Composites Science and Technology 102 (2014) 169–175
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Composites Science and Technology
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Improvement of the mechanical and electrical properties of polyamide 6nanocomposites by non-covalent functionalization of multi-walledcarbon nanotubes
http://dx.doi.org/10.1016/j.compscitech.2014.07.0220266-3538/� 2014 Elsevier Ltd. All rights reserved.
⇑ Corresponding author.
Jung Ryu a,b, Mijeong Han a,⇑a Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Koreab Department of Nanomaterials Science and Engineering, University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e i n f o
Article history:Received 7 March 2014Received in revised form 23 June 2014Accepted 17 July 2014Available online 29 July 2014
Keywords:A. Carbon nanotubesA. Nano compositesB. Electrical propertiesB. Mechanical propertiesA. Polymer–matrix composites (PMCs)
a b s t r a c t
Pyrene moieties containing amide functional groups with various alkyl chain lengths (butyl, octyl, anddodecyl) were synthesized and utilized in non-covalent functionalization of the surface of multi-walledcarbon nanotubes (MWNTs). The D/G ratio of the MWNTs (derived from the Raman spectra) remainedunchanged upon non-covalent functionalization of the surface of MWNTs, indicating that there wereno increases in the number of defect sites on the surface of the MWNTs. PA 6 (polyamide 6)/MWNT nano-composites were prepared by the melt-compounding method for evaluation of the mechanical and elec-trical properties. Flexural testing confirmed the much higher flexural modulus of the PA 6/non-covalentlyfunctionalized MWNTs relative to the PA 6/pristine MWNT nanocomposites. Furthermore, the PA 6/non-covalently functionalized MWNT nanocomposites exhibited better electrical properties due to preserva-tion of the intrinsic structure of the MWNTs as well as the uniform dispersion of the MWNTs in the PA 6matrix, which also played a role in the enhanced mechanical properties. It could be observed that thelonger alkyl chain length of the pyrene moieties led to further improvement of the sheet resistanceand mechanical properties, which is attributed to the enhanced compatibility with PA 6 derived fromfunctionalization.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction functional groups on the surface of the CNTs. Although this method
Numerous studies regarding carbon nanotubes (CNTs) havebeen conducted since their discovery, due to their attractive prop-erties such as high aspect ratio, low density, and excellent mechan-ical, thermal, and electrical properties, which may be exploited invarious applications [1–3]. In recent years, CNTs have been mostwidely utilized in polymeric nanocomposites [4,5]. The combina-tion of polymers and CNTs can be expected to lead to enhancedmechanical strength and modulus as well as electrical properties.Such desirable properties have led to the recognition of CNTs asan ideal potential candidate for the reinforcement of polymericmaterials. However, the strong self-interaction of CNTs due tovan der Waals forces and the weak interfacial adhesions with thepolymer matrix induce aggregation with consequently poor dis-persion in the matrix [6–9]. A number of techniques have beendeveloped in order to resolve these issues and obtain uniform ordesired dispersion in the polymeric matrix. One of the most recog-nized approaches is covalent functionalization by introducing
allows for strong interaction of the CNTs with the matrix [10–13],it simultaneously causes loss of the unique mechanical and electri-cal properties of the CNTs by disrupting the long-range p–p conju-gation of the CNTs, and introducing defects into the structure of theCNTs, resulting in inferior mechanical and electrical properties[14,15]. In recent decades, non-covalent functionalization methodsutilizing p–p interactions between the surface of the CNTs andorganic compounds have been attempted [16–20]. Aromaticgroup-containing moieties [21] and polymeric surface modifiers[22] have been used to functionalize the surface of CNTs in antici-pation of p–p interactions, as mentioned above, in order to main-tain the intrinsic structure of the CNTs. These non-covalentinteractions can effectively solubilize the CNTs in certain solventsand prevent their aggregation into bundles and ropes. Moreover,fewer defect sites such as pentagon–heptagon pairs called Stone–Wales, sp3-hybridized defects, and vacancies in the nanotubelattice may occur with this method than with covalent functional-ization. The enhancement of the mechanical and electrical proper-ties of polymer/CNT nanocomposites has been extensively studied.In preparing non-covalently functionalized CNTs, pyrene moietieshave been representatively used to modify the CNTs via p–p
170 J. Ryu, M. Han / Composites Science and Technology 102 (2014) 169–175
interaction [23,24]. Organic molecules and polymers containingpyrene have been successfully used to incorporate pyrene moietiesonto the surface of CNTs, and have been used in the preparation ofpolymeric nanocomposites, thereby conferring enhanced mechan-ical and electrical properties to the polymers. Polypropylene [25],polyethylene [26], and PA [27,28] have been evaluated as matrixmaterials with the expectation that the combination of the semi-crystalline material and CNTs should promote the formation ofcrystalline domains, thereby resulting in a higher electrical perco-lation threshold. Among the evaluated polymers, PA 6 has been fre-quently incorporated into nanocomposites with CNTs to achievehigh performance such as good mechanical properties, and thermaland abrasion resistance with low density [29,30].
