hardness of pure epoxy. This effect is caused by a weak interface between CNs and polymer. On the other hand, the favorable effect in the hardness reached whit 2 wt% of CNs, is attributed to the mesh-like structures formed when the concentration of CNs is increased. Authors of this research mentioned that the formation of this kind of structures is caused by the high aspect ratio of the nanotubes, which produces a entanglement that improves the resistance in the material to be scratched. These entanglements have been observed in different CNPN. However in spite of agglomerates and cluster formed in carbon nanotubes polymer nanocomposites tend to diminish some mechanical properties, like elastic modulus or rupture strength. Recently, it has been found that this phenomena could produce interesting applications, inasmuch as some anisotropic properties have been found in CNPN produced by solution and casting [40,8992]. In these nanocomposites interesting behavior related with their tribological properties is found, inasmuch as this kind of nanocomposites present different concentration on each side of the polymer matrix. Although these CNPN are very thin, (less than 0.5 mm) composites have different concentration in top and bottom as illustrated in Fig. (22), this causes different characteristics on the two side, inasmuch as, one side is opaque and other shiny. Thus, nanocomposites analyzed for wear tests present different behavior depending on which side is evaluated. CNPN and polymer samples are characterized on both sides. Fig. (23) and Fig. (24) shows scratch curves for carbon nanotube composites on both sides opaque and shiny respectively. Opaque side is noted with “a” letter and shiny side is distinguished with “b”. Numbers next to letters specifies the carbon nanotube concentration in weight percent. Consequently, CNT 1-a, represent the composite with 1 wt % of carbon nanotubes evaluated on the opaque side. Scratch curves confirm different behavior depending on the side evaluated. In Fig. (23) (opaque side) we can observe penetration depth for polymer sample and CNPN. It is clear that CNPN have variable wear resistance depending on the load. For instance, at



Fig. (22). Schematic representation of carbon nanotube polymer nanocomposite with different concentration in top and bottom side.

Fig. (24). Scratch curves in top side of carbon nanotube polymer nanocomposites.

the nanocomposites present the threshold at high loads where polymer matrix exhibits the same penetration depth at similar loads than CNPN. The points in which polymer sample have identical behavior that the CNPN are 10.5 N with 89 microns of depth for CNT-1b, 10.1 N with 88.3 microns for CNT-5b and 10.8 N and 90 microns for the sample CNT-7b. Nanocomposites with 10 wt % of carbon nanotube show lower wear resistance than polymer sample at any load. On both sides, once that threshold load is reached CNPN with 1, 5 and 7 wt % lower resistance to be worn than the polymer matrix was found. The samples present interesting anisotropic wear properties, at low and high load. However, near to the threshold zone at medium load composites show similar behavior [91,92].

Fig. (23). Scratch curves in bottom side of carbon nanotube polymer nanocomposites.

very low load (less than 1.8 N) all CNPN samples present better wear resistance than the polymer matrix. However, the sample with 10 wt% diminishes the resistance to be worn drastically, up to 2 N, presenting high penetration depth. The other CNPN samples present better wear resistance than the polymer samples, inasmuch as polymer matrix reach around 67 microns in penetration depth at 5 N of load while CNPN have lower penetration depths at the same load. Composite with 1 wt % of CNs has 45 microns and the composites with 5 and 7 wt % of CNs present 48 and 41 microns of penetration depth at 5 N of load. Nevertheless, these nanocomposites present an interesting threshold at high loads where polymer matrix exhibits the same penetration depth at similar loads than CNPN, except CNT-10a. CNT-1a presents this point at 9 N with 85 μm of penetration; CNT-5a samples have this situation around 8.3 N with 84 microns and CNT-7a samples at 11.7 with 93 microns in depth. Fig. (24) presents the results for shiny side, where, in the case of polymer sample the penetration depth is approximately 67 microns at a load of 5 N, conversely at same load CNPN with 1, 5 and 7 wt % show superior resistance to be worn. Sample containing 1 wt % of carbon nanotubes shows a penetration depth of around 49 microns at 5N of load. At the same load, CNT-5b and CNT-7b have depth penetrations around 45 and 43 microns respectively. However

