noncovalently functionalized carbon fiber by grafted self...
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Accepted Manuscript
Noncovalently functionalized carbon fiber by grafted self-assembled graphene
oxide and the synergistic effect on polymeric shape memory nanocomposites
Haibao Lu, Yongtao Yao, Wei Min Huang, David Hui
PII: S1359-8368(14)00302-3
DOI: http://dx.doi.org/10.1016/j.compositesb.2014.07.022
Reference: JCOMB 3108
To appear in: Composites: Part B
Received Date: 26 May 2014
Revised Date: 11 July 2014
Accepted Date: 20 July 2014
Please cite this article as: Lu, H., Yao, Y., Huang, W.M., Hui, D., Noncovalently functionalized carbon fiber by
grafted self-assembled graphene oxide and the synergistic effect on polymeric shape memory nanocomposites,
Composites: Part B (2014), doi: http://dx.doi.org/10.1016/j.compositesb.2014.07.022
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Noncovalently functionalized carbon fiber by grafted self-assembled graphene oxide and
the synergistic effect on polymeric shape memory nanocomposites
Haibao Lua,*, Yongtao Yaoa, Wei Min Huangb and David Huic
aScience and Technology on Advanced Composites in Special Environments Laboratory,
Harbin Institute of Technology, Harbin 150080, China
bSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798
cComposite Material Research Laboratory, Department of Mechanical Engineering,
University of New Orleans, LA 70148 USA.
*Corresponding author. E-mail address: [email protected] (H. Lu)
ABSTRACT: This paper presents an effective approach to significantly improve the
electrical properties and recovery performance of shape memory polymer (SMP)
nanocomposites that are able for Joule heating triggered shape recovery. Reduced graphene
oxide (GO) is self-assembled and grafted onto the carbon fibers to enhance the interfacial
bonding with the SMP matrix via van der Waals and covalent crosslink, respectively.
Experimental results verify that the electrical properties of SMP nanocomposites are
significantly improved via a synergistic effect of GO and carbon fiber. The morphology and
porous structure of GO on the carbon fiber are characterized by electron microscope and
optical microscopes, respectively. Furthermore, the behavior of electro-activated recovery and
the resultant temperature distribution within SMP nanocomposite are monitored and
characterized. We demonstrate that this simple way is able to produce electro-activated SMP
nanocomposites which are applicable for Joule heating at a lower electrical voltage.
Keywords: A: Polymer-matrix composites (PMCs), B: Electrical properties, B.
Interface/interphase, E. Assembly
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1. Introduction
Shape memory polymers (SMPs) are fascinating due to the feature of the shape memory
effect (SME) [1,2]. Most likely this feature was firstly termed in 1932 after such an effect was
discovered in an AuCd alloy [3]. After years of intensive research and development thereafter,
recently SMPs have started to attract great attention as a promising candidate for many
engineering applications in biomedicine, automobile, aerospace and textiles, etc, due to their
high elastic strain, tailorable transition temperature, ease in manufacturing and variable in
mechanical and physical properties within a wide range [4-9]. Consequently, they are now
considered as an important part in smart materials and structures [10,11]. Remarkable
progress in synthesis, characterization/analysis, modeling/simulation and structural design has
been made in a knowledge based approach [12-17]. While the SME in SMPs has been
achieved by applying a few different stimuli, via such as photoactive effect, magnetic-active
effect, water (or solvent)-active effect and electroactive effect, utilization of Joule heating to
drive the SME is desirable and convenient to apply in some practices, especially when direct
heating is not easily achievable. A variety of conductive fillers from zero dimensional
particles, one dimensional chains to two dimensional films, such as carbon nanotubes (CNTs)
[18,19], carbon nanofibers (CNFs) [20,21], carbon black [22], magnetic particles [23], short
carbon fibers [24,25], CNT alignment [26,27], CNF mat [28], nanopaper [29,30] etc have
been used as conductive fillers not only to enhance the electrical conductivity for Joule
heating, but also to improve the thermal conductivity [31-33].
