nylon 6,6 grafted multi-walled carbon nanotube composites fabricated by reactive extrusion

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Characteristics of nylon 6,6/nylon 6,6 grafted multi-walled carbon nanotube composites fabricated by reactive extrusion Sang Chul Roh, Jisun Kim, Chang Keun Kim

*

School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Huksuk-dong, Dongjak-gu, Seoul 156-756, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Nylon 6,6 composites containing nylon 6,6 grafted multi-walled carbon nanotubes (nylon

Received 23 January 2013

6,6-g-MWCNT) were fabricated from nylon 6,6 and acyl chloride grafted MWCNT

Accepted 13 April 2013

(MWCNT–COCl) by reactive extrusion. MWCNT–COCl was produced by reacting acid-trea-

Available online 19 April 2013

ted MWCNTs with thionyl chloride. Formation of nylon 6,6-g-MWCNT by reactive extrusion was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and scanning electron microscopy. To quantify the interfacial adhesion energies of nylon 6,6 and pristine and functionalized MWCNTs, the contact angles of cylindrical drop-on-fiber systems were determined using the generalized droplet shape analysis. The interfacial adhesion energy of the nylon 6,6/nylon 6,6-g-MWCNT composite was twice that of the nylon 6,6/pristine MWCNT composite. Nylon 6,6-g-MWCNTs exhibited excellent dispersion in the composite, whereas pristine MWCNTs exhibited poor dispersion when composite films were prepared by solvent casting. The reinforcement level of the composite increased with increasing MWCNT content. Among the composites examined, the nylon 6,6/nylon-g-MWCNT composite with a fixed MWCNT content exhibited the highest level of reinforcement.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNT), which were first reported by Iijima in 1991, have attracted considerable attention in the research and industrial communities due to their excellent mechanical, electrical, and thermal properties [1–7]. CNTs can have moduli and strength levels in the range of 200–1000 GPa and 200–900 MPa, respectively. CNTs with very large aspect ratios are capable of acting as metallic-like electrical and thermal conductors. These properties make CNTs excellent candidates for high strength and electrically conductive polymer composite applications. CNT polymer composites exhibit high stiffness, high strength, and good electrical conductivity at relatively low CNT concentrations due to the properties of the CNTs themselves [7–20].

Three methods are commonly used to produce polymer composites with CNTs: (1) film casting from a polymer solution containing suspended CNTs, (2) polymerization of monomer mixtures containing CNTs, and (3) melt mixing of a polymer with CNTs. Film casting has been used in most fundamental studies exploring the properties of polymer composites containing CNTs. The properties that are typically investigated include dispersion and orientation of CNTs, interfacial bonding, mechanical and dynamic mechanical properties, and electrical conductivity. Film casting has often been used owing to the limited availability of CNTs. On the other hand, melt processing is the preferred method for the commercial fabrication of composites. The tendency of CNTs to form aggregates could be reduced by the appropriate application of shear during melt processing [6,21].

* Corresponding author: Fax: +82 2 8205324. E-mail address: [email protected] (C.K. Kim). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.04.044

