Supramolecular Self-Assembly of Polymer-Functionalized Carbon Nanotubes on Surfaces

Supramolecular Self-Assembly of

Polymer-Functionalized Carbon Nanotubes on Surfaces

Chao Gao

College of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road,Shanghai 200240, P. R. China, and Macromolecular Chemistry II, Bayreuth University, D-95440 Bayreuth, GermanyFax: (þ86) 21 54741297; E-mail: [email protected]

Received: February 1, 2006; Revised: March 22, 2006; Accepted: March 24, 2006; DOI: 10.1002/marc.200600075

Keywords: atomic force microscopy; carbon nanotubes; functionalization; self-assembly; supramolecular; suprastructures;surface

Introduction

Carbon nanotubes (CNTs),[1] with unusual structural,

electronic, mechanical, and thermal properties, show great

potential in a wide range of applications ranging from

nanodevices to nanocomposites.[2,3] Functionalization of

CNTs by attachment of small molecules,[4] grafting of

polymers,[5–7] and conjugation of biomacromolecules[8,9]

presents a useful route to realize their full potential, making

the structure and properties of CNT-based materials and

devices tailorable. After tethering polymer chains to

the convex surface of the tubes, a core-shell structured

nanocable is formed, with the hard CNT as the core and the

soft polymer layer as the shell.[5] Such integrated core-shell

nanomaterials can be solubilized or individually dispersed

in solvents, resulting in properties different from those of

neat CNTs and polymers. The long, hard nanotubes may

play the role of a framework, and the short flexible polymer

grafts the role of building blocks. Therefore, the self-

assembly of such structures to suprastructures is possibly

expected. Sano and coworkers have reported that poly-

ethylene oxide (PEO)-grafted single-walled carbon nano-

tubes (SWNTs) can self-organize in solutions and in

Langmuir–Blodgett (LB) films.[10] Recently, Jung et al.

also reported the aggregation behavior of PEO-grafted

SWNTs; they found that ring-like structures in which the

aggregated PEO core was surrounded by SWNT bundles

were formed, when the freshly prepared PEO-grafted

SWNTs were cast from a benzene/tetrahydrofuran solvent

mixture.[11] These results imply that polymer-functiona-

Summary: Supramolecular self-assembly of poly(methylmethacrylate)-grafted multiwalled carbon nanotubes (MWNT-g-PMMA) was reported herein. The MWNT-g-PMMA (85 wt.-% PMMA) dispersed in tetrahydrofuran could self-assembleinto suprastructures on surfaces such as gold, mica, silicon,quartz, or carbon films. With decreasing concentration of theMWNT-g-PMMA from 3 to 0.1 mg � mL�1, the assembledstructures changed from cellular and basketwork-like forms to

multilayer cellular networks and individual needles. SEM,AFM, and TEM measurements confirmed the morphology ofthe assembled suprastructures, and revealed the assemblymechanism. Phase separation during evaporation of the solventdrives the MWNT-g-PMMA nanohybrids to assemble andform the suprastructures, and the rigid MWNTs stabilize thestructures.

SEM images of self-assembled suprastructures of basketwork (a), cellular network (b), andneedles (c) from the THF solution of the PMMA-grafted MWNTs on gold surface.

Macromol. Rapid Commun. 2006, 27, 841–847 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication DOI: 10.1002/marc.200600075 841

lized CNTs may play an important role in the self-assembly

field. However, more complex suprastructures assembled

from polymer-functionalized CNTs, especially multi-

walled carbon nanotubes (MWNTs), have not been

reported.

Self-assembly is accepted as a versatile method to create

novel materials and structures.[12] Amphiphilic and p-conjugated molecules may easily self-assemble to formsupramolecules, depending on electrostatic attraction,hydrogen bonds, van derWaals forces, andp-p stacking.Because of the lack of strong driving and stabilizingforces, however, self-assembly of structural materialsinto suprastructures remains a challenge. Interestingly,such phenomena are encountered frequently in nature.For instance, hard bones and elastic proteins constructlife by self-assembly, and rigid cellulose cell walls andflexible chloroplasts assemble plants. In these systems,the harder structure plays the role of the skeleton or theframework to support the assembly, and the softer therole of filler to connect and harmonize the assembly.Interactions among the hard frameworks and the softfillers stabilize the resulting suprastructures. Using thehard-soft structured model, nature creates inimitablyhard shells and fascinating pearls with inorganicminerals and organic biomacromolecules, whereasmankind builds many modern skyscrapers with steelbars and concrete. Inspired by such amodel, I show here

an example of supramolecular self-assembly ofpoly(methyl methacrylate) (PMMA)-functionalizedMWNTs into suprastructures on solid surfaces.

