Interface molecular engineering of single-walled carbon nanotube/epoxy composites

Article abstract of DOI:10.1039/c1jm14310g

Dispersion of large single-walled carbon nanotube (SWCNT) bundles into individual nanotubes or small bundles and thus strengthening of the nanotube/matrix interfacial interaction are prerequisites for taking full advantage of the remarkable multifunctional properties of SWCNTs in various carbon nanotube-based composites. Noncovalent functionalization of SWCNTs is an attractive option to simultaneously achieve these conditions. Toward this end, three reactive amino-containing pyrene derivatives (AmPys) with various spacer chain lengths were synthesized. One with the longest spacer length (12 methylene units, AmPy-12) shows the highest functionalization efficiency for SWCNTs in terms of dispersibility. Systematic characterization on a SWCNT/AmPy-12 hybrid suggests that ca. 10 wt% of AmPy-12 is strongly adsorbed on SWCNTs through π-π interactions, making them steadily dispersed into individual ones and/or small bundles without noticeable change in their electronic structure. AmPy-12-functionalized SWCNTs were then used for the preparation of epoxy composites. Since the SWCNT/epoxy interface was well engineered at a molecular level by application of AmPy-12, which interacts noncovalently with SWCNT but bonds chemically to the epoxy matrix, the composite with only 0.3 wt% SWCNTs displays an increase of 54% and 27% in tensile strength and Young's modulus, respectively, over neat resin. A low electrical percolation threshold of 0.1 wt% SWCNTs and improved thermal properties were also observed.

Full text of DOI:10.1039/c1jm14310g

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PAPER
Interface molecular engineering of single-walled carbon nanotube/epoxy
composites†
*
Yehai Yan, Jian Cui, Shuai Zhao, Jinfang Zhang, Jiwen Liu and Junmei Cheng
Received 1st September 2011, Accepted 28th October 2011
DOI: 10.1039/c1jm14310g
Dispersion of large single-walled carbon nanotube (SWCNT) bundles into individual nanotubes or
small bundles and thus strengthening of the nanotube/matrix interfacial interaction are prerequisites
for taking full advantage of the remarkable multifunctional properties of SWCNTs in various carbon
nanotube-based composites. Noncovalent functionalization of SWCNTs is an attractive option to
simultaneously achieve these conditions. Toward this end, three reactive amino-containing pyrene
derivatives (AmPys) with various spacer chain lengths were synthesized. One with the longest spacer
length (12 methylene units, AmPy-12) shows the highest functionalization efficiency for SWCNTs in
terms of dispersibility. Systematic characterization on a SWCNT/AmPy-12 hybrid suggests that ca. 10
wt% of AmPy-12 is strongly adsorbed on SWCNTs through p–p interactions, making them steadily
dispersed into individual ones and/or small bundles without noticeable change in their electronic
structure. AmPy-12-functionalized SWCNTs were then used for the preparation of epoxy composites.
Since the SWCNT/epoxy interface was well engineered at a molecular level by application of AmPy-12,
which interacts noncovalently with SWCNT but bonds chemically to the epoxy matrix, the composite
with only 0.3 wt% SWCNTs displays an increase of 54% and 27% in tensile strength and Young’s
modulus, respectively, over neat resin. A low electrical percolation threshold of 0.1 wt% SWCNTs and
improved thermal properties were also observed.
of CNTs and, more importantly, enable engineering of the
1. Introduction
nanotube/polymer interface at a molecular level for strong
interfacial interactions.5–9
A high aspect ratio (1000 or more) together with low density and
high strength render carbon nanotubes (CNTs) ideal reinforcing
fillers in polymer composites. More intriguing is that the excel-
lent mechanical, thermal and electrical properties of CNTs
afford great opportunities for multifunctional materials.1–4
However, an amount of work remains to be done before actu-
alizing the full potential of nanotube composites. The researchers
still face challenges, such as homogeneously dispersing CNTs
(perfectly down to an individual nanotube level) in the polymer
matrix and strengthening the CNT/matrix interfacial interaction.