Graphene, a one-atom-thick sheet of sp2-bonded carbon, has attracted enormous interest
because it has superior physical properties: an electrical conductivity of 106 S/cm, a thermal
conductivity of 5000 W/m·K, and a large specific surface area, and a high aspect ratio [34,35].
Bulk quantities of graphene can be produced effectively by the thermal exfoliation of graphite
oxide of which the sheets are reduced and exfoliated simultaneously upon rapid heating [36].
In order to turn graphite oxide into graphene oxide (GO), the most common techniques are
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sonication, stirring, or a combination of both. Thermally-exfoliated graphene is normally
comprised of few-layer graphene sheets with specific surface areas approximate to 1500 m2/g
[37-39]. Besides oxygen epoxide groups (bridging oxygen atoms), other functional groups
experimentally found are: carbonyl (=CO) and hydroxyl (-OH) prepared using sulphuric acid
(e.g. Hummers method) [40,41]. It is expected that GO should be able to fulfil its roles as a
fixed structure as well as a filler for reinforcement in epoxy-based polymer due to the
potential covalent bonds. Larger specific surface area provides a stronger connection between
CF and polymer. On the other hand, the uniform distribution of GO on CF surfaces enables
good dispersion of GO in the polymer around CF. Even at low volume fractions, the vast
interfacial area created by well-dispersed GO could affect the behavior of the surrounding
polymer matrix and create a co-continuous network to fundamentally change the physical
properties of the polymer matrix [42-44]. In this study, a synergistic effect of GO and carbon
fiber is introduced. The GO is self-assembled and grafted onto the carbon fibers to enhance
the reliability in bonding between carbon fiber and SMP matrix via van der Waals and
covalent crosslink, respectively. A combination of GO and carbon fibers is consequently
utilized to significantly improve the electrical properties to achieve actuation of SMP
composites at a low electrical voltage. A schematic illustration of the working mechanism of
the self-assembled GO grafted onto carbon fiber to improve the interfacial bonding with
epoxy-based SMP is shown in Figure 1.
2. Experimental details
2.1 Raw materials
Commercially available carbon-fibers (12K, 1.80 g/cm3), with an average diameter of 7 μm,
were used as reinforcing fillers in this study. The matrix used in this study is an epoxy-based
fully formable thermoset SMP resin, which was composed of a mixture of epoxy resin and
curing agent. After mixing and curing, this polymer has the thermo-responsive SME, in
addition to high toughness and strength. Its glass transition temperature is 110oC. Natural
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graphite flakes were received with an average diameter of 10μm. Sulfuric acid and
hydrochloric acid were supplied from Tianjin Chemical Factory, China. KMnO4 was obtained
from Sinopharm Chemical reagent Co., Ltd. (Shanghai, China).
2.2 Preparation of Graphene Oxide
GO was prepared and synthesized following a modified Hummers’ method. The raw
graphite, NaNO3, and sulfuric acid were stirred together for 15 min below 5oC. A certain
amount of KMnO4 was slowly added into the mixture while stirring maintained to prevent the
mixture temperature exceeding 20oC. And then the mixture was transferred into a 25oC water
bath and stirred for 30 min to form a thick paste. Consequently, 200 mL deionized water was
added into the mixture accompanied with strong mechanical stirring. Subsequently, the
mixture was further treated in a 700 mL of deionized water and 60 mL of 30% H2O2 solution.
The solution was then filtered and subjected to cyclic washing with deionized water. Finally,
the resulting graphite oxide was dispersed in deionized water and sonicated for 1 h to obtain
graphene oxide. Finally, the mixture was centrifuged at 7000 rpm for 15 min to get rid of the
unexfoliated graphite oxide.
