Novel UV initiator for functionalization of multiwalled carbon nanotubes by atom transfer radical polymerization applied on two different grades of nanotubes

Article abstract of DOI:10.1002/pola.24257

A novel nonoxidative method for preparation of functionalized multiwalled carbon nanotubes (MWCNT) has been developed based on a UV sensitive initiator for atom transfer radical polymerization (ATRP). The method has been investigated with respect to ligan

Full text of DOI:10.1002/pola.24257

Novel UV Initiator for Functionalization of Multiwalled
Carbon Nanotubes by Atom Transfer Radical Polymerization
Applied on Two Different Grades of Nanotubes
ANDERS E. DAUGAARD,1 KATJA JANKOVA,1 JESPER BØGELUND,2 JENS KROMANN NIELSEN,2 SØREN HVILSTED1
1Danish Polymer Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark,
Building 227, Sølvtofts Plads, DK-2800 Kgs. Lyngby, Denmark
2Danish Technological Institute, Taastrup, Gregersensvej 3, DK-2630 Taastrup, Denmark
Received 8 July 2010; accepted 23 July 2010
DOI: 10.1002/pola.24257
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A novel nonoxidative method for preparation of
functionalized multiwalled carbon nanotubes (MWCNT) has
been developed based on a UV sensitive initiator for atom
transfer radical polymerization (ATRP). The method has been
investigated with respect to ligands and polymerization time
for the preparation of polystyrene functionalized MWCNT. It
was found that pentamethyldiethylenetriamine (PMDETA) gave
superior results with higher loading in shorter polymerization
time. A comparative study of the method applied on two differ-
ent grades of nonoxidized MWCNT has been performed, illus-
trating large differences in reactivity and polymer loading,
underlining the importance of the choice of MWCNT starting
material. In addition to styrene, also poly(ethylene glycol)
methacrylate (PEGMA) was shown to polymerize from the sur-
face of the MWCNT. Finally, initial results from composites of
C
V
polystyrene or polyphenylenesulfide are presented.
2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
4594–4601, 2010
KEYWORDS: atom transfer radical polymerization; composites;
multiwalled carbon nanotubes; polystyrene; surface initiation;
ultra violet light
INTRODUCTION The use of carbon nanotubes (CNT) to rein-
force materials has received much attention in recent years.
The possibility of reinforcing polymers with these extremely
strong additives and obtaining composites of superior prop-
erties is very appealing. Many publications within the field
illustrate that the potential could be realized, however, so far
the improvements seen in mechanical properties are still
inferior to what is expected to be the potential of CNT
composites.1
results. Functionalization of CNTs can be done through a
number of methods.6,7 Often the reported methods are
combined with acid treatments to purify the tubes prior to
reaction. These methods are known to cause degradation
and generate defects in the CNT and to introduce chemical
groups such as acids, esters, anhydrides etc. in proximity to
the defects. Grafting with polymers from CNTs have primar-
ily been performed through ‘‘graft from’’ strategies by
controlled polymerization.8 These are mainly based on atom
transfer radical polymerizations9–19 (ATRP), but also nitro-
xide-mediated polymerizations20 (NMP), reversible addition
fragmentation chain-tranfer (RAFT) polymerizations,21,22 and
ring-opening polymerization23,24 have been applied. In addi-
tion to this, there are some examples of ‘‘graft to’’ strategies
by ATRP, NMP, and RAFT.11,12,25–27 However, in many cases,
both the polymerization and polymer grafting methods are
performed on CNTs that have been through a purification
step involving oxidative conditions resulting in a reduction
in the aspect ratio of the CNT. A few exceptions to this
procedure exist, and in these cases, either radical reactions
or Diels–Alder reactions have been applied for grafting to
pristine tubes.11,28
In preparation of nanocomposites, it is of crucial importance
to have a proper dispersion of the multiwalled carbon nano-
tubes (MWCNT) in the polymer matrix to gain the full effect
of the additive.2 It is one of the general beliefs that to obtain
a good incorporation of CNTs in a composite, it is necessary
to surface modify the CNT to obtain a good dispersion and
possibly load transfer from the matrix to the CNT. For com-
posites prepared through step growth polymerization, the
incorporation of an additive can be obtained by low-molecu-
lar weight prepolymerization functionalization of the CNT
material3,4 or by grafting of the CNT material during the
polymerization,5 enabling covalent incorporation into the
matrix during polymerization. If a direct covalent bond from
matrix to CNT is not possible, covalent anchoring of a similar
or compatible polymer would be expected to give the best
Herein, we present a nonoxidative method for functionaliza-
tion of MWCNT, that preserves the tubes and prevent
Correspondence to: S. Hvilsted (E-mail: sh@kt.dtu.dk)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4594–4601 (2010) 2010 Wiley Periodicals, Inc.
