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International Journal of NanoscienceVol. 8, No. 6 (2009) 533–541c© World Scientific Publishing CompanyDOI: 10.1142/S0219581X09006365

CARBON NANOTUBE-GRAFT-BLOCK COPOLYMERSCONTAINING SILVER NANOPARTICLES

MOHSEN ADELI∗,†,§, REZA SEPAHVAND‡, ALI BAHARI‡and BANDAR ASTINCHAP‡

∗Department of Chemistry, Faculty of ScienceLorestan University Khoramabad, Iran

†Department of ChemistrySharif University of Technology

P. O. Box 11155-3516, Tehran, Iran‡Department of Physics, Faculty of Science

Lorestan University, Khoramabad, Iran§mohadeli@yahoo.com

§adeli@sharif.edu

Revised 5 May 2009

Polycaprolactone-polylactide block copolymers (PCL-block-PLA) were grafted onto filled multi-wall carbon nanotubes (MWCNT) successfully. In this synthesis, MWCNTs were opened andfunctionalized, and then they were filled by silver nanoparticles. The filled MWCNT were usedas macroinitiator for ring opening polymerization of ε-caprolactone and l-lactide. Then the endhydroxyl functional groups of MWCNT-graft-PCL or MWCNT-graft-PLA were used as initiatorfor ring opening polymerization of lactide and ε-caprolactone and MWCNT-graft-PCL-block-PLA or MWCNT-graft-PLA-block-PCL were obtained, respectively. Length of grafted copolymerchains onto the MWCNT was controlled using CNT/monomer ratio. Nanocomposites’ propertiesdepend on the length of polymer blocks strongly. Structure of nanocomposites was evaluated byTEM and spectroscopy methods.

Keywords : Nanocomposites; MWCNT; poly(caprolactone); poly(lactide); nanoparticles.

1. Introduction

Carbon nanotubes (CNTs) are interesting materi-als with excellent physical, chemical and electricalproperties,1,2 but a big challenge to make nano-objects and devices from CNTs is their low pro-cessability. To overcome this disadvantage, CNTsare modified by different methodologies. There aretwo strategies to modify CNTs.

(I) The modification of the surface and tips ofCNTs. This strategy is based on chemical reac-tions on the outer wall of CNTs.1,3

A variety of molecules are conjugated ontothe convex and tips of CNTs by chemicalreactions.4,5 Modified CNTs are not onlysoluble in different solvents but also are con-taining functional groups which make themmultidisciplinary materials in order to use indifferent process.

Among different organic molecules,3

polymers are more interested to conjugateonto the surface of CNTs. Two methods areused to graft polymers onto the surface ofCNTs.

§Corresponding author.

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(i) “Grafting to” method, in which a poly-mer containing reactive functional groupsattached to a functionalized CNT by chem-ical reactions.6,7 Different polymers suchas poly(ethylene imine), poly(styrene),poly(ethylene oxide), poly(vinyl carb-azole), and poly(vinyl alcohol) are graftedonto the surface of CNTs using thismethod.8

(ii) “Grafting from” method, in which thefunctional groups of CNTs are usedas initiator for polymerization of amonomer.9–12

“Grafting from” method is more con-trollable and suitable than the “grafting to”method because in this method of purifica-tion is simple and the conversion is high.Adronov et al. have used different approachesand strategies such as ATRP and metathesisreaction for preparation of different CNT-graft-polymers.10,13 Gao et al. have also used “graft-ing from” method for conjugating of differentpolymers such as hyperbranched polymers,poly(caprolactone), nylon 1010, poly(styrene),poly(acrylates) and polyurea on CNTs.14 Theyalso have reported preparation of amphiphilicmaterials and polyelectrolytes using thismethod.15 Recently nanocomposites of CNTsand PLA have been prepared and their prop-erties have been investigated.16

(II) Second strategy to modify CNT properties andto make nano-objects is the filling of their cav-ities.17,18

Clearly the combination of the above twostrategies (i.e. modification of surface and cavity ofCNTs in the same time) lead to the novel nano-objects having advantages of both the procedures.Here the CNTs were functionalized and opened.Then the cavity of opened CNTs was filled by sil-ver nanoparticles. Conjugation of polymers ontothe surface of filled CNTs through “grafting from”method lead to new nano-objects.

