Journal of Materials Science and Chemical Engineering, 2014, 2, 7-12
Published Online January 2014 (
Microwave-Assisted Modification of Carbon Nanotubes
with Biocompatible Polylactic Acid
Qi Zhang, Shijun Zhang, Liying Zhang
Beijing Research Institute of Chemical Industry, SINOPEC, Beijing, China
Received October 2013
Polylactic acid (PLA) was successfully covalently grafted onto multi-walled carbon nanotubes (MWCNT) by mi-
crowave-assisted polymeriza tion of lactide monomers. The final products MWCNT-g-PLA were characterized
with Fourier-transform IR (FTIR), Raman spectroscopy, thermogravimetric analyses (TGA) and transmission
electron microscopy (TEM). The results indicated PLA chain was covalently attached to the MWCNT. The
grafted PLA was uniformly coated on the surface of MWCNT with a layer thickness of 2 ~ 6 nm. The grafted
PLA content could be controlled by microwave irradiation time and the concentrations of reactant. The product
with 60.5% grafted PLA content can be synthesized in one hour.
Mult i-Walled Carbon Nanotubes; Polylactic Acid; Microwave; Biocompatible
1. Introduction
Since the discovery of carbon nanotubes (CNTs) in the
early 1990 [1], their unique atomic structure, very high
aspect ratio and outstanding physical and chemical prop-
erties have attracted great attention and i magi na ti o n of
many scientists. As known to all, carbon nanotubes pos-
sess high flexibility, low mass density, and large aspect
ratio (typically > 1000), whereas they present extraordi-
nary high tensile strength and modul us , together with
excellent electrical properties [2,3]. The unique proper-
ties of CNT offered many opportunities for their applica-
tions such as reinforcement of fibers and nanocomposites,
field-emission displays and nanosize probe tips for
atomic force microscopy. However, the super amphipho-
bic property of carbon nanotubes is still the fundamental
and technical barriers toward expanding many of the
applicatio ns [4]. The inherent insolubilit y and aggrega-
tion of CNT in polymer composites prevent efficient
stress from transferring to individual nanotubes and have
a bad effect on efficiency usage of its properties [5].
Therefore, many researches on covalent and noncovalent
modification of CNT have been made to realize the we l l
dispersion of individual nanotubes and establish a strong
chemical affinity with the surrounding polymer matrix
[6-8]. The noncovalent approaches include surfactant
modification [9-14], polymer wrapping, [15,16] and
polymer absorption [17,1 8] . The covalent functionaliza-
tion of CNT is usually realized by grafting polymer or
long alkyl chains onto CNT [ 19 -23].
Since the environmental pollution and the exhaustion
of petroleum resources become critical issues, Polylactic
acid (PLA) has been spotlighted as a biodegradable, sus-
tainable and eco-friendly substituent for petroleum-based
polymers [24,25]. Accordingly, the combination of PLA
with CNT will broaden the novel application of carbon
nanotubes. Several researchers have prepared PLA-
functionalized CNTs via grafting fromand/or grafting
tomethods. Chen et al. [26,27] synthesized PLA func-
tionalized CNTs by reacting functional group COCl on
the surface of CNTs with different weight molecular
PLA or using the surface initiating ring-open polymeri-
zation of L-lactide. Song et al. have prepared CNT-g-
PLAs by polycondensation of L-lactic acid with carbox-
ylic acid-functionalized CNT in xylene solution. [28]
Feng et al. reported that the PLA was covalently grafted
onto the surface of magnetic multiwalled carbon nano-
tubes (m-MWCNTs) by in-situ ring-opening polymeriza-
tion of lactide [21]. However, the conventional heating
route to prepare the PLA grafted carbon nanotubes re-
quired strict conditions (vacuum, high temperature and
long reaction time) which are obviously not good for
commercially application.
In this paper, we present microwave-assisted method
to realize the grafting of PLA onto multi-wall carbon
nanotubes (MWCNT). The MWCNT was first treated by
acid to gain an active COOH group on the surface, and
then acid-functionalized MWCNT (MW C NT-COOH)
induced in-situ polycondensation of lactide together with
SnOct 2. The grafting and polymerization rate were much
faster than that of conventional heating. We investigated
the morphology and structure of the final grafting prod-
uct. In addition, the influences of reaction conditions
were discussed further.
