Magnetic alignment of SWCNTs decorated with Fe3O4 to enhance mechanical properties of SC-15 epoxy

We report signiﬁcant improvement in mechanical properties of SC-15 epoxy when reinforced with decorated nanotubes and cured in a modest magnetic ﬁeld. The chemical synthesis and ﬁeld curing process is a low cost and relatively easy technique to impose strong magnetic anisotropy into the system without the need of a supercon-ducting magnet. SWCNT(COOH)s were decorated with Fe 3 O 4 nanoparticles through a sonochemical oxidation process and then dispersed into SC-15 epoxy at 0.5 wt% loading. The admixture was cured for 6 hours in a magnetic ﬁeld of 10 kOe fol-lowed by an additional 24 hours of post curing at room temperature. Control samples were prepared in a similar manner but without the application of the magnetic ﬁeld. Mechanical tests performed on ﬁeld-cured samples indicated that tensile strength and modulus increased by 62% and 40%. Most importantly, modulus of toughness, fracture strain, and modulus of resilience improved by 346%, 165%% and 170%, respectively. Such enhancement in mechanical properties was attributed to changes in polymer morphology, partial alignment of nanotubes in the ﬁeld direction, and sliding at the polymer-nanotube interface. Detailed characterization of the system with XRD, TEM, DMA, and Magnetometry


I. INTRODUCTION
In recent years, carbon nanotube (CNT)/polymer composites have become very popular in engineering applications. 1, 2 They have replaced metals, such as aluminum alloy, due to their lightweight and low cost manufacturing. 3 Polymer matrices can be reinforced by introduction of CNTs [4][5][6] which have unique mechanical, electrical, optical and thermal properties. 2, 7 All these properties of the CNTs are more effective parallel to the tube axis than perpendicular to it. For that reason, alignment of CNTs in the matrix along a particular direction plays a predominant role in the enhancement of physical properties of nanocomposites. 8,9 Several methods of aligning CNTs in the polymers have been considered in the past 8, 10-16 but using a magnetic field is one particularly interesting technique. 9,[17][18][19] However, because of the extremely low magnetic susceptibility of CNTs (∼10 −6 emu/g at room temperature), the dipole moment induced in the magnetic field is not sufficient for rotating CNTs in the viscous matrix. Therefore CNTs need to be somehow magnetically activated. Different methods such as electrodeposition, 20 solution coating, 21 layer-by-layer (LbL) a Electronic mail: hmahfuz@fau.edu 2158-3226/2013/3(4)/042104/11 C Author(s) 2013 3, 042104-1 assembly, 22 ultrasonic irradiation, 23,24 sonochemical techniques 25 and other ultrasonic methods 26 have been investigated to decorate carbon nanofibers with magnetically active elements in the past.
In this work a sonochemical technique 27,28 is used to decorate the outer surface of chemicallyfunctionalized single-wall carbon nanotubes (SWCNT(COOH)s) with Fe 3 O 4 nanoparticles. 29 Due to the Fe 3 O 4 nanoparticles, the magnetic moment of SWCNT(COOH)s can be significantly increased. A modest uniform magnetic field of 10 kOe was used to align the SWCNT(COOH)s along the field direction in SC-15 epoxy. The induced anisotropy in the system that is introduced during the curing process will allow enhancement of structural and mechanical properties in the field direction of the resultant nanocomposites.
In our previous work, 30 we have observed that mechanical properties of SC-15 epoxy can be significantly increased by reinforcing it with nanofillers (Fe 2 O 3 ) and functionalized single-walled carbon nanotubes when cured in a modest magnetic field. Magnetic analysis showed that the iron oxide nanoparticles flocculate to form chains and create a structural anisotropy in the system. Measurements of tensile strength, tensile modulus, and compressive strength in both parallel and perpendicular direction of the magnetic field demonstrated such anisotropy. The idea of reinforcing SC-15 with Fe 2 O 3 particles in conjunction with nanotubes was to use particle alignment to enforce nanotube alignment. Although the resulting nanocomposites showed considerable improvement in mechanical properties, it was argued that since nanotubes are paramagnetic best way to align them would be to attach magnetic nanoparticles such as Fe 3 O 4 on to nanotube surface. This was the premise of the current work.