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reductions in the aspect ratio. A novel radical method
has been developed for CNT and used to couple an initiator
for ATRP onto the surface of MWCNT. The method has
been applied in ATRP on two different grades of MWCNT
having very different aspect ratios. Finally, initial results
from preparation of composites of polystyrene (PS) or poly-
phenylenesulfide (PPS) and the PS-grafted MWCNT are
presented.
MWCNT-Br General Procedure
MWCNT (0.500 g) was dispersed in a toluene (20 mL) solu-
tion of benzophenonyl 2-bromoisobutanoate29 (10 mg/mL,
0.03 M). The dispersion was stirred under UV (365 nm, 0.8
mW/cm2) for up to 60 min. The dispersion was then filtered
on a 0.4 lm PTFE filter, the isolated MWCNT-Br was rinsed
with a generous amount of toluene and CH2Cl2 and dried
in vacuo. The product was used without further purification.
MWCNT-PS General Procedure
MWCNT-Br (0.156 g) was dispersed in styrene (5 mL) with
CuBr (0.023 g, 0.16 mmol) and 4,40-dinonylbipyridine
(dNbiPy, 0.177 g, 0.40 mmol). The reaction mixture was
frozen in liquid nitrogen, and oxygen was removed through
three freeze-thaw cycleꢁs. The polymerizations were conducted
under nitrogen at 110 C for varied reaction times. After poly-
merization the grafted MWCNTs were reclaimed by filtration
and rinsed with copious amounts of toluene and CH2Cl2. The
isolated product was characterized by TGA.
EXPERIMENTAL
Materials and General Methods
All chemicals were acquired from Sigma-Aldrich and used as
received unless otherwise specified. PPS was purchased from
Witcom (Netherlands). Two batches of MWCNT prepared by
chemical vapor deposition have been used in these investi-
gations, a high-quality sample from Sigma-Aldrich (110–
170 nm ꢀ 5-9 lm) and an industrial grade MWCNT from
Nanocyl (Belgium) (9.5 nm ꢀ 1.5 lm).
Hydrolysis of MWCNT-PS
MWCNT-PS (38 mg, 47 wt % loaded) was hydrolyzed in a
mixture of KOH (1 g) and THF/ethanol (50 mL/5 mL) at
60 C for 16 h. The mixture was concentrated, and the hydro-
Scanning electron microscopy (SEM) was performed on a
Zeiss Ultra 55 SEM equipped with a field emission electron
source. High vacuum conditions were applied, and a second-
ary electron detector was used for image acquisition. The
samples were prepared on a carbon tape surface and coated
with a conductive layer of Au with approximately 20 nm
thickness. Thermogravimetric analysis (TGA) was performed
on a TGA Q500 from TA instruments under N2 with a heat-
ing rate of 10 K/min. For TGA-GC-MS a Mettler Toledo TGA/
SDTA 851e was applied for the TGA from 25 to 800 ꢁC at
10 K/min under nitrogen flow, while the off-gas was passed
through adsorption columns for subsequent analysis by
thermal desorption GC-MS. The collection of the degradation
products was divided over temperature intervals corre-
sponding to the weight loss steps detected in the TGA analy-
sis. Tenax TA 300 mg filters were analyzed by thermal
desorption (Perkin–Elmer ATD 400) in combination with gas
chromatography on a Varian CP Sil 8 CB capillary (30 m ꢀ
0.25 mm ꢀ 0.5 lm) with a mass spectrometry detector
(Perkin–Elmer Turbomass GC-MS). The amount of styrene
was quantified against external standards prepared on Tenax
filters. Differential scanning calorimetry (DSC) was per-
formed on a DSC Q1000 from TA Instruments. DSC analyses
were performed at a heating and cooling rate of 10 K/min.
