Multiwalled carbon nanotubes functionalized with maleated poly(propylene) by a dry mechano-chemical process

Polymer 53 (2012) 291e299

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Multiwalled carbon nanotubes functionalized with maleated poly(propylene) by a dry mechano-chemical process Veronica Ambrogi a, Gennaro Gentile b, *, Caterina Ducati c, Maria Cristina Oliva a, Cosimo Carfagna a, c a

Department of Materials and Production Engineering, University of Naples, Piazzale Tecchio, 80-80125 Naples, Italy Institute of Polymer Chemistry and Technology, National Council of Research of Italy, Via Campi Flegrei, 34-80078 Pozzuoli, Italy c Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2011 Received in revised form 30 October 2011 Accepted 23 November 2011 Available online 29 November 2011

Ball milling was used to graft maleated polypropylene (MAPP) on the surface of multiwalled carbon nanotubes (MWCNTs), with a view to preparing MWCNT/polypropylene composites with improved matrix/nanotube compatibility. The occurrence of the grafting reaction was evaluated by FTIR spectroscopy and the yield was quantified by thermogravimetric analysis, as a function of the milling time. Dispersion experiments confirmed the nanotube surface modification of the nanotubes since functionalized MWCNTs remained stably dispersed in an ethanol/xylene solution for more than 48 h after sonication. No evidences of significant structural damage after the mechano-chemical treatment were shown by Raman spectroscopy. Moreover, a layer attributable to the presence of grafted MAPP chains on MWCNT walls was clearly detected by transmission electron microscopy. The average thickness of this amorphous layer was evaluated and compared with quantitative TGA data. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Surface functionalization Carbon nanotubes Mechanochemistry

1. Introduction Polymer nanofiller composites based on carbon nanotubes (CNTs) have been extensively investigated by several research groups over the last decades [1]. CNTs exhibit outstanding mechanical, electrical and thermal properties due to their sp2hybridized carbon. On the other hand, one of the most important drawbacks lies in the fact that they show a great tendency to establish strong van der Waals and pep interactions which cause strong agglomeration phenomena. Hence, when mixed with polymer matrices, CNTs tend to segregate in tight bundles, precluding their effective distribution in composites [2]. Suitable strategies are strictly required to improve the CNT compatibility and dispersibility and to achieve the formation of homogeneous polymer based composites with improved polymer-filler interfacial adhesion. Different approaches have been used to improve the dispersion of carbon nanofillers into a polymer phase, including sonication [3], high-speed mixing [4e6] and calandering [7]. Other methods are based on non-covalent or covalent CNT surface functionalization. Among non-covalent, surfactant treatments [8], as well as polymer wrapping [9,10] have been adopted. However, these methods often require complicated organic treatment processes.

* Corresponding author. Tel.: þ39 081 8675057; fax: þ39 081 8675230. E-mail address: [email protected] (G. Gentile). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.11.048

Concerning covalent surface functionalization methods, these are usually based on the grafting of low molecular weight compounds or polymer chains directly on the surface of carbon nanofillers [11e13]. Other covalent modifications are based on the incorporation of oxygen-containing functional groups onto the CNT surface through a variety of methods, chiefly wet chemical oxidation [14e17] and plasma treatments [18]. Nevertheless covalent functionalization approaches have been found to deteriorate the CNT structure, altering the trigonal hybridization of carbon, and therefore decreasing nanotube opto-electrical properties [19]. A novel approach to functionalize the CNT surface through a simple solvent-free method consists in the solid-state mechanochemical treatment at room temperature. The mechano-chemical methodology consists of mixing CNTs with suitable precursor molecules and then proceeding the reaction through high-energy ball impacts. Ball milling processes have been successfully applied to the functionalization of multiwalled carbon nanotubes (MWCNTs) in reactive atmospheres like CO, Cl2, NH3, COCl2, H2S, and CH3SH using a low-impact vibration mill [20]. Further mechano-chemical applications include the functionalization of single-walled carbon nanotubes (SWCNTs) with alkyl-halides [21], the synthesis of water-soluble ‘‘nanotubols’’ in presence of KOH [22] and the fluorination of SWCNTs by high-energy ball milling [23]. Finally, Li et al. obtained alkyl and aryl-functionalized SWCNTs by high-speed vibration mill [24]. A possible drawback of ball milling techniques is that they can modify the morphology and the structure of CNTs, either by