Poly(l-lactide) (PLLA) and poly(ε-caprol-actone) (PCL) and their copolymers (PCLA) werechose to conjugate on the surface of CNTs becausethey occupy an important position in the familyof biodegradable polymers and have been widelyused in the medical field because of their excellentbiodegradabilities and biocompatibilities.19 Duringthe past decades, much research work focuses on thesynthesis, mechanical and degradation properties of

the PCLA copolymers.20 On the other hand silvernanoparticles are known as nanomaterials with highantibacterial properties.

Hence conjugation of PLLA, PCL and PLLA-block-PCL on the surface of CNT and filling thecavity of CNT by silver nanoparticles may leadto new nanocomposites with biodegradabilities andbiocompatibilities properties.

2. Experimental

2.1. Characterization1H NMR spectra were recorded in CDCl3 solu-tion on a Bruker DRX 400 (400 MHz) apparatuswith the solvent proton signal for reference. Allpolymer NMR spectra were recorded on 25 mg/mlof sample. IR spectra of samples as films on theKBr pellets were recorded using a Nicolet 320FT-IR. Differential scanning calorimeter diagramswere recorded using a Shimadzu DSC 60 appara-tus. Transmission electron microscopy (TEM) anal-yses were performed on an LEO 912 AB electronmicroscope.

2.2. Materials

The used multi-wall carbon nanotubes (MWCNT)were prepared by chemical vapor deposition pro-cedure in the presence of Co/Mo/MgO as cata-lyst at 900◦C. The outer diameter of MWCNTwas between 20–40 nm. ε-caprolactone and l-lactidewere purchased from Aldrich. ε-caprolactone waspurified by vacuum distillation and l-lactide waspurified by recrystallization from toluene. Reagentsstannous-2-ethylhexanoate and AgNO3 were pur-chased from Sigma and Merck and used asreceived.

2.3. Opening of MWCNTs

MWCNTs were opened according to the reportedprocedures in the literatures.17 Briefly, CNT wasmilled and dispersed in a 3/1 mixture of H2SO4

and HNO3. Mixture was refluxed for 10 h. Thenit was cooled, filtered and washed by distillatedwater up to pH 5. Opened CNT was dried at 120◦Cfor 6 h.

2.4. Filling of MWCNTs

One gram of opened MWCNTs was added to a solu-tion of AgNO3 in a water/ethanol 10/2 v/v. Mixture

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Carbon Nanotube-Graft-Block Copolymers Containing Silver Nanoparticles 535

was sonicated at room temperature for 30 min, thenit was stirred at room temperature for 72 h. Mix-ture was filtered and the product was washed bywater several times and dried by vacuum oven at150◦C.

2.5. Usual procedure forpreparation of CNT-graft-PCLor CNT-graft-PLAnanocomposites

A total of 1 ml of 1 × 10−3 M toluene solution ofSn(Oct)2 was added to a polymerization ampuleequipped with a magnetic stirrer and vacuum inlet.Toluene was evaporated by vacuum at 60◦C for30 min. One milliliter of ε-caprolactone or 1 g of lac-tide and MWCNT (amount of CNT for three dif-ferent reactions was 0.002, 0.01, 0.1 g, respectively)were added to polymerization ampule. Polymeriza-tion ampule was sonicated at 25◦C for 10 min. Thenit was left under vacuum for 1 h at 60◦C. Poly-merization ampule was sealed and it was stirred at120◦C for 10 h. Then it was cooled and ampule con-tents were dissolved in chloroform. Solution was fil-tered and product was precipitated in diethylether.

2.6. Usual procedure forpreparation ofCNT-graft-PCL-block-PLA orCNT-graft-PLA-block-PCLnanocomposites

One milliliter of 1 × 10−3 M toluene solution ofSn(Oct)2 was added to a polymerization ampuleequipped with a magnetic stirrer and vacuum inlet.Toluene was evaporated under vacuum at 60◦C for30 min. A total of 0.1 g of dried CNT-graft-PLA orCNT-graft-PCL and 1ml of ε-caprolactone or 1 g oflactide was added to polymerization ampule. Then,it was left under vacuum for 1 h at 60◦C. Poly-merization ampule was sealed and it was stirred at120◦C for 10 h. Then, it was cooled and ampule con-tents were dissolved in chloroform. Solution was fil-tered and product was precipitated in diethylether.