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Nylon 6,6 is widely used in many applications and is an important engineering thermoplastic material with outstanding properties such as high toughness, abrasion resistance, low density, and a low frictional coefficient [22,23]. However, its applications are often limited because of its low stiffness and strength. A variety of inorganic materials such as glass fiber, talc, and clay have been successfully used as additives and reinforcements to improve the stiffness and strength of nylon [23,24]. The extent of property enhancement depends on many factors, including the aspect ratio of the filler, the degree of dispersion and orientation in the polymer matrix, and the adhesion at the filler and matrix interface. CNTs are an inherently inert material, and they easily agglomerate and entangle due to their high aspect ratio and strong inter-tube van der Waals interactions [6,25,26]. Furthermore, CNTs do not interact well with organic polymers, and thus cannot achieve good dispersion or adequate adhesion. For CNTs to be effective nanofillers in polymer composites, proper dispersion of CNTs in the polymer matrix as well as good interfacial adhesion between CNTs and the polymer matrix must be achieved. Since the high interfacial adhesion between the polymer matrix and nanofillers guarantees better dispersion of nanofiller in the polymer matrix, the control of interfacial adhesion plays a crucial role in determining the physical performance of the composite. Surface treatments of CNTs are commonly used to enhance interfacial adhesion between CNTs and polymer matrices [10–20]. One of the most widely used methods is the covalent introduction of functional groups on the CNT surface. The incorporation of functionalized nanotubes is achieved by the use of a solvent processing method or by in situ polymerization in order to preserve the nanotube dispersion. To enhance the stiffness and strength of nylon, CNT composites with various nylons were produced [10–20]. Nylon 6,6 composite films containing single wall carbon nanotubes (SWCNTs) functionalized with a short alkyl chain were fabricated by in situ polymerization [10]. The in situ interfacial polymerization of nylon 6,6 in the presence of the functionalized SWCNTs was performed with adipoly chloride in toluene and 1,6-hexamethylene diamine in water. Kang et al. prepared nylon 6,10 composites containing MWCNTs by in situ interfacial polymerization of 1,6-hexamethylene diamine and sebacoyl chloride [14]. The fabrication of a series of nylon 10,10 and MWCNT composites by in situ polymerization of carboxylic acid functionalized MWCNTs and nylon monomer salts has also been reported [13]. The fibers of nylon 6/SWCNT composites were prepared by the in situ ring opening polymerization of caprolactam [27], while nylon 6,6/SWCNT composites were prepared by in situ polymerization of adipic acid–hexamethylene diamine salt [17]. Even though nylon composites containing well-dispersed CNTs can be produced by in situ polymerization techniques, a new, more simple route is still required to commercially produce nylon/CNT composites. In this study, nylon 6,6 composites with nylon 6,6 grafted MWCNTs (nylon 6,6-gMWCNTs) were fabricated by reactive extrusion. The MWCNTs functionalized with acyl chloride (MWCNT–COCl) were melt mixed with commercially available nylon 6,6 using a twin extruder with the expectation that a grafting reaction between the amine end groups of nylon 6,6 and MWCNT–

COCl might take place during melt extrusion. Formation of nylon 6,6-g-MWCNT by reactive extrusion was examined with FT-IR, XPS, TGA, and FE-SEM. The interfacial adhesion energies (or work of adhesion) between nylon 6,6 and MWCNT were quantified using the fiber-on-drop method [28–33] and the mechanical properties of the composites were explored.

2.

Materials and procedure

2.1.

Materials

Commercially available nylon 6,6 was supplied by LG Chemicals (Seoul, Republic of Korea). According to the supplier, the molecular weight as determined by gel permeation chromatography (GPC) using polystyrene standards was Mw = 43,000 and Mn = 22,000 g/mol. The MWCNTs used in this study were supplied by Nano Carbon Technologies Co., Ltd. (Kawasaki, Japan). The nitric acid (HNO3, HPLC grade) and sulfuric acid (H2SO4, HPLC grade) used for the acid treatment of MWCNTs and the formic acid (HCOOH, reagent grade) used as a nylon 6,6 solvent were purchased from Aldrich Chemicals (Milwaukee, WI, USA). Thionyl chloride (SOCl2), used as coupling agent to form acyl chloride on the MWCNT surface, was also purchased from Aldrich Chemicals. Nylon 6,6 was dehydrated under a vacuum at 100 C for 24 h before use.