For a high-quality composite, the former is a prerequisite, but
not yet substantial. This is because the atomically smooth surface
and chemical inertness of pristine CNTs induce weak interfacial
adhesion with the matrix, thus making load transfer from the
matrix to the nanotubes very poor. Functionalization of CNTs
has proven to be an effective means to improve the dispersibility
Of the available functionalization methods, covalent attach-
ment of functional groups on the nanotube surface is the most
commonly used strategy. Mechanical enhancements of the
resultant composites were generally achieved, however, this was
at the cost of the electrical and thermal properties of CNTs. On
the contrary, noncovalent methods based mainly on van de
Waals interactions are well-known to preserve nearly all of the
intrinsic features of CNTs. Although such methods are usually
not as powerful as covalent ones in terms of mechanical
enhancement of CNT/polymer composites, multifunctional
materials, e.g. electrically conductive polymer composites with
modestly enhanced mechanical performance, are available. In
a typical work, CNTs were noncovalently functionalized by poly
(vinylbenzyl oxyethyl naphthalene)-g-poly(methyl methacrylate)
and used in the matrix of poly(styrene-co-acrylonitrile).10 Since
the crucial issues of nanotube dispersion and interfacial inter-
action have been satisfactorily resolved without damaging the
nanotube’s electronic structure, the resultant composites dis-
played significantly improved mechanical and electrical proper-
ties as compared with neat polymer and composites filled with
pristine CNTs. The multifunctional composites have also been
achieved in the matrices of polycarbonate,11 Parmax8 and
Key Laboratory of Rubber-plastics, Ministry of Education/Shandong
Provincial Key Laboratory of Rubber-plastics, College of Polymer
Science and Engineering, Qingdao University of Science and Technology,
Qingdao, 266042, China. E-mail: yhyan@qust.edu.cn; Fax: +86-532-
8402 3977; Tel: +86-532-8402 2416
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c1jm14310g
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polyimide,12 where CNTs noncovalently functionalized by poly
(3-hexylthiophene)-g-polycaprolactones, poly(p-phenylene ethy-
nylene) and polyimide were used as fillers, respectively. To date,
noncovalent strategies have made much progress in thermo-
plastic matrices, but only a few studies addressed thermosetting
ones.13
seamlessly connected to the epoxy network and become an
integral part of the resultant composite. In order to implement
this novel design of a SWCNT/epoxy interface at a molecular
level through a noncovalent strategy, three AmPys with various
spacer chain lengths were first synthesized and characterized. The
functionalization effect of AmPy on SWCNTs was then
systematically assessed by diverse characterization techniques.
Finally, the SWCNT/epoxy composites were prepared by
a solution-based process and their mechanical, electrical and
thermal properties were measured as a function of nanotube
content. To the best of our knowledge, there are few reports thus
far on applying a pyrene derivative to the nanotube/epoxy
composite, although various pyrene derivatives have been
previously used in thermoplastic matrix composite systems.19–23
Compared with pristine SWCNTs, AmPy-functionalized
SWCNTs have shown higher efficacy for physical performance
enhancements. Multifunctional, but lightweight epoxy compos-
ites are hence at hand.
Epoxy resin is a well-established thermoset with a wide range
of applications from electronics to aerospace. Despite their
favorable mechanical performance and good chemical and
corrosion resistance, technological advances have put forward
multifunctional requirements on the epoxy-based materials and
there are practical demands on electrical conductivity and
improved mechanical properties. Filling epoxy resin with non-
covalently functionalized CNTs seems to be an attractive option
for such an objective. Surfactants,14,15 protein,16 gum Arabic17
and polyimide-g-bisphenol A diglyceryl acrylate13 have been
explored for the noncovalent functionalization of CNTs, which
were then used for the preparation of epoxy composites.
Unfortunately, the mechanical or electrical performance of these
materials have only been measured individually, and both have
not been assessed together in the same work. Recently, Grunlan
and co-workers18 have prepared epoxy composites using poly-
ethyleneimine as a noncovalent dispersant of CNTs. Their study
suggested that both the electrical conductivity and storage
modulus were similar to those of the composites based on pris-
tine CNTs and there was hardly any performance improvement.
The development of multifunctional CNT/epoxy composites is
still an open problem.
2. Experimental
2.1 Materials
SWCNTs (purity >90%, diameter 1–2 nm, length 1–3 mm)
produced by a conventional chemical vapor deposition (CVD)
process were purchased from Timesnano Inc., China. Epofix
from Struers was used as the epoxy matrix. It was supplied in two
parts: Epofix hardener (triethylenetetramine) and Epofix resin
containing 60–90% of bisphenol-A-diglycidylether ðMn#700Þ
and 10–40% of alkyl (C12–C14) glycidyl ether. 1-Pyrenebutyric
acid, N-hydroxysuccinimide and 1-ethyl-3-(3-dimethylamino-
propyl) carbodiimide hydrochloride (EDC$HCl) were bought
from Aldrich. Ethylenediamine, 1,6-diaminohexane, 1,12-dia-
minododecane and triethylamine were supplied by Aladdin
Reagent, China.
This paper presents the design, synthesis and application of
amino-containing pyrene derivative (AmPy) as a reactive non-
covalent functionalization agent for single-walled carbon nano-
tubes (SWCNTs). As depicted in Fig. 1, the pyrene units make
AmPy strongly interact with the nanotubes through a non-
covalent interaction, whereas the alkylamine species facilitate
dispersion of nanotubes and readily react with epoxy. Once the
functionalized SWCNTs are added into epoxy system (including
bisphenol-A-diglycidyl ether as resin and triethylenetetramine as
hardener), the amino groups in AmPy enable SWCNTs to be
2.2 Synthesis of pyrenebutyric acid N-hydroxysuccinimide
ester (PyHSE)
EDC$HCl (3.0126 g, 15.7 mmol) was dissolved in anhydrous
dichloromethane (100 ml) and then added dropwise into a solu-
tion of 1-pyrenebutyric acid (4.1670 g, 14.5 mmol) and
N-hydroxysuccinimide (1.9760 g, 17.2 mmol) in the same solvent
(150 ml) at 0 ꢀC. After being stirred for 30 min, the reaction
mixture was warmed up to room temperature and the reaction
was allowed to remain at this temperature for 24 h. The final
reaction solution was washed using ultrapure water (3 ꢁ 125 ml).