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reducing their length and inducing the formation of nanotubes with open and/or changed tips. Nevertheless, because of the large number of CNT/milling condition combinations, it is impossible to generalize about the kinetic of the degradation of CNTs during ball milling. Undamaged MWCNTs have been recovered after ball milling treatment carried out at room temperature for 120 h [25] and even for 200 h [26]. In general, it can be affirmed that SWCNTs are more sensitive to morphological and structural damages than MWCNTs during mechano-chemical treatments [27]. Starting from these considerations, in this work a novel approach based on the use of a ball milling technique has been optimised to graft maleated polypropylene on the surface of MWCNTs, with a view to preparing CNT/polypropylene composites with enhanced nanotube/matrix compatibility. In compliance with non polar chemical structure of polypropylene macromolecules, maleated polypropylene was used to provide the most efficient interfacial interaction in the resulting composites. The proposed approach is significant because it represents a simple, cheap, solvent-free CNT functionalization method that is easy to scale up also in a continuous industrial reactor. 2. Experimental 2.1. Materials Pristine CVD-grown MWCNTs (purity > 95%, diameter range 30e50 nm) were purchased from Cheap Tubes Inc. (Brattleboro, VT, USA). Isotactic polypropylene (PP), Moplen X30S (melt flow rate at 230  C, 2.16 kg: 8 g/10 min), and maleic anhydride-grafted polypropylene KA 805 (MAPP) containing approximately 0.7 wt% of grafted maleic anhydride [28], were kindly supplied by Basell Polyolefins (Ferrara, Italy). Xylene (>98.5%, isomeric mixture), and ethanol (96.0e96.9%) were purchased from Carlo Erba Reagents (Milano, Italy). 1,2Dichlorobenzene (>99.8%) was purchased by Romil Chemicals (Waterbeach, Cambridge, UK). All chemicals were used without further purification. 2.2. Ball milling process Ball milling of MWCNTs was performed in a Retsch PM100 planetary ball milling system (Haan, Germany). Before milling, MWCNTs and MAPP or PP were dried at 90  C under vacuum for 24 h. Therefore samples constituted by 2.0 g of MWCNTs and 2.0 g of MAPP were milled in a 125 mL steel milling cup with 25 spheres of 10 mm diameter at 650 rpm. The weight ratio spheres/MWCNTs was about 50:1. The ball milling process was carried out for 60, 120, 240, 480 and 1440 min. For comparison, a mixture constituted by 2.0 g of MWCNTs and 2.0 g of PP, as well as a sample constituted by 2.0 g of pure MWCNTs were ball milled for 480 min. In order to remove the ungrafted fraction of polymer from the MWCTN surface, purification of the milled samples was performed with xylene and 1,2-dichlorobenzene. Milled samples were exposed to 5 purification cycles for each solvent. Every purification cycle was carried out at T ¼ 135  C, under stirring, for 20 min. After that the mixture was filtered on paper. Finally, purified MWCNTs were dried at 150  C under vacuum for 48 h. Prepared samples are listed in Table 1.

Table 1 Codes, composition and ball milling time of the prepared samples. Code

Composition

MWCNT BM8 BM8-PP BM1-MAPP BM2-MAPP BM4-MAPP BM8-MAPP BM24-MAPP

MWCNTs 0 MWCNTs 480 MWCNT/PP 1:1 by weight 480 MWCNT/MAPP 1:1 by weight 60 MWCNT/MAPP 1:1 by weight 120 MWCNT/MAPP 1:1 by weight 240 MWCNT/MAPP 1:1 by weight 480 MWCNT/MAPP 1:1 by weight 1440

a

Ball milling time (min) Remarks as grown as obtained Purifieda Purifieda Purifieda Purifieda Purifieda Purifieda

As described in the experimental section.