Anisotropic properties presented above are caused by the presence of a gradient distribution in carbon nanotube polymer nanocomposite. Subsequently, although less carbon nanotubes are found in the top side (presented as shiny); these parts have better distribution in polymer matrix and therefore load transfer is achieved more effectively. Contradictory effect was found in bottom side (presented as opaque) where high concentration of nanotubes produces agglomerated which cause holes and poor distribution in different composite regions. However, although these clusters have produced reduced mechanical properties of CNPN, habitually in relatively elevated concentration. Management of the quantity and shape of these bundles may possibly produce remarkable anisotropic behavior for composites and other applications. Additionally, we have been found that these bundles contribute to generate intertube sliding when are incorporated in polymer matrix, producing interfacial slippage between polymer and nanotubes bundles. This would produce interesting damping property in carbon nanotube composites [93] and could allowed more possibilities for diversify CNPN features and multifunctional properties and applications. 4.2. Functionalized Carbon Nanotube Polymer Composites Chemical modification is not a new tool to improve the compatibility between reinforcements and matrix, inasmuch as chemical attack and graft have been used in natural fibers, synthetic fibers and particles [94-96]. However, the carbon nanotube nanocomposite field is relatively very recently and chemical functionalization offers an important tool to enhance interface interactions and take


full advantage of the outstanding properties that carbon nanotubes can provide. Furthermore, although different researches have taken part in this field, it is needed to understand the possible interactions in order to control and to improve the interface between carbon nanotubes and polymer matrix. The recent results in chemical modification of carbon nanotubes to incorporate these materials to polymer are promising and could be the way to form strong and functional new advanced materials. Some relevant results obtained in the area of functionalized carbon nanotube nanocomposites are described in this section. As we mentioned in previous sections, the carboxyl groups found in the tip and surface of CNs are useful for the interaction between CNs and other compounds. Consequently, the use of these moieties to improve the “link” in CN-polymer composites has been proposed either inserting other chemical groups or using the carboxyl groups produced in the oxidation [97]. 4.2.1. Properties and Interactions As we mentioned in the section processing by “in situ” polymerization; Jia et al. suggested that the initiator opens the bonds found in CNs and, in this manner, CNs take part in the polymerization [56]. However, although opening bonds in CNs allows to join them with other chemical groups, oxidation provides more possibilities to bond the nanotubes with the matrix, due to reactive chemical groups such as COOH, COO- and C=O which are found on the tip and on the wall. In another research Geng et al., use fluorinated single-walled nanotubes (fl-SWNTs) to enhance the uniformity and the nanotube dispersion using poly(ethylene oxide) (PEO) as matrix, and dissolving the nanotubes in methanol [44]. Velasco-Santos et al. [55] have proposed an effective and simple way to achieve interactions between the functional groups found in functionalized CNs and polymer chains, while the polymer is formed by “in situ” processing, inasmuch as the free radical created in monomer molecules by the initiator could either interact or react with the CN moieties easier than when the polymer is made and after 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 “in situ” polymerization using AIBN as initiator. The AIBN quantity, reaction time and temperature were controlled to have uniform molecular weights in all samples. Three samples in this research were manufactured with only polymer (poly (methyl methacrilate 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. In the research, the interaction of PMMA with f-MWNTs was studied by Infrared and Raman spectroscopy. In addition, mechanical result shown 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 temperature Tg by 40 °C, unlike with unfunctionalized nanotubes where E’ at 40°C increase by 50 % and the Tg by only 6 °C. Another important fact found in this paper in that fMWNTs composites, is the increase by more than 11-fold, at 90 °C, which is relatively high temperature for the kind of polymer using as matrix. Gojny et al. 2003 have shown that carboxyl-terminated carbon nanotubes treated with amines after included in epoxy matrix have better interaction than those not treated chemically [58]. In this study, TEM pull-out test were developed, showing a better interaction due to chemical groups on CNs surface, showing an important improvement in the interface links between these two materials. Goh et al. have developed in situ functionalized MWNTs in phenoxy composites by melt mixing with 1-(aminopropyl)imidazole. In this research, the composites with more than 4.8 wt % show higher