After drying over MgSO4 for 1 h, the organic solution was
filtered and dichloromethane was evaporated. The crude product
was dissolved in 5 ml tetrahydrofuran (THF) and recrystallized
by the addition of 20 ml hexane. After leaving in a ꢂ20 ꢀC freezer
for 12 h, the solid was collected by filtration. The recrystallization
process was repeated another two times. Finally, the collected
solid was dried in a 50 ꢀC vacuum oven for 6 h (yellow solid, 92%
yield). Proton nuclear magnetic resonance (1H-NMR) spectros-
copy (d, ppm, Fig. 2): 7.82–8.24 (a), 3.40–3.44 (b), 2.80 (c), 2.66–
2.69 (d), 2.23–2.27 (e); Fourier transform infrared (FTIR) spec-
troscopy (y, cmꢂ1): 1739 (ester carbonyl), 1780 (imide carbonyl).
Fig. 1 Interface molecular engineering of the SWCNT/epoxy composite
by application of AmPy.
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2.5 Preparation of composites
For the preparation of the SWCNT/EP–AmPy-12 composites,
Epofix resin was mixed with the SWCNT dispersion in a ratio to
yield the expected content of SWCNTs. The mixture was
strongly stirred for 30 min, followed by ultrasonication in a water
bath for 1 h. After removal of THF by rotary evaporation, the
hardener was added at a weight ratio of resin/hardener ¼ 25 : 3.
The mixture was stirred for 3 min, cast into silicon rubber mold
(60 mm ꢁ 60 mm ꢁ 1 mm) and degassed in a vacuum oven. The
ꢀ
ꢀ
final mixture was cured at 50 C for 12 h and 80 C for 2 h. As
indicated by the FTIR spectra of the cured samples, in which the
bands for epoxy groups (912 cmꢂ1) completely disappeared
(Fig. S1†), the curing cycle adopted here ensures that the epoxy
system and its composites are fully cured. The SWCNT/EP
composites were also prepared using the same process.
Fig. 2 The synthesis of AmPy.
2.3 Synthesis of AmPys
In a typical experiment, PyHSE (1.5012 g, 3.9 mmol) was dis-
solved in chloroform (85 ml) and then slowly added into the
solution of ethylenediamine (2.2 ml, 32.9 mmol) and triethyl-
amine (4.5 ml, 32.3 mmol) in the same solvent (15 ml) over 2.5 h
at 0 ꢀC. After being stirred for a further 30 min, the reaction
mixture was warmed up to room temperature and the reaction
was allowed to continue for 24 h. The reaction solution was
filtered and the filtrate was washed using ultrapure water (3 ꢁ
50 ml). After drying over MgSO4, the organic solution was
filtered and chloroform was evaporated. The crude product was
2.6 Characterization
1H-NMR (500 MHz, CDCl3/DMSO-d6) spectra were recorded
on a Bruker AV500 spectrometer. FTIR spectra were obtained
using a Bruker VERTEX 70 spectrometer in the Attenuated
Total Reflectance (ATR) mode. AFM measurements were per-
formed with a Multimode scanning probe microscope equipped
with a Nanoscope IIIa controller. UV-Vis and UV-Vis-NIR
spectra were collected using a TU-1800PC spectrophotometer
and a Varian Cary 500 spectrophotometer, respectively. Fluo-
rescence spectra and decay curves were recorded on a HITACHI
F-4600 spectrophotometer and an Edinburgh Instrument FLS
920 spectrophotometer, respectively, with an excitation wave-
length of 344 nm. Differential scanning calorimetry (DSC) was
conducted on a TA Instruments DSC Q20 in a nitrogen atmo-
sphere with a heating rate of 10 ꢀC minꢂ1. The fracture surface of
composites was observed using a JEOL JSM-7500F scanning
electron microscope (SEM). For the tensile test, the composites
were cut into rectangular specimens measuring 60 mm ꢁ 10 mm
ꢁ 1 mm. The tensile properties were evaluated using a Zwick/
Roell Z030 universal testing machine at a crosshead speed of
5 mm minꢂ1. All electrical conductivity measurements were
performed at room temperature and 50% relative humidity. The
volume conductivity lower than 10ꢂ5 S cmꢂ1 was measured using
a PC68 high resistance meter, while that higher than 10ꢂ5 S cmꢂ1
ꢀ
dissolved in 3 ml THF and recrystallized at ꢂ20 C overnight.
