Accepted Manuscript

Epoxy composites with carbon nanotubes and graphene nanoplatelets - Disper-

sion and synergy effects

Liang Yue, Gholamreza Pircheraghi, Seyed Ali Monemian, Ica Manas-

Zloczower

PII: S0008-6223(14)00626-5

DOI: http://dx.doi.org/10.1016/j.carbon.2014.07.003

Reference: CARBON 9123

To appear in: Carbon

Received Date: 10 February 2014

Accepted Date: 2 July 2014

Please cite this article as: Yue, L., Pircheraghi, G., Monemian, S.A., Manas-Zloczower, I., Epoxy composites with

carbon nanotubes and graphene nanoplatelets - Dispersion and synergy effects, Carbon (2014), doi: http://dx.doi.org/

10.1016/j.carbon.2014.07.003

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1

Epoxy composites with carbon nanotubes and graphene nanoplatelets - Dispersion and

synergy effects

Liang Yue1, Gholamreza Pircheraghi

1, Seyed Ali Monemian

1, Ica Manas-Zloczower*

,1

1 Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106

Abstract

Carbon Nanotubes (CNTs) and Graphene Nanoplatelets (GNPs) at different mix ratios were

dispersed by ultrasonication into an epoxy matrix and the effects of CNT: GNP ratios on the

mechanical and electrical properties of the hybrid composites were investigated. The

combination of CNT and GNP in a ratio 8:2 was observed to synergistically increase flexural

properties and to reduce the electrical percolation threshold for the epoxy composites,

indicating easier formation of a conductive network due to the improved state of CNT dispersion

in the presence of GNPs. The state of dispersion was evaluated at different length scales by using

optical microscopy, UV-Vis spectroscopy, rheological measurements, scanning electron

microscopy, transmission electron microscopy and sedimentation tests. The Fourier transform

infrared spectra for CNT and GNP indicate that the GNPs contain oxygen moieties responsible

for better interactions with the epoxy matrix.

* Corresponding author Tel: (216) 368-3596 E-mail: ixm@case.edu

2

1. Introduction

Carbon nanomaterials such as Carbon Nanotubes (CNTs) and Graphene Nanoplatelets (GNPs)

exhibit superior mechanical and electrical properties [1-3], which combined with their extremely

high aspect ratio make them ideal fillers for polymer nanocomposites used in advanced materials

applications [4, 5]. However, the strong van der Waals forces between the fillers result in uneven

dispersion and consequently, the composite properties fall short of the expectations associated

with the promise of individually dispersed nanofillers [6, 7]. Good dispersion of the nanofillers

leading to the formation of efficient networks for strain, electrical and thermal transfer in the

composite materials may enable the extraordinary potential of carbon nanofillers for making

advanced materials [8, 9]. Thus, homogeneous and stable dispersion of the nanofillers in the

polymer matrix besides enhanced interfacial filler/polymer interactions remain critical challenges

in this field [6]. Different dispersion methods as well as filler surface functionalization [10] and

the use of a variety of dispersing agents [6] were developed to overcome these challenges.

The state of dispersion at different length scales can be evaluated using a variety of techniques.

Microscopic methods such as optical microscopy, transmission electron microscopy (TEM),

atomic force microscopy (AFM) or scanning electron microscopy (SEM) can provide a direct

way to observe and qualitatively assess the state of dispersion at different length scales.

Individual or small bundles of CNTs can be detected in the UV spectra region while large

aggregates of CNTs and GNPs cannot be detected in this region [11, 12]. Thus, the UV-Vis

spectra provide a more quantitative way to evaluate CNT dispersion at nano scale [13, 14].

Also, rheological analysis of CNT suspensions can provide insight into their microstructure

and dispersion state [15, 16]. For an epoxy-CNT suspension, it was reported that as the state of

3

CNT dispersion improved, the CNT network became stronger as indicated by a higher storage

modulus, G', and complex viscosity, �∗ [15].