2.2. Preparation and of characterization nylon 6,6-gMWCNT Nylon 6,6-g-MWCNT was prepared for this work. The details of nylon 6,6-g-MWCNT synthesis and characterization are provided in the next section. The molecular structure of nylon 6,6-g-MWCNT was confirmed by Fourier transform infrared (FT-IR, Magna 750, Nicolet, WI, USA) analyses and X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA2000, UK) using a spectrometer with a Mg Ka X-ray source (1253.6 eV) and a hemispherical analyzer. The IR spectra were collected over 30 scans in the 4000–500 cm1 region using attenuated total reflection (ATR) mode at a resolution of 4 cm1. The XPS spectra were obtained in high-resolution mode with a 20 eV pass energy and a 0.1 eV step size. All binding energies were calibrated to carbon (C1s) at 284.5 eV. For curve fitting, the widths of the Gaussian peaks were kept constant in each spectrum. Scanning electron microscopy (FE-SEM, model: Sigma, Carl Zeiss, Germany) was employed to investigate the morphologies of the pristine MWNTs, nylon 6,6-g-MWCNT, and composites. Thermogravimetric analysis (TGA;, model: TGA-2050, TA Instruments, USA) of the samples was carried out to determine the amount of nylon 6,6 grafted on the MWCNTs. TGA analyses were performed under nitrogen at a heating rate of 10 C/min. The specimens for the TGA experiments were dried in a vacuum oven at 80 C for 1 day.

2.3. Preparation of composites and mechanical property testing Nylon 6,6 composites containing MWCNTs were prepared by melt mixing in a twin extruder (model BA-11, L/D ratio = 40, Bau Technology, Seoul, Republic of Korea). The composites

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were dried in an air-circulating oven at 100 C for one day before use. Specimens for tensile strength tests were prepared by compression molding. The mixture was poured into a mold and placed in a compression molding machine (model 25–12; Carver, Inc., USA) operating at a plate temperature of 280 C and a holding pressure of 12 MPa. The mixture was kept at 280 C for 10 min and was then cooled to room temperature for 2 h by natural convection. After being molded, the specimens were placed in a vacuum oven at 30 C prior to mechanical testing. Specimens for tensile testing were prepared in accordance with American Standards Testing Method (ASTM) specification D412. Tensile tests were performed using a universal testing machine (UTM, model UTM-301, R&B Corp, Daejon, Republic of Korea) at a cross-head speed of 5 mm/min. The tensile property values reported represent the averages of five specimens. The glass transition temperatures (Tg) and the storage moduli of the nylon 6,6 and its composites were measured by dynamic mechanical analysis (DMA, SS6100, Seico Instruments, Japan) at a constant frequency of 1 Hz. The specimens were cooled under liquid nitrogen and then heated from 100 to 200 C at a rate of 5 C/min under nitrogen. Composite films were also prepared by solvent casting from formic acid to examine the effects of functional groups grafted to the MWCNT surface on the dispersion of MWCNTs in the nylon 6,6 matrix. Nylon 6,6 (5 g) was dissolved in formic acid (100 ml), and MWCNTs (0.5 wt.% of nylon 6,6) were dispersed in the nylon 6,6 solution under sonication. Solutions cast onto a glass plate were dried in an oven at 80 C for 24 h until most of the solvent evaporated. The resulting films were finally dried for 24 h in a vacuum oven at 80 C.

2.4. Characterization of interfacial adhesion between nylon 6,6 and MWCNTs The interfacial adhesion energy (or work of adhesion) was characterized by determining the contact angles between MWCNTs and nylon 6,6. Symmetric barrel-type nylon 6,6 droplets formed on the MWCNT fibers, and the generalized drop shape analysis method was used for accurate evaluation of the contact angles. The details of droplet formation on the MWCNT fibers are provided in the next section.

3.

Results and discussion

3.1. Fabrication and characterization of nylon 6,6-gMWCNT The synthetic procedure for nylon 6,6-g-MWCNT is shown in Fig. 1. As shown, carboxylic-acid-terminated MWCNTs were synthesized. The pristine MWCNTs (1.0 g) were functionalized by heating (60 C) in a mixture of concentrated H2SO4 (concentration: 98 vol.%) and HNO3 (concentration: 70 vol.%) (2000 ml, mixing ratio of H2SO4/HNO3 = 3/2 by vol) at reflux. After stirring at 60 C for 24 h, the resulting mixture was diluted with deionized water (1000 ml) and then filtered through a 450 nm nylon membrane. The resulting MWCNTs were washed with deionized water (5 · 200 ml) and dried for 24 h in a vacuum oven at room temperature (hereafter referred to as ‘‘MWCNT–COOH’’).