2.7. Preparation of sample forTEM experiments

CNT-graft-polymer was dissolved in chloroform andstirred for 1 h. Then solution was sonicated for15 min. Sample was dropped on the grade and itwas left at room temperature. After evaporation thesolvent TEM experiments were done.

3. Result and Discussion

PLA and PCL are biodegradable, biocompatibleand nontoxic polymers which are used in thebiological systems ranging from drug delivery toscaffolds. However, the mechanical properties andfunctionality of PCL and PLA can be improvedusing nanomaterials. Carbon nanotubes, as newand interesting materials, are good candidates toimprove the properties of polymers. A routine wayto prepare CNT nanocomposites is to mix poly-mers with CNTs. Although this method lead to newnanocomposites with interesting properties but con-jugation of polymers to CNTs is preferred becausesome of problems such as aggregation and bundlingare avoided.

Preparation of CNT-graft-PCL nanocompositeswas reported by Gao et al. previously. They func-tionalized opened CNTs by ethylene glycol andused it as the macroinitiator for the ring open-ing polymerization of ε-caprolactone. Here, we usedthe opened CNTs as the macroinitiator for ringopening polymerization of ε-caprolactone and lac-tide without further functionalization which opena feasible way to synthesis of CNT-graft-PCL andCNT-graft-PLA nanocomposites in the large scale(Scheme 1).

The polymer chains of CNT-graft-PCL andCNT-graft-PLA are containing end hydroxyl func-tional groups which could be used for ring open-ing polymerization of another type of cyclicmonomer and preparation of CNT-graft-blockcopolymer. Herein, CNT-graft-PLA was used asthe macroinitiator for ring opening polymeriza-tion of ε-caprolactone and CNT-graft-PLA-block-PCL was obtained. CNT-graft-PCL was also usedas the macroinitiator for ring opening polymeriza-tion of l-lactide and CNT-graft-PCL-block-PLA wasobtained.

Processability directly depends on solubility;hence increasing the solubility of CNTs increasestheir processability directly.

Figures 1(a) and 1(b) are related to the chlo-roform solution of CNT-graft-PCL-block-PLA andopened CNT, respectively. As it can be seen CNT-graft-PCL-block-PLA is soluble in chloroform com-pletely and its solution is stable after several monthswhereas opened CNT is not soluble in chloroform.

Figures 2(a)–2(c) show the IR spectra of CNT-graft-PLA, CNT-graft-PCL and CNTs-graft-PLA-block-PCL. Absorbance band of carbonyl groupsof PLA chains appeared at 1780 cm−1 (Fig. 2(a)).

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COOH

COOH

COOH

COOH

OH

OH

OH

OH

Monomer, Sn(Oct)2, 120°C

Scheme 1. Synthesis of CNT-graft-copolymers.

(a) (b)

Fig. 1. Chloroform solution of (a) CNT-graft-PLA-block-PCL and (b) opened CNT.

Carbonyl groups of PCL and PCL-block-PLA chainsappeared at 1760 and 1765 cm−1, respectively(Figs. 2(b) and 2(c)). IR spectra of CNT-graft-PLA-block-PCL is similar to that for CNT-graft-PCL. This shows that the length of PCL block inthe PCL-block-PLA copolymer is longer than PLAblock.

Figures 3(A)–3(C) show the 1H NMR spec-tra of CNT-graft-PCL, CNT-graft-PLA and CNT-graft-PLA-block-PCL. In Fig. 3(A), two methylenegroups of PCL chains resonate at 4 ppm (–CH2–O–)and 2.2 ppm (–CO–CH2–), respectively. Signals at

1.3–1.6 ppm are assigned to the other methyleneprotons of PCL chains. In Fig. 3(B) signals at 5.2and 1.7 ppm are assigned to the methine and methylgroups of PLA arms, respectively. In Fig. 3(C) sig-nals of both PCL and PLA blocks can be seenclearly which prove the structure of PLA-block-PCLcopolymer. In this figure, the peak area ratio of PLAchain is lower than that for PCL block (peak arearatio at 5.2 ppm for PLA and 4ppm for PCL inFig. 3(C)). This again shows that the PCL blockis longer than PLA in the PLA-block-PCL arms.However, length of each block in the PLA-block-PCL arms could be controlled using feed ratio ofε-caprolactone. In this work 0.002/1, 0.01/1, 0.1/1CNT-graft-PLA (g)/ε-caprolactone (ml) ratios wereused to synthesize different nanocomposites withdifferent block lengths.