The strategy of using hybrid carbon nanofillers with different geometric shapes showed to be

an effective way to achieve better mechanical, electrical and thermal properties in composite

materials [17-22]. While, the mechanism of the synergetic effect is not totally understood, the

geometric shapes and the ratio of fillers in the mixture are important factors governing the

reinforcing capabilities of the composites. Recently, it was reported that the synergetic effect in

the thermal conductivity of epoxy composites with a hybrid of CNTs and GNPs has its origins in

bridging the planar GNP nanoplatelets by the flexible CNT rods, resulting in a 3D network [18].

A lower electrical percolation threshold and better mechanical performance were reported in

epoxy composites with 0.2 wt% CNTs and 0.2 wt% carbon black (CB) due to a more efficient

network formation [19]. A synergetic effect in terms of flexural modulus [22], stiffness and

hardness [20] was reported in CNT and GNP hybrid filled composites. More recently a

simulation model was developed to describe the synergistic behavior observed in the electrical

and thermal conductivity of epoxy composites filled with CNTs and GNPs [23]. However, in

spite of the recently renowned interest in the field, the mechanism of synergetic behavior is still

equivocal.

The objective of this study was to investigate the correlation between synergistic properties in

hybrid filler systems of epoxy with CNTs and GNPs and the state of filler dispersion at different

length scales.

2. Experimental

2.1 Materials

4

In this study the epoxy matrix used is a modified diglycidylether of bisphenol-A (DGEBA,

L135i, Hexion) with an amine hardener (RIMH 1366, Hexion) supplied by Momentive Specialty

Chemicals. The carbon nanotubes (CNT) employed are multi-walled carbon nanotubes Baytubes

C70P from Bayer Material Science with an average diameter of 13 nm and a length larger than 1

µm. The commercially available graphene nanoplatelets (GNP) N002-PDR GB from Angstron

Materials have a thickness ＜1 nm and a particle (lateral) size of 4~5 µm and are comprised of

stacks of 1to 3 monolayers graphene sheets. SEM images of the CNT and GNP powders are

shown in Figure 1. All the materials were used as received. Tetrahydrofuran (THF) from Fisher

Scientific was used for sample dilution for the UV-Vis absorption measurements.

Fig. 1 SEM images of (a) CNT, and (b) GNP at different magnifications

2.2 Processing of composites

Epoxy nanocomposites were prepared by dispersing the fillers CNT and GNP in the

appropriate ratio in the epoxy resin using simultaneous magnetic stirring and sonication in a

water bath at 30oC. The suspension was then degassed under vacuum for 0.5 hour to eliminate

5

the entrapped air and cooled to room temperature. Further, the suspension was mixed with the

hardener and cured at 30oC for 24h followed by post curing at 60

oC for 15h.

For mechanical testing, the concentration of filler was fixed at 0.1wt%, which is below the

theoretical percolation threshold for these fillers. Different ratios of CNT: GNP were considered

and were recorded as CNT0.8GNP0.2 (8:2), CNT0.6GNP0.4 (6:4), CNT0.4GNP0.6 (4:6) and

CNT0.2GNP0.8 (2:8). Control samples containing only CNTs or GNPs were prepared following

the same routine. In order to explore the electrical percolation threshold, samples with different

concentrations were also prepared using the same processing route.

2.3 Characterization

Optical microscopy analysis was carried out with an Olympus system BX51 in transmitted

light configuration. The analysis was done on a small droplet of the epoxy suspension placed on

a microscope glass without cover. Fracture surfaces of the cured composite samples were

observed using a SEM (JEOL JSM-6510LV) with an operating voltage of 5 kV. The samples

were obtained by fracture in liquid nitrogen. Transmission electron microscopy (TEM) images of

the samples were acquired using a field-emission gun, energy-filtering TEM (JEOL 1200EX)

instrument operating at 80 kV. The samples were microtomed on a Leica Ultramicrotome, UC6,

at room temperature with a Diatome diamond knife.

