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Composite Interfaces, Vol. 11, No. 8-9, pp. 567586 (2005) VSP
2005.Also available online - www.vsppub.com
Review
Carbon nanotube-polymer nanocomposites:The role of
interfaces
C. VELASCO-SANTOS 1,2,3, A. L. MARTINEZ-HERNANDEZ1,2,3and V. M.
CASTANO 1,1 Centro de Fsica Aplicada y Tecnologa Avanzada,
Universidad Nacional Autonoma de Mexico,
A. P. 1-1010 Quertaro, Quertaro 76000, Mexico2 Department of
Materials Science, University of North Texas, Denton, TX
76203-5310, USA3 Departamento de Materiales, Departamento de
Mecatronica, Instituto Tecnolgico de Quertaro,
Av. Tecnolgico, Col. Centro, Santiago de Quertaro, Quertaro
76000, Mexico
Received 25 June 2004; accepted 14 September 2004
AbstractResearch aimed at producing new nanocomposites with
improved properties has dramat-ically increased in the last decade,
especially on materials tailored at a nanometric level, such
asfullerenes and carbon nanotubes. The use of nanoforms as
reinforcement of organic polymers hasopened the possibility of
developing novel ultra-strong and conductive nanocomposites.
Neverthe-less, the challenge of manufacturing multifunctional
composite materials based on nanostructures isstill open, in
particular in the details of the corresponding interfacial
properties, which are particularlyrelevant in these systems. This
paper reviews the main technical activities in this field, focusing
on themost important parameters that influence the behavior of
their interface, discussing recent advances,as well as current and
future trends in research.
Keywords: Carbon nanotubes; polymer composites; interface;
chemical functionalization.
1. INTRODUCTION
Carbon nanotubes (CNs) are among the most interesting materials
discoveredduring the last twenty years; these materials have
diverse arrangements at ananometric level that lead to different
properties depending on the specific kindof nanotubes. In fact, a
number of different types of nanotubes, from single-walled
nanotubes (SWNTs), double-walled nanotubes (DWNTs), and
multiwallednanotubes (MWNTs) to their variants with helix and
bamboo shapes are alreadyknown to this date.
To whom correspondence should be addressed. E-mail:
[email protected]
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568 C. Velasco-Santos et al.
Also, a number of methods have been reported so far for
producing carbonnanotubes: arc discharge [1, 2], pyrolysis of
hydrocarbons over catalysts [3], laservaporization of graphite [4],
electrolysis of metal salts with graphite electrodes[5], and
hydrothermal methods [6]; also, nanotubes have been found recently
tobe produced under relatively mild natural environments [7].
CNs display different properties that suggest potential for use
in various advancedtechnological applications, such as tips for
Atomic Force Microscopy (AFM) [8],cells for hydrogen storage [911],
nanotransistors [12, 13], electrodes for electro-chemical
applications [14, 15], sensors of biological molecules [16],
catalysts [17],etc. In particular, one possible application that
has attracted a great deal of attentionin the materials engineering
community is the incorporation of CNs as reinforce-ment of
composite materials, mainly within an polymeric matrix. These
materialsare expected to exhibit outstanding mechanical properties,
such as extremely highYoungs modulus, stiffness and flexibility, as
has been demonstrated by both ex-perimental data and theoretical
models [1821]. In addition, the unique electronicproperties of
nanotubes suggest possibililities for use as either semiconductor
ormetallic conductive nanomaterials [22]. Also, these structures
possess high thermalstability [23], which could be advantageous for
aerospace applications. Addition-ally, the nearly perfect structure
of CNs, their small diameter, and their high surfacearea and high
aspect ratio, provide an amazing inorganic structure with unique
prop-erties extremely attractive to reinforcing organic
polymers.
However, in spite of the fact that CNs were originally
discovered in 1991 andthat the first report on their amazing
mechanical and electronic properties appearedin 1996 [18, 22], the
idea of the using CNs to produce new composite materialshas only
just begun to be explored in the past five years, when different
papershave reported the incorporation of CNs in various polymer
matrices, aiming tosynergetically increase diverse properties of
the material. Results so far have beencontrasting, since several
types of CNs have been used: some composites were madewith SWNTs,
others by using MWNTs, and the difference in the number of
wallscauses important variations on the diameter of the nanotubes,
which in turn producesdifferent characteristics in the composites.
It has also been found that the specificroute used to synthesize
CNs yields different graphitization degrees and, therefore,diverse
properties of the final material. Various polymers have been used
along withdifferent processing methods and, recently, some groups
have used functionalizedcarbon nanotubes (f-CNs) to improve
interfacial interaction between the inorganicfiller and the organic
matrix. In general, the field of developing strong, conductiveand
smart materials based on CNs is just beginning and promises to
become a veryimportant area of R&D.
Accordingly, in this review we have compiled and discussed the
leading resultsavailable in this emerging field of materials
science; with special emphasis on therole of interfaces, aiming not
only to provide an insight into the field, but also to tryto
foresee the main areas of activity in the coming years.
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Carbon nanotube-polymer nanocomposites: The role of interfaces
569
2. PROPERTIES OF CARBON NANOTUBES
Carbon nanotubes possess many features that make them unique
materials: forexample, although standard carbon fibers have long
been utilized in industrialcomposites as reinforcements for
polymers that produce excellent properties, suchas low density with
high specific strength and specific modulus, CNs have
superiorproperties as compared to any carbon fiber, in spite of
being chemically equivalent.
The very first paper reporting the Youngs modulus of CNs-based
composites waspublished by Treacy et al. [18], where a figure of
1.8 TPa was found as an average,and these authors mentioned there
that Youngs modulus in SWNTs can reachvalues as high as 5 TPa.