Elmer Spectrum One FTIR equipped with a universal ATR sampling accessory (Wellesley, MA, USA). Spectra were collected using a resolution of 4 cm1 and 32 scan collections. Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer Pyris Diamond thermogravimetric analyser (Wellesley, MA, USA). Each sample (about 2 mg) was analysed at 10  C min1 heating rate in air flux (50 mL min1). Dispersion experiments were performed on pure MWCNTs and ball milled samples after purification cycles. 7 mg of each samples were ultrasonicated in a 30 mL of ethanol/xylene solution (3:1 wt/ wt) using a Sonics vibracell (500 W, 20 kHz) ultrasonic processor (Sonics & Materials, Newtown, CT, USA). Three ultrasonication cycles, each lasting 2 min, were carried out a 40, 50 and 60% of amplitude respectively, cooling the mixture in an ice bath. Energy dispersive X-ray spectroscopy (EDX) analysis were performed on a FEI Quanta 200 FEG SEM (Eindhoven, The Netherlands) equipped with an Oxford Inca Energy System 250 and an Inca-X-act LN2-free analytical silicon drift detector. To prepare samples for EDX, disks of about 0.2 mm thickness, 13 mm diameter were obtained by pressing MWCNTs in an evacuable KBr die. Samples were then mounted onto SEM stubs by means of carbon adhesive disks and analysed at 10 kV acceleration voltage. Prior to the analysis, samples were dried overnight in the SEM chamber under high vacuum (106 torr). Average results and standard deviation values are based on three consecutive measurements on different areas of each sample. Confocal Raman spectra were acquired on powder samples by a Horiba-Jobin Yvon Aramis Raman spectrometer (Kyoto, Japan) operating with a diode laser excitation source limiting at 532 nm and a grating with 1200 grooves/mm. The 180 back-scattered radiation was collected by an Olympus metallurgical objective (MPlan 50X, NA ¼ 0.75) and with confocal and slit apertures both set to 200 mm. The radiation was focused onto a Peltier-cooled CCD detector (Synapse Mod. 354308) in the Raman-shift range 3200e100 cm1. To separate the individual peaks in the case of unresolved, multicomponent profiles, spectral deconvolution was performed using the software Grams/8.0AI, Thermo Scientific (Waltham, MA, USA), using a Voigt function line shape. By a non-linear curvefitting of the data, height, area and position of the individual components were calculated [29]. High resolution transmission electron microscopy (HR-TEM) experiments were performed on a JEOL JEM-4000EX II (400 kV, LaB6) microscope with a spherical aberration coefficient Cs of 1.07 mm and point resolution of 0.17 nm. Prior to observations, the specimens were dispersed onto holey carbon copper grids. 3. Results and discussion

2.3. Characterization Fourier transform infrared spectra were obtained through the attenuated total reflectance (ATR-FTIR) method using a Perkin

Molecular solid-state processes have been recently defined as physicalechemical processes in which contact between molecular solids is promoted by the mechanical action in order to favour

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mutual approach of the reacting centres [30]. The undeniable advantage of these processes is that they are solvent-free and are easy to scale up. Starting from these considerations, a dry ball milling process has been adopted to graft MAPP on MWCNT surface to improve the compatibility of the nanotubes with polypropylene. The proposed surface modification is a critical step with a view to realizing composites characterized by a homogeneous dispersion of MWCNTs into the PP matrix and by an improved interfacial adhesion between the phases. In order to evaluate the effect of the treatment on structural and morphological features of MWCNTs, the ball milling process was carried out on pristine MWCNTs, on MWCNTs with unmodified polypropylene, as well as on MWCNTs with maleated polypropylene. Moreover, the effect of the ball milling time on the grafting yield of MAPP was also investigated. In Table 1 codes, composition and ball milling time of treatments are reported for the investigated samples. Preliminary tests (see Table S1, electronic supplementary material) carried out on neat polypropylene, have shown that the polymer does not undergo to chain scission phenomena during prolonged ball milling, as the low shear melt viscosity, h0, was found unchanged after 24 h of ball milling. The weight average molecular weight Mw was estimated as about 250.000 Da for both untreated and ball milled polypropylene [31]. Fig. 1 presents the ATR-FTIR spectra obtained on pristine MWCNTs, BM8-MAPP and BM24-MAPP powder samples. For comparison, the ATR-FTIR spectrum of a maleated polypropylene film is also reported (Fig. 1d). Consistently with previously reported data [32], no detectable transmission bands were observed for the pristine MWCNTs (Fig. 1a) in the wavenumber range between 650 and 4000 cm1. ATR-FTIR spectrum of BM8-PP, not reported in Fig. 1, was similar to that of as grown MWCNTs. No absorption bands were observed, thus indicating that polypropylene did not react with nanotubes during the ball milling process and could be completely removed during the purification process. Both BM8-MAPP and BM24-MAPP showed remarkably different spectra from that of untreated MWCNTs (Fig. 1b and c), indicating that nanotubes reacted with MAPP. In particular, the successful

