COMPOSITES

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Composites Science and Technology 67 (2007) 806–810

SCIENCE ANDTECHNOLOGY

Functionalization of carbon nanofibres by 1,3-dipolarcycloaddition reactions and its effect on composite properties

Rui Araujo a, M.C. Paiva b,*, M.F. Proenca a, Carlos J.R. Silva a

a Department of Chemistry, University of Minho, 4710-057 Braga, Portugalb Department of Polymer Engineering, University of Minho, 4800-058 Guimaraes, Portugal

Received 1 November 2005; accepted 5 January 2006Available online 24 October 2006

Abstract

The present paper reports the functionalization of carbon nanofibres through the reaction of 1,3-dipoles, generated by condensationof an a-amino acid and an aldehyde, with the CNF surface. The reaction was carried out in solvent and in dry conditions. The organicfunctionalization of the CNFs was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Thesestudies contributed to the understanding of the chemical functionalization processes, the identification of the synthetic conditions andthe evaluation of the degree of functionalization of the carbon nanofibres.

Composite samples were formed using polyamide 6 and CNFs, before and after functionalization under wet and dry conditions. Themechanical tests show that functionalization, under both conditions, plays an important role on the properties of the composites.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon fibres; A. Polymer-matrix composites (PMCs); B. Interface; D. Differential scanning calorimetry (DSC); D. Thermogravimetricanalysis (TGA)

1. Introduction

Carbon nanotubes (CNT) and nanofibres (CNF) exhibitunique mechanical, electric and thermal properties [1–3].Their physical form is not convenient for most applicationsdemanding their combination with a matrix material. Thepreparation of a composite requires an efficient dispersionof the reinforcing phase and its good interaction with thematrix. However, the chemical inertness and low surfaceenergy of the graphitic structure of the CNTs and CNFsmakes this process difficult. Thus, the surface modificationof CNTs and CNFs, aiming at the improvement of theirinteraction with other materials, is becoming an area ofincreasing importance. This modification can be achievedby attaching functional groups to the CNT/CNF surface[4–8], that will interact strongly with the matrix material,

0266-3538/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2006.01.040

* Corresponding author. Tel.: +351 253510327; fax: +351 253510339.E-mail address: mcpaiva@dep.uminho.pt (M.C. Paiva).

thus originating composites with exceptional structuralproperties at low reinforcement loads [9,10].

A possible route to organic functionalization of carbonnanotubes involves the use of 1,3-dipoles, generated bycondensation of an a-amino acid and an aldehyde. Theprocess was previously studied by other authors [8], andis represented in Scheme 1.

The present paper reports the results obtained for thefunctionalization of carbon nanofibres using the 1,3-dipo-lar cycloaddition reaction, under the conditions describedin the literature [8] and in solvent-free conditions.

2. Experimental conditions

The carbon nanofibres used were PR-24-PS from AppliedSciences Inc.. These fibres are hollow, present in a wide diam-eter range varying from 60 to 150 nm, and a length that canreach 100 lm. A transmission electron micrograph (TEM)of a PR-24-PS carbon nanofibre is presented in Fig. 1.

O

OHN

R1

OH

H

H

H HN

OO

O

H

H

HHR1

HH

NR1

H

H

H

H- CO2- H2O

Scheme 1. Formation of the 1,3 dipole by reaction of N-benzyloxycarbonylglycine and paraformaldehyde.

Fig. 1. TEM micrograph of a P-24 carbon nanofibre.

R. Araujo et al. / Composites Science and Technology 67 (2007) 806–810 807

The amino acid used was N-benzyloxycarbonylglycine(Z-gly-OH, 99%, from Aldrich), and the paraformalde-hyde, reagent grade, from Sigma.

The reaction was carried out under two different condi-tions: (a) refluxing a suspension of the CNF in a solution ofZ-gly-OH and paraformaldehyde (1:5 molar ratio) in DMFfor 120 h, according to the conditions described in [5], and(b) mixing the reagents, in the same proportion as in (a),homogeneously in a mortar and heating in a round bottomflask, at 180–210 �C, for 30 h. The resulting fibres werewashed with ethanol, filtered and dried. For the reactionin solution a blank experiment was run, where both

Fig. 2. (a) Solution obtained on the blank test. (b) CNF suspension after functi(b).

reagents were combined in the absence of CNFs, reproduc-ing the experimental conditions referred in (a) above.