Recently, Yu et al. [21] found smaller Youngs modulus forMWNTs.
However, the results varied depending on the specific approach used
toperform the mechanical probes and also on the particular type of
NTs. For instance,Wong et al. [24] calculated the Youngs modulus
through atomic force microscopy(AFM) and found a value of 1.28 TPa
for MWNTs. The method was applied tosilicon carbide nanorods which
showed a Youngs modulus of around half of that ofMWNTs. There, the
modulus was calculated from bending force vs.
displacementcurves.
Other studies have evaluated the mechanical properties of carbon
nanotubesproduced by arc-discharge and catalytic methods, since
these two kinds of CNspresent different advantages over each other.
For example, catalytic CNs canbe produced in aligned patterns,
which produces useful results like well-definedarrangements and
uniform electrical properties in the composites. In addition,the
principle used promises to become an important tool to produce CNs
in largequantities. However, the degree of graphitization in these
structures is poor and thisis detrimental to the mechanical and
electronic properties.
On the other hand, CNs produced by arc-discharge methods show a
high degreeof graphitization and excellent mechanical and
electrical properties, which areideal when these nanomaterials are
used in polymeric matrix composites. Thedisadvantage is that they
have a high content of impurities, which limits theirpossible uses.
The arc-discharge approach to produce SWNTs requires
catalyzedreactions, which induce defects in some regions where the
bonds between the carbonmaterial and the metallic element of the
catalytic particle are formed, in the sameway as those produced by
catalytic methods. However, SWNTs produced by thismethod are the
strongest fibers known to date, though the SWNT bundles formed
bythis process could cause some problems in the dispersion of
nanotubes in polymercomposites.
In this context, Salvetat et al. [25] evaluated the mechanical
properties ofarc-discharge MWNTs (arc-MWNTs) and grown catalytic
MWNTs (cat-MWNTs)using AFM. The results show an important range in
the elastic modulus betweentwo forms of CNs: the average elastic
modulus (E) for arc-MWNTs has a valueof 0.87 TPa while E for
cat-MWNTs is around 0.027 TPa [25]. However, anotherrecent report
indicates that the tensile strengths of these CNs might be
relativelyhigh, inasmuch as the defects could assist in stress
transfer between layers [26].
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570 C. Velasco-Santos et al.
Another team has measured the elastic modulus by nanoindentation
in vertically-aligned multiwalled carbon nanotubes produced by
catalytic method with PlasmaEnhanced Chemical Vapor Deposition
(PECVD) [27]. The parameters evaluated inthese CNs are: effective
bending modulus (Eb), axial modulus (Ea) and nanotubewall modulus
(Ew). The mechanical results for these catalytic CNs are
morepromising than others with the aim of producing polymer
composites, since Ebvaries from 0.9 TPa to 1.24 TPa, Ea is in the
range of 0.9 to 1.23 TPa and Ew hasa value between 4.14 and 5.61
TPa. The latter figure is in agreement with thoseobtained by
theoretical calculations with the local density approximation
model[28] and with the finite element approach [29], which report
effective moduli of4.7 TPa and 4.84 TPa, respectively. Other
important reports have obtained differentresults in simulations
with respect to the dependence of the elastic properties
ofnanotubes; Lu found that these properties in CNs are insensible
to helicity, numberof walls and size, with an elastic modulus
around 1 TPa for all nanotubes with aradius larger than 1 nm [30].
However, other research with similar calculations hasfound that
elastic modulus is dependent on the structure and tube diameter
with avalue for E of 1.24 TPa [31]. As mentioned before, the vast
majority of theoreticaland experimental results agree that CNs
produce the strongest fibers in the world.
Table 1.Comparison of diameter, mechanical and electrical
conductive properties in different carbon materials
Carbon fibersa Youngs modulus Electrical resistivity
Diameter(precursor) (GPa) (104 cm) (nm)Rayon 390 10
70009000Polyacrylonitrileb 230 18 50007000L.T.c pitch mesophase 41
100 11 000H.T.d pitch mesophase 41 50 11 000Fibras tipo carburo 340
9 5000resina L.T.cFibras tipo carburo 690 1.8 5000resina
H.T.dGraphitee 1000 0.4 hCarbon nanotubes De 800 a 5000f 0.05 a 2g
0.490
0.05 a 1.2ga Reference [32].b In the reference [33] are cited
different carbon fibers that were developed using
polyacrylonitrile
as precursor. The elastic modulus differs from 220 to 390 GPa in
different commercial fibers such as:AVCO, Celanese, Courtaulds,
Great Lakes, Carbon, Hercules, Union Carbide, USA, Toray.
c Low temperature.d High temperature.e The values were
calculated in the sheet plane.f The majority of the elastic modulus
values in different researches fluctuate between these ranges.g
Values of different researches Ref. [22] and cited in the reference
[34]. Note: The values fluctuate
depending on the arrangement in CNs.h Does not apply because the
graphite is a sheet.
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Carbon nanotube-polymer nanocomposites: The role of interfaces
571
Table 1 shows a comparison of the mechanical properties and
other features betweencarbon fibers, graphite and carbon nanotubes
of different sources.
There is no doubt that another important reason to incorporate
CNs in nanocom-posites is the electronic arrangement of these
materials, inasmuch as different workshave shown that CNs can be
metallic or semiconducting depending on their diam-eter and
structure [3537] and another report has shown that the properties
changein each tube depending on its structure [22]. Thus, the
electrical conduction range israther broad, which diversifies
potential uses of CNs. Table 1 also shows a compar-ison of
electrical properties of CNs with other carbon materials. These
propertiescould be very useful in order to use CNs in
multifunctional composites. However,other features need to be taken
into account if CNs are to be effectively incorpo-rated in polymer
composites, since CNs must take part in the load transfer througha
good interfacial interaction, to truly take advantage of their
excellent mechanicalproperties.