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Fig. 2. TGA traces in air flow of:a) MWCNT; b) BM8-PP; c) BM2-MAPP; d) BM8-MAPP; e) BM24-MAPP.

functionalization of MWCNT with MAPP was confirmed by the presence of absorption bands in the range 2800e3000 cm1, characteristic of CeH symmetrical antisymmetrical stretching. Absorption bands of the maleic anhydride group in the range 1650e1800 cm1, whose intensity was already very low in the spectrum of MAPP (see the arrow in Fig. 1d), were clearly not detectable in the spectra of functionalized nanotubes. Moreover, in the spectrum of BM24-MAPP (Fig. 1c), weak absorption bands at 1747 and 1030 cm1 could be observed. These bands were assigned to the stretching of C]O and CeO groups, respectively, and they suggest that oxidation phenomena occurred on the nanotube surface during prolonged ball milling in presence of MAPP, inducing the formation of carbonyl and carboxyl groups that can eventually interact with maleic anhydride groups via hydrogen bonds or other dipoleedipole interactions. The existence of hydrogen bonds between maleic anhydride and modified nanotubes was often hypothesized in PP based composites reinforced with carbon nanotubes in which maleated polypropylene was used as compatibilizer. Lee et al. [33], evidenced these interactions by FTIR analysis, showing that the absorption band of polymaleic

Fig. 1. ATR-FTIR spectra of: a) MWCNT; b) BM8-MAPP; c) BM24-MAPP; d) MAPP.

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V. Ambrogi et al. / Polymer 53 (2012) 291e299 Table 2 EDX results of pure and ball milled multiwalled carbon nanotube samples. Sample

Elemental composition (wt%) C

MWCNT BM8 BM8-PP BM8-MAPP

Fig. 3. Weight loss at 450  C vs. milling time of purified MWCNT/MAPP samples.

anhydride stretching (centred at about 1785 cm1) was down shifted in PP/CNT composites and its relative intensity was reduced due to hydrogen bond formation. In the cited paper, maleated polypropylene containing about 4 wt% of maleic anhydride was used. Nevertheless, MAPP used is the present work was characterized by only about 0.7 wt% of maleic anhydride. Such a low content was responsible for the low intensity of the absorption bands characteristic of the maleic anhydride group not only for the sample BM24-MAPP but also for MAPP, preventing the possibility of evaluating the hydrogen bonding formation by FTIR analysis, as the absorption band of the maleic anhydride stretching is characterized by a very low intensity. The assessment of the MWCNT surface functionalization was obtained by TGA analysis performed in air flow. Fig. 2 shows TGA traces of MWCNT, BM8-PP, BM2-MAPP, BM8-MAPP and BM24MAPP. For temperatures lower than 200  C all the samples did not exhibit any significant degradation phenomena. At higher temperatures they showed different thermo-oxidative behaviour. In particular, in the range between 300 and 500  C the TGA trace of MWCNT showed a weight uptake that can be ascribed to the oxidation of metal particles used as catalyst for their synthesis. This phenomenon has been previously reported in literature [34]. The maximum weight gain, observed at 500  C, was 1.3 wt%. On the contrary, no weight increase was detected during thermo-oxidative treatment of ball milled MWCNT. In this case, as

97.47 97.49 97.58 96.32

O    

0,08 0,10 0,19 0,10

0.53 0.44 0.60 2.02

Fe    

0,04 0,07 0,14 0,07

0.41 0.40 0.36 0.32

Ni    

0,03 0,05 0,04 0,02

1.59 1.67 1.46 1.34

   

