Microwave-Assisted Modification of Carbon Nanotubes with Biocompatible Polylactic Acid

doi:10.4236/msce.2014.21002

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Journal of Materials Science and Chemical Engineering, 2014, 2, 7-12

Published Online January 2014 (http://www.scirp.org/journal/msce)

http://dx.doi.org/10.4236/msce.2014.21002

OPEN ACCESS MSCE

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

Email: [email protected]

Received October 2013

ABSTRACT

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.

KEYWORDS

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 from” and/or “grafting

to” methods. 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

Q. ZHANG ET AL.

OPEN ACCESS MSCE

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

MWCNT.

2.3. Microw ave-Assisted Functionalization of

MWCNT-COOH with PLA

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 cm−1. 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

MWCNT-COOH.

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 cm−1

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

cm−1 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

Q. ZHANG ET AL.

OPEN ACCESS MSCE

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 cm−1 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.

Q. ZHANG ET AL.

OPEN ACCESS MSCE

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] =

2000.

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

Q. ZHANG ET AL.

OPEN ACCESS MSCE

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 didn’t 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-

sites.

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