It is expected that a deformation in the CNs could modify their
electrical prop-erties in some parts of the nanotube and change the
behavior of the composite.Accordingly, the effect of the
deformation in SWNTs has been calculated, wherethe electronic
structure, and therefore electrical conduction, is affected when
thenanotubes are deformed [38]. The authors found that the
conducting SWNTs maybecome semiconducting once they were
mechanically deformed, but semiconduct-ing SWNTs will never become
conducting when they are mechanically deformed.This could be useful
to build novel smart polymeric composite materials, since
adifferent electrical response could be expected depending on the
mechanical defor-mation.
Other studies of SWNTs, produced by approaches such as laser
ablation andarc-discharge, were aimed to form ropes and bundles
[39], and some theoreticalresearches have studied the mechanical
properties of these bundles in differentconditions, with the object
of predicting mechanical and electrical properties ofthese
aggregates of CN groups. These models could be useful in
predictingproperties and employing CNs in composites and other
applications [40, 41].
In addition to the relevant CN properties discussed above, there
exist otherimportant features that position CNs well above any
other existing reinforcementfibers for polymer composites. Indeed,
in nanotubes, the nanometric scale diameteris a key factor in
improving the interface, since their very small size producesclose
interaction at the molecular scale, useful in minimizing the zones
withoutcontact at the interface, enhancing the interactions and
thus providing compatibilitybetween two quite different materials:
inorganic CNs and organic polymer matrix.CN diameter plays an
important role in providing a near-molecular interaction, andthe
corresponding high aspect ratio is key to reinforce composites.
Another important characteristic of CNs becomes evident as they
are first de-formed and then the stress released: they recover
their original shape. This flexibil-ity gives nanotubes unique
opportunities as reinforcements, for neither carbon fibersnor other
known reinforcements exhibit this property [26]. It is known that
the syn-
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572 C. Velasco-Santos et al.
thetic fibers used nowadays are more sensitive to fracture when
they are deformed.In addition, the mechanical deflections in CNs
can be electrically induced [42].
All the properties described above make CNs exciting materials
to developcompletely new polymer composite materials. However, the
development ofthose novel composites still has important barriers
to overcome, mainly relatedto the understanding of the phenomena at
the interface between CNs and organicpolymers. The path to develop
these new materials began only a few years ago,and the results
available at this time are scattered and partial and
sometimesmisleading, for some studies promise that these materials
are ready for technologicalapplications, while other reports reveal
still a poor adhesion and poorly performingcomposites. Thus, it is
extremely important to analyze and discuss the resultsreported so
far in the light of the underlying fundamental scientific problem:
theinterfaces.
3. CARBON NANOTUBE POLYMER NANOCOMPOSITE
Different approaches have been used to produce carbon nanotube
polymer nanocom-posites (CNPN) from melt blend [43], dissolving and
casting [44], in situ polymer-ization [45] and extrusion [46] among
others. Nevertheless, the first studies thatincorporated nanotubes
in polymers were developed to align and to observe thepossible
interaction of CNs with the polymer. The first report where CNs
wereincorporated in polymer dates back ten years. In this study
Ajayan et al. [47]aligned CNs in an epoxy matrix, then cut small
slides to obtain a partially alignedsegment. Aligned carbon
nanotubes in polymer matrices can be considered as adesigned
arrangement, inasmuch as this would produce possibilities of
control ofthe properties along the direction of the carbon
nanotubes in the nanocomposites.This first evidence of aligned CNs
in a polymer matrix applies only to small areasof these
nanocomposites. Jin et al. [48] report alignment of CNs in polymer
ma-trix by mechanical stretching. They used MWNTs synthesized by
the arc-dischargemethod and the thermoplastic polymer
polyhydroxyaminoether that was dissolvedin chloroform and later
molded in Teflon. The results show a partial alignment thatdepends
on the stretching ratios and on the carbon nanotube fraction.
In another report, SWNTs are aligned by a combination of solvent
casting andmelt processing methods: there the CNs were dispersed in
polymethylmethacrylate(PMMA) using dimethylformamide as solvent.
Alignment in composites provideshigher conductivity along the flow
direction. Elastic modulus (E) and yieldstrength (Y ) were measured
in melt-spun fibers: both E and Y increase withnanotube content and
draw ratio. Although these increases are not uniform, in
bothparameters higher values are reached with 5 wt% and 8 wt% of
SWNTs [49].
Wagner et al. developed an interesting study where stress
transfer ability wasshown in carbon nanotube polymer nanocomposite.
In this report, the fragmentationof MWNTs under tensile stresses in
thin films was observed. The results showthat the efficiency to
transfer stress in this composite is at least one order of
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Carbon nanotube-polymer nanocomposites: The role of interfaces
573
magnitude larger than in conventional fiber composites [50]. A
later report byLourie and Wagner indicates that the polymer
nanotube interface is not inert andload is transferred to the
nanotubes via the surrounding matrix. In this paper, itwas
suggested that, in spite of the huge difference in scale between
traditional fibercomposites and carbon nanotube nanocomposites, the
fundamental concepts of themechanics of conventional composites
could hold at the nanometric level [51].
Deformation studies by transmission electron microscopy (TEM) of
polymerCN composites were developed with a nanocomposite containing
MWNTs in apolystyrene matrix. The studies were realized using a TEM
strain holder and a filmwith a notch. The results show that
nanotubes tend to align and bridge the crackwhen a tensile stress
was applied to the composite, showing load transfer across
thenanotubepolystyrene interface [52].