A. Materials synthesis
Commercially prepared, chemically-functionalized single-wall carbon nanotubes (SWCNT(COOH)s) were used in the project. The nanotubes were obtained from Cheap Tubes Inc. and had a quoted outer diameter of 1-2 nm, a length of 0.5-2 μm, a purity >90% by weight and a functional content of 2.73% by weight. The outer surface of the SWCNT(COOH)s were decorated with ferrimagnetic Fe 3 O 4 nanoparticles using a sonochemical technique. The details of the sonochemical oxidation process are: 500 mg of 99.995% Iron (II) acetate (C 4 H 6 FeO 4 ), 250 mg of cetyl trimethyl ammonium bromide (CTAB, a surfactant) and 50 mg of SWCNT(COOH) were ultrasonically mixed in a graduated glass beaker in 50 mL of double distilled water for 3 hours at 80 • C. The mixture was then centrifuged in distilled water for 30 minutes at 13,000 rpm three times and then again in ethanol at 13,000 rpm for 20 minutes. The resulting product was vacuum dried overnight at standard room conditions.

B. Fabrication of nanocomposites
First, the SWCNT(COOH)/Fe 3 O 4 fillers were mixed for 1 hour at room temperature with part A of the SC-15 epoxy (60-70% diglycidylether of bisphenol A, 10-20% aliphatic diglycidylether, and 10-20% epoxy toughener) using a sonicator. The ultrasonic mixing was carried out in a Sonics Vibra Cell liquid processor with Ti-horn, at 20 KHz frequency, and with an intensity of 100W/cm 2 . Acoustic cavitation is one of the efficient ways to disperse nanoparticles into low viscosity liquids. 31,32 Application of alternating acoustic pressure above the cavitation threshold creates numerous cavities in the liquid. Some of these cavities oscillate at a frequency of the applied field (usually at 20 kHz) while the gas content inside these cavities remain constant. It is not anticipated nor did we observe before 33 any destruction of nanotubes due to sonication.
CNTs easily agglomerate, bundle together and entangle, leading to many defect sites in the composites. 34 That is why ultrasonic mixing was done to uniformly distribute the SWCNT(COOH)s/Fe 3 O 4 in the low-viscosity fluid and prevent agglomeration. 35 This mixture was externally cooled by keeping specimen inside of 6 0 C water during the mixing process to prevent an increase in temperature of the sample. Second, a high-speed mechanical stirrer was used to mix the modified epoxy with hardener (part B) of the epoxy (70-90% cycloaliphatic amine and 10-30% of polyoxylakylamine). The hardener was added at a volume ratio of 3:10 and the mixture was subsequently vacuum-degassed using a desiccator source for 20 min. Following this, the mixture was divided and poured into two plastic molds for initial curing (casting). One of the samples was cured at standard room conditions (samples labeled RC) while the other was placed into the most uniform portion of a 10 kOe magnetic field for 6 hours (samples labeled FC). The magnet had a 2 cm gap and a field variation of 10 kOe to 9.8 kOe, center to edge. Previous research has shown that this is sufficient time for the resin to gel completely. 36 During polymerization, the high-molecular-weight polymer increases the solution viscosity, restricts Brownian motion and sedimentation of nanofillers and stabilizes the nanofiller dispersion against agglomeration. Finally, each of these samples is cured for an additional 24 hours at standard room conditions. A schematic view of the fabrication process is shown in Fig. 1. The figure shows the steps of fabrication of epoxy-based nanocomposites with nanofillers. SWCNT(COOH)s were first decorated with Fe 3 O 4 nanoparticles using a sonochemical technique. Then a solvent casting method was used to add decorated nanotubes into polymer SC-15 epoxy. While curing in the magnetic field, the decorated nanotubes were aligned inside the polymer matrix. Once cured, samples were machined for magnetic, mechanical and thermal characterization. Characterization of field cured samples are done both along the direction of the curing field (labeled FC ) as well as perpendicular to the curing field (labeled FC⊥).