In the present study, the effects of non-covalent functionaliza-tion of MWNTs on the mechanical, electrical, and morphologicalproperties of PA 6/MWNT nanocomposites are investigated to eval-uate the surface-functionalization of MWNTs with pyrene-contain-ing amide functional groups of various alkyl chain lengths (butyl,octyl, and dodecyl). The specimens for the measurements ofmechanical and electrical properties of the PA 6/MWNT nanocom-posites were prepared by melt-compounding.
2. Experimental
2.1. Materials
MWNT powder (S.A. NC 7000, purity 90%) was purchased fromNanocyl Co., Belgium. PA 6; Cheil Industries Inc., Korea wasselected as a matrix. 1-Pyrenebutyric acid, oxalyl chloride, triethyl-amine (TEA), tetrahydrofuran (THF), and dichloromethane werepurchased from Sigma–Aldrich and used as received. n-Butyl-amine, n-octylamine, and n-dodecylamine were obtained fromSamchun Chemical Co., Ltd., Korea, and used without furtherpurification.
2.2. Syntheses of organic modifiers
2.2.1. Synthesis of amide like-1, 2, and 31-Pyrenebutyric acid (1 g, 3.4 mmol) was dissolved in dichloro-
methane (150 ml), and oxalyl chloride (0.66 g, 5.2 mmol) wasadded to the solution at 0 �C. The reaction mixture was stirred atroom temperature (r.t.) for 3 h. After completion of the reaction,the solvent was evaporated and the product was dried under vac-uum for 12 h to give 1-pyrenebutyric acid chloride (yield: 95%). Toindependent solution of 1-pyrenebutyric acid chloride (1 g,3.2 mmol) in dichloromethane (200 ml) were added triethylamine(1.32 g, 13 mmol), followed by the addition of each 2 eq of then-alkylamine (alkyl = butyl, octyl, and dodecyl) at 0 �C under nitro-gen. The temperature was increased to r.t. and stirring wascontinued for 4 h. The crude product was extracted with dichloro-methane and brine and subsequently purified by column chroma-tography (silica gel, n-hexane:ethyl acetate = 1:1). The targetproducts amide like-1 (alkyl-butyl), amide like-2 (alkyl = octyl),and amide like-3 (alkyl = dodecyl) each as a white solid in yieldsas below) were obtained by evaporating the solvent and dryingunder vacuum.
Amide like-1 (79%)
1H NMR (CDCl3): 0.89 (t, 3H), 1.29–1.43 (m, 4H), 2.17–2.27 (m,4H), 3.22 (q, 2H), 3.38 (t, 2H), 5.34 (brd s, 1H), 7.84 (d, 1H,J = 7.8 Hz), 7.96–8.17 (m, 7H), 8.29 (d, 1H, J = 9.3 Hz).
13C NMR (CDCl3): d (13.75, 20.07, 27.47, 31.72, 32.75, 36.10,39.25, 123.40, 124.76, 124.90, 124.96, 125.85, 125.06, 126.70,127.37, 127.46, 128.77, 129.91, 130.88, 131.39, 135.88, 172.46.
HRMS: calculated 343.1936 C24H25NO, observed 343.1939C24H25NO.
Amide like-2 (83%)
1H NMR (CDCl3): 0.89 (t, 3H), 1.29–1.52 (m, 8H), 2.22–2.31 (m,4H), 3.25 (q, 2H), 3.42 (t, 2H), 5.35 (brd s, 1H), 7.88 (d, 1H,J = 7.8 Hz), 7.99–8.20 (m, 7H), 8.33 (d, 1H, J = 9.3 Hz).