Other PMMA-based studies have explored other parameters, such as
CN distrib-ution and the possible modification of CNs as they are
incorporated in a polymermatrix. The studies were realized with
SWNTs obtained from arc-discharge methodand not purified. The
investigation shows that low concentrations of CNs in polymergive a
uniform distribution with this kind of CN [53, 54].
Although some papers claim a good distribution and load transfer
at the interface,this should be reflected in the properties of
nanocomposites. However, contradictoryresults have been obtained in
this kind of composite, mainly with respect tothe mechanical
properties, showing that many factors affect the production,
theinterface and, therefore, the successful development of carbon
nanotube polymernanocomposites.
Schadler et al. developed MWNTs-epoxy composite cured with
triethylene tetra-amine. They evaluated both tensile and
compression strength. The results show thatcompression modulus is
higher than tensile modulus in this composite. The loadtransfer was
evaluated through Raman spectroscopy, and it was found that strain
incarbon bonds only shifts significantly under compression. The
authors speculatethat all the walls in MWNTs only participate in
compression, unlike the case whenthe composite is subjected to
tensile stress, where the outer walls participate [55].
Shaffer and Windle formed CN-polyvinyl-alcohol nanocomposite
with differentloads of catalytically grown nanotubes: the content
of CN was varied from 10 wt%to 60 wt% and dynamical mechanical
parameters were measured. The values ofstorage modulus (E)
increased over two-fold, but only by using a huge quantityof CN. At
the same time, the onset of thermal degradation was retarded in
thiscomposite. Therefore, these authors suggest the application of
CNs as modifiers.Also, the conductive properties in composites were
measured, and a percolationthreshold was found at a concentration
between 5 and 10% of CN [56]. In otherstudy, MWNT-PMMA composites
were developed and tested mechanically. Twotypes of MWNTs were
incorporated in the polymer matrix: as-synthesized andtreated MWNTs
(t-MWNTs: CNs ground in a high rate ball mill for 20 min andthen
boiled for 0.5 h in nitric acid); the method used to form the
compositeswas polymerization using azoiso-butyronitrile (AIBN) as
initiator. Satisfactory
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574 C. Velasco-Santos et al.
increases in the tensile strength and hardness were reached with
3 wt% and 5 wt%of t-MWNTs; however, with higher loads the tensile
strength decreased. The resultsare attributed by the authors to the
good interaction between CNs and polymermatrix, due to the fact
that the initiator could break the bonds of CNs and, inthis way, it
is possible to form a chemically bonded interface. However,
manyother factors could also be responsible for this success: for
example, the fact thatt-MWNTs were treated with nitric acid
solution could cause a slight oxidationin some zones of the
nanotubes, where the defects might be increased by theinitiator
action, producing functionalized nanotubes that would interact with
thepolymer chains. More research is needed to confirm this and to
understand the roleplayed by the polymeric initiator in CN bonds
and on the development of nano-composites [57].
PMMA is one of the most studied polymers for these composites.
Cooper et al.evaluated mechanical properties of nanocomposites
formed with MWNTs, SWNTsand nanofibrils. The results of these
analyses present a tensile modulus almostinsensitive to the
presence of CNs; however, the impact properties are
significantlyimproved by both MWNTs and SNWTs [58]. Tatro et al.,
in another recentstudy with PMMA and MWNTs, have evaluated thermal
and electrical properties,Vickers microhardness and dynamical
mechanical analysis in nanocompositesirradiated with a Cesium-137
source, with the aim of determining the effect ofcarbon nanotubes
when the sample is irradiated. The results show that MWNTsmay
improve radiation hardness of mechanical properties and glass
transitiontemperature when a composite is formed. The effect is
attributed to conjugatedbonds in CNs, which absorb some part of the
radiation energy, limiting the damageof PMMA molecules. More aging
studies with radiation and evaluation of theinterface are proposed
and currently being developed by these authors [59].
Another recent report evaluates the dynamical mechanical
properties of purePMMA and nanocomposites formed with SWNTs and
PMMA matrix at twodifferent frequencies. The study was developed
with the aim of understanding theeffect produced by the
incorporation of the nanotubes on the ability of PMMAto store and
dissipate the mechanical energy. SWNTs used in this researchwere
produced by laser ablation method and the nanocomposites were
preparedby in situ polymerization using a solution of
dimethylformamide (DMF) and2,2-azobisisobutyronitrile (AIBN) as
initiator. -Relaxation, related with theonset of the glass
transition temperature (Tg) and -transition associated withhindered
rotation of the side chain, were evaluated in PMMA and
SWNT-PMMAnanocomposites. Transitions were detected in both tan
curves and elastic moduluscurves (E) at low and high temperatures.
Results at two different frequenciesshow that PMMA and
nanocomposites formed with slight quantity of SWNTsexhibit similar
-transition temperature and identical Tgs. However, E at
lowtemperature (150C) in the nanocomposite with a tiny quantity
(0.014 wt%) ofSWNTs exhibits an important increase in comparison
with E of the pure PMMA,from 5.1 GPa in PMMA to 7.2 GPa for the
nanocomposite, both evaluated with a
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Carbon nanotube-polymer nanocomposites: The role of interfaces
575
frequency of 100 rad/s. This represents a significant increase,
considering the smallquantity of SWNTs used.
Thermogravimetric analysis (TGA) and nuclear magnetic resonance
(NMR)studies have been used to evaluate possible changes in the
structure of the polymernanocomposite caused by the addition of
SWNTs. The authors cite that neithermicrostructure nor thermal
degradation changes were localized in nanocomposites,as compared to
pure PMMA. This indicates that these parameters could not beliable
for the change in the mechanical properties of the nanocomposites.