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storage modulus than the polymer matrix [98], however the behavior in high temperature is not common for these materials, inasmuch as f-MWNTs tend to increase the modulus when normally the polymer matrix tends to flow. In this research the modulus at high temperature is diminished with the incorporation of in situ functionalized nanotubes. Liu L. and D. Wagner recently produced two CNPN developed with two types of epoxy resins using f-MWNT and MWNTs. fMWNTs consisting of carboxyl-terminated nanotubes [59]. Viscosity is a determining point for the synthesis of these composites, inasmuch as better dispersion is reached in rubbery epoxy resin than in glassy epoxy. However, in both cases, functionalized carbon nanotube nanocomposites present better tensile modulus than those produced with MWNTs and than pure resins, using 1 wt % of nanotubes. In addition, very good impact properties are reached when functionalized nanotubes are incorporated to epoxy resin. Zhao et al. [61] have used carboxylated functionalized nanotubes in order to reinforce polyamide 6 and Bhattacharyya et al. [43] use MWNTs modified with ferritin protein molecules to reinforce Poly (vinyl-alcohol) PVA. In both cases, un-functionalized nanotubes were used. The storage modulus and glass transition temperature are increased notably when functionalized nanotubes are incorporated in polymer matrix. Composites of PVA are synthesized with ferritin functionalized MWNTs and the increases are around 100-110% in the modulus with the addition of 1.5 wt% of functionalized nanotubes, tan delta related with glass transition temperature is increased 79 °C for pure PVA to 86 °C and 102 °C for acid treatment carbon nanotube composite and functionalized ferritin carbon nanotube composite, respectively. Fig. (25) shows the elastic modulus E’ and Fig. (26) tan delta results for the three samples.

Fig. (25). Storage Modulus curves for Poly (vinyl-alcohol) PVA, oxidized MWNTs-PVA Composite (PVA-MWCNT) and ferritin functionalized Multiwalled carbon nanotube-PVA composite (PVA-MWCNT+Ferritin). Copyright 2005 American Institute of Physics.

In other work involving polyamide as polymer matrix, acidtreated CNs were incorporated during in situ polymerization of Nylon 610. Thermogravimetric analysis of these materials shows that thermal stability is improved, since Nylon 610 begins to degrade at approximately 400 °C and was completely decomposed at 525 °C. However, 1.5 wt% of the MWNTs in the composites remained, even above 550 °C, which indicates a higher decomposition temperature. In addition, mechanical properties CNPN present



from 58.7 (MCPA6) to 68.0 MPa (MCPA6/MWNTs nanocomposites) and from 815 (MCPA6) to 923 MPa (MCPA6/MWNTs nanocomposites), respectively, by the incorporation of 1.5 wt % MWNTs into a MCPA6 matrix. However, the elongation at break decreased gradually when the amount of MWNTs increased. The enhancement of the tensile strength and modulus of these nanocomposites is attributed to the reinforcement effect of MWNTs and their uniform dispersion into the polymer. The elongation at break (about 91%) slightly decreased, indicating that the composite became somewhat brittle, as compared to neat MCPA6. Also, in these composites the thermal stability is improved when CNs were added to the polymer matrix. The onset temperature and the temperatures at maximum mass loss rate obtained by thermogravimetric analysis (TGA) are higher in composites than those of the pure MCPA6 [63].

Fig. (26). Tangente delta (tan ) vs temperature curves for Poly (vinylalcohol) PVA, oxidized MWNTs-PVA Composite (PVA-MWCNT) and ferritin functionalized Multiwalled carbon nanotube-PVA composite (PVAMWCNT+Ferritin). Copyright 2005 American Institute of Physics.

PVA matrix also has been used by Paiva et al. to form CNPN [45]. In this research, previously-functionalized PVA-SWNTs are used as reinforcement, sample with pure SWNTs also is used to produce carbon nanotube polymer nanocomposites (CNPN). Results show that all samples that contain CNs improving the Young modulus for all composites. The strength increased considerably for the composites with functionalized SWNT, but decreased for the composites with pure SWNTs, as compared to the pure PVA. Fig. (27) shows typical stress vs. strain curves for these nanocomposites and pure PVA. Authors conclude that the functionalization facilitate the dispersion of the nanotubes within the polymer, forming homo-

Fig. (27). Stress-strain curves for PVA and functionalized single walled carbon nanotubes composites. Copyright 2004 Elsevier.