13C NMR (CDCl3): d (14.08, 22.61, 26.90, 27.43, 29.18, 29.23,29.62, 31.76, 32.71, 36.04, 39.53, 123.37, 124.73, 124.87, 124.92,125.02, 125.82, 126.67, 127.32, 127.43, 128.73, 129.87, 130.85,131.35, 135.86, 172.44.
HRMS: calculated 399.2562 C28H33NO, observed 399.2556C28H33NO.
Amide like-3 (78%)
1H NMR (CDCl3): 0.89 (t, 3H), 1.25–1.49 (m, 20H), 2.21–2.28 (m,4H), 3.25 (q, 2H), 3.41 (t, 2H), 5.37 (brd s, 1H), 7.88 (d, 1H,J = 7.8 Hz), 7.98–8.19 (m, 7H), 8.32 (d, 1H, J = 9.3 Hz).
13C NMR (CDCl3): d (22.68, 26.91, 27.43, 29.28, 29.34, 29.53,29.58, 29.63, 30.30, 30.34, 31.90, 32.72, 36.04, 39.53, 123.37,124.73, 124.87, 124.93, 125.03, 125.82, 126.67, 127.33, 127.43,128.74, 129.87, 130.38, 131.36, 135.87, 172.43.
HRMS: calculated 455.3188 C32H41NO, observed 455.3182C32H41NO.
2.2.2. Non-covalent functionalization of MWNTs with amide like-1, 2,and 3
Respective 1 g samples of amide like-1, 2, and 3 were dissolvedin 300 ml of THF, and the MWNTs (1 g) were dispersed in eachsolution. Sonication was carried out for 30 min using a typicalbath-type sonicator. The solution was then filtered and washedwith sufficient amount of THF (3 L) and dried under vacuum for12 h.
2.3. Preparation of PA 6/MWNT nanocomposites by melt-compounding
Samples of the PA 6/MWNT nanocomposites with variousMWNT loadings (1, 2, 3, and 5 phr) were prepared via the melt-compounding method by using a DSM Xplore micro-compounder(twin-screw mixer, X-polo, Netherlands) operated at 250 �C for10 min with a screw speed of 100 rpm. The blended mixtures wereinjected into a test-specimen mold for flexural testing according toASTM D 760. The injection molding temperature and pressure wereset to 260 �C and 15 bar.
2.4. Characterization
1H NMR (nuclear magnetic resonance, Bruker ARX-300 spec-trometer) was used to confirm the chemical structures of thesynthesized pyrene moieties. UV–Vis absorption spectroscopy(UV-2550 spectrophotometer SHIMADZU) was used to identifythe presence of pyrene moieties on the surface of MWNTs afterwashing the functionalized MWNTs with excess THF. To quantifythe surface modifiers non-covalently bonded the MWNTs, thermo-gravimetric analysis (TGA) measurements were performed undernitrogen atmosphere, from r.t. to 800 �C, at a heating rate of10 �C/min, using a model TA3100 Instrument. Raman spectra(HORIBA Jobin Yvon) were acquired to confirm the D/G ratio inorder to identify the formation of defect sites on the MWNTs fromfunctionalization of the surface of MWNTs. Dynamic mechanicalanalysis (DMA, DMA 2980 TA instrument) was performed on asample (50 mm length � 12.7 mm width � 1.6 mm thickness)using a DMA multi-frequency dual cantilever. The tests were
0.2 0.4 0.60.2
0.4
0.6
Ab
s.
Wavelength nm.
Amide like-3 Amide like-3 MWNTs
Fig. 2. UV–Vis absorption spectra of amide like-3 and amide like-3 MWNTs.