2.3 Fabrication of SMP nanocomposite
GO has been regarded as a hydrophilic material since it was discovered because of its
excellent water solubility. Distilled water was used as the solvent. As prepared GO of 0.02,
0.03, 0.04, 0.05 and 0.06 g was mixed with 60 mL of distilled water to form suspension. GO
suspension was then sonicated for 30 min. After that, the suspension was filtrated under a
high pressure to enabled GO for self-assemble and subsequently grafted onto the virgin
carbon fibers with the aid of a hydrophilic polycarbonate filter membrane. The suspension
and 60 mL of distilled water were then filtered through the filter. In the next step, the carbon
fibers grafted with GO were dried in an oven at 120 oC for 2 h to further remove the
remaining water. Finally, six pieces of nanopaper with different weight ratio of GO: carbon
fiber (namely, 0: 0.14, 0.02: 0.14, 0.03: 0.14, 0.04: 0.14, 0.05: 0.14 and 0.06: 0.14) were
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obtained to prepare nanocomposite samples. These six pieces of mat were individually placed
on the bottom of a metallic mold. Resin transfer molding technique was applied to make SMP
nanocomposite. The relative pressure of the resin transfer molding was kept at a constant level
of 6 bar. After the mold was filled, the mixture was cured at a ramp of approximately 1oC/min
from room temperature (22oC) to 100oC and kept for 5 h before being ramped to 120oC at
20oC per 180 min. Finally, it was ramped to 150oC at 30oC per 120 min to produce the final
SMP nanocomposites.
3. Results and discussion
3.1 Morphology of CNT assemblies onto carbon fibers
The surface morphologies and structure of GO functionalized carbon fibers were observed
by field emission scanning electron microscopy (FESEM) at two scales of 2 µm and 10 µm,
respectively, as shown in Figure 2. Figures 2(a), (b) and (c) reveal the surface morphology of
self-assembled GO in which many wide and deep grooves are visible along the axial direction
of carbon fiber. It is shown that different sized GO sheets are attached to the surface and stick
to the carbon fibers along the outer edges of the grooves with different angles due to the high
specific surface area of GO, forming a new hierarchical structure. The GO was grafted onto
and uniformly deposited on the carbon fiber via the membrane filtration process. With an
increase in the weight fraction of GO, GO sheets homogeneously cover the entire surface of
carbon fibers (Figures 2(b) and (c)). Both images reveal that GO sheets can be easily
introduced into the interfacial regions of carbon fiber.
On the other hand, optical microscope observation of self-assembled GO sheets was
performed on using a three-dimensional optical-resolution photoacoustic microscopy (VHX-
900) and Olympus stereomicroscope (OLS3100) equipped with a CCD camera. The
maximum magnifications of the ocular and objective are 10× and 100×, respectively, and thus
the overall magnification is 1000×. Optical microscope provides an effective approach to
characterize and provide three-dimensional visualization of the internal morphology and
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structure of GO sheets. The GO assembly structure has pores with an average size of around
50 µm, as shown in Figures 3(a) and (b). Therefore the polymer resin is able to penetrate into
this porous structure to closely attach to the GO. It is expected and has been verified that the
GO can significantly improve the interfacial shear strength and interlaminar shear strength of
carbon fiber reinforced polymer composite [45]. A stereomicroscopic three-dimension
observation was conducted on the self-assembled GO to further confirm the internal
morphology and structure. In the testing, the thickness readings were taken from the back of
the mat to determine the thickness of the GO grafted onto carbon fiber mat. The thickness
distribution of the GO is shown in Figure 3(c). The three-dimensional graph reveals the
thickness distribution of the GO on the carbon fiber mat within an area of 480 μm × 640 μm.
It is concluded that the thickness is in a range of 0 to 178.3 μm due to the random layered
structure. At the same time, it is shown that the self-assembled GO is uniformly dispersed on
the carbon fiber.