2. Experimental Section
2.1. Mat eri als
The multi-walled carbon nanotubes (MWCNT s) (>98%,
with an outside diameter of 20 - 30 nm, an inside diame-
ter of 5 - 10 nm, and lengths of 10 - 30 mm) were pur-
chased from Cheaptubes Co.Ltd, USA, and used as re-
cived. D, L-Lactide (Aldrich) and Stannous octoate
(Sn(Oc t) 2, 98%, Sinopharm Chemical Reagent Co. Ltd.)
were used as received. Sulfuric acid, nitric acid, metha-
nol, chloroform, and N, N-dimethylformatide (DMF) as
well as other chemicals were purchased from Aldrich and
used without further purification.
2.2. Acid Treatment of MWCNT
The acid-funct i o nal ized multi-wall carbon nanotube
(MW CNT -COOH) was prepared from the pristine
MWCNT as the previous report [29]. MWCNTs were
treated with a mixture of concentrated sulfuric and nitric
acids (volume 3:1). The mixture was ultrasonicated
(power of 60 W and nominal frequency of 40 kHz at am-
bient temperature) for 30 min, and then magnetically
stirred at 80˚C for 4 h. After the mixture was cooled to
room temperature, it was diluted with deionized water
and then vacuum-filtered through 0.22 mm Millipore
polypropylene membrane, and washed with distilled wa-
ter until the pH value was moderate (~ ca. 7). The filtered
product was dried under vacuum for 24 h at 60˚C, giving
the acid-functionalized MWCNT-COOH. The amount of
carboxylic acid groups of MWCNT-COOH prepared in
this study was determined to be 0.0023 mol per 1.0 g
2.3. Microw ave-Assisted Functionalization of
The apparatus used for the polymerization was a do me-
stic microwave oven (Me ilin g, China, 2450 MHz and
800 W). A mixture of purified MWCNT-COOH, La cti de
and catalyst stannous chloride as well as a certain amount
solvent DMF in an open beaker was first ultraso nicate d
for 30 min and then irradiated at the microwave power of
200 W for some period of time. Then the crude product
was cooled to room temperature and washed several
times by chloroform and precipitated in methanol, and
the precipitate was dried under vacuum for 12 h at 60˚C
to gain the final product MWCNT-g-PLA.
2.4. Characterization of MWCNT-g-PLA
Fourier transform infrared (FTIR) spectra (collected from
a Bruker Tensor 27 FTIR system) were used to chara-
cterize the molecular structure. The samples of nano-
composites were imbedded in KBr disks. Raman spectra
used to confirm the structure of MWCNTs operating at
514 nm with a resolution of 1.5 cm1. Transmission elec-
tron microscopy (TEM) images were examine d using a
FEI Tecnai-20 instrument operated at a 100 kV accele-
rating voltage to observe the nanoscale structures of the
various MWCNT -g-PLA. Thermogravimetric analysis
(TGA) was conducted in nitrogen atmosphere. MWCNT
and MWCNT-g-PLA were heated at a heating rate of
10˚C/min from 50˚C to 500˚C, to determine the graft
content of PLA.
3. Result and Discussion
PLA is grafted from the surface of MWCNT -COOH by
the microwa ve-assisted ring opening polymerization of
lactide monomers. It’s reported that, in the pristine state,
Sn(Oc t) 2 does not contain any reactive alkoxide groups
and that the alcohol employed usually as coinitiator sub-
stitutes at least one of the octanoate groups in a rapid
equilibration, and the resulting Sn alkoxide is then the
true initiator of the polymerization process [18]. There-
fore, in our study, the polymerization is initiated by
Sn(Oc t) 2 together with hydroxyl which is actually con-
tained by carboxylic acid group on the surface of
3.1. FT-IR Spectra
In our investigation, we focused on the structure analysis
and characterization and the effect of reaction conditions
on the grafting rate of final product MWCNT-g-PLA .
The FTIR spectra of MWCNT and MWCNT-g-PLA
were shown in Figure 1. The spectrum of MWCNT-
COOH presented a relatively weak peak at 1750 cm1
which corresponds to the incorporated carboxylic acid
groups as a result of the acid treatment process. After
polymerization, the spectrum of MWCNT-g-PLA sample
showe d relatively strong absorption peaks around 1750
cm1 assigned to the C=O stretching, which clearly indi-
cated that the PLA molecules were covalently bound to
the MWCNTs.