However,the interface in these nanocomposites plays an important
role and the propertiesof the polymer changed in the vicinity of
the SWNTs, probably by cohesiveinteractions produced by the large
surface area of CNs [60]. This researchalso involved discussion and
possible adaptation of the traditional theory of CNnanocomposites;
the authors found that nanotubes have a greater effect on
theproperties of composites than those expected from traditional
composite theory [60].Therefore, new models will be needed to
adjust and predict CN nanocompositeproperties because the interface
produced with polymers and nanomaterials isgenerating unexpected
properties.
Dufresne et al. processed and characterized nanocomposites that
were developedwith catalyzed MWNTs and poly(styrene-co-butyl
acrylate) [61]. The compositeswere prepared by casting, after
stirring with different loads of CNs from 0 to15 wt%. Mechanical
properties of these composites were evaluated by
dynamicalmechanical analysis (DMA) and tensile test: increases in
the tensile modulus withthe content of CN were found in almost all
samples. However, an unexpecteddecrease in the modulus was observed
in the sample with 7 wt% of CNs. Althoughgood distribution was
achieved in most samples, some areas with nanotubesappeared like
clusters causing unpredictable behavior in those samples. This
hasbeen observed in other composites prepared with CNs when the
content is higherthan 5 or 7 wt% and explains the unexpected
behavior in this particular study.
Wong et al. have analyzed two kinds of nanocomposite that show
agglomeratesthat cause similar effects to those just mentioned.
Although it was shown by fieldemission scanning electron microscopy
(FESEM) and TEM that CNs have goodinteraction and adhesion with the
polymers, other regions show agglomerates ratherthan even
distribution and cause a deterioration in the nanocomposite
properties. Inthis research, CNs reinforcing polystyrene rod and
epoxy thin film were analyzed.In the case of CNpolystyrene
composites, tensile strength increases from 22.1 MPa(for
polystyrene) to 24.4 MPa (for the sample with 0.1 wt% of CN);
however, whenthe content of CNs was increased at 5 wt% the strength
was diminished to 17.9 MPa.Furthermore, the samples with 1 wt% and
2 wt% CN showed lower values in tensilestrength than the sample of
pure polystyrene [62].
Also in this study, epoxy thin films were evaluated
mechanically. Microscopyresults of these samples revealed the two
mentioned regions. One corresponds tothe failure of the matrix but
not at the CN-epoxy interface. Nevertheless, in otherregions, CN
agglomerates were observed showing poor dispersion locally.
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576 C. Velasco-Santos et al.
Finally, the paper published by Wong et al. gives an interesting
analysis involvingmolecular simulations of CN pullout. Molecular
models of SWNTs in polystyrenematrix and SWNTs in epoxy matrix were
generated, and then total work requiredfor fiber pullout was
calculated. The results obtained for CN-epoxy are very similarto
those obtained in experimental pullout for the same system. The
authors concludethat the results of this simulation and the
previous experimental studies suggest thatthe interfacial shear
stress of CNpolymer systems is at least one order of
magnitudehigher than micro fiber reinforced composites and it is
assumed that the surface ofthe CN is important to achieve intimate
contact at nanometric level between CNsand polymer matrix [62].
Thus, these results show that a strong interface can beachieved
between inorganic CNs and organic matrix when the polymer
interactswith nanotubes. However, once more, large quantities cause
problems by formingagglomerates.
An opposite effect to those discussed above has been reported in
CN nanocom-posites when the hardness has been measured; here, the
increase in the quantityof nanotubes incorporated in the polymer
has a favorable effect. The addition of2 wt% of CNs improves the
hardness of an epoxy matrix by some 20%; however,when quantities
higher than 1.5 wt% were used, the hardness diminished as com-pared
to the hardness of pure epoxy. The authors have speculated that
this effectis caused by a weak interface between CNs and the
polymer. On the other hand,a favorable effect on the hardness with
2 wt% of CNs is attributed to the mesh-like structures formed when
the concentration of CNs is increased. The authorsmentioned that
the formation of these structures is caused by the high aspect
ratioof the nanotubes, which produces a tangle arrangement that
enhances the scratchresistance. In this report, different
temperatures were used to treat CN nanocom-posites and to test
their failure after flexural strength in diverse conditions;
sinceCNs have been proposed for new composites for the space
industry these variable-temperature states are needed to simulate
this environment. Results indicate thatthe mesh-like structures
mentioned do not improve the flexure strength under thedifferent
conditions studied. The authors conclude that, in this case, the
cause ofthe reduction could be due to structural non-homogeneity or
the presence of a weakbonding interface [63].
Epoxy matrix is another polymer used in many studies in CN
nanocomposites,showing excellent electrical properties with small
amounts of carbon nanotubes.Allaoui et al. report a percolation
threshold between 0.5 wt% and 1 wt% of carbonnanotubes whereas the
tensile strength improved significantly with 1 wt% of CNs inyield
point and in 10% in strain [64]. However, only a small increase was
observedwhen 4 wt% of CNs was added. As we mentioned, this effect
has been observedby other researchers, with the addition of
unpurified CNs. In fact, the majority ofthe results demonstrate
that the spatial distribution is an open question and that it
ispossible that the inhomogeneity of the samples has a strong
influence, as importantas the type of polymer used [44].