0,06 0,04 0,06 0,04

already reported [34], metal particles were encapsulated by CNTs during the ball milling process, thus resulting protected against oxidation. Furthermore, the mechano-chemical treatment did not induce the formation of any appreciable amount of amorphous carbon, whose presence should be revealed by an oxidative degradation step at lower temperature due to the reduced stability of amorphous carbon with respect to nanotubes [35]. The whole TGA curve of BM8 was shifted to lower temperatures with respect to that of untreated nanotubes. The temperature corresponding to 90 wt% of residual weight was 579  C for BM8 and 596  C for pure MWCNTs, indicating that the ball milling treatment affects the nanotube thermo-oxidation, reducing their stability. In the case of BM8-PP, the TGA trace, not reported in Fig. 2, overlapped that of BM8 in the whole range of investigated temperatures. The purified sample did not show any weight loss step attributable to the presence of polypropylene, confirming that the polymer was completely removed by purification treatments, because it did not react with MWCNT during the ball milling process. On the contrary, all the MWCNT samples ball milled with MAPP exhibited a weight loss step in the range between 300 and 500  C. As shown in Fig. 3, the extent of this mass loss step increased with the milling time. After ball milling of 2 h, purified sample showed a weight loss of 2.7 wt% at 450  C. At the same temperature this amount increased to 4.8 wt% for BM8-MAPP and up to 5.7 wt% in the case of BM24-MAPP. This phenomenon is related to the degradation of a maleated polypropylene fraction that has not been removed after purification in xylene and 1,2-dichlorobenzene, thus supporting the indication that the ball milling treatment is able to induce a grafting of MAPP chains on the CNT surface. In order to further confirm the functionalization of MWCNT with MAPP and try to clarify the effect of the ball milling treatment

Fig. 4. Dispersion stability of samples in ethanol/xylene after 48 h from the sonication: a) MWCNT; b) BM8; c) BM8-PP; d) BM8-MAPP.

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Fig. 5. Scheme of the possible interactions between maleic anhydride groups of MAPP multiwall carbon nanotubes.

on the nanotubes, dispersion experiments in ethanol/xylene solution (3:1 wt/wt) were performed on MWCNT, BM8, BM8-PP and BM8-MAPP. Both BM8 and BM8-PP settled 24 h after sonication. Pure MWCNT were found to precipitate completely just after 36 h, while the sample containing functionalized CNTs remained stably dispersed for more than 48 h after sonication (Fig. 4).

As reported in literature [27], the ball milling process induces the formation of large aggregates of entangled MWCNTs. The sonication procedure was not able to wholly disrupt CNT agglomerates and this can explain the poor dispersibility of both BM8 and BM8-PP, found to be less stable than pristine MWCNTs in the ethanol/xylene solution. On the other hand, for BM8-MAPP the effect of agglomeration phenomena is reduced by the functionalization of the nanotubes: that is, the presence of maleated polypropylene grafted on the surface of the nanotubes significantly improves the stability of the dispersion. Further supporting evidence of the MWCNT functionalization with maleated polypropylene was provided by the results of EDX analysis, reported in Table 2. As concerning pure MWCNTs, the oxygen percentage was 0.53 wt%. This result can be explained by considering that even in “oxygen free” nanotubes, there is an oxygen amount measurable by EDX. This amount has been recently attributed to oxygen atoms of residual metal oxide particles used as catalysts for MWCNT production [1] rather than to absorbed water vapour, since CNTs underwent extensive drying before the analysis, as described in the experimental section. As expected, for BM8 and BM8-PP the oxygen amount, within the experimental errors, was comparable to that recorded for MWCNTs. Instead, a significant increase in the oxygen content, up to about 2 wt%, was detected for BM8-MAPP, this amount being significantly higher than that expected considering the oxygen stoichiometric amount of the anhydride groups of MAPP chains grafted to the CNT surface, evaluated by TGA. In order to explain this unpredicted result, it can be considered that maleated polypropylene is industrially obtained by reactive extrusion of polypropylene with maleic anhydride in presence of organic peroxides as initiators. It can be hypothesized that these

Table 3 IG/ID intensity ratio for pristine and treated multiwalled carbon nanotube samples and coefficient of determination (R2) of the spectral deconvolution.

1

Fig. 6. Raman spectra in the range 1200e1700 cm of: a) MWCNT; b) BM8; c) BM8PP; d) BM8-MAPP; a’) resulting profile obtained by spectral deconvolution of a) and individual components D, G and D0 .