J. Ryu, M. Han / Composites Science and Technology 102 (2014) 169–175 171
carried out in three point bending mode, and the temperature wasincreased from r.t. to 220 �C with a heating rate of 10 �C/min at5 Hz. The flexural modulus of the PA 6/MWNTs, and amide like-1, 2, and 3 MWNT nanocomposites was measured by using a uni-versal testing machine (UTM). The specimens for the flexural testwere made according to ASTM D 760 (dimensions: 127 mmlength � 12.7 mm width � 1.6 mm thickness) and loaded inthree-point bending mode with a support span of 50 mm, using aconstant cross-head speed of 1.0 mm/min. The standard four pointprobe was used with KEIYHLEY 195A digital multimeter and pro-grammable current source for sheet resistance measurement.And the sample of PA 6 nanocompsoites (dimensions: 30 mmlength � 30 mm width � 1.0 mm thickness) were prepared bycompression molding after melt compounding. The dispersion ofthe MWNTs in PA 6 was observed using transmission electron-microscopy (TEM, Tecnai G2 20 microscope). The samples ofnanocomposites by melt-compounding were prepared by an ultra-microtoming with 50–100 nm thickness and the cut samples weremounted on a carbon-coated copper grid. X-ray diffraction (XRD,using a Rigaku Ultima IV Diffractometer at 40 kV, 40 mA) analysiswas conducted at 40 �C to investigate the effects of inclusion on theMWNTs on the crystallinity of PA 6.
3. Results and discussion
3.1. Non-covalent functionalization of MWNTs with pyrene moieties
Organic modifiers containing pyrene moieties were synthesizedto functionalize the surface of the MWNTs. The chemical structuresand the preparation of the organic modifiers are shown in Fig. 1.The surface of the MWNTs was functionalized via the non-covalentmethod using a typical bath-type sonicator. Equivalent amounts(w/w) of the organic modifier and MWNTs were added to tetrahy-drofuran and sonicated for 30 min at r.t., filtered, and washed withfresh tetrahydrofuran to remove excess organic modifier.
UV–Vis absorption spectra were acquired to verify functionali-zation of the MWNTs via the non-covalent method; the represen-tative absorption spectra of pristine MWNTs, amide like-3, andamide like-3 MWNTs are presented in Fig. 2. The spectrum of
Fig. 1. The preparation of amide like-1, 2, and 3 and the non-
amide like-3 MWNTs after washing with excess THF shows similarabsorption peaks which are characteristic absorption bands ofamide like-3, indicating the presence of amide like-3 on the surfaceof the MWNTs.
Quantitative analysis of the amount of amide like-1, 2, and 3 onthe surface of the MWNTs was performed using TGA (Fig. 3). Basedon the weight loss at 800 �C observed for the pristine MWNTs (ca.less than 4.2%), the weight loss for amide like-1, 2, and 3 MWNTscould be evaluated to be more than 8.5%, 7.0%, and 6.8%, respec-tively. The main reason for the weight loss of amide like-1, 2, 3MWNTs being greater that of the pristine MWNTs is the thermaldecomposition of the organic component; that is also indicativeof the presence of amide like-1, 2, and 3 on the surface of theMWNTs, which is consistent with the results of UV–Vis absorptionspectroscopy.
Raman spectra were used to determine the relative changes inthe electronic structure of the pristine MWNTs, amide like-1, 2,and 3 MWNTs in Fig. 4. The D/G band ratios from the Raman spec-tra for amide like-1, 2, and 3 MWNTs were ca. 1.190, 1.187, and1.189, respectively, which is similar to the value of 1.186 for the
covalently functionalized amide like-1, 2, and 3 MWNTs.
0 100 200 300 400 500 600 700 800 9000
20
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100
0 100 200 300 400 500 600 700 800 900
90
95
100
Wei
gh
t (%
)
Temperature (oC)
Pristine MWNTs Amide like-1 MWNTs Amide like-2 MWNTs Amide like-3 MWNTs
Fig. 3. TGA thermograms of pristine, amide like-1, 2, and 3 MWNTs.
1200 1300 1400 1500 1600 1700
0
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Co
un
ts
Raman Shift (cm-1)
Pristine MWNTs Amide like-1 MWNTs Amide like-2 MWNTs Amide like-3 MWNTs
Fig. 4. Raman spectra and D/G ratios of pristine, amide like-1, 2, and 3 MWNTs.
0 1 2 3 4 5
60
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110
Fle
xura
l Str
eng
hts
(M
Pa)
Contents (Phr)
PA6/MWNTs PA6/amide like-1 MWNT PA6/amide like-2 MWNT PA6/amide like-3 MWNT
(a)
0 1 2 3 4 51.5
2.0
2.5
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xura
l Mo
du
lus
(GP
a)
Contents (Phr)
PA6/MWNTs PA6/amide like-1 MWNT PA6/amide like-2 MWNT PA6/amide like-3 MWNT
(b)
Fig. 5. Variation of flexural strengths (a) and modulus (b) of PA 6/pristine MWNTand PA 6/amide like-1, 2, and 3 as a function of MWNTs contents.