3.2 Raman spectra characterization
Raman spectroscopy (Horiba HR 800 spectrometer) was used to characterize disorder and
defects in GO. 632 nm radiation from a 20 mW air-cooled Argon ion laser was used as the
exciting source. The graphene and GO could manifest themselves in Raman spectra by means
of changing in relative intensity of two main peaks: D and G bands. Figure 4 shows the
Raman spectra of graphene and GO. The D peak of graphene located at 1348 cm-1 and at 1346
cm-1 for GO streams from a defect-induced breathing mode of sp2 rings. Here, Raman
spectroscopy is shown to provide a powerful tool to differentiate sp2 carbon nanostructures of
graphene and GO which have many properties in common and others that differ. The intensity
of D band is related to the size of the in-plane sp2 domains [46,47]. The decrease of the D
peak intensity indicates the formation of less sp2 domains. The G bonding of C=C shifts from
1591 cm−1 to 1593 cm−1. All sp2 carbon lattice arises from the stretching of C-C bond. The G
peaks are at approximate 1593 cm−1 for GO and at 1591 cm−1 for graphene. The relative
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intensity ratio of both peaks (ID/IG) is a measure of the degree of disorder and is inversely
proportional to the average size of the sp2 clusters [48]. The D/G intensity ratio for GO is of
0.95 that is larger than that of the graphene of 0.85. This suggest that more graphitic domains
are formed and the sp2 cluster number increases.
3.3 Electrical resistivity measurement
The electrical resistivity of the resultant composites was determined based on the measured
data using a van der Pauw four-point probe apparatus. The apparatus has four probes with an
adjustable inter-probe spacing. A constant current passed through two outer probes and an
output voltage is measured across the inner probes with a voltmeter. In order to reduce
experimental errors, a symmetrical tested sample is preferable. The specific resistivity and
Hall effect of an electrically conductive material is measured by cutting a sample into a
symmetrical shape [49]. Current contact points A and B and voltage contacts points C and D
as shown in Figure 5(a). The electrical resistivity of the carbon fiber mats grafted with
different GO contents (in weight) plotted in Figure 5(b). Figure 5(c) reveals the discrepancy
of measured results. It was found that the average value of electrical resistivity decreases from
1.46, 1.28, 1.23 to 1.16 Ω·cm, as the weight content of GO increases from 0.03, 0.04, 0.05 to
0.06 g, while the weight of the carbon fiber is kept at a constant of 0.14 g. And the
experimental error in the testing is limited to 4%. While the electrical resistivity of 0.05g
reduced graphene oxide being reduced as measurement times increase is resulted from the
more nonuniform dispersion of GOs. The electrical resistivity of tested samples is improved
and lowered with an increase in the weight concentration of GO. It is expected that the more
GO being assembled and invloved, both the amplitude of electric current and current carrying
capability increase, resulted by a lower electrical resistivity. And it should be noted that the
improved electrical properties are resulted by the synergitc effect of GO and carbon fiber.
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3.4 Electrically triggered shape recovery and temperature distribution
As experimentally demonstrated above, the electrical property of SMP nanocomposite is
improved. Therefore, it is expected that these nanocomposites could show the electrically
induced SME. The tested SMP nanocomposite had a permanent flat shape (60×6×1.5 mm3).
After the nanocomposite was heated above 120oC, it was deformed into a “U” shape upon
application of an external force. After cooling back to room temperature, the freestanding
nanocomposite largely maintained the deformed shape. The synergistic effect of carbon fiber
(0.14 g in weight) and self-assembled GO (0.03 g in weight) on the electrical actuation was
investigated on SMP nanocomposites (1.2 g in weight). A constant 5.5 V DC voltage was
applied to the SMP nanocomposite. While the applied electric current was 0.45 A. The
electro-responsive shape recovery was recorded by a video camera. Snapshots of the shape
recovery sequence are shown in Figure 6. Experimental results reveal that it took 80 s to
complete the shape recovery. There was not much recovery during the first 20 s, but
remarkable recovery was recorded at 60 s and virtually no more further recovery after that till
80 s. Finally, the SMP nanocomposite regained its permanent shape and the recovery ratio is
close to 95%. Simultaneously, an infrared video camera was also used to record and monitor
the shape recovery behavior and temperature distribution in the recovery process. Snap-shot
of the tested SMP nanocomposite is presented in Figure 7. As we can see, higher temperature
is observed where internal strain is higher during the shape recovery process, which is
attributed to the higher local resistive heating loss. When an electrical current is passed
through Joule heating effect results in temperature increased, and thus the shape recovery of
SMP nanocomposite.