Sample

IG/ID intensity ratio

Coefficient of determination (R2)

MWCNT BM8 BM8-MAPP BM24-MAPP

1.29 1.32 1.37 1.46

0.993 0.990 0.991 0.990

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Fig. 7. Bright-field TEM images of: a) MWCNT; b) BM8-MAPP.

traces of organic peroxides, in combination with the maleic anhydride groups, contribute to the increase of the oxygen content by inducing oxidation on the nanotube surface during the ball milling process. The presence of oxidized groups onto the nanotube surface may also explain the occurrence of the grafting of maleated polypropylene. In fact, it has been already reported that anhydride groups or carboxyl groups deriving from the opening of the anhydride ring can form hydrogen bonds with carbonyl or carboxyl groups eventually present on the surface of carbonaceous materials [36], as schematized in Fig. 5. Raman spectroscopy is a fundamental tool for the characterization of carbon nanotubes. Raman spectra in the range 1200e1700 cm1 of pristine and treated carbon nanotubes are shown in Fig. 6. As an example, the result of the spectral deconvolution carried out on the sample MWCNT was also reported (Fig. 6, curves a’). Each spectrum consists of a band at w1335 cm1

(D-band) and a complex band whose components are centred at w1565 cm1 (G-band) and w1610 cm1 (D0 -band). Furthermore, two additional bands at w2670 cm1 (D*-band, overtone of the Dband) and at w2900 cm1 (D þ G combination mode band), not shown in Fig. 6, were also evidenced in the investigated range of Raman-shifts [15,37]. The D-band is usually attributed to amorphous or disordered carbon in the carbon nanotubes. Although a systematic work to evaluate the various contributions of the D-band is still needed, this structural disorder depends on the presence of finite or nanosized graphitic planes, defects of the nanotube walls, vacancies, heptagonepentagon pairs, kinks and heteroatoms. The G-band originates from the in-plane tangential stretching of carbonecarbon bonds in graphene-like sheets. Finally, the D0 band, evidenced in the Raman spectra as a shoulder of the G-band at higher frequencies, is another feature induced by disorder and defects in the nanotubes.

Fig. 8. TEM images of a) MWCNT; b) enlarged view of the walls of MWCNT and c) FFT of b; d) BM8-MAPP; e) enlarged view of the walls of BM8-MAPP and f) FFT of e.

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By performing the spectral deconvolution it was possible to calculate the intensity ratio IG/ID, a parameter that can indicate a degree of order in the nanotube structure. Results are reported in Table 3. As it can be observed, similar values were found for the IG/ID ratio of MWCNT and BM8, showing that the ball milling process did not induce further defects in the nanotube structure in the selected conditions and time scale. The progressive increase of the IG/ID ratio, evidenced for nanotube samples milled for 8 and 24 h in presence of MAPP, can be attributed to the washing and filtration procedure that removed part of the disordered graphitic nanoparticles present in the original sample. Transmission electron microscopy was used as a tool to further confirm that the ball milling did not induce significant morphological and structural changes on carbon nanotubes and to further corroborate the occurrence of the grafting of MAPP chains onto their surface. Low magnification bright-field TEM analysis carried out on MWCNT, BM8 and BM24-MAPP previously dispersed in ethanol by sonication (Fig. S1, electronic supplementary material), showed comparable nanotube lengths for all the samples, demonstrating that the nanotube structure was not compromised either when the ball milling was carried out on the pristine multiwalled nanotubes, either when the mechano-chemical process was performed in presence of MAPP. Similar results were obtained by SEM analysis (Fig. S2, electronic supplementary material). In Fig. 7 bright-field TEM images of pure MWCNTs and BM8MAPP are reported. Catalytic metal nanoparticles are well evident in MWCNTs, (see Fig. 7a) thus confirming that they were exposed

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and susceptible to oxidation during the thermal treatment in air flow, as revealed by TGA. The most evident change deriving from the ball milling process is that MWCNTs milled for 480 min formed large agglomerates, whose dimensions were about 2 mm (see Fig. 7b). The average dimension of these agglomerates was analogous for all the ball milled samples, independently of the presence of the polymer fraction. Within these agglomerates it is difficult to distinguish catalytic metal nanoparticles, which are often encapsulated in graphitic shells and therefore protected against oxygen during the thermal treatment. This finding supports the results obtained by TGA experiments concerning the weight uptake recorded for untreated MWCNTs. In Fig. 8 HR-TEM images of pure MWCNTs and BM8-MAPP are reported. Slight deformations of the nanotubes morphology are evident for the ball milled sample, as indicated by the arrow in Fig. 6d. Moreover, no significant structural damage was evident as a consequence of the mechano-chemical treatment. For both MWCNTs and BM8-MAPP the spacing between adjacent layers (d002) has been evaluated as 0.35  0.01 nm, and a comparable crystallographic order has been recorded along [100], as it can be observed from the fast Fourier Transform (FFT) of significant areas of interest of both the samples (see Fig. 8cef) [38]. Similar results were obtained also for BM8 and BM8-PP. Nevertheless, the main difference observed for BM8-MAPP with respect to the other samples is related to the external surface, which showed a well evident layer attributable to the presence of grafted MAPP chains (see Fig. 9). More specifically, the average thickness of this layer, measured along the external surface of the