172 J. Ryu, M. Han / Composites Science and Technology 102 (2014) 169–175
pristine MWNTs, as summarized in Table. This result revealed thatthe non-covalent functionalization of the surface of MWNTs didnot increase in the number of defects into the structure of theMWNTs, resulting in an unchanged D/G ratio.
MWNTs
D/G ratio Pristine MWNTs 1.186 Amide like-1 MWNTs 1.190 Amide like-2 MWNTs 1.18713
Amide like-3 MWNTs 1.18911
1012
10
sq)
PA 6/pristine MWNTs PA 6/amide like-1 MWNTs PA 6/amide like-2 MWNTs
0 1 2 3 4 5
102
103
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108
109
1010
10
Sh
eet
Res
ista
nce
(o
hm
/
Contents (phr)
PA 6/amide like-3 MWNTs
Fig. 6. Sheet resistance of PA 6/MWNT nanocomposites by melt-compounding.
3.2. PA 6/MWNT nanocomposites prepared by melt-compounding
3.2.1. Mechanical properties of PA 6/MWNT nanocompositesThe flexural properties of the PA 6 nanocomposites based on the
pristine and amide like-1, 2, and 3 MWNT nanocomposites areshown in Fig. 5. It was found that the flexural strengths and mod-ulus of PA 6/amide like-1, 2, and 3 MWNT nanocmoposites exhib-ited a much higher than those of the PA 6/pristine MWNTnanocomposites. Significant enhancement of the flexural strengthsand modulus were observed at a MWNT content of 1 phr for theamide like-1, 2, and 3 MWNT nanocomposites as well as animproved G0 value which is attributed to effective dispersion ofMWNTs in PA 6 which we will mention later. The flexural modulus
of the PA 6/pristine MWNT nanocomposites improved by only 35%compared to that of neat PA 6, while the PA 6/amide like-1, 2, and 3MWNT nanocomposites exhibited respective increases of 44%, 49%,and 55% at 1 phr, with continuous enhancement up to the maxi-mum content of 5 phr with a value of 3.76 GPa. Thus, amide like-1, 2, and 3 were successfully used as compatibilizers, facilitatingeffective dispersion of the MWNTs in PA 6, resulting in higher flex-ural strengths and modulus than the pristine MWNTs.
10 15 20 25 30 35 400
500
1000
1500
2000
2500
3000
3500
Inte
nsi
ty (
qo
s)
2-theta (deg)
PA 6 PA 6/pristine MWNTs PA 6/amide like-1 MWNTs PA 6/amide like-2 MWNTs PA 6/amide like-3 MWNTs
Fig. 8. XRD patterns of neat PA 6 and PA 6/pristine, PA 6/amide like-1, 2, and 3MWNT nanocomposites (5 phr).