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Fig. 1 – Synthetic route for the fabrication of nylon 6,6 grafted MWCNT (nylon 6,6-g-MWCNT) by reactive extrusion.

The obtained MWCNT–COOH was dispersed in SOCl2 (500 ml) in a sonicator for 2 h and then stirred for 24 h at 70 C. Next, the suspension was vacuum-filtered through a 450-nm nylon membrane. The resulting MWCNTs were washed with tetrahydrofuran (3 · 200 ml) and ethanol (3 · 200 ml) and dried for 12 h in a vacuum oven at room temperature (hereafter referred to as ‘‘MWCNT–COCl’’). Nylon 6,6-g-MWCNTs were prepared from nylon 6,6 and MWCNT–COCl by reactive extrusion in a twin extruder. The temperatures of the feeding zone, melting zone, mixing zone, and exit die of the twin extruder were 265, 280, 280, and 260 C, respectively. The material feed rate and the extrusion speed were held constant at 10 g/min and 300 rpm, respectively. Melt-mixed composites were immediately quenched in a water bath after extrusion. To obtain nylon 6,6-gMWCNTs and remove the unreacted nylon 6,6, the composite (3 g) was dissolved in formic acid (200 ml) and then nylon6,6g-MWCNTs were collected using a centrifuge. The collected nylon6,6-g-MWCNTs were dispersed in formic acid (200 ml) and nylon6,6-g-MWCNTs were again collected using a centrifuge. This procedure was repeated five times to remove unreacted nylon 6,6. The resulting product was washed with formic acid (300 ml · 3) and dried for 12 h in a vacuum oven at 30 C. The formation of nylon 6,6-g-MWCNT was confirmed by FT-IR, XPS, FE-SEM, and TGA. Fig. 2 shows FT-IR spectra of pristine MWCNTs (Fig. 2a), MWCNT–COOH (Fig. 2b), and nylon 6,6-g-MWCNT (Fig. 2c). As shown in Fig. 2b, the FT-IR spectrum of MWCNT–COOH exhibits stretching peaks at about 3400 and 1716 cm1, corresponding to the hydroxyl groups and carbonyl groups in MWCNT–COOH, respectively. Stretching peaks corresponding to methylene groups (2850–3000 cm1) and to the amide groups (3400 and 1640 cm1) in nylon 6,6 were observed in the FT-IR spectrum of nylon 6,6-g-MWCNT. XPS analyses were also performed to confirm the formation of the MWCNT–COOH, MWCNT–COCl, and nylon 6,6-gMWCNT. Fig. 3 shows XPS wide scans of pristine MWCNT, MWCNT–COOH, MWCNT–COCl, and nylon 6,6-g-MWCNT (Fig. 3a), as well as fits of the C1s bands of pristine MWCNT (Fig. 3b), MWCNT–COOH (Fig. 3c), and nylon 6,6-g-MWCNT (Fig. 3d). The Cl2p peak of MWCNT–COCl (Fig. 3e) and the N1s peak of nylon 6,6-g-MWCNT (Fig. 3f) are also provided. As shown in Fig. 3a, a N1s peak originating from amide groups in nylon 6,6-g-MWCNTwas observed, while it was not observed

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Fig. 2 – FT-IR spectra of (a) pristine MWCNT, (b) MWCNT– COOH and (c) nylon 6,6-g-MWCNT.

