18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS Functionalization of SWCNTs: Impact on Tensile Properties and Morphology of CNT-Paper S. Steiner, S. Busato, P. Ermanni* Centre of Structure Technologies, Swiss Federal Institute of Technology Zurich, Switzerland * Corresponding author (permanni@ethz.ch) Keywords : CNT-paper, functionalization, tensile strength, morphology carbon nanotubes (SWCNTs) and the resulting Abstract material processed into CNT paper, a thin mat Carbon nanotube papers prepared from chemically consisting of randomly in-plane oriented CNTs. functionalized SWCNTs are investigated for their CNT paper is easily prepared by vacuum filtration of mechanical tensile properties. The Young’s moduli CNT suspensions. The groups selected for surface are found to be unaffected by the functionalization functionalization were aniline ( 1 ), p-amino-N- with diazonium salts of aniline or aromatic mono- phenylbenzamide ( 2 ) and p-[(p-aminobenzoyl)- and bis-amides. Tensile strengths of CNT papers, amino]-N-phenylbenzamide ( 3 ). 2 and 3 are however, are found to increase with increasing aromatic amides with one and two amide links, degree of functionalization, and more pronouncedly respectively. The presence of such groups promises with increasing number of amide groups capable of enhanced intertubular interactions due to the hydrogen bonding. The importance of hydrogen formation of hydrogen bonds and π - π interactions. bonding becomes evident after its inhibition through Also, steric effects are expected to result in higher N-methylation of the amide groups, resulting in a resistance against lateral displacement of adjacent distinct reduction of strength values. SEM analysis CNTs and CNT bundles. Overall, increased indicates that a high degree of functionalization or a intertubular interactions are expected to result in high number of amide groups results in the higher mechanical strength of such CNT-paper as formation of domains with aligned CNTs. compared to non-functionalized reference CNT- paper. 1 Introduction The exceptional properties reported for individual 2 Experimental carbon nanotubes (CNTs) have raised hopes for their 2.1 Functionalization of SWCNTs application as high-performance structural and functional materials. However, macroscopic High purity single-walled carbon nanotubes (>90% materials presented to date have exploited no more SWCNTs; HeJi Inc., Hong Kong) and commercially than a small fraction of the individual CNT’s obtained reagents were used as received. Aromatic theoretical potential. Even fibers of aligned CNTs amines 2 and 3 were synthesized stepwise starting show disappointing tensile strengths, unless the from aniline 1 (Fig. 1). A similar reaction sequence gauge length in the measurement is chosen to be has been published by others [2]. Melting ranges and small [1]. Because of the weak intertubular NMR spectra suggested high purity of the crude interactions, small structural defects such as products, which were employed for CNT inclusions, voids or entanglements are sufficient to functionalization without further purification. significantly reduce the performance of CNT- 100 mg of SWCNTs together with 1 mol eq. (per materials. In order to create stronger CNT-materials, mol CNT carbon atoms) aromatic amine 1 - 3 and it is necessary to increase the intertubular 20 ml NMP were added in a 100 ml flask and interactions and thus enhance load transfer. homogenized in an ultrasonic bath with gradual In this study, short and stiff sidegroups were heating to 60°C. Isopentyl nitrite (1.17 g, 1.2 mol chemically grafted to the surface of single-walled eq.) was added dropwise to the dispersion and
O O CNT CNT O HN HN H 2 N HN 3-CNT 2-CNT 1 Sodium hydride, iodomethane, DMF, r.t. 93 % a, b O O O H 2 N CNT CNT O HN 2 N N Me Me N Me m2-CNT m3-CNT 82 % a, b O Fig. 3. N-methylation of functionalized CNTs H 2 N O HN HN 3 temperature for 10 min. The NMP-moist 2- or 3- 4-nitrobenzoyl chloride, N,N -dimethylaniline, a: CNT- paste prepared from 100 mg SWCNTs (as acetone, 0°C r.t.; b: Pd (10% on activated charcoal), ammonium formate, DMF, 0°C r.t. described above) was added all at once. After stirring for 30 min, iodomethane (0.31 ml, 5 mmol) Fig. 1. Synthesis of amines 2 and 3 was added to the suspension. After 1.5 h remaining iodomethane was removed from the reaction mixture by evacuation for 10 min. Excess sodium hydride functionalized was decomposed by addition of ethanol. CNTs were with 1 : R= filtered off and washed with NMP before CNT paper 1-CNT preparation. N-methylated CNTs are designated m2- O and m3-CNT for N- methylated 2 - and 3 - R with 2 : R= functionalized SWCNTs, respectively. HN 2-CNT O 2.3 Preparation of CNT papers with 3 : R= O 3-CNT HN SWCNT Functionalized CNTs prepared from 100 mg HN SWCNTs (as described above) were sonicated in Fig. 2. Sidewall functional groups grafted to CNTs NMP (250 ml) for 45 min with a Hielscher UP 200S ultrasonic processor and cooled with ice during the process. Subsequently, the dispersion was sonication continued for another 120 min at 60°C. centrifuged for 30 min at 4000 RCF in order to After cooling to room temperature, the CNTs were remove remaining agglomerates. 80 vol% of the filtered off with a microporous membrane and supernatant dispersion was carefully removed by washed with NMP until the filtrate was colourless. pipette and filtered through a microporous The NMP-wet CNTs were removed from the filter membrane with the aid of vacuum. The filtration and used for CNT paper preparation without further was stopped before the filter ran dry and the treatment. The corresponding functionalized CNTs remaining layer of approx. 5 mm suspension was are designated 1- , 2- and 3-CNT in the following discarded and the CNT paper washed with 2- (Fig. 2). propanol. While still wet, the CNT paper was peeled off the filter and placed between silicone rubber 2.2 N-methylation of amide links sheets and dried overnight in vacuum at 80°C. Batches of 2- and 3-CNT were N-methylated with sodium hydride and iodomethane in dry DMF 2.4 Analysis of CNT papers (Fig. 3). 55% sodium hydride in oil (0.15 g, The degree of chemical functionalization of the 6.3 mmol) and dry DMF (7 ml) were stirred at room SWCNTs was assessed by thermo-gravimetric
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