J. Ryu, M. Han / Composites Science and Technology 102 (2014) 169–175 173
3.2.2. Electrical characterization of PA 6/MWNT nanocompositesThe variation of the sheet resistance of the PA 6 nanocomposites
depending on the functionalization of the MWNTs is shown inFig. 6. Samples with dimensions of 1 � 1 � 0.1 cm3 were preparedusing the compression molding method. The sheet resistance of PA6/pristine and amide like-1, 2, and 3 MWNT nanocompositesdecreased with increasing MWNT contents. The sheet resistanceof PA 6 exceeded over 109 X/h, presenting difficulty to measureat out measurement. So we used the value of PA 6 for sheet resis-tance from other studies [31]. As the MWNT content was varied to2, 3, and 5 phr, the sheet resistance of the PA 6/pristine MWNTnanocomposites smoothly decreased with increasing the contentof MWNTs to 2.17 � 108, 3.76 � 106, and 2.26 � 105 X/h, respec-tively. In comparison with the pristine MWNTs, the sheet resis-tance of PA 6 nanocomposites was dramatically decreased whenthe functionalized MWNTs with amide like-1, 2, and 3 were used.The considerable decrease in the sheet resistance was observedwith the addition of 2 phr of the functionalized MWNTs. And, thesheet resistance for PA 6/amide like-1, 2, and 3 MWNT nanocom-posites with 5 phr content further decreases to 5.3 � 103 X/h,3.99 � 103 X/h, and 3.36 � 103 X/h, respectively. Therefore, wecould observe the improvement of the electrical conductivity ofPA 6 by incorporation of MWNT and could obtain further enhance-ment of electrical conductivity of PA 6/the non-covalent function-alized MWNTs nanocomposites, possibly attributed to the betterdispersion with the PA 6 matrix. The PA 6/amide like-1, 2, and 3MWNT nanocomposites exhibited reduced sheet resistance, and
0 1 2 3 4 5
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rag
e M
od
ulu
s (G
Pa)
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PA 6/pristine MWNTs PA 6/amide like-1 MWNTs PA 6/amide like-2 MWNTs PA 6/amide like-3 MWNTs
(a)
-50 0 50 100 150 200
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Temperature (oC)
PA 6 PA 6/pristine MWNTs PA 6/amide like-1 MWNTs PA 6/amide like-2 MWNTs PA 6/amide like-3 MWNTs
56
6265
6768
(b)
Fig. 7. Storage modulus (G0) depending on contents of MWNTs for PA 6/pristineMWNTs, PA 6/amide like-1, 2, and 3 MWNT nanocomposites (a) and loss tangent(tand) depending on temperature of PA 6/pristine MWNTs, PA 6/amide like-1, 2,and 3 MWNT nanocomposites (b).
Fig. 9. TEM images of PA 6/pristine MWNT (5 phr) (a) and PA 6/amide like-3 MWNTnanocomposites (5 phr) (b) by melt-compounding.
the formation of a MWNT network was initiated at the relativelylower MWNT content of 1 phr compared to the pristine MWNTs.
3.2.3. Rheological measurements of PA 6/MWNT nanocompositesFig. 7 presents the storage modulus (G0) of PA 6/MWNT nano-
composites depending on the different contents of MWNTs. TheG0 of both the PA 6/pristine MWNT and PA 6/amide like-1, 2, and
Fig. 10. SEM images of PA 6/pristine MWNTs (5 phr) (a and b), PA 6/amide like-3 MWNT (5 phr) nanocomposites (c and d) by melt-compounding.
174 J. Ryu, M. Han / Composites Science and Technology 102 (2014) 169–175
3 MWNT nanocomposites increased up to the maximum MWNTcontent of 5 phr. Notably, the G0 values of the PA 6/amide like-1,2, and 3 MWNT nanocomposites increased to a greater extent thanthose of the PA 6/pristine MWNTs. Interestingly, the G0 value of thePA 6/pristine MWNT nanocomposites is to be 1.23 GPa at a MWNTloading of 5 phr, compared to neat PA 6, whereas the G0 value of thePA 6/amide like-1, 2, and 3 MWNT nanocomposites are to be 1.31,1.33, and 1.38 GPa, respectively. The mechanical percolationbehavior of the nanocomposites is presented. Even though no clearpercolation behavior of the nanocomposites was observed for thePA 6/pristine MWNTs, the PA 6/amide like-1, 2, and 3 MWNT nano-composites exhibited a considerable increase of G0 at a MWNTloading of 1 phr, which indicates that the MWNT network wasformed in the PA 6 matrix. And also, the glass transition tempera-ture (Tg) of PA 6 nanocomposites increase according to longer alkylchain lengths as seen in loss tand in Fig. 7(b). The values of Tg
increase from 56 for the neat PA 6 to 62, 65, 67 and, 68 for thePA 6/pristine MWNT nanocomposites and PA 6/amide like-1, 2,and 3 MWNT nanocomposites, respectively. The existence ofMWNTs in the PA 6 matrix hinders the mobility of PA 6 chain,resulting in increased Tg [32]. And, the enhanced dispersion ofamide like-1, 2, and 3 MWNTs in the PA 6 matrix as the alkyl chainlengths resulting in decreasing facilitate more confinement of PA 6chain, leading to higher Tg value of PA 6 nanocomposites.