Glass transition temperatures (Tgs) were measured at the
inflection point. Size exclusion chromatography (SEC) was
performed in THF on a Viscotec model 200 operating in THF
on two Polymer Laboratories PLgel 5 lm MIXED-D columns
at a flow rate of 1 mL/min. A refractive index/viscometry
detector and a PS calibration were used to determine Mn
and PDI. Raman spectra were recorded on a Thermo DXR
dispersive Raman microscope with a 780 nm laser. Small
amplitude oscillatory shear measurements were made on an
AR2000 rheometer from TA Instruments using parallel plate
geometry with a diameter of 25 mm and a plate separation
ꢁ
lyzed polymer was dissolved in THF and analyzed by SEC.
MWCNT-PPEGMA
MWCNT-Br (0.156 g) and poly(ethylene glycol) methacrylate
(PEGMA, Mn ¼ 360 g/mol, 2.0 mL, 5.3 mmol) was dispersed
in methanol (2.5 mL) with CuCl (0.008 g, 0.08 mmol) and
dNbiPy (0.032 g, 0.08 mmol). The reaction mixture was
frozen in liquid nitrogen, and oxygen was removed through
five freeze-thaw cycles. The polymerization was conducted
under nitrogen at 30 ꢁC for 2 h. After polymerization, the
grafted MWCNTs were recovered by filtration and rinsed
with copious amounts of H2O and methanol. The isolated
product was characterized by TGA.
Nanocomposite Preparation
Before processing, the PPS was dried at 160 ꢁC for 4 h.
Masterbatches were prepared by mixing in a corotating twin
screw extruder Bernstorff ø 25 mm, L/D ¼ 48 with the
temperatures of heating ꢁzones 310, 310, 315, 315, 320, 320,
320, 330, 330, and 335 C and at 100 rpm. Compositions of
the masterbatches were 97.5/2.5 wt % (PPS/MWCNT).
RESULTS AND DISCUSSION
Two batches of MWCNTs were investigated with SEM and
Raman spectroscopy and as can be seen from Figure 1, the
two materials are very different. In the scanning electron
micrograph of the MWCNT from Sigma-Aldrich (A), it is clear
that these tubes have the lowest aspect ratio with an average
diameter of 110–170 nm and an average length of 5–9 lm.
Compared with these, the MWCNT from Nanocyl (B) are
much smaller with an average diameter and length of
9.5 nm and 1.5 lm, respectively. Similarly, the Raman spectra
of the two batches also illustrate large differences. Both
show the characteristic peaks of MWCNT with a G-band indi-
cating in-plane stretching at around 1590 cmꢂ1 and a D-
band corresponding to the sp3 hybridized carbon or defects
ꢁ
of 1 mm. The measurements were performed at 200 C and
frequency sweep between 0.01 rad/s and 100 rad/s were
made at a low strain of 2%.
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FIGURE 1 Raman spectra and scanning electron micrographs of the two different grades of MWCNT. The left column shows data
on the pristine MWCNT from Sigma-Aldrich (A), the right column shows data on the pristine MWCNT from Nanocyl (B).
at 1310 cmꢂ1. However, for the MWCNT from Sigma-Aldrich,
the ratio of the D and G-band was found to be 0.43, whereas
for the MWCNT from Nanocyl, it was found to be 1.94. This
difference indicates that there are a large number of defects
in the industrial grade MWCNT from Nanocyl, which would
be expected of a lower quality material.
concept developed by Huang et al.29 for surface grafting of
polypropylene was adopted to functionalization of MWCNT
as shown in Scheme 1.