After filtration, the solid was thoroughly washed using hexane
ꢀ
and dried in a 50 C vacuum oven for 6 h. All three AmPys are
pale yellow solids with yields of 85%, 65% and 68% for AmPy-2,
AmPy-6 and AmPy-12, respectively.
2.4 Dispersion of pristine SWCNTs
SWCNTs (0.2 mg mlꢂ1) were dispersed in THF by mixing with
a calculated content of AmPy followed by ultrasonication
(Yuhua KQ250B, 40 kHz) for 2 h with the water bath temper-
ature maintained at around 20 ꢀC. The dispersion was allowed to
settle for 24 h and 3/4 supernatant was carefully drawn out and
passed through densely packed glass wool to remove the big
aggregates. The uniform and dark dispersion thus collected was
used in the optimization study of AmPys, i.e., for the measure-
ments of nanotube dispersibility and dispersion stability.
was measured by
a standard four-probe method using
a MODEL SZT-2000 instrument.
For other work, SWCNTs (74 mg Lꢂ1) were dispersed in
THF by mixing with AmPy-12 (0.08 mg mlꢂ1) followed by
ultrasonication in a water bath for 2 h. The thus-obtained
dispersion can be directly used for the composite preparation.
For assessing the effect of AmPy-12 on the dispersion of
SWCNTs (i.e., for the characterization of atomic force
microscopy (AFM), ultraviolet-visible-near infrared (UV-Vis-
NIR) and fluorescence spectroscopy), however, the dispersion
should undergo the following process to remove the free AmPy-
12. After filtration over a 1.0 mm PTFE membrane (Millipore),
the filter cake was redispersed in fresh THF by ultrasonication
for 5 min. The dispersion was then filtrated again. The process
was repeated several times until no AmPy-12 could be detected
by UV spectroscopy in the filtrate. The collected black solid was
3. Results and discussion
AmPys were synthesized through an amidation reaction between
1-pyrenebutyric acid and diamine (Fig. 2). The FTIR and
1H-NMR spectra for a typical AmPy, i.e. AmPy-2, are given in
Fig. 3. The peaks centered at 3291, 1655 and 1544 cmꢂ1 in the
FTIR spectrum correspond to –NH– stretching, C]O stretching
and C–N–H bending, respectively, evidencing the formation of
a secondary amide group. The presence of the characteristic band
of –NH2 (3415 cmꢂ1) reveals that the desired asymmetric ami-
dation was realized by adding the intermediate of PyHSE into
equimolar diamine as slowly as possible. Moreover, the multi-
1
peaks at 7.95–8.41 ppm in the H-NMR spectrum make clear
ꢀ
dried in a 50 C vacuum oven for 6 h and named as SWCNT/
that the pyrene unit is truly included in the molecular structure of
1
AmPy-2. Similar FTIR and H-NMR spectral features are also
AmPy-12 hybrid.
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Fig. 4 Dispersibility of SWCNTs as a function of the mass concentra-
tion of AmPy.
its optimum causes a decrease of the dispersibility. A similar
tendency was also observed using sodium dodecyl sulfate (SDS)
as a dispersant for SWCNTs in water.26 Van der Waals- and
depletion-induced aggregation mechanisms have been suggested
there to explain the experimental fact that the SWCNT dis-
persibilities are relatively lower at low or high SDS concentra-
tions. In the present case, the AmPy molecules are adsorbed on
the nanotube surface and their adsorption and desorption
behaviors in dispersions are parallel and competitive. At low
concentrations, adsorption predominates over desorption so that
the amount of AmPy adsorbed on the nanotube surface quickly
increases with increasing AmPy concentration. The adsorbed
AmPy molecules induce steric repulsion against van der Waals
interactions among nanotubes, thus improving the nanotube
dispersibility. Beyond a critical concentration (0.4 mg mlꢂ1 for
AmPy-2, 0.2 mg mlꢂ1 for AmPy-6 and 0.08 mg mlꢂ1 for AmPy-
12), the excessive AmPy molecules may possibly self-organize
into some supramolecular structures due to strong p-stacking
interaction between pyrene units (Fig. S3†).27 These structures
prefer to approach each other to maximize the entropy of the
whole system, rather than fit into two adjacent individual
nanotubes or bundles. As a result, the approach generates
a depletion attraction28 that is presumably responsible for the
decreased dispersibility at the high AmPy concentration.
Fig. 3 (a) FTIR and (b) 1H-NMR spectra of AmPy-2.
observed for AmPy-6 and AmPy-12, thus suggesting that the
amino-containing pyrene derivatives were successfully synthe-
sized. It should be noted that the process for the synthesis of
AmPys presented here is relatively complex and could be
a drawback for a large scale production. As a promising alter-
native, in future work, 1-pyrenecarbaldehyde may be used to
replace 1-pyrenebutyric acid, so that the AmPy can be simply
synthesized in one step (Fig. S2†).