C. Material characterization
X-ray diffraction (XRD) patterns of the samples were collected with a Siemens D5000 x-ray powder diffractometer operating at 45 kV and 40 mA. It uses Ni-filtered, Cu-K α radiation and a diffracted-beam graphite monochromator to provide radiation with a 1.54060 Å wavelength. Images for the SC-15/SWCNT(COOH)/Fe 3 O 4 RC and FC systems were taken by energyfiltered transmission electron microscopy (EF-TEM) using a Zeiss Libra 120 with a beam energy of 120 keV and current of 4 μA. Samples were cut using a microtome with a diamond knife. Samples with thickness in the range of 50-150 nm were placed on Cu grids with 400 and 600 mesh.
Magnetic measurements on the SWCNT(COOH)/Fe 3 O 4 epoxy-based nanocomposite samples were performed by SQUID magnetometry (Quantum Design MPMS-5) at 300 K and applied magnetic fields |H| < 2 kOe. For each sample, measured magnetic moments were normalized with respect to the nominal mass of the Fe 3 O 4 nanoparticles.
Tensile tests were performed on a Deben Tensile Tester with the load limited to 200 N. The test specimen was not standard due to small sample size (18 mm × 5 mm × 1 mm). Sandpaper was used at both ends to prevent slipping and breaking of the sample inside the gripping jaw. Load was applied parallel and perpendicular to the direction of the original curing field and three samples from each category were examined. Force and displacement were recorded and converted to stress and strain, respectively.
The Q800 dynamic mechanical analyzer (DMA) was used to test viscoelastic properties of the samples. Interchangeable clamps with a zero offset guage were used for measurements taken in tensile mode with a span length of 12 mm and an oscillation frequency of 1 Hz. The sample size was 18 mm × 5 mm × 1.5 mm. The oscillation amplitude was kept small (1 μm) to maintain linear viscoelastic response. Experimental data were taken from 20 • C to 120 • C at a constant scanning rate of 3 • C min −1 and a soak time of 5 min. While heating, the material was deformed under the tensile force and the viscoelastic properties were determined from the deformation. implies that the magnetite particles are well crystallized, as confirmed in the TEM images. The peak intensities of the SWCNT(COOH) pattern diminish after coating them with Fe 3 O 4 nanoparticles, since the wt% of the SWCNT(COOH)s in the irradiated sample is smaller than before.

III. RESULTS AND DISCUSSION
TEM images of SWCNT(COOH)/Fe 3 O 4 powder were taken in order to understand the dispersion of Fe 3 O 4 nanoparticles on outer surface of the SWCNT(COOH)s. Fig. 3(a) shows the TEM image of as-received SWCNT(COOH)s. In Fig. 3(b) it is obvious that an overwhelming number of Fe 3 O 4 particles are clinging around the network of SWCNTs. To further demonstrate attachment of particles high resolution TEM images were taken. Fig. 3(c) illustrates that Fe 3 O 4 particles are within 1-2 nm distances of SWCNTs suggesting a close attachment. This micrograph shows non-uniform decoration of the SWCNT(COOH)s with nanoparticles. Coating does not need to be uniform because Fe 3 O 4 is ferrimagnetic and carries an induced moment sufficient for magnetic alignment. Also, the roughness of the Fe 3 O 4 coating contributes to significant interface interaction between the COOH chemical group and SC-15 matrix, enhancing mechanical properties. Fig. 3(c) was also used to estimate the size of Fe 3 O 4 nanoparticles. Particles have roughly spherical shape, approximately 20 nm in diameter and are attached to the surface of the SWCNT(COOH)s. We have found two references 37,38 where it has been reported that It is to be noted that Figure 3(a) and 3(b) are unfiltered TEM images. Both figures show high contrast without energy filtering because there is no epoxy matrix surrounding them. Figures 3(c), 4 and 5 (presented below) are all energy filtered TEM images taken at 0 eV. Elastic image (0 eV) is known to increase contrast by removing inelastically scattered electrons. 39 We find it especially useful in distinguishing the SWCNTs from the epoxy matrix, since both the epoxy and SWCNTs are largely made up of carbon atoms and their contrast differences in unfiltered TEM are rather limited.