notable enhancement with respect to pure nylon 610, since the Young’s modulus and tensile strength of the composite films increase. E for nylon is 0.9±0.1 GPa and E for nylon reinforced with CNs is 2.4± 0.3 GPa, representing a 170%increase. Also, tensile strength and elongation are higher in this composite than those of nylon. These authors conclude that the composite is apparently stronger than nylon 610 as a result of the incorporation of only a small amount (1.5 wt%) of MWNTs [62]. Nanotubes functionalized with isocyanate groups have been used to modify a polyamide matrix. The evaluation of these composites (MCPA6/ MWNTs, MCPA6) show significantly improvement of the tensile properties of the CNPN in comparison with the MCPA6 matrix. The tensile strength and modulus were increased

geneous composites and leading to a considerable improvement in the film mechanical properties. Grafted Poly(Methyl MethAcrylate) Multiwalled Carbon Nanotubes (PMMA-MWNTs) have been used to reinforce poly(Styreneco-AcryloNitrile) (SAN) matrix. Composites are synthesized by solution casting from tetrahydrofuran. The solubility features of both polymers allow good dispersion of grafted PMMA-MWNTs in SAN. The nomenclature of samples A, B, and C correspond to the composites with 0.5, 1 and 2 wt%, of PMMA-MWNTs, respectively. For a comparison authors synthesized an SAN/MWNT composite containing 1 wt% as-received MWNTs designated as composite D. Fig. (28) and Fig. (29) show the results of Storage Modulus and tan delta, respectively. It is clear that modified nano-



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Fig. (28). Storage Modulis curves for poly(styrene-co-acrylonitrile) SAN (), composite A () 0.5 wt% of PMMA grafted carbon nanotubes, composite B () 1 wt% of PMMA grafted carbon nanotubes, composite C () 2 wt% of PMMA grafted carbon nanotubes, composite D () 1 wt% of as received carbon nanotubes. Copyright 2005 Elsevier.

Fig. (29). Tangente delta curves for poly(styrene-co-acrylonitrile) SAN (), composite A () 0.5 wt% of PMMA grafted carbon nanotubes, composite B () 1 wt% of PMMA grafted carbon nanotubes, composite C () 2 wt% of PMMA grafted carbon nanotubes, composite D () 1 wt% of as received carbon nanotubes. Copyright 2005 Elsevier.

tubes increase the storage modulus at 40 °C more than as-received MWNTs. However, at high temperature the storage modulus decreases for all samples. Tan delta maximum related with Tg tend to diminish for all samples that contain functionalized nanotubes, in comparison with SAN and as-received MWNTs. Authors attribute this effect to reduced interface between SAN and MWNT, as a result of the covering of the nanotubes by PMMA and also the miscibility between SAN and PMMA [46]. However, interactions between grafted nanotubes and SAN influence the storage modulus; therefore it’s possible that these interactions are lost with interme-

diate temperature, inasmuch as the synthesis approach not produces functional groups in the polymer matrix. The influence of interactions at room temperature is also probed by results obtained for Young’s modulii, tensile strength, ultimate strain, and toughness of PMMA-MWNTs SAN composites, the increments in these properties are up to 51, 99, 184, and 614%, respectively, compared to the pristine SAN. Polycarbonate is other polymer used with functionalized nanotubes. Zhang et al. have produced CNPN using oxidized SWNTs [99]. Authors indicate that carboxyl groups assist in the exfoliation



Fig. (30). (a) Storage moduli of PMMA ( | ) and composites with PMMA:MWNTs:PEO-NH2 ratios of: () 90:10:0; () 95:5:0; () 98:2:0; () 98:0:2. (b) Storage moduli of PMMA ( | ) and composites with PMMA:MWNTs:PEO-NH2 ratios of: () 85:10:5; () 94:1:5; () 96:2:2; () 97:1:2; (+) 97:2:1; () 93:2:5; () 93:5:2; () 88:10:2. Copyright 2006, Elsevier.