in the spectra of pristine MWCNT, MWCNT–COOH, or MWCNT–COCl. It should be noted that a Cl2p peak from the

acyl halide groups was observed in the spectrum of MWCNT– COCl. The atomic ratios of pristine MWCNT, MWCNT–COOH, MWCNT–COCl, and nylon 6,6-g-MWCNT obtained from the data in Fig. 3a are listed in Table 1. The C1s/O1s ratio of pristine MWCNTs (97.8/2.2) was larger than that of MWCNT–COOH (83.3/16.7). This indicates that carboxylic acid groups were formed on MWCNTs by acid treatment. The C1s peaks of pristine MWCNTs, MWCNT–COOH, and nylon 6,6-g-MWCNT can be deconvoluted into component peaks using Gaussian fitting. As shown in Fig. 2b and c, the C1s peak of pristine MWCNT and MWCNT–COOH was deconvoluted into three binding energies at 284.6 (–C@C– or –C–C–), 286.0 (–C–O–), and 287.2 (>C@O), while that of MWCNT–COOH was deconvoluted into four binding energies at 284.6 (–C@C– or –C–C–), 286.0 (–C–O–), 287.2 (>C@O), and 289.0 eV (–COOH). The deconvoluted peak at 289.0 eV in MWCNT–COOH, which was not observed in the spectrum of pristine MWCNTs, originated from carboxylic acid groups on MWCNT–COOH. The C1s peak of nylon 6,6-g-MWCNT (Fig. 3d) could be deconvoluted into three binding energies at 284.6 (–C = C– or –C–C–), 285.7 (–C–NH–), and 287.8 eV (–NH–C@O). The deconvoluted peaks at 285.7 and 287.8 eV in the C1s of nylon 6,6-g-MWCNT, which correspond to –C–NH– and –NH–C@O, respectively, indicated that nylon 6,6 was grafted onto the MWCNTs. The Cl2p peak of the MWCNT–COCl (Fig. 3e) could be curve-fitted

Fig. 3 – XPS scan spectra of MWCNTs (a) wide scan spectra of pristine MWCNT, MWCNT–COOH, MWCNT–COCl, and nylon 6,6g-MWCNT (b) C1s curve fitting for pristine MWCNT; (c) C1s curve fitting for MWCNT–COOH; (d) C1s curve fitting for nylon 6,6g-MWCNT; (e) Cl 2p curve fitting for MWCNT–COCl; (f) N1s curve fitting for nylon 6,6-g-MWCNT.

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Table 1 – Chemical compositions of pristine MWCNTs and surface-treated MWCNTs.

Pristine MWCNT MWCNT–COOH MWCNT–COCl Nylon 6,6-g-MWCNT

C1s

O1s

Cl2p

N1s

97.8 83.3 84.4 78.8

2.2 16.7 12.5 12.9

0 0 3.1 0

0 0 0 8.3

with two component peaks. The bonding energy peaks at 200.3 and 201.9 eV (Fig. 3e), which can be assigned to Cl 2p1/2 and Cl 2p3/2, respectively, indicate the presence of Cl in the form of C–Cl bonds. The N1s peaks of nylon 6,6-g-MWCNT (Fig. 3f) could be curve-fitted with a single component peak; specifically, the binding energy (BE) peak at 400.11 eV was attributed to the amide nitrogen. The changes in the C1s and N1s spectra reflected the formation of nylon 6,6-gMWCNT. Fig. 4 shows FE-SEM images of pristine MWCNTs (Fig. 4a), MWCNT–COOH (Fig. 4b), MWCNT–COCl (Fig. 4c), and nylon 6,6-g-MWCNT (Fig. 4d). Fig. 4a shows pristine MWCNTs with diameters from 40 to 50 nm. The surface of the pristine MWCNTs became rough when carboxylic acid or acyl chloride formed on the MWCNT surface. The diameters of the nylon 6,6-g-MWCNTs (Fig. 4d) were greater than those of pristine MWCNTs, and their surfaces became rough after nylon 6,6 grafting. TGA analysis provided further evidence for the quantity of introduced nylon 6,6 based on the difference between the thermal degradation temperatures of nylon 6,6 and MWCNTs. Fig. 5 shows TGA thermograms of nylon 6,6 pristine MWCNT and nylon 6,6-g-MWCNT. Thermal degrada-

Fig. 4 – FE-SEM photomicrographs of (a) pristine MWCNTs, (b) MWCNT–COOH, (c) MWCNT–COCl and (d) nylon 6,6-gMWCNT.