An energy-filtered TEM (EF-TEM) image (0 eV) of room cured (RC) nanocomposite with a 0.5 wt.% SWCNT(COOH)/Fe 3 O 4 concentration is illustrated in Fig. 4. Nanotubes are randomly orientated inside the epoxy matrix, shown by arrows in Figure 4. An EF-TEM image (0 eV) of a field to what was seen in an earlier work. 30 The magnetization curve in Fig. 6 illustrates the magnetic anisotropy of the sample with 0.5 wt.% SWCNT(COOH)/Fe 3 O 4 powder in SC-15 epoxy. Three measurement conditions are consideredroom-cured (RC), field-cured in direction perpendicular to the curing field (FC⊥) and field-cured in direction parallel to the curing field (FC ). It is clear from Fig. 6 that the low-field magnetic susceptibility for FC is much higher than that of the RC or FC⊥. This implies that there is an easy magnetic axis along the orientation of field curing.
By decorating the SWCNT(COOH)s with Fe 3 O 4 nanoparticles, we allow for interaction between the applied curing field and the nanotube/nanoparticle fillers. There are two ways that this interaction could lead to structural anisotropy in the composite. First, it is possible that the magnetic anisotropy of the individual fillers could lead to a minimum-energy configuration when aligned with the applied curing field. Uneven coating of the nanotubes by particles makes this an unlikely condition though. More likely, a slight gradient in the field/particle potential energy leads to a force that drags the nanoparticles through the epoxy in the direction of the field, similar in effect to what happens in a field-gradient magnetometer. 29 In either case, this is an effective way to get nanotubes alignment in the epoxy matrix.
Typical stress-strain response for the system with 0.5 wt.% SWCNT(COOH)/Fe 3 O 4 powder in SC-15 epoxy is shown in Fig. 7. As in the magnetic analysis, FC samples were studied with load applied parallel and perpendicular to the curing field. Tensile strength has increased in all samples with respect to the neat epoxy. The maximum enhancements of 62% in tensile strength and 40% in tensile modulus are observed in the case with load applied along the curing field. These results can be explained as carbon nanotubes have higher strength and modulus parallel to the tube axis. 8,9 Once they are oriented in a particular direction, the nanofillers enhance mechanical properties of the epoxy in that particular direction-even more than just regular reinforcement of the epoxy with carbon nanotubes. When nanotubes are aligned in the epoxy, they share load according to their modulii. Since modulus is governed by the rule of mixture at the macroscopic scale, modulus of the reinforced system is increased. Another interesting phenomenon observed in Fig. 7 is that three parameters related to the fracture toughness of the FC⊥ and FC samples have significantly increased. As seen in Table I, improvements in modulus of toughness, failure strain and modulus of resilience are 346%, 165% and 170%, respectively, with FC samples. These enhancements are simply phenomenal and demonstrate the usefulness of employing a magnetic field in the cure process. It is to be noted here that "modulus of toughness" represents the integral of the stress-strain function while "modulus of resilience" is the integral up to the yield point. Both parameters are a measure of absorption of strain energy when the specimens are loaded. One disadvantage with nanotube inclusion in a polymer is that the nanocomposite system becomes more brittle and the system loses fracture strain. 6 Although in Fig. 7 we see a slight increase in fracture strain for RC samples, it is likely due to functionalization and coating of CNTs with Fe 3 O 4 . However, as the samples are cured under a magnetic field, fracture strain increases dramatically, especially in the case of FC . In this case, we observe a simultaneous increase in both strength and fracture strain. We attribute such increases to the phenomena taking place at the nanotube/polymer interface. At the initial stage of load being applied, the interface between the CNTs and epoxy is intact. This allows efficient load transfer and causes the strength and modulus to rise. Both CNTs and epoxy co-continuously deform and share the load. Since CNTs are aligned in the direction of loading, they can carry a substantial part of the load as their modulii is much higher compared to that of epoxy. Once the load reaches near yield stress, we see a prolonged deformation of the specimen, almost without any increase in load. This extended stretching