UV and visible light absorption spectra were obtained using a UV–Vis spectrophotometer

(V670, Jasco Inc.) operating in the range of 250–800 nm. Epoxy suspensions after sonication

were diluted with THF to a concentration of 0.01mg/ml in order to obtain a detectable

transmission during the measurements. All samples were diluted to the same concentration.

Considering the similar solubility parameters of epoxy and THF [24], it is reasonable to consider

that the state of dispersion was not significantly affected by the dilution process.

6

Dynamic rheological properties of epoxy suspensions were measured using a Thermo

Scientific Haake Mars III rheometer at 25°C. Experiments were carried out in oscillatory shear

mode using a parallel plate geometry with a diameter of 35 mm and gap of 1 mm. Frequency

sweeps from 100 to 0.01 Hz were performed at stress level of 0.1 Pa within the linear

viscoelastic regime. The specimens were placed between the plates and were allowed to

equilibrate for approximately 4 min prior to each run, which was found to be satisfactory for

measurement reproducibility.

Fourier transform infrared spectroscopy (FT-IR) analyses were carried out using a Perkin

Elmer System series 2000 spectrophotometer in a frequency range of 4000-500 cm-1

to identify

the functional groups of CNT and GNP.

Mechanical properties of the prepared samples were investigated according to ASTM D790,

using an Instron 1011 universal tensile tester. We report data based on 7 tests for each

composition.

The electrical conductivity of the composite samples was characterized by a standard two-

probe station (Signatone 1160) with a DC current [25]. Silver paint was applied to the test face

for better contact of the copper lead and test surface. Reported values were the average of 5 tests.

3. Results and discussion

3.1 Mechanical properties

Studies have shown that in CNT reinforced composites, the mechanical reinforcement

efficiency changes above a critical filler concentration where a percolating filler network is

formed [26]. For CNTs the percolation threshold, P� (CNT), is estimated to be [26]:

���CNT� = .�

� [1]

7

where η = ��� is the aspect ratio of the CNTs, and l and d are the average length and diameter

of the nanotubes, respectively. Considering l=10 µm and d=13 nm the �� (CNT) is 0.065 vol%.

This corresponds to a weight fraction of 0.113 wt% when considering the density of CNTs (2.1

g/cm3) and the density of epoxy (1.14g/cm

3).

The percolation threshold for GNP, ��(GNP), can be calculated from [27]:

���GNP� =��.�

� [2]

where, � is the aspect ratio of the GNP. According to the technical data sheet for the GNP used

in this study,� = 4000 and thus the calculated ��(GNP) is 0.53 vol%, which corresponds to a

weight concentration of 0.68 wt%. In our study, the overall filler concentration was chosen at 0.1

wt%, below the estimated percolation thresholds for both fillers.

Fig. 2 Flexural modulus ratio and flexural strength ratio of epoxy composites with a fixed concentration of 0.1wt%;

dotted line shows mixing rule.

Figure 2 shows the flexural modulus and strength ratios of the epoxy composites. Obviously

the sample CNT0.8GNP0.2 shows significant improvement in both modulus and strength over the

CNT and GNP single filler composites. Thus, a synergetic effect on flexural properties is

8

achieved by the hybrid filler CNT0.8GNP0.2, whereas at other concentration ratios, the data follow

the mixing rule.

3.2 Electrical Properties

The electrical conductivity of the single filler nanocomposites as a function of the filler content

is shown in Figure 3a. In this filler content range, percolation thresholds for CNT and GNP are

clearly observed. Typical percolation behavior for DC conductivity follows a power law [28]:

���� ∝ �� − ��� [3]

where σ is the DC conductivity, p is the concentration of conductive filler, pc the percolation

threshold and t is a critical exponent. The percolation threshold is known to be largely dependent

on the microstructure of the conductive filler, whereas the critical exponent t depends only on

system dimensionality [29]. Percolation thresholds of about 0.84 wt% for the CNT and 0.88

wt% for the GNP single filler epoxy composites were calculated based on the best fit to the

experimentally measured conductivity data plotted as log���� versus log�� − ��� for CNT and

GNP filled epoxy composites (Figure S-1 in supplementary data).