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Carbon nanotube-polymer nanocomposites: The role of interfaces
577Table 2.Comparative results (E, % increase in E) of Carbon
nanotubes composites obtained in differentpolymers matrices at 40C
1 Hz of frequency [44]a
Polymer E (40C, 1 Hz) E (40C) CNs wt%d Increase % Ee
Reference(matrix) MPab CN composite
MPacPMMA =800 =1600 26 100 [43]PS =2400 =3500 5 44 [65]PSBA
=0.681 =1.584 7 132 [61]PVA =5000 =11 200 60 124 [56]MEMA 708 2340
1 230 [44]
a Modified table of the reference [44].b Storage modulus at 40C
of the polymer matrix.c Storage modulus at 40C of the carbon
nanotube composite.d Weight percent of carbon nanotubes in the
composite.e Increase percent in storage modulus of composite with
respect to polymer matrix.
The alignment of carbon nanotubes in nanocomposites has an
important influenceon the electrical properties. Mechanical
properties also change when CNs arealigned. Thostenson and Chou
found relevant improvements in tensile stressproperties and
toughness when 5 wt% of CNs were incorporated into a
polystyrenematrix, up to five times with respect to the randomly
oriented composite, when thesame quantities of CNs were aligned by
extruder [65].
Other important topic that helps to enhance the compatibility at
the interfacelevel between CNs and an organic polymer matrix is the
incorporation of additivesand other polymers that could assist in
the dispersion and compatibility. In thiscontext, different
behaviors have been observed when an additive is incorporatedinto
nanocomposites. Gong et al. [66] incorporated nonionic surfactants
inthermosetting polymer to improve the interfacial interaction
between nanotubes andthe polymer. The results show higher increases
in storage modulus (E) and in glasstransition temperature when both
surfactant and CNs were used, with respect tothe composite where
only carbon nanotubes were incorporated. In both cases only1 wt% of
CNs was employed; however, E with surfactant is increased by more
than30% and glass transition increases by 25C. In contrast, the
addition of CNs withoutsurfactant produced only a moderate increase
in the temperature and mechanicalproperties. In other reports,
better results have been reached without additives,providing great
care is taken during mixing [44]. A non-ionic surfactant was
alsoused but with thermoplastic copolymer, methylethylmethacrylate
(MEMA). Thisstudy is one of the first that compares their own
results with other researchers whereboth only carbon nanotubes and
additives are used. An important increase in E bymore than 200% at
40C is reached with only 1 wt% of CNs, without additives.
Thecomparative results presented involving different groups are
shown in Table 2, forCN nanocomposites where additives were used,
and in Table 3 for studies whereonly CNs were used. The authors
suppose that, although the non-ionic surfactantutilized is a good
dispersant for carbon, the wetting could occur with the
impurities
-
578 C. Velasco-Santos et al.
Table 3.Comparative results of DMA on carbon nanotubes
composites using a surfactant in two differentpolymersa
Sample Epoxy (66) MEMA (44)E40C Tg C (tan )c E 40C Tg C (tan
)c(MPa)b (MPa)b
Polymer 1390 63 708 92Polymer + Surf 1150 62 774 89Polymer 1 wt%
CN 1550 72 2340 99Polymer 1 wt% CN, 1750 88 1983 1021 wt% surf
a Modified table of the reference [44].b Storage modulus at
40C.c Glass transition temperature (tan ).
in the CNs surroundings and an interface is created with only
with some part ofthe CNs. This would produce interfaces that are
not so effective, since they form alittle barrier that produces
different conductivity behavior [67]. The results of theconductive
behavior in these films are currently in preparation.
Reports where other substances take part in the interfacial and
dispersion be-havior of CN nanocomposites were published by Jin et
al. [68]. In this study,MWNTs were sonicated in a dimethylformamide
solution of poly(vinylidene flu-oride) (PVDF). The mix was melt
blended with PMMA to form a nanocompos-ite. The results show that
there is a threshold in the addition of PVDF, since slightamounts
of the PVDF improve the storage modulus, whereas higher PVDF
contentslower the storage modulus.
4. CHEMICAL FUNCTIONALIZATION OF CARBON NANOTUBES
The results analyzed in the previous section show that CNs
obtained from methodsthat include impurities together with the CNs
lead to unpredictable properties, eventhough the composites in some
cases show an important increase in mechanicalproperties below a
content of 5% of CNs. Other papers report a poor interface dueto
poor wettability [69], which could be influenced by the impurities.
Accordingly,different methods have been proposed to eliminate
impurities in CNs. The mostimportant impact has been produced by
oxidation methods which, in addition toreducing impurities, cause
chemical modifications of CNs. This has proven usefulfor composites
and other applications of these materials. This section
describessome important studies in the field of chemical
modification.
The oxidation technique has opened a new gate in this field, by
producingcarboxyl and carboxylate groups on the tips and surfaces
of CNs. These moietieshave been used to attach different chains in
CNs that allow different possible usesfor these materials [70].
Oxidation approaches were developed few years afterCNs were
discovered. The theoretical basis is the different resistance to
oxidation
-
Carbon nanotube-polymer nanocomposites: The role of interfaces
579
(a)
(b)Figure 1. A schematic representation (a) carbon nanotube (b)
oxidized and functionalized carbonnanotube.
between CNs and other nanoparticles formed during the
production. Thus, it wasobserved that the nanotubes when oxidized
are consumed from the tips inward atthe same time [71] and the
surface wall is modified if the oxidation is severe.
Although the first oxidations were developed with the aim of
opening andfilling CNs, the carboxyl, hydroxyl and carboxylate
groups generated through theoxidation on the surfaces of these
materials are useful moieties in order to bondnew reactive chains
that improve solubility, processability and compatibility withother
materials and, therefore, improve the interfacial interactions of
CNs with othersubstances. This route has become one of the most
important: it is the focus of manycurrent researches because of the
outstanding results it has produced [72].