The recognized crystalline forms of PA 6 are the a and c crystal-line forms, and their formation is affected by the crystallizationconditions and the presence of fillers [32]. Fig. 8 shows the XRDpatterns of the PA 6/pristine and PA 6/amide-like 1, 2, and 3 MWNTnanocomposites at 5 phr. The main peaks of neat PA 6 wereobserved at 2h values of 21.2� and 23�, corresponding to thec-crystalline form and less of the a-crystalline form. The additionof MWNTs to PA 6 resulted in a significant change in the ratio ofthe a and c crystalline forms of PA 6. Furthermore, the crystallinityof the PA 6/MWNT nanocomposites increased with increasingMWNT content, which is attributed to the well-established factthat the MWNTs influenced the crystallization behavior of PA 6by acting as heterogeneous crystallization nuclei for the PA 6
polymer molecules during the melt-blending process [7]. Notably,the longer alkyl chain of the amide like-1, 2, and 3 MWNTs resultedin higher intensity and a shift to higher 2h values of the peaks orig-inally appearing at 23.3�, 23.8�, and 23.4�, indicating the presenceof a larger quantity of the a-crystalline form. It is proposed thatamide like-1, 2, and 3 MWNTs with amide-like alkyl chains influ-enced the formation of the a-crystalline form and dispersion stateof MWNTs in PA 6. To elucidate our issues, the dispersion sated ofMWNTs was investigated by SEM and TEM image.
3.3. Morphology characterization of PA 6/MWNT nanocomposites
Figs. 9 and 10 present the morphology of PA 6/pristine MWNTsand PA 6/amide like 3 MWNTs nanocomposites at 5 phr. It iscleared seen that different stated of MWNTs in PA 6 dependingon the functionalization of MWNTs. Well-dispersed MWNTsobserved in the PA 6/amide like-3 MWNT nanocomposites inFig. 9(b), in contrast to the PA 6/pristine MWNT nanocompositesbeing aggregated in Fig. 9(a). This could be explained that the sim-ilar chemical structures of amide like-1, 2, and 3 with PA 6 play animportant role in compatibility in polymer matrix, leading to bet-ter dispersion of the MWNTs in the PA 6 matrix. We prepared thespecimens with the freeze-fractured surfaces of PA 6/MWNT nano-composites at 5�phr for the SEM analysis. The SEM images of PA 6/MWNT nanocomposites provide more clear evidence on the effectsof the functionalization of MWNTs with amide like-3 on the dis-persion in PA 6 matrix. The pristine MWNTs nanocomposites havea relatively smooth fracture surface with big size of hole inFig. 10(a), represented as the circle, those holes seem to be tornout is attributed to existence of void in PA 6 in Fig. 10(b). Theaggregated MWNTs hinder the permeation of PA 6 molecules andbesiege individual MWNTs. On the other hands, the PA 6/amidelike-3 MWNT nanocomposites have undulating and rough surfaceswith somewhat elongated crack patterns in Fig. 10(c). This elon-gated crack patterns possess higher crack growth resistance ofthe composites [25] and we could consider such formation of thosecrack patterns as to well-dispersed MWNTs in PA 6 in Fig. 10(d).
J. Ryu, M. Han / Composites Science and Technology 102 (2014) 169–175 175
4. Conclusions
Pyrene moieties containing amide functional groups with vari-ous alkyl chain lengths (butyl, octyl, and dodecyl) were synthe-sized and used for non-covalent functionalization of MWNTs.Raman spectroscopy demonstrated that the D/G ratio of theMWNTs remained unchanged upon non-covalent functionaliza-tion. The mechanical and electrical properties of PA 6/pristineand PA 6/amide like-1, 2, and 3 MWNT nanocomposites preparedby the melt-compounding method revealed that the PA 6/amidelike-1, 2, and 3 MWNT nanocomposites had a significantly higherG0 even with the addition of 1�phr of MWNTs. The PA 6/amidelike-1, 2, and 3 MWNT nanocomposites with longer alkyl chainlengths exhibited higher flexural modulus, which is attributed totheir better compatibility with the PA 6 matrix. The PA 6/amidelike-1, 2, and 3 MWNT nanocomposites exhibited reduced sheetresistance, and the formation of a MWNT network was initiatedat the relatively lower MWNT content of 1 phr compared to thepristine MWNTs. TEM analysis of the PA 6/amide like-3 MWNTnanocomposites at 5 phr demonstrated effective dispersion of theMWNTs in the PA 6 matrix.