At a concentration of 4 wt%, the GNP/epoxy nanocomposites show an electrical conductivity

of 2.1×10-5

S/m, which is an increase of almost 7 orders of magnitude over the conductivity of

the neat epoxy. The CNT/epoxy nanocomposites exhibit even better performance with an

electrical conductivity of 4.3×10-3

S/m, an increase of 9 orders of magnitude over the neat epoxy.

These results confirm that CNT and GNP are excellent conductive fillers for epoxy composites at

low concentration loadings. GNP and CNT inherently high conductivity and high aspect ratios

allow them to easily form electrical pathways in the matrix [30].

In order to investigate the potential synergetic effect of CNTs and GNPs with respect to the

epoxy composites electrical properties, experiments were designed to explore both the

9

percolation threshold and the electrical conductivity for systems with hybrid fillers. For the

percolation threshold, the total filler concentration was fixed as 0.8 wt%, close to the percolation

threshold of the single filler systems, and the mix ratio was varied. In order to investigate the

potential synergetic effect on conductivity, a total filler concentration of 4 wt% was selected and

the mix ratio was varied.

Fig. 3 a) Electrical conductivities of CNT, GNP and CNT0.8GNP0.2 epoxy composites as a function of the filler

content. Insert shows the plot of log σ versus log (p-pc) for the CNT0.8GNP0.2 filler systems (R2 is the R-square of

this fitting); b) Electrical conductivity of composite samples with different CNT: GNP ratios at constant overall

concentrations of 0.8wt% and 4.0wt%;

In Figure 3b electrical conductivities of hybrid filler nanocomposites are plotted as a function

of the mix ratios at the selected overall filler concentrations. The CNT/GNP hybrid system

shows a significant increase in conductivity at the lower overall filler concentration when the

ratio of CNT: GNP is 8:2. The enhancement may be potentially attributed to the formation of a

conductive percolation network. To further investigate this hypothesis, various overall filler

concentrations at this ratio were tested. Figure 3a shows the conductivity for the CNT0.8GNP0.2

hybrid system at various overall filler concentration. Also shown are the conductivities for the

CNT and GNP single filled composites. The insert in Figure 3a displays the best fit to the

(a) (b)

10

experimentally measured conductivity data plotted as log ���� versus log �� − ��� for the

CNT0.8GNP0.2 system, confirming a lower percolation threshold, of 0.62 wt% for the hybrid filler

system. Also noteworthy is the higher value for the critical exponent t=2.41 by comparison with

the single filler systems (t=1.56 for the CNT composites and t=1.31 for the GNP composites,

Figure S-1in supplementary data), indicative of a hybrid filler network closer to the 3D type

architecture [29].

It is commonly believed that the conductivity of filled conductive polymeric systems derives

from the formation of a conductive network by the fillers in the matrix [31]. In the CNT/GNP

hybrid system, the conductive network was formed at a lower overall filler concentration than for

the single CNT and GNP filled systems. This can be explained by the formation of conductive

pathways more efficiently when combining 1D CNTs with 2D GNPs.

In the following we try to find an explanation for the synergy observed in flexural and

electrical properties of the hybrid filler system by analyzing the microstructure and state of

dispersion in these composite materials.

3.3. Dispersion characterization

3.3.1 Optical microscopy

Optical microscopy images can be used to characterize the state of aggregation at micrometer

scale. Optical micrographs at different magnification levels for the filler/epoxy suspensions are

presented in Figure 4. Images were taken 72h after - sonication. Micrographs for the CNT/epoxy

suspensions (Figure 4a) show loosely packed isolated aggregates of CNTs. Such aggregates form

due to CNT unfavorable interactions with the epoxy matrix [32] and van der Waals attraction

forces between the CNTs. By contrast, the GNP/epoxy suspensions (Figure 4d) show more

homogenously distributed aggregates due to more favorable GNP interactions with the epoxy

11

matrix [33]. At high magnification the presence of large size GNP aggregates can be observed,

primarily due to van der Waals forces and strong π-π interactions between the GNP sheets [34].