The defects caused by oxidation have been studied and it has
been foundthat nanotubes tolerate a limited quantity of them,
before a macroscopic sampleloses its special mechanical and
electronic properties [7376]. This kind offunctionalization is
currently being researched in order to understand and to improvethe
utilization of these chemical modifications. The organic carboxyl
groupsformed in the nanotube surface are localized on the defects
on functionalizedsingle-walled and multi-walled nanotubes (f-SWNTs
and f-MWNTs, respectively);suitable reactive organic groups for
other chemical chains tend to react in this zone.An idealize scheme
of unfunctionalized and functionalized CNs with these
organicmoieties is shown in Fig. 1.
Next, different reports have focused on the chemical
functionalization afteroxidation. In these papers, the COOH groups
generated in the oxidation processare used to attach different
molecules useful to improve surface compatibility ofCNs with other
materials. The relevance of these groups to join CNs with
organicpolymer matrices is also mentioned.
-
580 C. Velasco-Santos et al.
One of the routes used as complementary procedure to insert long
organic chain isthe use of SOCl2 (thionyl chloride) in dimethyl
formamide. In this way, a long chainof alkylamine (octadecylamine)
has been coupled to activated f-SWNTs [77]. Thismethod is very
useful to insert different chains. The approach also has been
usedto enhance the solubility properties of SWNTs with the
mentioned chains and othermoieties as alkyl-aryl amine and
4-dodecyl-aniline [78]. The same method with asubsequent reaction
has been used to add other organic moieties. For instance, ithas
been followed by a reaction with monoamine terminated poly(ethylene
oxide)(PEO), which was useful to facilitate the linkage of this
chain in a grafted PEO-SWNTs system [79]. Also, the system (f-SWNTS
+ SOCl2) has been used inMWNTs to functionalize these structures
via addition of an aminopolymer, such
aspoly(propionylethylemine-co-ethylenemine) (PPEI-EI) with the
purpose of forminghydrophilic MWNTs.
Direct reactions have been carried out with molten
octadecylamine in carboxyl-terminated SWNTs, in the range of
120130C, forming a carboxylate group inthe SWNT, which interact
with an octadecylamine functionalized free radical [73].This
produces greater solubility in SWNT (s-SWNTs) than those formed
usingSOCl2 [77]. Other researchers have developed a wide variety of
aliphatic aminescoupled to carboxyl groups in the SWNT surface
[80], showing that amines ingeneral can be attached to CNs
successfully. The insertion of these groups in theCN surface could
be useful in improving compatibility between these nanomaterialsand
aminopolymer matrices.
A comprehensive study of SWNTs and MWNTs using carboxyl organic
groupstreated with SOCl2, with further functionalization through
esterification reactionswith lipophilic and hydrophilic dendra, has
been presented recently [81]. Defunc-tionalization reactions of
these nanostructures under alkaline and acid catalyzedhydrolysis
have been carried out to provide evidence of the ester linkages
betweenthe carboxyl-terminated carbon nanotubes and the mentioned
organic moieties [82].
Dodecylamine groups have been bonded to CNs through carboxyl
moieties via theSOCl2 reaction. The functionalized carbon nanotubes
have shown an improvementin the solubility with the polymer matrix
and organic solvents and have beenused to develop carbon
nanotube-oxotitanium phthalocyanine composite. Thematerial showed
an important improvement in photosensitivity with that content
offunctionalized CNs and an important five-fold increase over that
with oxotitaniumphthlocyanine alone [83].
The carboxylic terminated organic groups on SWNTs have been used
by Quinet al. [84] in order to attach n-butylmethacrylate via atom
transfer radicalpolymerization. In this reaction, the
polymerization is controlled and mightbe useful to graft copolymer
blocks and other polymers, thus improving thecompatibility between
nanotubes and synthetic macromolecules.
A recent research study promotes the insertion of many
functional groups onthe CN surface. Using the same reaction,
different organosilanes were added toMWNT via carboxyl groups on
modified CNs. The method allows diversifica-
-
Carbon nanotube-polymer nanocomposites: The role of interfaces
581
tion of CN properties depending on the terminal chain of the
coupled organosi-lane. The oxidation and silanization achieved in
this research yield functionalizedmoieties, which contain
terminated organosilane groups designated as R groupsthat are
bonded to the nanotubes by silanol groups. This has allowed
modifica-tion of organic-terminated CNs with many different chains,
chemically-bonded toMWNTs, with the silanization reaction, where
the R group (methacryl, glycidoxy,etc.) in the organofunctional
silanes can be changed depending on the specific poly-meric matrix
employed. The behavior of these MWNTs changes as the R
terminalchanges, so it has been found that
3-methacryl-trimethoxysilane (3-MAT) producessoluble MWNTs in
organic solvent such as acetone or ethanol [67, 85], since
themethacryl chain attached as R group on the MWNTs surface changes
the chemi-cal character of this material, increasing their
solubility. This does not occur when3-methacryl-mercaptosilane
(3-MPT) is attached in MWNT, for the terminal thiolgroup in the
chain attached to CNs produces different properties on the
MWNTsurface. Figure 2 shows a scheme of the reaction proposed in
these studies.
Chemical functionalization has reached an important position in
the CN field, asdifferent chemical processes have been developed to
diversify CN properties. Themodifications described above represent
only an intermediate stage in the route toimprove the interactions
between nanotubes and organic polymer matrices. Indeed,other
chemical reactions have already been explored to couple these
materials toproteins [86] and other substances. Also, the carboxyl
groups in the CN surfacehave been shown to improve the
compatibility of CNs with biomolecules. This hasbeen employed for
nanotechnology applications [87] and might be an important toolto
develop good compatibility between organic biopolymers and CNs,
which couldfind applications in biomaterials and biomedicine.