For the AmPy-functionalized SWCNTs, the highest dis-
persibility takes place in the presence of AmPy-2, but the optimal
AmPy-2 mass concentration is also the highest among the three
AmPys (Fig. 4 and Table 1). Once the mass concentration of
AmPy is converted to the molar concentration of pyrene unit and
the dispersibility is redefined as nanotube content per molar
pyrene unit, the renewed dispersibility was found in an ascending
order of AmPy-2, AmPy-6 and AmPy-12. Namely, AmPy-12
holds the highest functionalization efficiency for SWCNTs in
terms of dispersibility. Furthermore, the dispersion-containing
AmPy-12 displays the most excellent stability. After being
centrifuged at 3000 rpm for 30 min, the nanotube content in the
upper clear liquid phase was measured to be 63.2 mg Lꢂ1 using
the UV-Vis spectroscopy-based method mentioned above. For
the dispersions based on AmPy-6 and AmPy-2, however, the
corresponding values are 37.2 and 15.0 mg Lꢂ1, respectively. The
longest methylene chain length of AmPy-12 provides the biggest
The AmPy compounds were then used for noncovalent func-
tionalization of SWCNTs. A well developed UV-Vis spectros-
copy-based method with a previously determined specific
extinction coefficient of 3500 ¼ 0.021 is adopted here for
measuring the dispersibility of SWCNTs in THF.24,25 As shown
in Fig. 4, the dispersibility shows a strong dependence on the
concentration of AmPy. Only 4.5 mg Lꢂ1 of uniform dispersion
can be formed in pure THF. In the presence of AmPy, however,
this value increases up to a maximum of 91.0 mg Lꢂ1 for AmPy-2
at a concentration of 0.4 mg mlꢂ1, 68.4 mg Lꢂ1 for AmPy-6 at
0.2 mg mlꢂ1, or 75.4 mg Lꢂ1 for AmPy-12 at 0.08 mg mlꢂ1
.
Interestingly, further increasing the AmPy concentration beyond
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Table 1 The dispersibility of SWCNTs at the optimal concentration of AmPy
Optimal mass concentration
of AmPy/mg mlꢂ1
Dispersibility
of SWCNTs/mg Lꢂ1
Molar concentration
of pyrene unit/mmol Lꢂ1
SWCNT content per
molar pyrene unit/g molꢂ1
AmPy
AmPy-2
AmPy-6
AmPy-12
0.4
0.2
0.08
91.0
68.4
75.4
1.21
0.52
0.17
75.2
132.3
443.4
spatial volume in dispersion to impart strongly repulsive forces
onto nanotubes via steric hindrance against re-agglomeration. In
consideration of both functionalization efficiency and dispersion
stability, the dispersion containing 75.4 mg Lꢂ1 of SWCNTs and
0.08 mg mlꢂ1 of AmPy-12 is preferred in the subsequent experi-
mental work.
AmPy-12 interaction? As a powerful tool, fluorescence spec-
troscopy was used to address this issue. After being excited at
a maximum adsorption wavelength of 344 nm, the AmPy-12
solution in THF (1 mg mlꢂ1) exhibits the strong characteristic
fluorescence of pyrene with distinct peaks at 376, 387, 397 and
417 nm (Fig. 6a). Similar spectral features are also observed in
the dispersion of the SWCNT/AmPy-12 hybrid (1 mg mlꢂ1 of
AmPy-12, Fig. 6a inset). For molecular pyrene, the intensity
ratio of the peak centered at 376 nm to that at 387 nm, commonly
known as I1/I3 ratio, has been suggested to provide information
on the polarity of the medium surrounding the pyrene.29,30 It is
thus expected that the I1/I3 ratio of the SWCNT/AmPy-12 hybrid
should be different from that of AmPy-12, reflecting the change
To further assess the effect of AmPy on the dispersion of
SWCNTs, the free AmPy-12 molecules that are not adsorbed on
the nanotube surface were eliminated by a repeated process of
ultrafiltration and filter cake re-dispersion. The collected solid is
hereafter named a SWCNT/AmPy-12 hybrid. For AFM obser-
vation, the hybrid was redispersed in fresh THF and several
drops of the dilute dispersion were spin-coated (4000 rpm/2 min)
onto a cleaned silicon wafer. As revealed by Fig. 5, the nanotubes
are reliably involved in the dispersion and are well-dispersed. The
near-round particles in the AFM image are impurities, such as
amorphous carbon material and metal catalysts used for the
SWCNT synthesis. Statistics on the height of 30 various tubular
structures in Fig. 5 indicate that the hybrid is a mixture of indi-
vidual nanotubes (1.6 ꢃ 0.6 nm of diameter) and their 3–5 nm
bundles decorated with AmPy-12. The AmPy-12 content in the
hybrid was quantitatively determined by the UV-Vis spectros-
copy-based method. For this purpose, the molar extinction
coefficient of AmPy-12 at 344 nm in THF was first established to
be 4.96 ꢁ 104 Mꢂ1 cmꢂ1 according to Lambert-Beer law. By
comparing the concentrations of AmPy-12 in the feed and the
filtrate liquid, the AmPy-12 content in hybrid is finally calculated
to be 9.6 wt%.