Here, a UV sensitive ATRP initiator (1) was prepared directly
from commercially available starting materials by an ester
synthesis.29 This initiator could then be covalently bound to
the pristine MWCNT in dispersion by exposure to UV light
(365 nm). Here, it is believed that a ketyl radical is formed
through abstraction of a proton from either solvent (toluene)
For modification of the MWCNT, it was decided to use a radi-
cal method, because these are independent of the former-
mentioned acid treatments. A functional initiator based on a
SCHEME 1 Preparation of the UV sensitive ATRP initiator and subsequent reaction with MWCNTs under UV irradiation at 365 nm.
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SCHEME 2 ATRP of styrene from the initiator functionalized MWCNT-Br. TGA of such a sample is shown in Figure 2, where A is
Sigma-Aldrich and B is Nanocyl.
or the defects on the nanotubes. Recombination or reaction
with the nanotube affords the initiator functionalized mate-
rial. Raman spectroscopy corroborates the introduction of
the initiator onto the MWCNT. In the Raman spectra of the
MWCNT-Br, there is an increase in the D-band relative to the
G-band, indicating an increase in sp3 hybridized carbon. On
the Sigma-Aldrich MWCNT, this resulted in an increase in the
peak intensity ratio ID/IG from 0.43 to 0.49, whereas for
Nanocyl’s MWCNT the ID/IG ratio was increased from 1.94
to 2.14. This indicates that the reactions are occurring on
the sidewall and not only on defects, although it must be
assumed that the reaction will occur at both defects and on
the sidewall. If only defects were reacted no change would
be observed in the Raman spectra from pristine to MWCNT-
Br. Unfortunately, the loading of the initiator is too low to
be detected from TGA analyses. Therefore, the prepared
MWCNT-Br was used directly to initiate polymerizations of
styrene in dispersion under standard ATRP conditions as
shown in Scheme 2.
although the increase is substantially higher for the indus-
trial grade from Nanocyl. Pristine MWCNT exposed to the
equivalent polymerization conditions afforded no increase in
weight loss as determined by TGA for both grades of
MWCNT. This clearly confirms that the increase in loading is
due to polymer formed on or in proximity to the MWCNT,
and thus corroborates the presence of the initiator on the
tubes. If the initiator was only adsorbed and not covalently
bound, the bulk of such free polymer would have been dis-
solved during the purification process. This would result in a
much lower loading of polymer than what is observed on
the MWCNT from Nanocyl and in no loading of polymer on
the MWCNT from Sigma-Aldrich. Adsorption experiments
with free PS on both grades of MWCNT showed that the
tubes from Sigma-Aldrich do not adsorb any free polymer
from solution, whereas the MWCNT from Nanocyl shows
some adsorption. However, even after long exposure times
with a large excess of free polymer, the maximally possible
adsorption was found to be 5–8 wt % PS. With the loading
observed from TGA in Figure 2, it is clear that this can only
be a result of the presence of covalently bound initiator on
the MWCNT.
As can be seen, none of the pristine tubes show noteworthy
weight loss in the analyzed region from room temperature
to 700 ꢁC, which indicates that the defects observed from
Raman spectroscopy are not low-molecular weight carbon
material. After the polymerizations have been performed,
there is an increase in weight loss observed for both grades,
The chemical composition of the formed polymer was inves-
tigated by TGA-GC-MS of a small sample of MWCNT grafted
with polymer. The sample was degraded by TGA, and
FIGURE 2 TGA analyses of the two grades of MWCNT before and after introduction of the initiator and subsequent ATRP of
styrene.
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TABLE 1 TGA Results Showing PS Loading Resulting From
Variation in Polymerization Time and Ligands on the Two
Different MWCNT-Br Batches
The combined analysis results show that the method works
and enables the preparation of MWCNT-PS through appli-
cation of the UV initiator. After establishing the method,
the polymerization time and the choice of ligands were
optimized. Polymerizations were conducted with dNbiPy or
pentamethyldiethylenetriamine (PMDETA) on both grades of
tubes for different polymerization times. TGA of the prepared
MWCNT-PS were used to determine the optimal reaction
conditions as shown in Table 1.