Experimental Part

Materials

MWNTs, made by the method of chemical vapor deposition,with diameters of 10–25 nm were purchased from Tsinghua-Nanfine Nano-Powder Commercialization Engineering Cen-tre, with a purity higher than 95%. The MWNT-basedmacroinitiator (MWNT-Br) was prepared according to proce-dures published previously.[13] PMMA-grafted MWNTs(MWNT-g-PMMA) were synthesized by surface-initiatedatom transfer radical polymerization (ATRP).[5a] The graftedpolymer content, determined by TGA, was 85 wt.-%. The

Scheme 1. Structure of PMMA-grafted MWNT.

Figure 1. Representative SEM images of the as-prepared MWNT-g-PMMA bulk material (a), andself-assembled structures of MWNT-g-PMMA on surfaces of gold (b) and quartz (c, d). The goldsurface was prepared by sputter-coating a homogeneous gold layer with 2–3 nm thickness on a cleanquartz surface. The concentration of the solution is 1 mg MWNT-g-PMMA per 1 mL THF.

842 C. Gao

Macromol. Rapid Commun. 2006, 27, 841–847 www.mrc-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

PMMA cleaved from the nanotubes by refluxing in CH3ONa/CH3OH solution (24 h) has a number-average molecularweight (Mn) 8 700 and polydispersity index (PDI) 2.5. Otherreagents, unless otherwise stated, were purchased fromAldrich.

Self-Assembly of MWNT-g-PMMA on Surfaces

A THF solution of the MWNT-g-PMMA of a givenconcentration was dropped on a flat surface (e.g., siliconwafer, quartz, gold, carbon, or mica). The substrate-supportedsample was obtained after the evaporation of the solvent atroom temperature (about 30 min), and used for characteriza-tion.

Characterization

The samples were investigated by scanning electron micro-scopy (SEM, LEO-5000, operated at 10 keV), transmissionelectron microscopy (TEM, Hitachi H7100, operated at 100keV), high-resolution transmission electron microscopy(HRTEM, JEOL JEM 2200FS, operated at 200 keV), andatomic force microscopy (AFM, Digital Instrument NanoscapeIIIa SPM) operating in the tapping mode.

Results and Discussion

To explore the self-assembly model of core-shell hard-soft

nanohybrids, the PMMA-grafted multiwalled carbon

nanotubes (MWNT-g-PMMA)[5a] with 85 wt.-% of poly-

mer were first synthesized by nanotube surface-initiated

ATRP.[14] The schematic structure of a PMMA-immobi-

lized MWNT is shown in Scheme 1.

PMMA was selected for the grafts because of its

universality. A high content of PMMA is needed to ensure

good solubility of the nanohybrids; and a strong force of

chain entanglement during organization. The used CNTs

are MWNTs made by the method of chemical vapor

deposition, which is easily available on a large scale. The

bulk morphology of the as-prepared MWNT-g-PMMA is

shown in Figure 1(a), indicating the common structure of

general CNT-polymer nanohybrids, with the nanotubes

embedded in the polymer phase.

In the assembly experiments, the THF solution of the

MWNT-g-PMMAwas dropped onto a clean surface such as

gold, quartz, mica, or a carbon film. After evaporation of the

solvent, the sample was characterized by microscopy.

Figure 1(b) shows a representative SEM image of the

sample on a gold surface. A porous network structure is

clearly observed. The pores have a size of micrometers and

the nets a width of 100–500 nm. Moreover, the cellular

network is multilayered, which can be recognized from the

different contrast and brightness of the local domains. A

representative SEM image of the structure formed on a

quartz surface is shown in Figure 1(c). The structure of the

assembled cellular foam is similar to that shown in

Figure 1(b). Under a higher magnification, the multilayered

Figure 2. SEM images of the self-assembled patterns or structures of the MWNT-g-PMMA on a goldsurface with concentrations of 3 (a, b), 2 (c), and 0.1 (d) mg �mL�1.

Supramolecular Self-Assembly of Polymer-Functionalized Carbon Nanotubes on Surfaces 843


structure is also evident [Figure 1(d)], as marked by arrows.

Similar cellular structures were observed for samples on

other surfaces, such as silicon wafers and carbon films.

The solution concentration has a significant influence on

the assembled structures or patterns. Figure 2 shows SEM

images of samples on gold surface at different concen-

trations. For a highly concentrated solution, the function-

alized nanotubes tended to form patterns with a continuous

phase. Typically, two kinds of patterns, closed-cellular and

basketwork-like, are shown in Figure 2(a) and (c). At a high

magnification, a rod-like morphology can be observed

[Figure 2(b)], indicating the existence of nanotubes.