The organic functionalization of the CNFs was studiedby calorimetric analysis (DSC), using a Mettler TC11 con-troller and a DSC20 Mettler oven, and thermogravimetricanalysis (TGA), on a TGA-7 from Perkin–Elmer.

Composite samples were formed using polyamide andCNFs, before and after functionalization. The sampleswere prepared by wet-casting, by dissolution of the poly-mer in formic acid. The CNF suspension in polyamidesolution was mixed in a magnetic stirrer for 3 h, cast ona flat glass surface and dried in a vacuum oven for 8 h at90 �C. The composite obtained was shopped in smallerpieces with a scissors. Composite plaques of approximately200 lm thickness were prepared by compression moulding,on a heated press. Tensile tests were conducted at roomtemperature on a PL Minimat, Miniature Materials Tester.The samples were rectangular with approximate planardimensions of 30 mm · 5 mm. Films were cut into test sam-ples using a razor blade. The samples were kept in a desic-cator prior to testing.

3. Results and discussion

The heterogeneous dispersion of CNFs in the solutionof Z-gly-OH, paraformaldehyde and DMF led to a homo-geneous suspension of the CNFs, after the heating processwas completed (Fig. 2b). The blank test, using the sameconcentration of Z-gly-OH and paraformaldehyde inDMF, led to a coloured solution after refluxing for 120 h(Fig. 2a). The same coloration was obtained for the mother

onalization reaction. (c) Solution obtained after filtration of the CNF from

NR1

H

H

H

HNR1

H

= PhCH2COO-R1

Scheme 2. Formation of a tertiary amine through the addition reaction ofthe 1,3-dipole.

Fig. 4. TGA scan of a mixture of paraformaldehyde, Z-gly-OH andCNFs. The DSC scan of a similar mixture is superimposed.

808 R. Araujo et al. / Composites Science and Technology 67 (2007) 806–810

liquor, recovered by filtration of the CNFs in the function-alization experiment (Fig. 2c).

The solid mixture of Z-gly-OH, paraformaldehyde andCNFs was heated at 180–200 �C for 30 h. A portion ofthe initial mixture (before reaction) was reserved for DSCand TGA studies.

Fig. 3 represents the DSC scans obtained for Z-gly-OH(a), paraformaldehyde (b), these solid compounds mixed ina mortar (c) and, finally, the mixture of paraformaldehyde,Z-gly-OH and the CNFs (d). The thermogram shown in (a)represents the melting peak of the amino acid, atapproximately 120 �C; (b) corresponds to the thermaldecomposition of paraformaldehyde into formaldehyde.The reaction starts at approximately 80 �C, with a peakmaximum within the range reported in the literature(118–122 �C); (c) represents the reaction of paraformalde-hyde with Z-gly-OH, in the same proportion as used forthe functionalization reaction. The thermogram indicatesthat Z-gly-OH reacts with formaldehyde as soon as thiscompound is formed, generating the 1,3-dipole, as repre-sented in Scheme 1. Fig. 3c shows the complete absenceof the melting peak of Z-gly-OH when the DSC isperformed on a combination of these reagents. The ther-mogram corresponding to the reaction with the CNFs isshown in (d). The reaction with the CNF surface isexpected to proceed as soon as the 1,3-dipole is formed,as represented in Scheme 2.

These observations were compared with the resultsobtained by TGA (Fig. 4). The DSC scan for the samereagent combination was superimposed to the TGA scan.The weight loss observed in the interval 90–120 �C maybe due to the formation of the 1,3-dipole with eliminationof H2O and CO2, and simultaneous loss of part of theformaldehyde formed. This reaction leads to the formationof the tertiary amine represented in Scheme 2.

The results suggest that, at higher temperature (180–200 �C ), cleavage of the R1 group takes place (compatible

Fig. 3. DSC scans of Z-gly-OH (a), paraformaldehyde (b), and theircombination without (c) and with (d) CNFs.

with the large weight loss observed by TGA), with forma-tion of the secondary amine (Scheme 2, R1 = H). This reac-tion may take place in more than one step, and depends onthe reaction conditions. Further studies on this subject arein progress. The groups remaining on the CNF surfacedecompose at a temperature above 350 �C with a smallerweight loss.