The hybrid filled epoxy suspensions show more homogenously dispersed aggregates compared

with the CNT/epoxy systems and smaller cluster size at high magnification compared with the

GNP/epoxy suspensions. These results seem to indicate that graphene platelets could improve

CNT dispersion and suppress re-aggregation, due to their large surface area and space hindrance

effects, while rod like CNTs could also reduce the formation of large size stacked GNP sheets.

Fig. 4 Optical microscope images of suspensions 72h after sonication, (a) CNT/epoxy (b)CNT0.8GNP0.2/epoxy (c)

CNT0.2GNP0.8/epoxy (d) GNP/epoxy

3.3.2 UV-Vis spectra

The UV-Vis spectroscopy has been successfully employed to investigate the dispersion

behavior of CNTs and GNPs [35, 36]. This technique provides an effective method to evaluate

the dispersion state of graphene and carbon nanotubes at nanoscale. Individual CNTs and

12

graphene sheets are more absorptive in the UV-Vis region [37], while, bundled CNTs and

stacked GNPs are hardly active in this range. Furthermore, it was shown that the maximum

absorption which occurs around 250~270 nm will increase with increasing the concentration of

individual CNTs [13, 38]. Figure 5a shows the UV-Vis absorption of epoxy/filler suspensions

diluted in THF. Both CNTs and GNPs show maximum absorption around 260~270 nm

wavelength, in good agreement with the literature [38]. With the decrease of the CNT content in

the system, the absorption peak decreases, indicating that CNTs dominate the absorption in the

UV region for the hybrid filler system. However, in the visible region, absorption and scattering

of bundled CNTs and stacked GNPs are more important. Interestingly, sample CNT0.8GNP0.2

showed a higher absorption at 500 nm than the CNTs or other hybrid filler combinations (Figure

5b), most likely due to a change in the dispersion state. The results seem to indicate that GNP

platelets could prevent re-aggregation of the CNT bundles. Thus in the presence of GNPs, there

are more CNT bundle units present in the system, resulting in a higher absorption and scattering

in this region (~500 nm). This result is consistent with the optical microscopy results showing

that the dispersion state was improved in the hybrid filler system, especially at a ratio CNT: GNP

of 8:2.

Fig. 5 (a) UV-Vis absorption spectra of the epoxy nanocomposite suspensions diluted in THF solvent, (b)

Absorption at 500 nm versus GNP/CNT ratio.

13

Furthermore, the time dependence of the UV-Vis absorption peak at 260~270 nm was used to

evaluate dispersion stability. It is widely accepted that the absorption intensity is proportional to

the concentration of individual CNTs and small bundles dispersed in the suspension [13]. Figure

6 shows the UV absorption peak (around 260 nm) measured at different times after sonication.

All the samples show a decrease in the absorption peak with time due to the re-aggregation of

individual and small bundles of CNTs and GNP platelets. After 200 hours, the maximum

absorption intensity was displayed by the CNT0.8GNP0.2 hybrid system confirming better

dispersion stability in the hybrid system. The dispersion state at nanoscale, measured by UV-Vis

spectroscopy, correlates well with the dispersion state at micro scale, observed by optical

microscopy.

Fig. 6 Time-resolved UV-Vis absorption at ~263 nm for the suspensions at different times after sonication.

3.3.3 Rheological measurements

It is generally accepted that rheological measurements can be used for a comprehensive

evaluation of the state of CNT dispersion, considering that rheological properties are dependent

on CNT concentration, aspect ratio and polymer-CNT interactions [15]. The dependence of

complex viscosity (η*) and storage modulus (G’) on the oscillatory frequency (ω) are shown in

(b) (a)

14

Figure 7. The neat epoxy sample shows Newtonian behavior in a wide range of frequencies due

to its low molecular weight. While dispersion of 0.1 wt% GNP in epoxy did not affect the

suspension Newtonian behavior, dispersion of CNTs at similar filler concentration leads to

viscosity upturn at low frequencies. The CNT0.8GNP0.2 hybrid sample shows an even higher

complex viscosity than the CNT filled sample especially at low frequencies. Figure 7b shows the

variation of storage modulus with frequency for the different samples. In the low frequency

region the neat epoxy sample exhibits the typical terminal behavior [39] with scaling of G' ~ ω2.