3.2. Raman Spectroscopy
Raman spectroscopy is a powerful tool used for the
characterization of functionalized CNTs. As shown in
Figure 1 FTIR spectra of MWCNT-COOH and MWCNT-
g-PLA. The IR sample of MWCNT-g-PLA was prepared by
irradiating at the microwave power of 200 W for 60 min,
[LA]/[COOH] = 20,[LA]/[Sn(Oct)2] = 2000 .
Figure 2, the D and G bands of the MWCNTs at 1285
and 1600 cm1 which are attributed to the defects and
disorder-induced peaks and tangential-mode peaks were
clearly observed in both MWCNT-COOH and MWCNT-
g-PLA spectrums. The intensity of MWCNT-g-PLA was
greatly decreased than that of MWCNT-COOH. Mea n-
while, the Raman signals of neat PLA were not found in
the spectrum of MWCNT-g-PLA. That result possibly
can be related to the structure change of MWCNT [27].
After covalently fictionaliza tion, the polymer chains
grafted to the MWCNT surface formed nanometer-scale
layer. The energy transfer between the MWCNT and
PLA layer as well as the effect of grafted PLA chains on
the electronic properties of the MWCNT induced the
changes of the Rama n s i g nals.
3.3. Mor p hologies
A further certification of grafting PLA onto MWCNT
was determined by transmission electron microscopy.
The fine nanostructures of the as-prepared MWCNT-g-
PLA as well as the acid-functionalized CNT were shown
in Figure 3. The images clearly show that the surface
morphology of MWCNT-g-PLA is significantly different,
by contrasting with MWCNT-COOH. The diameter
marked in the image is ~30 nm. In the TEM images of
MWCNT-COOH (Figure 3 (a), (b)), the CNT wall was
relatively smooth and clean and was not obviously cov-
ered with an extra phase. After grafting PLA from
MWCNTs, a relatively rough polymer layer can be easily
distinguished , with a thick ness of 2 ~ 6 nm. With high
magnifications, it could be seen that a homogeneous shell
had formed on all of the surface s of MWCNTs. Fur-
thermore, the original length and pattern of MWCNTs
remain intact after a series of chemical treatments, which
Figure 2. Raman spectra of PLA, MWCNT-COOH and
MWCNT-g-PLA samples.
Figure 3. TEM images of MWCNT-COOH and MWCNT-
g-PLA at different magnifications. (a) MWCNT-COOH
(high magnification), (b) MWCNT-COOH (low magnifica-
tion), (c) MWCNT-g-PLA (high magnification) and (d)
MWCNT-g-PLA (low magnification).
can be identified in Figure 3(d).
3.4. Grafting Content of MWCNT-g-PLA
To evaluate the content of grafted PLA, thermogr avime-
tric analysis (TGA) of MWCNT -COOH and MWCNT -
g-PLA was performed in Figure 4. For MWCNT-COOH,
more than 95 wt% was retained when heated over a tem-
perature range of 50˚C ~ 500˚C. However, the MWCNT-
g-PLA exhibited major weight loss in the temperature
range of 300˚C ~ 400˚C, this is approximately caused by
the degradation of the PLA grafted on to the MWCNTs.
Figure 4. Thermogravimetric analysis (TGA) of MWCNT-
COOH and MWCNT-g-PLA. The sample of MWCNT-
g-PLA was prepared by irradiating at the microwave power
of 200 W for 45 min, [LA]/[COOH] = 20, [LA]/[Sn(Oct)2] =
Comparing the mass gain after the grafting reaction
with the thickness of the PLA layer observed in the TEM
images of the MWCNT-g-PLA samples, we proposed
that the larger the amount of grafted PLA, the thicker the
polymer shell. In the study, the degree of weight loss at
500˚C in TGA was taken as the content of grafted PLA.
With the increasing reaction time, the gained PLA
content presents an increasing tendency (Figure 5). Ac-
cor d ingly, by considering that MWCNT-COOH in this
study possess carboxylic acid groups of 0.0023 mol for
MWCNT of 1 g and by assuming that all the carboxylic
acid groups of MWCNT-COOH are reacted with lactide
during the ring-opening polymerization, the apparent
number average molecular weights of grafted PLA
chains of MWCNT -g-PLAs synthesized at various irra-
diation time could be evaluated, as can be seen in Table
1 and Figure 5. As shown in Figure 5 and Table 1, the
PLA content was increased with the prolonged irradia-
tion time; meanwhile the increasing tendency gradually
became flat. The number-average molecular weight of
grafted PLA chains could reach up to 1430 by only 90
minutes. According to the results, the fine products were
obtained in a very short irradiation time. Different from
conventional heating, the microwave-assisted modifica-
tion process dismissed vacuum because of the acceler-
ated reaction rate. Comparing the results under micro-
wave irradiation with conventio na l hea ti ng method, it
was considered that the enhanced polymerization rate
originated not only from thermal effects but also from
microwave effects.