of the nanotube bundles, producing intermolecular forces caused by dipole-dipole interactions between polar groups of functionalized nanotubes and polar carbonate moieties. This provides better dispersion of CNs in the polymer matrix. Results show increeases in tensile elastic modulus for composites that include 1 wt% of oxidized nanotubes with respect to the Polycarbonate sample, in addition sample with 2wt% increase even more tensile modulus. Wang et al., worked with oxidized MWNTs produced by CVD and incorporated to PMMA as polymer matrix, using as compatibilizer amine-terminated poly(ethylene oxide) (PEO-NH2) [54]. Infrared analysis of PEO-NH2 and PEO-NH2 mixed with oxidized MWNTs suggest interaction between COOH groups of oxidized nanotubes with PEO-NH2 via protonation of amine groups in PEONH2 by carboxylic acid groups of nanotubes. This interaction was corroborated by XPS, showing that PEO-NH2 molecules are ionically attached to nanotubes surface. Dynamical Mechanical Analy-

sis (DMA) of different samples that present variations in weight ratio between PMMA, MWNTs and PEO-NH2 are presented in Fig. (30). The results show contribution of PEO-NH2 and nanotubes in storage modulus, therefore PEO-NH2 can serve as a compatibilizer to improve interfacial adhesion between MWNTs and PMMA. However, the improvement on storage modulus by PEO-NH2 is not dramatic. Authors point out that this moderate effect is due to the low-Tg nature of the PEO chains and suggest the use of a high-Tg polymeric compatibilizer to produce a more dramatic reinforcing effect on the storage modulus of the composites. Yaping et al. reinforce epoxy resin with modified MWNTs treated with amines, amine functionalized multiwalled nanotubes MWNT-NH2 used in this research were prepared with triethylenetetramine [60]. Infrared spectroscopy, used for the analysis of untreated and modified MWNTs, showed that MWNTs were modified and contain NH2 moieties. Composites were prepared at differ-



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Fig. (31). Impact strength of carbon nanotube composites, where is observed the effect in different concentration of both MWNTs and functionalized MWNTs-NH2, in impact properties for composites. Copyright 2006 Elsevier.

Fig. (32). Stress vs. Strain curves of poly(vinyl alcohol) PVA, composite of poly(vinyl alcohol)-Sodium Dodecyl Sulfate-SWNTs (PVA/SDS/SWNT) and composites poly(vinyl alcohol)-Sodium Dodecyl Sulfate and modified collagen SWNTs PVA/SDS/SWNT/Collagen. Copyright 2006 American Institute of Physics.

ent loads of functionalized carbon nanotubes, containing 0.2%, 0.4%, 0.6% and 1.0 wt % of MWNTs and MWNTs-NH2, separately. Impact properties of these samples show important contribution of nanotubes and functionalized MWNT-NH2 polymer composites show better impact properties at 0.2, 0.4 and 0.6 wt% of CNs than polymer composites that contain unfunctionalized nanotubes at the same concentrations. Fig. (31) presents impact strength results for these samples. In addition, authors show that bending strength and flexural modulus can be increased 100% and 58%, respectively, by using functionalized nanotubes as load. Other CNPN developed using functionalized nanotubes have modified these nanomaterials with biomolecules; in recently research Bhattacharyya et al. functionalized SWNTs with denatured collagen and using PVA as polymer matrix [100]. Collagen is dena-

ture onto sidewalls of SWNTs, for this procedure sodium dodecyl sulfate (SDS) is used to produce denaturation in proteins. The functionalization in these materials improve compatibility with matrix. Also, the collagen layer was found to increase the PVA matrix crystallinity, which results in a dramatic enhancement of the Young modulus (260%), tensile strength (300%) and toughness (700%). Typical stress vs. strain curves from PVA and composites with different loads of both SWNTs and collagen functionalized SWNTs are shown in Fig. (32). In those curves, it is clear the influence of functionalized nanotubes on mechanical properties of these nanocomposites. In addition, DSC was performed to evaluate and quantify the PVA crystalline fraction. Typical DSC for PVA and different PVA based collagen films are shown in Fig. (33). A considerable increase was observed in the area under the melting curve for



Fig. (33). Differential Scanning Calorimetry (DSC) curves of poly(vinyl alcohol) PVA, composite of poly(vinyl alcohol)-Sodium Dodecyl Sulfate-SWNTs (PVA/SDS/SWNT), Composite poly(vinyl alcohol)-Sodium Dodecyl Sulfate-Collagen (PVA/SDS/collagen) and composite poly(vinyl alcohol)-Sodium Dodecyl Sulfate and modified collagen SWNTs (PVA/SDS/SWNT/Collagen). Copyright 2006 American Institute of Physics.