The same behavior is observed for the GNP/epoxy suspension indicating that small amounts of

GNP cannot change the rheological behavior of the neat epoxy. In other words, the rheological

percolation of GNP is much higher than the filler concentration in the sample analyzed. The

storage modulus is highly increased in the CNT/epoxy suspension and its dependence on ω in

the low frequency region is weakened. This nonterminal low frequency behavior can be

attributed to the formation of a CNT network as a result of fine dispersion [39]. These results are

in agreement with reported rheological percolation thresholds for GNP and CNT suspensions in

which CNTs exhibit much lower rheological percolation threshold than GNPs [40]. Moreover,

the epoxy suspension with the hybrid filler shows an increase in the viscosity and storage

modulus compared to the CNT/epoxy suspension.

In order to further explore the microstructure characteristics in different systems, a transient

shear test was performed. During shearing in the non-linear flow regime (high deformation), not

only does the microstructure of the dispersed phase take time to respond to the applied shear, but

it may also undergo time dependent changes [41]. The time-dependent stress response after

sample shearing at a shear rate of 5s-1

is shown in Figure 7c. The hybrid sample shows the

highest steady shear stress among the samples, as reported also in Table 1, indicating higher

15

shear viscosity mainly due to better dispersion of the CNTs in the presence of GNPs. These

results also indicate that small amounts of graphene platelets can act as a dispersing agent and

improve CNT dispersion.

Fig. 7 Complex viscosity (a) and storage modulus (b) as function of frequency and stress versus time (c) for the

samples at an overall filler concentration of 0.1 wt%.

Table 1. Steady shear properties of the neat epoxy and different suspensions after applying a step shear rate of 5s-1.

Sample Epoxy Epoxy/CNT Epoxy/GNP Epoxy/CNT0.8GNP0.2

Steady shear stress (Pa) 2.74 4.03 2.98 4.35

Steady shear viscosity (Pa.s) 0.55 0.80 0.59 0.87

(a) (b)

(c)

16

3.3.4 Colloidal Stability

The colloidal stability of various fillers in THF was investigated by sedimentation tests and the

images of the suspensions taken 72 hours after sonication in front of a light source are shown in

Figure 8.

Fig. 8 Images of carbon fillers/THF suspensions, 72h after sonication. From left to right are CNT, CNT0.8GNP0.2,

CNT0.4GNP0.6, CNT0.2GNP0.8, and GNP at filler concentration of 0.1mg/ml. Schematic illustration for the sediment

packing in various filler suspensions.

CNTs easily precipitate at the bottom of the vial due to re-agglomeration after sonication, and

the transparent supernatant indicates low colloidal stability of CNTs in THF. GNPs showed

better colloidal stability in THF with a dark non-transparent liquid supernatant and a small highly

packed sediment volume. These results can be explained in terms of the different surface

CNT CNT0.8

GNP0.2

GNP

17

properties of the fillers as revealed by the FTIR data shown in Figure 9. The most noticeable

differences are the broad and intense peaks at 3430 cm-1

(O-H stretching vibrations) and 1090

cm-1

(C-O) stretching vibration) in the GNP and the sharp peak at 1588 cm-1

in CNT due to

vibrations of the skeletal graphitic structure [42]. Thus, the GNP surface decorated by polar,

oxygen containing functional groups can better interact with the THF polar solvent resulting in

better colloidal stability of the GNP suspensions.