It is unquestionable that AmPy-12 is adsorbed on the nano-
tube surface in a noncovalent fashion. However, this naturally
raises the question of what is the exact nature of the nanotube/
Fig. 6 (a) Fluorescence spectra (excited at 344 nm) and (b) decay curves
of emission recorded at 397 nm of (1) AmPy-12 solution and (2) SWCNT/
AmPy-12 dispersion in THF with the same AmPy-12 concentration of 1
Fig. 5 A typical AFM image of the SWCNT/AmPy-12 hybrid.
1932 | J. Mater. Chem., 2012, 22, 1928–1936
mg mlꢂ1
.
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of the local environment of the pyrene moiety. However, AmPy-
12 and the SWCNT/AmPy-12 hybrid show the almost same I1/I3
values of 1.29 and 1.31, respectively, which are quite close to I1/I3
¼ 1.35 for molecular pyrene29 in the same solvent of THF.
Although the mechanism for producing such a puzzling result is
not yet clear, it does suggest that the I1/I3 ratio is not sensitive to
the noncovalent interaction of SWCNTs with pyrene moieties. It
is fortunate that the fluorescence intensity is strongly dependent
on this interaction. Determined from the emission peak area,
95% of the fluorescence emission is quenched in the SWCNT/
AmPy-12 hybrid with respect to that of AmPy-12. Actually, such
a quenching effect has also appeared in other CNT/pyrene
derivative systems and is widely accepted as evidence of the p–p
interaction between nanotubes and pyrene derivatives.22,23,31–34
In order to provide further evidence, emission lifetime
measurements were carried out on the same samples. Fig. 6b
gives the temporal profiles of emission recorded at 397 nm upon
excitation at 344 nm. Compared with AmPy-12, the hybrid
shows a much faster emission decay and its fluorescence lifetime
has shortened to 7.8 ns from 24.6 ns for AmPy-12, again sug-
gesting that the pyrene emission is quenched by SWCNTs. The
kinetic behavior, hence, provides a firm indication that there
exists a strong molecular level interaction, i.e. p–p interactions,
between SWCNT and AmPy-12.
electronic structure, thus implying that the electrical property of
the SWCNTs remains basically unchanged after the functional-
ization with AmPy-12.
The optimized SWCNT dispersion, which contains 75.4 mg
Lꢂ1 of SWCNTs and 0.08 mg mlꢂ1 of AmPy-12,36 was used for
the preparation of the epoxy composites. For comparison, the
dispersion with an absence of AmPy-12 was also employed.
Table 2 summarizes the mechanical properties of neat epoxy
resin and its composites. Each datum reported here is an average
of at least five measurements. With increasing SWCNT content,
both the tensile strength and Young’s modulus of the composites
filled with AmPy-12-functionalized SWCNTs (SWCNT/EP–
AmPy-12) increase and reach the maxima at a content of 0.3 wt
%. Further increase of the SWCNT content leads to a weakening
of tensile properties. The maximal tensile strength and Young’s
modulus were 51.55 ꢃ 3.61 MPa and 2.01 ꢃ 0.18 GPa, respec-
tively, representing 54% and 27% increments relative to those of
neat resin. In contrast, the SWCNT/EP composite acquires its
optimal tensile strength (40.76 ꢃ 4.71 MPa) and Young’s
modulus (1.77 ꢃ 0.15 GPa) at 0.5 wt% nanotube content, which
are 22% and 12% above neat resin and fall within a reasonable
range considering the smooth and inert surface, as well as the
intrinsically bundled nature of pristine SWCNTs. Moreover, it is
observed that the addition of SWCNTs slightly increases the
elongation at break, indicating that the toughness of EP is not
weakened.
The remarkable electrical conductivity of SWCNTs stemming
from their unique electronic structure is highly desired for
multifunctional composites. UV-Vis-NIR measurements were
carried out to examine the electronic structure of the SWCNTs
before and after the adsorption of AmPy-12 (Fig. 7). The
absorption features within the recorded wavelength range of
400–1600 nm are usually interpreted as the interband electronic
transitions of SWCNTs:35 first transitions for metals (M11, 400–
550 nm) and first and second transitions for semiconductors
(S11, 850–1600 nm; S22, 550–850 nm). No evident spectral
differences were noticed between the SWCNTs and hybrid. The
strong p–p interactions between the SWCNTs and AmPy-12,
which has been attributed to the disruption of p-conjugation by
a conformational change,34 does not change the nanotubes’
For exploring the reinforcement mechanism, the SEM study
was performed on the fracture surface of SWCNT/EP–AmPy-12
composite with 0.3 wt% SWCNT content. As observed in
Fig. 8a, the AmPy-12-functionalized SWCNTs are uniformly
dispersed and tightly embedded in the epoxy matrix. A magnified
observation shows that some nanotubes span the microcracks
(Fig. 8b), suggesting strong interfacial adhesion between the
epoxy matrix and SWCNTs. The strong interfacial interaction is
further argued by nanotubes sheathed with epoxy resin (Fig. 8c).