Fig. 9. TEM images of BM8-MAPP showing the MAPP phase grafted on the external walls (a and b) and on the nanotube ends (c and d).

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nanotubes through a careful image analysis, is 1.02  0.25 nm. In the case of MWCNTs and BM8-PP the average thickness of the amorphous layer is below 0.30 nm, suggesting that the average thickness of the MAPP coating on the external surface of the nanotubes is about 0.7 nm. This value can be explained taking TGA results into account. Considering an average radius of the nanotubes of 15 nm, an average length of 10 mm, and density values of 2.1 g cm3 for MWCNTs and 0.95 g cm3 for MAPP, and simplifying the morphology of the nanotubes to a perfectly cylindrical shape, an average thickness value of about 0.7 nm corresponds to a MAPP content of 4.1 wt%, which is in good agreement with the value (4.8 wt%) obtained from thermogravimetric analysis. A unique feature of the BM8-MAPP specimen is the presence of small amorphous aggregates, visible in some of the concave regions of the MWCNTs. One of these aggregates is shown at the end of the MWCNT in Fig. 9c, magnified in Fig. 9d. From the contrast in the image, and supported by the data discussed above, it is reasonable to assume that the aggregate is a larger residue of MAPP anchored to the graphitic walls due to the curvature of the surface. 4. Conclusions In this work, a dry ball milling process was set up to functionalize MWCNT surface with maleated polypropylene, with a view to realizing composites characterized by a homogeneous dispersion of MWCNTs into the PP matrix and by an improved interfacial adhesion between the phases. In order to evaluate the effect of the mechano-chemical treatment on MWCNTs, this process was performed on pristine MWCNTs, on MWCNTs with unmodified polypropylene, as well as on MWCNTs with maleated polypropylene. Moreover, the grafting yield of MAPP was also investigated as a function of the ball milling time. ATR-FTIR analysis of purified samples confirmed that MAPP was grafted onto the nanotube surface, whereas MWCNTs ball milled in presence of neat polypropylene did not show any spectral difference with respect to untreated nanotubes. Furthermore, for prolonged ball milling in presence of MAPP, consistent oxidation phenomena occurred on the nanotube surface, as revealed by the onset of new absorption bands related to carbonyl and carboxyl groups. TGA curves confirmed and quantified the occurrence of the functionalization. After purification, only MWCNT samples ball milled with MAPP exhibited a weight loss step in the range between 300 and 500  C, whose extent increased up to 5.7 wt% with the milling time. This phenomenon is related to the degradation of a maleated polypropylene fraction that has not been removed after purification, thus corroborating the ability of the ball milling treatment to induce the modification of the CNT surface. Further evidence of the occurred grafting was obtained by dispersion experiments in ethanol/xylene solution: pure MWCNT resulted completely precipitated after 36 h, while the sample containing MAPP-grafted CNTs remained stably dispersed for more than 48 h after the sonication. EDX analysis allowed to clarify the grafting mechanism. Functionalized nanotubes showed an oxygen amount higher than that calculated taking into account the anhydride groups of MAPP grafted to the CNT surface, as inferred from TGA results. This suggests that residual organic peroxide catalysts deriving from the maleinization process of PP, in combination with the maleic anhydride groups, increased the oxygen content by inducing oxidation on the nanotube surface during the ball mill process. The presence of oxidized groups would explain the grafting mechanism of maleated polypropylene onto the nanotube surface, as anhydride groups can form hydrogen bonds with carbonyl and carboxyl groups present on the surface of carbonaceous materials.