the PVA/SDS/SWNT/collagen composite, indicating higher crystallinity. This effect contributes to the mechanical properties along with interactions at interface level produced by functionalized nanotubes. Also, pretreated CNs, incorporated to PBO to produce fiberreinforced CNPN have shown important improvements by the addition of small loads of CNs. Stress–strain curves for the PBO and CNs, (2 wt%)/PBO fibres show that the tensile strength of PBO composite fiber was improved by 20–50%. In addition, the thermal stability, as analyzed by TGA, is enhanced. These authors mention that the improvement of mechanical and thermal properties is apparently the result of some special interactions between CNTs and PBO [64]. Other functionalized CNs have been added to PBO. Fractured surface SEM observations of U-MWNTs/PBO composites, show individual U-MWNTs enwrapped by the PBO matrix. This confirms that functionalized nanotubes improve the interactions between polymer matrices and CNs. In this specific case, U-MWNTs and PBO matrix have interactions at the interface level, so good interfacial adhesion between U-MWNTs and PBO matrix is produced [65]. In addition, SOH-terminated nanotubes have been used by Liu et al. to reinforce PVA polymer matrix, improving mechanical properties [101] and Cho et al. use carboxyl terminated nanotubes in polyurethane to produce interesting electroactive shape recovery, promising for the field of actuator materials [102]. Also, it is well known the outstanding conductive properties that CNPN posses, a review of these properties and applications are presented by Velasco-Santos et al. [91,92]. Recently non-covalent functionalization in carbon nanotubes was used for create a new ultra strong composite employed to create the first bicycle where the frame have as reinforcement carbon nanotubes. BMC is the company that produced the bicycle employed polymer composite material, developed between Zyvex,

which provide functionalized nanotubes and Easton 1. This represents the first CNPN that is developed for a special application, in concordance with all mentioned in this section. This practically show how chemical-functionalized nanotube nanocomposites have open different important field to diversify and improve carbon nanotube polymer composite properties and applications. 4.2.2. Tribological Properties of Chemically Modified Carbon Nanotube Polymer Nanocomposites In fact, functionalized carbon nanotube polymer nanocomposites are developed by different approaches and with different chemical moieties. Results have shown important increases in storage modulus and glass transition temperature, as we reviewed in the previous section, however other important parameters such as hardness have been evaluate recently. Owens used fluorinated and non fluorinated SWNTs included in Polyacrylonitrile [47]; Results of fluorinated nanotube composite present higher hardness than non fluorinated nanotube composite. For instance, a sample with 12 wt % of fluorinated nanotubes has a hardness of around 100, according with ASTM D2240 while the composite with the same quantity of non-fluorinated nanotubes presents a hardness less than 80. Similar effects were obtained with less concentration of functionalized and non functionalized nanotubes in composites. Not only pure polymer has been studied to make nanocomposites, since also polymer blends as the reported by Yang Xue et al. [103]. They experimented also with UHMWPE but, since this has an extremely high melt viscosity, they added High Density PolyEthylene (HDPE) to reduce that characteristic. The resulting mix can be processed in common processing machines and the lower viscosity allows a better dispersion of the MWNTs within the matrix. MWNTs with high purity (more than 95%) were used asreceived but also with a nitric acid treatment. The results of this work show that the wear resistance of the UHMWPE/HDPE com1