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tion of nylon 6,6 started at about 360 C, while that of nylon 6,6 grafted onto MWCNTs started at about 330 C. Mass loss was not observed up to 800 C for pristine MWCNTs, whereas a mass loss of about 25 wt.% was observed for nylon 6,6-g-MWCNTs due to thermal degradation of the nylon 6,6 grafted onto the MWCNTs. When the average length of the pristine MWCNTs as received (6.0 lm) is compared with that of the nylon 6,6-g-MWCNTs (4.2 lm), reduction in the average length was approximately 30%. Reduction in the average length of MWCNTs after melt extrusion might come from shear stress forced on the MWCNTs during melt processing.

3.2. Changes in interfacial adhesion energies with surface treatment of MWCNT The interfacial adhesion between MWCNTs and nylon 6,6 was characterized using a quantitative approach for the determination of contact angles [28–33]. Common methods which work for planar solid surfaces may not be able to be applied to MWCNTs because of the cylindrical shape and the small diameter of MWCNTs. Droplet shape analysis is used to determine the contact angle of cylindrically symmetric drop-on-fiber systems [28–33]. It is known that accurate evaluation of contact angles is possible with this method when the effects of gravity are negligible and the droplet dimension on the fiber can be precisely determined [29]. The former condition is usually fulfilled when fiber diameter is smaller than 50 lm [29,33]. To form nylon 6,6 drops on MWCNTs, a dilute solution containing nylon 6,6 (20 mg), MWCNT (200 mg), and formic acid (200 ml) was first prepared, and nylon 6,6 agglomerates were then precipitated on MWCNTs from the formic acid solution using diethyl ether as a nonsolvent. Pristine MWCNT coated with nylon 6,6 agglomerates by rapid precipitation exhibited a coarse textured structure (Fig. 6a). Note that MWCNT–COOH, MWCNT–COCl, and nylon 6,6-g-MWCNT

Fig. 5 – TGA thermograms of pristine MWCNTs, nylon 6,6 and nylon 6,6-g-MWCNT.

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Fig. 6 – FE-SEM photomicrographs of MWCNTs: (a) pristine MWCNTs coated with nylon 6,6 agglomerates by nonsolvent precipitation, (b) nylon 6,6 drops formed on pristine MWCNT after annealing, (c) nylon 6,6 drops formed on MWCNT–COOH after annealing, (d) MWCNT–COCl encapsulated with nylon 6,6 after annealing, (e) nylon 6,6-g-MWCNT encapsulated with nylon 6,6 after annealing. Note that MWCNT–COOH, MWCNT–COCl and nylon 6,6-g-MWCNT coated with nylon 6,6 agglomerates by nonsolvent precipitation exhibited similar morphology to Fig. 6a.

coated with nylon 6,6 agglomerates by nonsolvent precipitation exhibited a similar morphology with rapid precipitation. The nylon 6,6 coated MWNCTs were then annealed 15 C above the melting temperature of nylon 6,6 (Tm = 265 C) for 1 h to produce an equilibrium interfacial geometry. Fig. 6 shows FE-SEM microphotographs of nylon 6,6 drops on pristine MWCNTs (Fig. 6b), MWCNT–COOH (Fig. 6c), MWCNT– COCl (Fig. 6d), and nylon 6,6-g-MWCNT (Fig. 6e) after annealing. Axisymmetric nylon 6,6 nanodrops enveloping the MWCNTs were formed on pristine MWCNTs and MWCNT–COOH. The interfacial adhesion energy (or work of adhesion) between a solid and liquid phase is given by Young’s equation [34] Wa ¼ cs  cSL þ cL ¼ cL ð1 þ cos hÞ