Raman, TEM and SEM analysis revealed that the mechanochemical treatment did not induce significant structural damages to the nanotubes. Moreover, as observed by HR-TEM analysis, the external surface of nanotubes ball milled with MAPP for 24 h showed an amorphous layer, whose thickness was estimated about 0.7 nm, attributable to the presence of grafted MAPP chains. On the basis of a simplified geometrical model, this value corresponds to a MAPP content of 4.1 wt%, which is in good agreement with the value (4.8 wt%) obtained from thermogravimetric analysis. All the experimental results allow to conclude that the ball milling process represents an effective method to graft compatibilizing agents onto the surfaces of carbonaceous materials. The high added value of this process is that it is cost-effective, ecosustainable, simple and easy to scale up. Acknowledgements The authors are grateful to Dr. P Musto and co-workers (ICTPCNR, Italy) for their valuable scientific and technical support in Raman spectroscopy. This work was funded partly by the IP3 project of the 6th Framework Programme of the European Commission: ESTEEM (Enabling Science and Technology for European Electron Microscopy) - Contract number 0260019. Caterina Ducati would like to acknowledge the Royal Society for funding. Appendix. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.polymer.2011.11.048. References [1] Wepasnick KA, Smith BA, Schrote KE, Wilson HK, Diegelmann SR, Fairbrother DH. Surface and structural characterization of multi-walled carbon nanotubes following different oxidative treatments. Carbon 2011; 49(1):24e36. [2] Georgakilas V, Bourlinos A, Gournis D, Tsoufis T, Trapalis C, Mateo-Alonso A, et al. Multipurpose organically modified carbon nanotubes: from functionalization to nanotube composites. J Am Chem Soc 2008;130:8733e40. [3] Wepasnick KA, Smith BA, Bitter J, Fairbrother DH. Chemical and structural characterization of carbon nanotube surfaces. Anal Bioanal Chem 2010; 396(3):1003e14. [4] Matarredona O, Rhoads H, Li Z, Harwell J, Balzano L, Resasco D. Dispersion of single-walled carbon nanotubes in aqueous solutions of the anionic surfactant NaDDBS. J Phys Chem B 2003;107:13357e67. [5] Cheng J, Fernando KAS, Veca LM, Sun Y-P, Lamond AI, Lam YW, et al. Reversible accumulation of PEGylated singlewalled carbon nanotubes in the mammalian nucleus. ACS Nano 2008;2(10):2085e94. [6] Chang TE, Jensen LR, Kisliuk A, Pipes RB, Pyrz R, Sokolov AP. Microscopic mechanism of reinforcement in single-wall carbon nanotube/polypropylene nanocomposite. Polymer 2005;46(2):439e44. [7] Chandra B, Bhattacharjee J, Purewal M, Son Y-W, Wu Y, Huang M, et al. Molecular-scale quantum dots from carbon nanotube heterojunctions. Nano Lett 2009;9(4):1544e8. [8] Vaisman L, Marom G, Wagner HD. Dispersions of surface-modified carbon nanotubes in water-soluble and water-insoluble polymers. Adv Funct Mater 2006;16(3):357e63. [9] Satake A, Miyajima Y, Kobuke Y. Porphyrin-carbon nanotube composites formed by noncovalent polymer wrapping. Chem Mater 2005;17(4):716e24. [10] Xin X, Xu G, Zhao T, Zhu Y, Shi X, Gong H, et al. Dispersing carbon nanotubes in aqueous solutions by a starlike block copolymer. J Phys Chem C 2008; 112(42):16377e84. [11] Deng J, Cao J, Li J, Tan H, Zhang Q, Fu Q. Mechanical and surface properties of polyurethane/fluorinated multi-walled carbon nanotubes composites. J Appl Polym Sci 2008;108(3):2023e8. [12] Men XH, Zhang ZZ, Song HJ, Wang K, Jiang W. Functionalization of carbon nanotubes to improve the tribological properties of poly(furfuryl alcohol) composite coatings. Compos Sci Technol 2008;68(3e4):1042e9. [13] Yang B-X, Shi J-H, Pramoda KP, Goh SH. Enhancement of stiffness, strength, ductility and toughness of poly(ethylene oxide) using phenoxygrafted multiwalled carbon nanotubes. Nanotechnology 2007;18(12): 125606. [14] Rosca ID, Watari F, Uo M, Akasaka T. Oxidation of multiwalled carbon nanotubes by nitric acid. Carbon 2005;43(15):3124e31.

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