The irreversible nature of the reactions is evident fromthe results presented in Fig. 5, where an homogeneous mix-ture of paraformaldehyde, Z-gly-OH and CNFs was stud-ied by DSC. The first scan was run from room temperatureto 300 �C showing an endothermic peak around 120 �C,due to the decomposition of paraformaldehyde and the for-mation of the dipole, and the exothermic peaks corre-sponding to the decomposition of the groups formed byreaction of the CNF with the 1,3-dipole. A second scanwas run on the same sample from room temperature to500 �C, showing stability of the sample up to 300 �C, andan exothermic peak above this temperature correspondingto the thermal decomposition of the remaining chemicalgroups. The third run on the same sample was done fromroom temperature to 500 �C, showing a stable systemwithin this temperature range.

Larger amounts of CNFs were functionalized in thepresence of solvent and in dry conditions for the prepara-tion of composites. After washing and drying of the func-

Fig. 5. DSC scans of a mixture of CNFs, Z-gly-OH and parafor-maldehyde.

Fig. 6. SEM micrographs of: (a) non-functionalized CNFs and (b) CNFsfunctionalized in solution.

R. Araujo et al. / Composites Science and Technology 67 (2007) 806–810 809

tionalized CNFs the resulting fibres were analysed by SEMand TGA. SEM observation of the functionalized CNFsshow a higher degree of disentanglement of these fibrescompared to the non-functionalized CNFs, as shown inFig. 6a and b.

The functionalized CNFs were analysed by TGA show-ing weight losses around 4%. Fig. 7 presents the TGA ofCNFs functionalized in dry conditions in the temperaturerange of 180–210 �C for 34 h, compared to the TGA ofnon-functionalized CNFs.

Composites with 3% of CNFs, functionalized and non-functionalized, and polyamide 6 (PA6), were prepared.The dispersion of the functionalized CNFs in the polymerwas notably easier than the dispersion of the pure CNFs(for both types of functionalized fibres, in solution or indry conditions). The analysis of the cross-section of thecomposites formed shows that the dispersion of the CNFsthrough the polymer matrix was reasonable for the non-functionalized, and good for the functionalized CNFs. It

Fig. 7. TGA curves of non-function

was observed that, in the former case, the fibres tendedto agglomerate in small bundles, while in the later case theywere homogeneously and individually dispersed, as shownin Fig. 8a and b.

The mechanical properties of the composite films with3% of CNF, pure and functionalized, were measured.

alized and functionalized CNFs.

Fig. 8. SEM micrographs of the composite cross-sections formed with: (a)non-functionalized CNFs and (b) solution functionalized CNFs.

0

10

20

30

40

50

0 0.05 0.1 0.15 0.2

Strain (mm/mm)

Str

ess

(MP

a)

PACNF pureCNF functionalized in solution (CNF1)CNF functionalised in the solid (CNF2)

Fig. 9. Typical stress–strain curves obtained for the composites prepared.

810 R. Araujo et al. / Composites Science and Technology 67 (2007) 806–810

The fibres were functionalized in solution (CNF1) and indry conditions (CNF2). An increase of 50–75% in elasticmodulus and 15% in tensile strength was observed, aspresented in Table 1. Fig. 9 shows typical stress-straincurves obtained for composites prepared with functional-ized and non-functionalized CNFs. The results show thatfunctionalization plays an important role on thedispersion of the CNFs in the polymer matrix, and onthe mechanical properties of the composites formed. Itwas also shown that the reaction described in thispaper can be satisfactorily achieved in solvent-freeconditions.

Table 1Tensile properties of the PA6/CNF composites prepared

Elasticmodulus (GPa)

Tensilestrength (MPa)

Elongation atbreak (%)

PA6 0.88 ± 0.06 34 ± 5 12 ± 5Untreated CNF 0.95 ± 0.10 35 ± 2 12 ± 3CNF1 1.67 ± 0.14 41 ± 3 12 ± 6CNF2 1.45 ± 0.19 41 ± 4 7 ± 3

4. Conclusions

The functionalization of the carbon nanofibres was donevia the 1,3-dipolar cycloaddition reaction. The 1,3-dipolewas formed by reaction of N-benzyloxycarbonylglycine(Z-gly-OH) and formaldehyde, formed in situ by thermaldecomposition of paraformaldehyde. The reaction was suc-cessfully done in DMF solution and in the solid state. Evi-dence of the occurrence of functionalization of the CNFswas obtained from DSC and TGA studies. The CNFs werefunctionalized with comparable efficiency by the two differ-ent routes.

The fibres functionalized with and without solventshowed improved dispersion in a polyamide 6 matrix,and the mechanical properties were enhanced relative tothe properties of the composites with non-functionalizedCNFs.

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