ð1Þ

where c represents the surface tension of the solid (cS), the solid–liquid interface (cSL), and the liquid (cL), and h is the contact angle between the solid and the liquid phase. Since the surface free energy of nylon 6,6 (cL = 47.0 mJ/m2) is known [35], the contact angle between nylon 6,6 and MWCNT can be used to obtain the work of adhesion. The contact angles between nylon 6,6 and pristine MWCNTs (and MWCNT– COOH) were determined from the geometries of the droplets shown in Figs. 6b and 5c using a generalized drop length– height method [29]. The contact angles of nylon 6,6 on pristine MWCNT and MWCNT–COOH were determined to be 36 and 10, respectively. The works of adhesion of nylon 6,6 with pristine MWCNT and MWCNT–COOH calculated from Equation (1) were 85.0 and 93.3 mJ/m2, respectively. The higher interfacial adhesion of nylon 6,6/MWCNT–COOH relative to nylon 6,6/pristine MWCNT might originate from hydrogen

Fig. 7 – Photographs of nylon 6,6/MWCNT composites prepared by solvent casting: (a) nylon 6,6/pristine MWCNT composite, (b) nylon 6,6/MWCNT–COOH composite, (c) nylon 6,6/MWCNT–COCl composite, (d) nylon 6,6/nylon 6,6-gMWCNT composite. Note that composites contained 0.5 wt.% MWCNTs.

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Fig. 8 – Changes in the (a) tensile modulus and (b) yield strength as a function of MWCNT content in the composite.

Fig. 9 – The storage moduli of nylon 6,6, and nylon 6,6 composites containing 0.5 wt.% of MWCNTs observed with DMA.

that the reactions between MWCNT–COCl and nylon 6,6 agglomerates occurred during annealing, and as a result, nylon 6,6-g MWCNTs were formed. In summary, the interfacial adhesion between nylon 6,6 and MWCNT was enhanced in the order of nylon 6,6/pristine MWCNT < nylon 6,6/MWCNT– COOH < nylon 6,6/nylon 6,6-g-MWCNT. To explore the changes in MWCNT dispersion in the nylon 6.6 matrix with surface treatment, composite films containing nylon 6,6 and various MWCNTs were prepared by solvent casting. As shown in Fig. 7, the dispersity of MWCNTs was enhanced in the order of nylon 6,6/pristine MWCNT < nylon 6,6/ MWCNT–COCl < nylon 6,6/MWCNT–COOH < nylon 6,6/nylon 6,6-g-MWCNT. Note that reactions between the amine end groups of nylon 6,6 and acyl chloride of MWCNT–COCl could not occur without annealing at 15 C higher than the melting temperature of nylon 6,6. The better dispersity of MWCNT– COOH in the nylon 6,6 matrix than pristine MWCNT (or MWCNT–COCl) stemmed from hydrogen bonding between carboxylic acid and amide groups in nylon 6,6. Nylon 6,6/nylon 6,6-g-MWCNT composites, which had the highest interfacial adhesion among the composites, exhibited the best dispersion of MWCNTs in the nylon 6,6 matrix.

3.3. bonding between carboxylic acids and amide groups in nylon 6,6. When MWCNT–COCl and nylon 6,6-g-MWCNT coated with nylon 6,6 agglomerates were annealed above the melting temperature of nylon 6,6, MWCNTs were encapsulated with nylon 6,6, as shown in Fig. 5d and e. This indicates that the contact angle between nylon 6,6 and nylon 6,6-g-MWNCT is 0, and thus the work of adhesion between them is the same as the cohesion energy of nylon 6,6 (94 mJ/m2) [35]. It is worth noting that the contact angle between nylon 6,6 and MWCNT–COCl observed after annealing was also 0o. This result indicates

Characteristics of nylon 6,6/MWCNT composites

Fig. 8 shows the effects of MWCNT content and the surface treatment of the MWCNTs on composite modulus (Fig. 8a) and yield strength (Fig. 8b). The tensile modulus and yield strength of the composite increased with increasing MWCNT content over the MWCNT content range examined. When composites contain the same amount of MWCNTs, the tensile moduli and yield strengths of the composites increase in the order of nylon 6,6/pristine MWCNT < nylon 6,6/MWCNT– COOH < nylon 6,6/nylon 6,6-g-MWCNT. Note that, for composites containing 2 wt.% pristine MWCNTs, specimens failed