In the control study, SWCNTs form into agglomerates in the
SWCNT/EP composite containing the same SWCNT content,
displaying a very poor dispersing state (Fig. 8d). In addition, due
to weak interfacial interactions, many nanotubes were pulled out
from the matrix and float over the fracture surface (Figs 8d and
e). It is well-known that good dispersion decreases the stress
concentration centers, affords more uniform stress distribution
and increases the interfacial area for efficient stress transfer.1 A
good interfacial adhesion between the nanotubes and matrix
further enhances these effects.37 From the SEM observation,
application of AmPy-12 realizes both a uniform dispersion of
nanotubes and their strong adhesion to the epoxy matrix, thus
ensuring superior mechanical properties of the resultant
composites. Even so, it should be pointed out that there exists an
upper limit for AmPy-12-functionalized SWCNTs to be well-
dispersed in the matrix. Fig. 8f reveals that SWCNTs tend to
agglomerate, while the content increases to 0.5 wt%. Such
observation makes a good response to the mechanical test
results.
In the presence of AmPy-12, enhanced interfacial adhesion is
attributed to the strong p–p interactions between the SWCNT
and AmPy-12, as well as the covalent bonding of AmPy-12 with
the epoxy matrix (Fig. 1). The formation of new bonds was
confirmed by FTIR analyses. For easily detecting the reaction,
Fig. 7 UV-Vis-NIR spectra of the dispersions of (a) the SWCNTs and
(b) SWCNT/AmPy-12 hybrid in THF.
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Table 2 Mechanical and thermal properties of neat epoxy resin and its composites. Datum within parentheses is % increase relative to neat resin.
SWNT content/wt (%)
0
0.1
0.2
0.3
0.5
1.0
Tensile Strength/
MPa
Young’s Modulus/ SWCNT/EP
SWCNT/EP
SWCNT/EP–AmPy-12
33.4 ꢃ 2.9 34.9 ꢃ 3.0 (4)
35.4 ꢃ 3.7 (6)
36.3 ꢃ 3.1 (9)
39.8 ꢃ 4.3 (19) 40.8 ꢃ 4.7 (22) 37.5 ꢃ 4.5 (12)
42.4 ꢃ 2.7 (27) 51.6 ꢃ 3.6 (54) 47.2 ꢃ 4.2 (41) 46.1 ꢃ 4.4 (38)
1.68 ꢃ 0.15 1.68 ꢃ 0.17 (6) 1.67 ꢃ 0.14 (6) 1.73 ꢃ 0.16 (9) 1.77 ꢃ 0.15 (12) 1.72 ꢃ 0.17 (9)
1.74 ꢃ 0.15 (10) 1.80 ꢃ 0.17 (14) 2.01 ꢃ 0.18 (27) 1.89 ꢃ 0.17 (20) 1.82 ꢃ 0.18 (15)
GPa
SWCNT/EP–AmPy-12
SWCNT/EP
Elongation
at Break/%
Tg/ꢀC
2.6 ꢃ 0.2
2.6 ꢃ 0.3 (0)
2.7 ꢃ 0.2 (4)
2.8 ꢃ 0.3 (8)
2.8 ꢃ 0.3 (8)
2.8 ꢃ 0.4 (8)
SWCNT/EP–AmPy-12
SWCNT/EP
SWCNT/EP–AmPy-12
2.7 ꢃ 0.3 (4)
2.8 ꢃ 0.3 (8)
3.4 ꢃ 0.3 (31)
3.1 ꢃ 0.3 (20)
3.1 ꢃ 0.3 (20)
70
70
75
68
73
56
70
57
62
–
–
behavior. The dependency of conductivities (s) on SWCNT
content (m) can be described using the scaling law38
s ¼ s0(m ꢂ mc)b, for m > mc
(1)
where mc is the mass fraction at the percolation threshold, s0 and
b are constants, and b should be close to 2 for a three dimen-
sional (3D) percolating system.39 To easily calculate the perco-
lation threshold, eqn (1) is transformed to the logarithmic form
of eqn (2):
logs ¼ blog(m ꢂ mc) + logs0
(2)
By evaluating the linearity of the logs vs log(m ꢂ mc) graph
until the lowest possible SWCNT content, the mc values are
determined to be 0.1 wt% (b ¼ 2.02) for SWCNT/EP–AmPy-12
composites (Fig. 10b) and 0.4 wt% (b ¼ 1.99) for SWCNT/EP
composites (Fig. 10c). These values suggest that in the presence
of AmPy-12, 0.1 wt% of SWCNTs in the epoxy matrix is already
enough to form a 3D network as an effective conductive path. In
the absence of AmPy-12, however, the dispersion quality of the
SWCNTs becomes poor and more nanotubes (0.4 wt%) are
required to construct the similar conductive path. One of the
Fig. 8 SEM images of (a–c) SWCNT/EP–AmPy-12 composite with
0.3 wt% SWCNTs, (d) and (e) SWCNT/EP composite with 0.3 wt%
SWCNTs and (f) SWCNT/EP–AmPy-12 composite with 0.5 wt%
SWCNTs.