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posites can be significantly improved by adding MWNTs, in fact the wear rate decreases with increasing the MWNTs content in the tested range from 0 to 2 weight %. However, addition of MWNTs has a minimum effect or even diminishes the creep resistance, since, in spite of the penetration depth after 60 hours of the nanocomposites being smaller than that of the pure UHMWPE, it was mainly the HDPE content that improve the creep resistance of the polymer blend composites. Also, the chemical treatment of the MWNTs has an unfavorable effect on the creep resistance; the authors conclude that the improvement in the CNT/matrix bonding is insignificant or covered by shortening of MWNTs. Although the authors do not discuss the reason of this effect, we consider that in this work the processing of nanocomposites does not involve the presence of reactive sites or free radicals in the polymeric chains where the functionalized MWNTs (treated by nitric acid) can be attached, since the polymeric chains were already synthesized prior to the incorporation of MWNTs. In this case, effectively, there is no reason for improve the creep resistance or other tribological or mechanical properties since nitric acid treatment affects the integrity of the outside walls of the carbon nanotubes and does not exist the possibility to improve the interface between MWNTs and polymer, more over than the results achieved with the untreated MWNTs. Yang et al. [81, 103], also studied the tribological properties in nanocomposites, but they used SWNTs purified by a cleaner treatment with concentrated hydrochloric acid and nitric acid. The polymer matrix was PolyStyrene (PS). In this case, the nanocomposites were synthesized by an in-situ polymerization using benzoyl peroxide as initiator in a free radical polymerization, as consequence, they show better tribological properties. It is expected that, when the carbon nanotubes used to make nanocomposites were chemically treated, some reactive sites are formed, onto whcih the growing polymer chains can be attached, joining the NTs and the polymer, making a true interface that allow interesting mechanical and tribological properties. In their results, Yang et al., show that microhardness of SWNTs/PS nanocomposite increases sharply when the SWNTs content is below 1.5 weight %, but the value decreases when the content is above 1.5 weight %, this is attributed by the authors to the agglomeration of SWNTs in the polymeric matrix. The same behavior is observed in the friction coefficients and also the wear rate in these nanocomposites decreases from 1.3 X 10-4 for the pure PS to 8.0 X 10-6 mm3 N-1 m-1 for PS with 1.5 weight % of chemically treated SWNTs. Above this content of SWNTs the wear rate increases gradually with the content of nanotubes. These results suggests that compatibility between carbon nanotubes and polymers is improved with the chemical treatment on the nanotubes, in addition if the polymer is formed by in situ polymerization the properties could are much better in some cases since a true interface is achieved. CONCLUDING REMARKS Carbon nanotubes (CNTs) have been analyzed and chemically modified aiming to diversify their properties and be incorporated into an engineering polymer matrix successfully. The oxidation approaches in these materials have been an important technique to attach or link diverse chemical moieties. In addition, other routes that involve chemical functionalization are reported frequently and are promise to generate new possible applications in different fields. In this context, the nature of the chemical compounds linked to the nanotubes have allowed to diversify the behavior of carbon nanotube surface, and then produce important changes in the solubility properties of CNTs. Depending on the chemical groups found in the nanometric material, the nanotubes present different solubility behavior in water, organic solvent or polymer dissolved. The functionalization also has contributed to improve compatibility of CNTs with different organic and inorganic materials. Thus, novel techniques are developed constantly. Grafting is a relatively new technique that has allowed incorporating diverse polymer chains in


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nanotube surface using wrapping or bonding. The latter, collectively with the other chemical routes of modification, have created a new research branch related to the chemical modification of carbon nanotubes. Numerous chemical moieties and polymer grafted attached to the nanotubes are added to nanomaterials surfaces in order to enhance the compatibility between diverse polymers and nanotubes. An important amount of research focus to modified nanotubes using chemical functionalization have improved successfully the dispersion and compatibility of CNTs with different polymer matrix, in order to develop novel carbon nanotube polymer nanocomposites (CNPN). The results in this field indicate that nanotubes improve notably the composite properties when these nanomaterials are incorporated to polymer matrices in low loads, even when nanotubes do not present chemical modification. However, chemical functionalization in CNTs have led to better properties in many cases, since chemical groups at molecular level improve the links in the nanocomposite interface. The functionalization improves dispersion but better results have been obtained when, in combination with the synthesis of the polymer, one produces free radicals during the polymer growing and in the chemical groups on the nanotube surface. Thus, different parameters are considered to be important for the properties control of CNPN and then to scale these materials to multifunctional composites. Some of these parameters reviewed here are: chemical modification on carbon nanotubes, processing methods for CNPN, dispersion techniques, alignment methods for carbon nanotubes in polymer composites, use of compatibilizers depending of applications, production approaches for carbon nanotubes, among others. In addition, it is important to indicate that there are other research fields where theoretical studies are related to the some properties of CNPN reviewed in this article, at the same time different studies have found new models to predict engineering parameters depending on size, load, and form of carbon nanotubes and other nanomaterials and the relation with interface in polymer matrices [104-106], also mathematical modeling treating to make a correlation of these parameters with processing of nanocomposites in direction of understanding and predicting properties in CNPN [107,108]. Thus, more research is needed to integrate and resolve all parameters that play an important role in the development of new age of multifunctional polymer nanocomposites. REFERENCES [1] [2]

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Accepted: August 13, 2009