We used the 1H NMR spectra for evalua-tion the relation between length of blocks andfeed ratio of ε-caprolactone. Hence above differ-ent nanocomposites with mentioned ratios wereprepared and their 1H NMR spectra were com-pared together. Figures 4(a)–4(c) show the 1H NMRspectra of CNT-graft-PLA-block-PCL synthesizedusing 0.002/1, 0.01/1, 0.1/1 CNT-graft-PLA (g)/ε-caprolactone (ml) ratios, respectively. In all spec-tra signals of PLA and PCL are presented. Inthese spectra methine group of PLA resonates at5.2 ppm and two methylene groups of PCL chainsresonate at 4 (–CH2–O–) and 2.2 ppm (–CO–CH2–),respectively. Signals at 1.3–1.7 ppm are assigned tothe other methylene protons of PCL and methylprotons of PLA chains. Clearly from spectra 4(a)–4(c) the peak area ratio of PLA protons increasewhich proves increasing the length of PCL block inthe PLA-block-PCL copolymers decreases the CNT-graft-PLA (g)/ ε-caprolactone (ml) feed ratios.

Figure 5 shows the DSC thermograms ofCNT-graft-PLA, CNT-graft-PCL and CNT-graft-PLA-block-PCL. In the DSC thermogram ofCNT-graft-PLA the glass transition and meltingtemperature of PLA can be seen at 58◦C and 164◦C,respectively. In the DSC thermogram of CNT-graft-PCL the melting and decomposition temperaturesof PCL are presented at 61◦C and 205◦C, respec-tively. In the DSC thermogram of CNT-graft-PLA-block-PCL glass transition and melting temperatureof both PLA and PCL blocks are observed.

TEM experiments were done to evaluate thestructure of solid films of nanocomposites. Theseexperiments show that the structure of films

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Carbon Nanotube-Graft-Block Copolymers Containing Silver Nanoparticles 537

Fig. 2. IR spectra of (a) CNT-graft-PLA, (b) CNT-graft-PCL and (c) CNT-graft-PLA-block-PCL.

(C)

(A)

(B)

OO

O

O

na

b

c

d

e

OO

O

O

O

Men

a

bMe O

Me O

OO

O

O

n

a

bMe O

Me OO

O

mc

d

e

f

g

ae b, c, d

a b

ppm (δ)

a

g c b, d, e, f

Fig. 3. 1H NMR spectra of (A) CNT-graft-PCL, (B) CNT-graft-PLA and (C) CNT-graft-PCL-block-PLA nanocom-posites.

strongly depends on the type and thickness of poly-mer shell.

Figures 6(a) and 6(b) show the TEM ima-ges of solid films of CNT-graft-PLA (synthesized

ppm (δ)

a

b

c

Fig. 4. 1H NMR spectra of CNT-graft-PLA-block-PCLnanocomposites prepared using (a) 0.002/1, (b) 0.01/1 and(c) 0.1/1 CNT-PLA (g)/ε-caprolactone (ml) ratios.

using 0.1/1 CNT (g)/lactide (g) ratio) andCNT-graft-PLA-block-PCL (synthesized using 0.1/1CNT-graft-PLA (g)/ε-caprolactone (ml) ratio),respectively. For preparation of these films CNT-graft-polymer or CNT-graft-block copolymer weredissolved in the chloroform and stirred for 1 h.Then, they were sonicated and dropped onto thegrade and they left to evaporate the solvent and

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20

24

mW/mg

26

28

CNT-PLA-PCL

CNT-PLA

CNT-PCL

Fig. 5. DSC thermograms of CNT-graft-PCL, CNT-graft-PLA and CNT-graft-PLA-block-PCL nancomposites.