TGA Mass Loss @ 460 ꢁC
After Different Polymerization
Times (wt %)
MWCNT-Br
Ligand
2.0 h
6.5 h
16.5 h
a
Sigma-Aldrich
Nanocyl
dNbiPy
0.7
–
4.0
Based on earlier experience where dNbiPy has shown supe-
rior properties in surface initiated ATRP,31 it was decided to
use this ligand for the first polymerizations. Even after
repeated experiments and longer reaction times, it was
found to be impossible to obtain more than a 4.0 wt %
loading of polymer on the MWCNTs from Sigma-Aldrich
(determined by TGA). Even though this was disappointing,
the experiment was repeated for comparative reasons on the
MWCNTs from Nanocyl. Surprisingly, the exact same method
resulted in a loading of up to 47.6 wt % after 16.5 h under
the same conditions. Therefore, it was decided to test these
tubes with a different ligand, PMDETA, and otherwise un-
changed reaction conditions. With PMDETA and Nanocyl’s
tubes, it was possible to obtain the highest loading of all
cases after only 6.5 h of reaction with 54.0 wt % polymer.
The comparative experiment on the tubes from Sigma-
Aldrich resulted only in a 2.0 wt % loading of polymer. It is
quite clear that the industrial grade tubes from Nanocyl give
a much higher loading compared to the tubes from Sigma-
Aldrich under all conditions. This could be an effect of both
the much higher aspect ratio in these tubes (160 versus 50),
resulting in a larger surface area or an effect of the amount
of defects as illustrated by Raman spectroscopy in Figure 1.
This large difference in loading illustrates the importance of
characterization of the starting material before functionaliza-
tion to be able to evaluate the efficiency of different methods
of grafting, a property that is often neglected. In addition to
the effect of different tubes, the choice of ligand has also
been found to have a substantial influence on the polymer
dNbiPy
5.3
19.0
54.0
2.0
47.6
b
Nanocyl
PMDETA
PMDETA
20.2
–
a
a
Sigma-Aldrich
–
–
a
Samples were omitted as they were not necessary to compare with
the equivalent Nanocyl sample.
b
It was not possible to continue the polymerization due to high
viscosity.
the corresponding off-gas was passed through adsorption
columns for analysis by thermal desorption GC-MS. The
collection of the degradation products was divided over
temperature intervals corresponding to the weight loss steps
detected in the TGA analysis. These investigations directly
correlated the observed weight loss to PS from the grafting.
The PS-grafted MWCNT (MWCNT-PS) were additionally
attempted investigated by nuclear magnetic resonance spec-
troscopy (NMR), attenuated total reflection Fourier transform
infrared spectroscopy (ATR-FT-IR), KBr pellets in transmission
FT-IR and finally by Fourier transform photoacoustic infrared
spectroscopy (FT-PAS-IR) to corroborate the structure. How-
ever, in our hands, these investigations were unsuccess-
ful and yielded no useful analysis data. Especially, PAS-IR
is a technique that has been known to give very good
results on samples containing high amounts of carbon black
and was expected to be well suited for analysis of functional-
ized CNT.30 However, in this case even this method was
unsuccessful.
FIGURE 3 Scanning electron micrographs of polystyrene-grafted MWCNT, A: Sigma-Aldrich MWCNT-PS, B: Nanocyl MWCNT-PS.
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a Mn ¼ 39,600 g/mol (PDI ¼ 1.5) for a sample with 47 wt
% polymer.
The general method is also applicable for other monomers,
which was illustrated by a polymerization of the hydrophilic
monomer PEGMA. This resulted in a grafting of 4.6 wt % of
poly(PEGMA) onto the MWCNT from Sigma-Aldrich, which
corresponds to loading obtained with styrene. The poly
(PEGMA) grafting had an obvious influence on the solubility,
where the functionalized MWCNT could be dispersed in
MeOH, which is in clear contrast to the pristine and initiator
functionalized MWCNT that were indispersable in MeOH.
FIGURE 4 Dispersion experiments of MWCNT-PS (Nanocyl, A-
C) compared with pristine MWCNT (Nanocyl, D-F) 1.5 h after
dispersion. The dispersions have been prepared at 20 ꢁC in
cyclohexane (A,D), dichloromethane (B,E), and dioxane (C,F)
with a concentration of 0.5 mg/mL.
Finally, the prepared MWCNT-PS was applied in new
composite materials based on polyphenylenesulfide (PPS).