3.5. Effect of Reactant Rati o
In order to further investigate the influence of reaction
Figure 5. Changes of grafted PLA content with different
microwave irradiation time. Condition: Microwave power
200 W, [LA]/[COOH] = 20, [LA]/[Sn(Oct)2] = 2000.
Table 1. Weight contents and number average molecular
weights of grafted PLA chains for MWCNT-g-PLA.
Irr a d i ation
time (min) The grafted
PLA contenta (%) Number-average molecular
weight of the grafted PLAb
10 12.1 240
20 22.5 440
30 28.7 675
45 39.4 1100
60 45.6 1350
90 48.2 1430
aThe grafted PLA content in MWCNT-g-P LA is evaluated from TGA ther-
mograms. bThe concentration of carboxylic acid groups is 0.0023 mol per
MWCNT of 1.0 g.
conditions on the final product, we also studied the varia-
tion of gained grafted PLA content in MWCNT-g-PLA
which prepared by different catalyst concentrations.
Figure 6 shows the effect of varied Sn(Oc t ) 2 concentra-
tion and different [LA]/[COOH] ratio on the grafted PLA
conte nt . The grafted polymer content in MWCNT-g-P LA
was strongly dependent on the amount of Sn(Oct)2 and
the ration of [LA]/[COOH]. As shown in Figure 6(a) ,
with the increasing Sn(Oct)2 concentration from 0.01 to
0.2%, the content of PLA in MWCNT-g-PLA increased
until it reached a maximum at 0.05%. However, the con-
centration of Sn(O ct) 2 which is larger than 0.05 mol% is
not good for PLA gr a ft i ng. This phenomena of inhibition
can be explained by that Sn(Oct ) 2 catalyzes not only the
polymerization of LA but also the decomposition of
polymer. Too much Sn(Oct)2 might facilitated decompo-
sition than polymerization. On the other hand, the reac-
tion that PLA grafting to MWCNT was induced by active
species that generated by carboxylic acid groups and
Figure 6 The effect of Sn(Oct)2 concentration and [LA]/
[COOH] ratio on the grafted PLA conte nt. (a) Microwave
pow e r 200 W, irradiation time 60 min, [LA]/[COOH] = 20.
(b) Microwave power 200 W, irradiation time 60 min, [LA]/
[ Sn(Oct)2] = 2000.
Sn(Oc t) 2, with fixed concentration of carboxylic acid
group s , higher concentration of Sn(Oc t) 2 didnt increase
the number of active species. The different ratio of [LA]/
[COOH] also has great influence on the grafted PLA
content. The result (Figure 6(b)) suggested that larger
ratio can generate higher grafted PLA content. With the
[LA]/[COOH] ratio of 50, the gained PLA amount in
MWCNT-g-PLA could reach up to 60.5%. By contr o l-
ling the reaction condition, we can get the final product
with different grafting rate.
4. Conclusion
The biocompatible Pol ylactic acid were directly grafted
to acid functionalized MWCNT by microwave irradia-
tion. The final grafted product MWCNT-g-PLA with
different PLA grafting rate can be synthe siz e d in only 60
min. The TEM images of coated PLA layer clearly indi-
cated the grafted PLA was uniform on the surface of
MWCNT. Moreover, the molecular weight and grafted
content can be well controlled by altering Sn( O c t) 2 con-
centration and irradiation time as well as [LA]/[COOH]
ratio. With the conditions which the microwave power is
200 W and the ratio of [LA]/[COOH] and [LA]/[Sn(Oc t )2]
is 50 and 2000 respectively, the MWCNT-g-PLA which
the grafted PLA chains could reach up to 60.5 wt% can
be prepared in only one hour. Different from conven-
tional heating, the microwave-assisted modification
process dismissed vacuum because of the accelerated
reaction rate. The micr owave-assisted method shows the
advantages of rapid speed and benign conditions over
conventional heating and should be favored in industri-
alization for co mmercially biocompatible nano compo-
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