is only possible if polymer ligaments begin to slide over the nanotubes. Earlier studies 40 have shown that polymer chains wrap around nanotubes, form an interface with the bulk and get oriented along the nanotube direction during the cure process. This suggests that oriented polymer chains can deform further than their counterparts when they are entangled without the presence of aligned or partially aligned nanotubes. The interface condition at this stage of the loading is such that it allows sliding of the polymer, giving rise to fracture strain while still allowing load transfer to carry this high load. As the test continues, elongation of the chains reaches its limit and the specimen ruptures. In the case of FC⊥ samples, we also observe a significant increase in fracture strain. This is due to the fact that all nanotubes did not align exactly parallel to the axis of the magnetic field but somewhat skewed as evident in Fig. 5. Comparing the current system with that described in our previous work 30 it is observed that tensile strength and modulus are higher than the previous system. But most importantly, modulus of toughness and fracture strain increased by 346%, 165%, respectively in the current system as opposed to 269% and 27.9% in the previous one. Dynamic Mechanical Analysis (DMA) measurements of tan δ for the system with 0.5 wt.% SWCNT(COOH)/Fe 3 O 4 powder for RC and FC samples are shown in Fig. 8 with respect to neat epoxy. DMA results show that Tg (glass transition temperature) has increased by 10.4% for RC and FC⊥ samples and by 30% for FC samples. Experimental results of this system are tabulated in Table II. Figure 8 and Table II suggests that Tg increases with the inclusion of SWCNTs and the largest shift in Tg is found with FC samples. Changes in Tg is correlated with the chain relaxation behavior of the polymer. 41,42 Specifically, both experimental and theoretical studies have indicated that Tg is influenced by nanoparticle (NP) boundary interactions on the dynamics of polymers within an interfacial layer near the NP surfaces. In particular, polymer chains in direct contact with the NP show a slowing down behavior meaning increased Tg. It is also reported in the literature 43 that tanδ peak originates from the movement of molecular chains in the amorphous region, corresponding to Tg of the nanocomposites. With higher nanoparticle (silica) loading, it was shown that tanδ peak shifted to higher Tg. Authors commented that this indicated good adhesion between modified reinforcement and the surrounding polymer matrix. In our case, although there is no direct evidence of strong interfacial bond, large shift in Tg with FC samples suggests more restrictions in chain movement imposed during field curing in the parallel direction. According to above references, this points to increased interface adhesion. The majority of polymers are locally constrained only by other polymers. However, during cure in a magnetic field while the polymer is still viscous, presence of nanotubes and their movement can impede the motion of polymer molecules. As a result, Tg can increase.

IV. CONCLUSION
In summary, nonmagnetic SWCNT(COOH)s were decorated with magnetic Fe 3 O 4 nanoparticles by using a sonochemical technique. These nanotubes were then aligned in the SC-15 epoxy under a modest magnetic field of 10 kOe. XRD spectra show that SWCNT(COOH)s get decorated with the magnetite without any changes in the SWCNT(COOH)s structure. TEM micrographs demonstrate uniform distribution of the Fe 3 O 4 nanoparticles over the surface of SWCNT(COOH)s. Magnetic analysis reveals that FC samples have a magnetic easy axis in the direction parallel to the curing field. Large enhancements in mechanical properties were observed parallel to the curing field direction. In this orientation, we found 62%, 40% and 165% improvement in tensile strength, modulus and failure strain, respectively. Also, 346% and 170% enhancement in the modulus of toughness and modulus of resilience were recorded, respectively.

ACKNOWLEDGMENTS
This work was partially supported by National Science Foundation (grant HRD 976871). Portions of this research were conducted at the Center for Nanophase Materials Sciences and the SHaRE User Facility, which are sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. The authors would also like to thank Dr. Mahmoud Madani at Florida Atlantic University for his assistance in performing mechanical test and Dr. Andreas Kyriacou at Florida Atlantic University for the assistance in XRD test.