Because PS and PPS are miscible, it was intended to exploit
the PS functionalization of the MWCNT to compatibilize the
nanotubes to the matrix. Because PPS is insoluble such a
composite needs to be prepared by extrusion, and it was
decided to test the dispersion of the MWCNT-PS in a soluble
polymer (PS) first to simplify the system.
loading. This is not surprising because it is well known from
ATRP that the ligand has a large influence on polymeriza-
tions. However, it is interesting that dNbiPy that has been
found to show superior performance on other surfaces in
this case performs worse than PMDETA.
Model composites were solution casted from toluene and
dioxane and investigated by rheology at elevated tempera-
tures. The degree of CNT dispersion in PS was found by eval-
uation of the low-frequency behavior of the storage modulus,
Samples of the PS-grafted MWCNTs were investigated by
SEM, resulting in the micrograph shown in Figure 3.
Because of the low loading on the Sigma-Aldrich tubes,
the micrograph shows no substantial difference compared
with the unfunctionalized one in Figure 1. In the case of the
Nanocyl tubes, it is clear that a substantial loading has been
obtained resulting in a matrix of polystyrene enveloping
the tubes. This was corroborated through a dispersion
experiment in cyclohexane, dichloromethane, and dioxane as
shown in Figure 4.
As can clearly be seen, the grafting of the tubes have had a
substantial influence on solubility. The functionalized mate-
rial is completely indispersable in cyclohexane at 20 ꢁC and
dispersible in dichloromethane in contrary to the pristine
material, whereas the dispersability in dioxane was un-
changed. Similar results were observed for dispersion experi-
ments on the PS-grafted MWCNT from Sigma-Aldrich.
DSC investigations of the prepared MWCNT-PS showed glass
transition temperatures in the range of 107–110 ꢁC. These
high glass transition temperatures indicate that the PS is
attached to the surface of the MWCNT where the proximity
to the rigid MWCNT restricts rotation and thereby results in
a higher glass transition temperature compared with free
PS prepared by ATRP.32 This has also been observed by
Viswanathan et al.,33 who showed that only covalently bound
PS gave rise to such an increase in glass transition tempera-
ture on SWCNT.
To estimate the molecular weight of the grafted chains, a
hydrolysis experiment was performed on the MWCNT-PS.
Because of the low loading observed on Sigma-Aldrich tubes,
it was not possible to do this on these samples. However, it
was done on the MWCNT-PS prepared from Nanocyl’s tubes.
Here SEC analysis showed that the hydrolyzed polymer had
FIGURE 5 Extruded samples of a standard PPS (a), a PPS com-
posite with pristine MWCNT (b) and a PPS composite with
MWCNT-PS (c).
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G0, measured from small amplitude oscillatory shear. Samples
with well-dispersed nanotubes are known to display a non-
terminal G0 at low frequencies, as compared with samples
with poorly dispersed nanotubes and pure polymer melts.
This effect is caused by a formation of nanotube networks
that restrains the motions of the polymer chains.34 The
oscillatory shear rheology measurements confirmed a good
dispersion of the MWNCT-PS in the PS matrix, and the
nanotubes were consequently applied for preparation of the
PPS-(MWCNT-PS) composite prepared by mixing during
extrusion. Three different samples were extruded as shown
in Figure 5 where an ordinary PPS, a masterbatch with
2.5 wt % of pristine MWCNT and a masterbatch with 2.5 wt %
of PS-grafted MWCNTs are compared.
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The authors acknowledge the Innovation Consortium
‘‘Extreme Materials for Extreme Environments’’ funded by
the Danish Ministry of Science Technology and Innovation
under contract 08-034101 for financial support. Louise
Kjærulff and Ulrich Berner are acknowledged for synthesis
work as part of internships at the Danish Polymer Centre,
Department of Chemical and Biochemical Engineering at
the Technical University of Denmark (DTU). Laboratory
technician Kim Chi Szabo performed the thermal analyses
at DTU. Kathrine Bjørnboe acquired SEM and Eva Pedersen
performed the GC-MS analyses at the Danish Technological
Institute.
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