Figure 2. Sequence of silanization reactions on nanotubes
surfaces [72].
-
582 C. Velasco-Santos et al.
5. FUNCTIONALIZED CARBON NANOTUBE-POLYMER NANOCOMPOSITES
Chemical modification is not a new tool to enhance the
compatibility between areinforcement and a matrix, since chemical
attack and grafting have long been usedin natural fibers, synthetic
fibers and particles [8890]. However, in the field ofthe
nanocomposites, chemical functionalization offers an important tool
in order toimprove the interface and achieve the amazing properties
that carbon nanotubescan provide. Although different groups are
very active in this field, interfacialinteractions still need to be
fully understood to improve the interface between carbonnanotubes
and polymer matrix. The recent results in chemical modification
ofcarbon nanotubes are promising and could be the way to form
strong and functionalnew advanced materials. Some results obtained
in the area of functionalized carbonnanotube nanocomposites will be
reviewed in this section.
It was described above how the carboxyl groups found in the tip
and surfaceof CNs are useful for the interaction between CNs and
other compounds. Thus,the use of these moieties to improve the link
in CNpolymer composites havebeen proposed, either by inserting
other chemical groups or by using the carboxylgroups produced
during the oxidation [85, 91]. As mentioned before, Jia et al.[57]
suggested that the initiator opens the bonds found in CNs and, in
this way,CNs take part in the polymerization. However, although
opening the bonds inCNs would allow them to join to other chemical
groups, oxidation provides morepossibilities to bond the nanotubes
to the matrix, due to reactive chemical groupssuch as COOH, COO and
C O that are found on the tip and on the wall surface.In another
report Geng et al. [92] use fluorinated single-walled nanotubes
(fl-SWNTs) to enhance the uniformity and the nanotube dispersion
using poly(ethyleneoxide) (PEO) as matrix, and dissolving the
nanotubes in methanol. Nevertheless, arecent report [45] proposes
that the best moment to achieve interactions between thefunctional
groups (found in tips and walls in CNs) and polymer chains is
definitelywhen the polymer is created by in situ processing,
inasmuch as the free radicalformed in monomer molecules by the
initiator could either interact or react with theCN moieties more
easily than when the polymer is made and after it is dissolvedor
melted to produce the composites. In that research, methyl
methacrylatemonomer (MMA), 2-2 azobisisobutyronitrile (AIBN) and
oxidized f-MWNT wereused, and CN composites were produced by in
situ polymerization using AIBNas initiator. The AIBN quantity,
reaction time and temperature were controlled toyield uniform
molecular weights in all samples. Three samples in this research
weremanufactured with only polymer (poly(methyl methacrylate
PMMA)): polymerwith 1 wt% of unfunctionalized MWNTs (u-MWNTs), and
polymer with 1 wt%of functionalized MWNTs (f-MWNTs), additional
composite with polymer and1.5 wt% of f-MWNTs was also produced to
observe the behavior when the additionof f-MWNTs was increased
slightly. Thee interaction of PMMA with f-MWNTswas analyzed by
infrared and Raman spectroscopy. In addition, mechanical
resultsshow an important increase in E at 40C by 66% and 88%, with
1 wt% and1.5 wt% of f-MWNTs, respectively, and both samples
increase the glass transition
-
Carbon nanotube-polymer nanocomposites: The role of interfaces
583
Figure 3. SEM image after tensile test of f-MNWT composite where
it is possible to observe thedispersion of the f-MNWTs and the
wetting of f-MNWTs with the polymer. Copyright AmericanChemical
Society [45].
temperature Tg by some 40C. This is different from the case with
unfunctionalizednanotubes, where E at 40C increased by 50% and the
Tg by only 6C. Anotherimportant fact is that the performance of
f-MWNT composites increases more than11-fold, at relatively high
temperature. A comparison of different properties ofthis and other
f-CN composites is presented in Ref. [72]. A scanning
electronmicrograph (SEM) of functionalized carbon nanotube
nanocomposites is presentedin Fig. 3.
Other recent papers have shown that carboxyl-terminated carbon
nanotubestreated with amines and incorporated into epoxy matrix
have better interactionthan those that were not treated chemically,
showing an important improvementin the interface links between
these two materials. In this study, TEM pullout testswere carried
out, showing a better interaction due to chemical groups on the
CNsurface [93]. Other authors have develop in situ functionalized
MWNTs in phenoxycomposites by melt mixing with
1-(aminopropyl)imidazole [94]. In this research,the composites with
more than 4.8 wt% show higher storage modulus than thepolymer
matrix. In this research the modulus at high temperature is
diminishedwith the incorporation of in situ functionalized
nanotubes.
6. FUTURE PROSPECTS IN CARBON NANOTUBE-POLYMERNANOCOMPOSITES
CNs promise to open up a new age of advanced multifunctional
materials; however,the wide variety of results available today
demonstrate that the addition of largequantities of CN in
polymer-based composites is not favorable in all cases, andthe
reason for this behavior is not completely understood yet. Several
of the
-
584 C. Velasco-Santos et al.
analyzed reports agree that CNs in large amounts form clusters.
This diminishes theinteraction and, therefore, the interface is not
optimized. Nevertheless, when smallquantities of nanotubes are
incorporated into the polymer, the electrical, optical
andmechanical properties improve significantly, reaching important
values unseen inother materials.
The remarkable properties obtained when f-CNs are incorporated
into polymericcomposites represent a promising route to design
ideal materials for aerospace-related structural applications.
However, the field requires much deeper fundamen-tal research.
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