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Fig. 10 – Cross-sectional photomicrographs of nylon 6,6 composites containing 2 wt.% of (a) pristine MWCNT, (b) MWCNT– COOH and (c) nylon 6,6-g-MWCNT.

before yield. The storage moduli (E 0 ) of the nylon 6,6 and nylon 6,6/MWCNT composites obtained from DMA as a function of temperature are shown in Fig. 9. The storage modulus of the composite is always higher than that of nylon 6,6 both below and above the glass transition temperature. The E 0 of the composite also increases in the same order as that observed for the tensile modulus. The fractured cross-sectional morphology of the composites, including the dispersion of the MWCNTs and the interface state between the MWCNTs and the nylon 6,6 matrix, was observed with FE-SEM. As shown in Fig. 10, even though MWCNTs are uniformly dispersed in the nylon 6,6 matrix regardless of their surface modification, there exists a significant difference in the detailed nanostructure. In composites containing pristine MWCNTs (Fig. 10a), fractures occurred primarily at the interface between the nylon 6,6 matrix and the pristine MWCNT due to poor interfacial adhesion between nylon 6,6 and the pristine MWCNTs. However, when the interfacial adhesion between the nylon 6,6 matrix and the MWCNTs was strong due to surface treatment of the MWCNTs, interfacial debonding between the nylon 6,6 matrix and the surface-treated MWCNTs was not observed (see Fig. 10b and c). High-magnification SEM photomicrographs also showed that the interfacial adhesion of the composites increased in the order 6,6/pristine MWCNT < nylon 6,6/ MWCNT–COOH < nylon 6,6/nylon 6,6-g-MWCNT. The high level of reinforcement observed in the nylon 6,6g-MWCNT composites compared to the other composites examined is likely caused by the high interfacial adhesion, as indicated by the work of adhesion data, and chain entanglement. Nonpolar, pristine MWCNTs do not have specific interactions with the nylon 6,6 matrix, while MWCNT–COOH hydrogen bonds with the amide groups in the nylon 6,6

matrix. Because of this, nylon 6,6/MWCNT–COOH composites have better interfacial adhesion than nylon 6,6/pristine MWCNT composites. In nylon 6,6/nylon 6,6-g-MWCNT composites, both hydrogen bonding and chain entanglement between the nylon 6,6 matrix and the nylon 6,6 grafted on the MWCNTs lead to the best interfacial adhesion and the highest level of reinforcement among the composites examined.

4.

Conclusion

Nylon 6,6 composites containing nylon 6,6-g-MWCNT were fabricated by reactive extrusion. Nylon 6,6 and MWCNT–COCl were melt mixed in a twin extruder to form nylon 6,6/nylon6,6-g-MWCNT composites by reacting COCl grafted on MWCNTs with the amine end-groups of nylon 6,6. The formation of nylon 6,6-g-MWCNTwas confirmed by FT-IR, XPS, TGA, and FE-SEM. Changes in the interfacial adhesion energies with the organic moieties grafted on MWCNTs were quantified using the generalized droplet shape analysis method. The interfacial adhesion energy increased in the order of nylon 6,6/pristine MWCNT < nylon6,6/MWCNT–COOH < nylon6,6/nylon6,6-g-MWCNT. The interfacial adhesion energy of the nylon 6,6/nylon 6,6-g-MWCNT composite was two times larger than that of nylon 6,6/pristine MWCNT composites. The highest level of reinforcement was also observed in the nylon 6,6-g-MWCNT composite when MWCNT content in the composite was fixed. The high interfacial adhesion and the high level of reinforcement of nylon 6,6/nylon 6,6-g-MWCNT compared to the other composites examined originated from hydrogen bonding and chain entanglement between the nylon 6,6 matrix and the nylon 6,6 grafted on the MWCNTs.

CARBON

6 0 (2 0 1 3) 3 1 7–32 5

Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

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