Acknowledgement
This research was supported by a grant from the FundamentalR&D Program for Technology of World Premier Materials fundedby the Ministry of Science, ICT, and Future Planning, Republic ofKorea.
References
[1] Popov VN. Mater Sci Eng 2004;43:61–102.[2] Lanticse LJ, Tanabe Y, Matsui K, Kaburagi Y, Katsumi S, Hoteida M, et al. Carbon
2006;44:3078–86.
[3] Eder D. Chem Rev 2010;110:1348–85.[4] Ma PC, Kim JK, Tang BZ. Compos Sci Technol 2007;67:2965–75.[5] Zhao W, Liu YT, Feng QP, Xie XM, Wang XH, Ye XY. J Appl Polym Sci
2008;109:3525–32.[6] Andrews R, Weisenberger MC. Curr Opin Solid State Mater Sci 2004;8:31–7.[7] Jose MV, Steinert BW, Thomas V, dean DR, Abdalla MA, Price G, et al. Polymer
2007;48:1096–104.[8] Vijayakumar C, Balan B, Kim MJ, Takeuchi M. J Phys Chem C 2011;115:4533–9.[9] Niyogi S, Hamon MA, Hu H, Zhao B, Bhowmik P, Sen R, et al. Acc Chem Res
2002;35:1105–13.[10] Moniruzzaman M, Karen I. Macromolecules 2006;39:5194–205.[11] Zhao YL, Stoddart JF. J Acc Chem Res 2009;42:1161–71.[12] Usrey ML, Strano MS. J Am Chem Soc 2009;113:12443–53.[13] Rahmat M, Hubert P. Compos Sci Technol 2011;2011(72):72–84.[14] Holzinger M, Abraham J, Whelan P, Graupner R, Ley L, Hennrich F, et al. J Am
Chem Soc 2003;125:8566–80.[15] Tunckol M, Durand J, Serp P. Carbon 2012;50(4):4303–34.[16] Carol L, Niamh G, Andrew IM, Richard OK, Gordon W. Carbon
2009;47:2337–43.[17] Fu JW, Huang XB, Huang YW, Zhang JW, Tang XZ. Chem Commun
2009;1051:1049–51.[18] Sluzarenko N, Heurtefeu B, Maugey M, Zakri C, Poulin P, Lecommandoux S.
Carbon 2006;44:3207–12.[19] Petrov P, Stassin F, Pagnoulle C, Jerome R. Chem Commun 2003:2904–5.[20] Steuerman DW, Star A, Narizzano R, Choi H, Ries RS, Nicolini C, et al. J Phys
Chem B 2002;106ro:3124–30.[21] Star A, Liu Y, Grant K, Ridvan L, Stoddart F, Steuerman DW, Heath JR, et al.
Macromolecules 2003;36:553–60.[22] Ou YY, Huang MH. J Phys Chem B 2006;110(5):2031–6.[23] Nakashima N, Tomonari Y, Murakami H, Yoshinaga K. Chem Lett
2002;31:638–9.[24] Luan J, Zhang A, Zheng Y, Sun L. Compos A Appl Sci Manuf 2012;43:1032–7.[25] Seo MK, Park SJ. Chem Phys Lett 2004;395(4):4–48.[26] McNally T, Potschke P, Halley P, Murphy M, Martin D, Bell SEJ, Quinn JP, et al.
Polymer 2005;46:8222–32.[27] Bose S, Bhattacharyya AR, Kulkarni AR, Potschke P. Compos Sci Technol
2009;69:365–72.[28] Krause B, Potschke P, Haußler L. Compos Sci Technol 2009;69:1505–15.[29] Kodgire PV, Bhattacharyya AR, Bose S, Gupta N, Kulkarni AR, Misra A. Chem
Phys Lett 2006;432:480–5.[30] Zhang L, Wan C, Zhang Y. Compos Sci Technol 2009;69:2212–7.[31] Krause B, Potschke P, Haubler L. Compos Sci Technol 2009;69:1505–15.[32] Gerber I, Oubenali M, Bacsa R, Durand J, Goncalves A, Pereira MFR, et al. Chem
A Eur J 2011;17:11467–77.