4. Conclusion
A series of experiments were conducted to study the synergistic effect of carbon fiber and
GO on SMP nanocomposites, of which the actuation was triggered by means of Joule heating.
The GO was grafted onto the carbon fiber to significantly enhance the reliability in bonding,
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and further helped to transfer the resistive Joule heating between the carbon fiber and the
SMP. As demonstrated, the electrically driven shape recovery was carried out by means of
applying an electric voltage of 10 V and a power of 2.5 W. Furthermore, temperature
distribution of the SMP nancomposite was monitored during the recovery process to present
the synergistic effect of carbon fiber and rGO on the shape recovery induced by Joule heating.
We demonstrated a simple way to produce electro-activated SMP nanocomposites by
application of deposition of rGO onto carbon fibers, and thus Joule heating was possible at a
lower electrical voltage.
Notes
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC)
(Grant No. 51103032) and Fundamental Research Funds for the Central Universities (Grant
No. HIT.BRETIV.201304).
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Figure captions
Fig. 1. (a) Schematic illustration of the graphite stack is oxidized to separate the individual
layers of GO. (b) The role of GO in the interfacial bonding between carbon fiber and epoxy-
based SMP matrix via Van der Waals bonding and covalent crosslink, respectively.
Fig. 2. The typical surface profile of the GO grafted onto the carbon fibers at a scale of (a) 2
µm, (b) 10 µm and (c) 20 µm, respectively.
Fig. 3. (a) and (b) internal morphology and structure of self-assembled GO on the carbon fiber
mat revealed by optical microscopies. (c) 3D graph of thickness distribution of GO.
Fig. 4. Raman spectra of graphene and GO were recorded with 632 nm laser and at a laser
power of 20 mW.
Fig. 5. Electrical resistivity of carbon fiber mat with different weights of GO. (a) Schematic
measurement mode. (b) Electrical resistivity as a function of masurement time for the carbon
fiber mat with 0.03 g, 0.04 g, 0.05 g and 0.06 g of GO. (c) Deviation of measured results.
Fig. 6. Joule heating-induced shape recovery of SMP nanocomposite incorporated with 0.14 g
of carbon fiber and 0.03 g of GO.
Fig. 7. Snapshot of Joule heating-induced shape recovery in SMP nanocomposite recorded by
infrared video camera (VarioCAM HiRessl, JENOPTIK Infra Tec.).
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Fig. 1. (a) Schematic illustration of the graphite stack is oxidized to separate the individual
layers of GO. (b) The role of GO in the interfacial bonding between carbon fiber and epoxy-
based SMP matrix via Van der Waals bonding and covalent crosslink, respectively.
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Fig. 2. The typical surface profile of the GO grafted onto the carbon fibers at a scale of (a) 2
µm, (b) 10 µm and (c) 20 µm, respectively.
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Fig. 3. (a) and (b) internal morphology and structure of self-assembled GO on the carbon fiber
mat revealed by optical microscopies. (c) 3D graph of thickness distribution of GO.
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Fig. 4. Raman spectra of graphene and GO were recorded with 632 nm laser and at a laser
power of 20 mW.
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Fig. 5. Electrical resistivity of carbon fiber mat with different weights of GO. (a) Schematic
measurement mode. (b) Electrical resistivity as a function of masurement time for the carbon
fiber mat with 0.03 g, 0.04 g, 0.05 g and 0.06 g of GO. (c) Deviation of measured results.
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Fig. 6. Joule heating-induced shape recovery of SMP nanocomposite incorporated with 0.14 g
of carbon fiber and 0.03 g of GO.
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Fig. 7. Snapshot of Joule heating-induced shape recovery in SMP nanocomposite recorded by
infrared video camera (VarioCAM HiRessl, JENOPTIK Infra Tec.).