a blend containing a larger weight ratio of AmPy-12 to Epofix
resin (60/40) was heated at the curing condition of composites.
After heating, the viscous blend becomes a hard, but brittle solid.
The FTIR spectra show that the bands for epoxy group
(912 cmꢂ1) in Epofix resin (Fig. 9a) and for –NH2 (3415 cmꢂ1
)
and –NH– (3290 cmꢂ1) species in AmPy-12 (Fig. 9b) still appear
in the as-prepared blend (Fig. 9c), but cannot be detected
anymore in the thermally treated sample (Fig. 9d). A new and
broad peak centered at 3365 cmꢂ1, assigned to –OH stretching,
presents in the spectrum of the treated sample (Fig. 9d), sug-
gesting the reaction of AmPy-12 with Epofix resin.
The electrical conductivities of the composites were also
characterized. As shown in Fig. 10a, the conductivities of both
SWCNT/EP–AmPy-12 and SWCNT/EP significantly improve
with increasing SWCNT content, showing a typical percolation
Fig. 9 FTIR spectra of (a) Epofix resin, (b) AmPy-12, (c) the as-
prepared blend of AmPy-12 and Epofix resin (weight ratio of 60/40)
and (d) thermally treated sample (blend in c) at 50 ꢀC for 12 h and 80 ꢀC
for 2 h.
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criterion, however, about 8–20 wt% of carbon black should be
added into epoxy resin.41 This means that antistatic, but much
lightweight epoxy composites are in prospect.
With addition of AmPy-12, the thermal properties (typically
glass transition temperature (Tg) determined by DSC) of
SWCNT/EP–AmPy-12 composites are improved relative to neat
resin and SWCNT/EP composites (Table 2). The increased Tg
results from the good nanotube dispersion and strong interfacial
interaction, which restricts the segmental motion of epoxy-
hardener molecules. Beyond 0.3 wt%, a further increase of the
SWCNT content leads to the reduction of Tg, indicating the
lower curing degree in the composite. This may possibly be due
to the formation of SWCNT agglomerates (Fig. 8f) that in turn
hinder the local curing reaction of epoxy with hardener.
4. Conclusions
Three reactive AmPys with various spacer chain lengths, i.e.
AmPy-2, AmPy-6 and AmPy-12, were synthesized for engi-
neering a SWCNT/epoxy matrix interface at a molecular level.
The particularly designed amino group enables AmPy to be
covalently bonded to the epoxy network, whereas the pyrene unit
provides AmPy with the ability to be strongly adsorbed on the
nanotube surface through p–p interactions. By setting two
criterions of (1) the functionalization efficiency for pristine
SWCNTs and (2) the dispersion stability, AmPy-12 was opti-
mized for the preparation of epoxy composites. Due to homo-
geneous dispersion and enhanced interfacial interaction, only 0.3
wt% of SWCNTs increased the tensile strength and Young’s
modulus of the composite by 54% and 27%, respectively, over
that of neat epoxy resin. A low electrical percolation threshold of
0.1 wt% SWCNTs and an improved thermal property were also
observed. In comparison, the composite with an absence of
AmPy-12 shows a 22% increment of the tensile strength and 12%
of Young’s modulus at an optimum SWCNT content of 0.5 wt%.
The corresponding percolation threshold was up to 0.4 wt%. The
application of AmPy-12 thus affords multifunctional, but light-
weight epoxy composites. Finally, the concept of Interface
Molecular Engineering of the CNT/epoxy composite system
demonstrated here may be easily extended to other CNT/poly-
mer systems, providing a general strategy for developing high-
quality CNT/polymer composites.
Acknowledgements
The financial support from NSF of China (No. 20704024 and
51173091), NSF of Shandong province (No. Q2007F08) and
Taishan Mountain Scholar Constructive Engineering Founda-
tion are gratefully acknowledged. Y. H. Yan would also like to
thank the Alexander-von-Humboldt-Stiftung (Germany) for
a fellowship.
Fig. 10 (a) Electrical conductivity of the epoxy composites as a function
of the SWCNT content (m) and the linear behavior of conductivity
against (m ꢂ mc) with mc of (b) 0.098 wt% for the SWCNT/EP–AmPy-12
composite and (c) 0.39 wt% for the SWCNT/EP composite.
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