Fig. 9 FTIR spectra of GNP (upper, black) and CNT (lower, red) nanoparticles

All hybrid suspensions exhibit better colloidal stability in THF than the CNTs as shown by

their darker supernatant. GNP/CNT " − " stacking interactions [43], and GNP better colloidal

stability in polar media may explain the hybrid filler better dispersion in matrices with some

polar groups such as epoxy resins.

Schematics of filler packing in the sediments are shown in Figure 8. For the flexible entangled

CNTs the junction can be visualized as a point contact, whereas for the GNPs, contact is

established by a planar surface. In the hybrid filler system, packing is more complex, with the 1D

CNTs bridging adjacent 2D GNPs. These point contacts can prevent the planar contacts of GNP

18

nanoplatelets [18]. At the same time, the GNP nanoplatelets may provide steric and electrostatic

stabilization, preventing CNT re-agglomeration. Thus, one can imagine that the hybrid filler is

more prone to form a loosely packed 3D nanoparticle network in the solvent. Such ease of

forming a 3D network in the sediment may also explain the lower electrical percolation threshold

observed in the hybrid filler epoxy system.

3.3.5 Filler dispersion in epoxy composites

To further investigate filler dispersion in the cured epoxy composites with different fillers,

TEM images of microtomed samples at an overall filler concentration of 0.1 wt. % are shown in

Figure 10. Large aggregate domains in size of 600 nm are clearly seen in the CNT/epoxy sample

(Figure 10a). For the GNP/epoxy systems, the presence of voids in the stacked nanoplatelet

structures can be detected at higher magnification in Figure 10b. Filler dispersion is clearly

improved for the hybrid system as illustrated in Figure 10c. The 2D graphene nanoplatelets are

intercalated between the 1D nanotubes which can lead to the formation of a 3D filler network,

conducive of better mechanical and electrical properties.

The fracture surfaces of epoxy composites were investigated by scanning electron microscopy

(SEM) and the micrographs are shown in Figure 11. The fracture surface of the neat epoxy

system displays long crack propagation with a relatively smooth surface, typical of brittle

fracture behavior (Figure 11a).

In Figure 11b, rich CNT domain areas are observed. Figure 11c shows that the GNP/epoxy

composite displays a rougher fracture surface, suggestive of many micro-cracks. Agglomerated

GNPs can also form steric obstacles, restricting polymer flow into the agglomerates and resulting

in the formation of holes and voids between GNPs and epoxy [44, 45]. Figure 11d for the

composite with the hybrid filler system shows better dispersion of the CNTs in the matrix with

19

no rich CNT domains present on the surface. This result indicates that the dispersion and state of

aggregation of CNTs were significantly changed using a small amount of GNPs. Moreover, no

voids or holes were detected on the fracture surface. The well-dispersed CNTs can inhibit

stacking of the GNP sheets, preventing void formation.

Fig. 10 TEM images of: (a) CNT/epoxy composite, (b) GNP/epoxy composite, (c) CNT0.8GNP0.2/epoxy composite.

20

Fig. 11 SEM images of the fracture surface: (a) neat epoxy (b) CNT/epoxy composite, (c) GNP/epoxy composite

(arrows point to voids present on the surface) (d) CNT0.8GNP0.2/epoxy composite.

Filler dispersion in the cured epoxy systems as observed by TEM and SEM images corroborate

the results obtained in the experiments involving the corresponding epoxy suspensions.

4. Conclusions

The combination of CNT and GNP in a ratio 8:2 was observed to synergistically increase

flexural properties and to reduce the electrical percolation threshold for epoxy composites, most

likely due to the improved CNT dispersion in epoxy in the presence of GNP. Our results show

21

better dispersion in the hybrid filler system CNT/GNP 8/2 at all length scales. With the addition

of GNP to the system, both the dispersion state and stability of CNTs in epoxy improved, as

confirmed by optical microscopy and UV-Vis spectra at the micro and nano scale. Moreover, the

hybrid filler forms a more robust nanoparticle network, attested in sedimentation tests and

corroborated by rheology measurements. Filler dispersion is preserved in the cured composites

as attested by TEM and SEM data.

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