dried in the room temperature. In the TEM imageof CNT-graft-PLA, a dendritic self assembly isclearly observed. Although the CNT-graft-PLA iscompletely soluble in the organic solvents such aschloroform, in the solid state the affinity of polymer-to-polymer and CNT-to-CNT cause a dendriticself assembly. TEM image of CNT-graft-PLA-block-PCL is not containing dendritic self assembly. HereCNTs are looped by block copolymers (Fig. 6(b)).It seems the long PCL blocks prevent a strongaffinity between CNTs and lead to the more flex-ible and homogenous films. Figure 6(c) also showsthe TEM image of solid film of CNT-graft-PLA-block-PCL prepared using 0.002/1 CNT-graft-PLA(g)/ε-caprolactone (ml) ratio in which the lengthof PCL block is longer than that of the samenanocomposite prepared using 0.1/1 CNT-graft-PLA (g)/ε-caprolactone (ml) ratio (Fig. 6(b)). Thisfigure presents a flexible and homogeneous two-dimensional film. As a result, the structure ofnanocomposite in the solid state deeply depends onthe length of copolymer blocks.

Figures 7(a) and 7(b) show the TEM imagesof CNT-graft-PLA (prepared using 0.1/1 CNT(g)/lactide (g) ratio) and CNT-graft-PLA-block-PCL (prepared using 0.1/1 CNT-graft-PLA (g)/ε-caprolactone (ml) ratio), respectively. The brightlayer on the CNT in Fig. 7(a) is related to the PLAshell which is relatively crystalline. The thicknessof the grafted PLA shell on the surface of CNT isalmost 3 nm. In Fig. 7(b) PLA-block-PCL copoly-mer as an unsmooth layer is grafted on the surfaceof CNT. The average thickness of grafted copolymeron the surface of CNT is around 7nm.

(a)

(b)

(c)

Fig. 6. TEM images of solid films of (a) CNT-graft-PLA(b) CNT-graft-PLA-block-PCl and (c) CNT-graft-PLA-block-PCL with longer polymer chains.

As mentioned, another strategies to modifyCNTs is filling their cavity by different nanopar-ticles. Here the cavity of opened MWCNTs wasfilled by silver nanoparticles through wet chem-ical method. Silver nanoparticles are universally

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Carbon Nanotube-Graft-Block Copolymers Containing Silver Nanoparticles 539

PLA

(a)

PLA-block-PCL

(b)

Fig. 7. TEM images of (a) CNT-graft-PLA and (b) CNT-graft-PLA-block-PCL nanocomposites.

Silver nanoparticle

Fig. 8. TEM image of a filled CNT by silver nanoparticles.

known as antibacterial materials hence filling ofCNTs by silver nanoparticles may be inducted bythe antibacterial property in these nanomaterials.Figure 8 shows the TEM image of CNT filled by

Fig. 9. TEM image of CNT-graft-PLA-block-PCL contain-ing silver nanoparticles inside the CNT.

silver nanoparticles. In this figure encapsulated sil-ver nanoparticles in the cavity of CNT can be seenclearly. The size of silver nanoparticles is between10 and 35 nm and the thickness of CNT wall isabout 4 nm.

To obtain high processable nanomaterialswhich can be soluble in different solvents, filledCNTs by silver nanoparticles were used topolymerize lactide and ε-caprolactone and CNT-graft-PLA-block-PCL containing encapsulated silvernanoparticles were obtained. Figure 9 shows theTEM image of CNT-graft-PLA-block-PCL contain-ing encapsulated silver nanoparticles. The copoly-mer shell on the surface and silver nanoparticles inthe cavity of CNT are represented in this figure. Theaverage size of encapsulated silver nanoparticles is10 nm and thickness of the grafted block copolymeron the surface of CNT is 7 nm. This figure shows thepossibility of preparation of new nanomaterials con-taining different segments through the combinationthe above two strategies.

4. Conclusion

Combination two strategies for modification ofCNTs lead to new nanomaterials with combinedproperties. It is a promising way to preparenew nano-devices containing a collection of nano-materials which can be used in different fields.For example, synthesized CNT-graft-PLA-block-PCL containing encapsulated silver nanoparticles

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are promising materials for application in the bio-logical systems.

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