Interestingly, assembled straight needles with a length of

around 1mm and width of 100–400 nm were observed when

the solution concentration was low [Figure 2(d)]. Judging

from the size, one needle contains several functionalized

nanotubes. Similar assembled structures were found on

other surfaces when the sample was prepared at a similar

concentration. From the variation of the structures obtained

under different preparation conditions, the assembly

mechanism can be deduced. With the evaporation of

solvent, the polymer chains tend to aggregate, driving the

tubes to move on the surface and become organized. On the

other hand, the existence of long tubes makes such

movement difficult. If the concentration is high, the

viscosity is high and since the interactions among the

functionalized tubes are strong, the aggregation is incom-

plete. Thus, continuous phase structures with many voids

would appear. The porous or basketwork-like patterns

provide direct evidence for the tube movement or

aggregation [Figure 2(a) and (c)]. At a suitable concen-

tration, the interactions among polymers and tubes make

the aggregation more complete, but the long and aligned

tubes inhibit the formation of individual domains, giving

rise to the porous network. Certainly, multilayered net-

works would be fabricated due to the deposition effect of

functionalized tubes. With decreasing concentration, sin-

gle-layered networks and nanofiber or other structured

Figure 3. AFM height images of MWNT-g-PMMA on quartzwith concentrations of 1 (a), 0.2 (b), and 0.04 (c) mg �mL�1. (d, e)Corresponding section analysis of images (a) and (c). The imagesizes are 18� 18 mm (a), 5� 5 mm (b), and 3� 3 mm (c).

Figure 3. (Continued)

844 C. Gao


domains might be formed. According to this model,

individual functionalized nanotubes lying on the surface

could be encountered for an extremely dilute solution. This

‘‘solvent evaporation’’-induced self-assembly mechanism

is further confirmed by the analysis given in the next

paragraph. Obviously, the dispersibility of the functional-

ized nanotubes, mainly influenced by the grafted polymer

content and the tube length, is a key factor in this process.

Such an assembly mechanism for the core-shell hard-soft

hybrid materials is also in agreement with the aggregation

phenomenon observed in the system of PEO-grafted

SWNTs.[11]

The resulting structures were further characterized by

AFM since the height of the structure can be determined.

Figure 3 displays the AFM images formed on a

quartz surface. At a concentration of 1 mg �mL�1, cellular

network structures were also detected here [Figure 3(a)].

Similar structures were also observed on other surfaces

(figures not shown). With decreasing concentration, the

thickness of the assembled nets decreased. At a concen-

tration of 0.2 mg �mL�1, a network composed of single- to

double-layered functionalized nanotubes appeared. In

this case, the morphology of nanotubes can be seen

[Figure 3(b)]. Individual nanotubes scattered on the surface

Figure 4. Low-voltage (100 kV) TEM images of MWNT-g-PMMA on a carbon-coated TEM sample grid (a–c),high-voltage (200 kV) TEM image of MWNT-g-PMMA on a multipore lacey carbon TEM sample grid (d), and high-resolution high-voltage TEM images of MWNT-g-PMMA on the carbon-coated TEM sample grid (e–g), and SEMimages of the same sample measured by TEM [shown in image (a)] of assembling on the TEM grid (h, i). Theconcentration of solution is 1 mg MWNT-g-PMMA per 1 mLTHF. The scale bars are 3 mm (a), 200 nm (b), 50 nm (c),0.5 mm (d), 50 nm (e), 50 nm (f), 10 nm (g), 5 mm (h), and 1 mm (i).



were detected when the concentration was further decreased

to 0.04 mg �mL�1 [Figure 3(c)]. Section height analysis

indicates that a multilayered network with a thickness of

250–500 nm was constructed with around 8–20 layers of

individual functionalized nanotubes [Figure 3(d)], with the

reference of individual tubes [Figure 3(e)].

It is noteworthy that a mixture of pristine or oxidized

CNTs and PMMA did not show any self-assembly in my

experiments, and only a simple composite film was formed.

Furthermore, for MWNT-g-PMMA, no self-assembled

structures were found either, if the grafted polymer content

was lower than 50 wt.-%. This is most likely due to the

poor solubility of CNTs and weak interactions between

the polymer and the nanotubes. A detailed study on the

influence of polymer content and composition on

the resulting morphology of the self-assembled structures

is in progress, and will be reported later.

To inspect and confirm the polymer-grafted nanotube

structure and the fine net structure of assembled networks,

TEM was used. Figure 4 shows representative images of

both low-voltage (100 kV) and high-voltage (200 kV)

TEM. The sample was prepared by dropping directly a THF

solution of MWNT-g-PMMA (85 wt.-% of PMMA) onto a

carbon-coated TEM sample grid (Agar S160-4 Carbon Film

400 Mesh Cu). Similar network structures are observed on

the TEM grid, demonstrating the good reproducibility and

the generality of the supramolecular self-assembly of

polymer-functionalized CNTs [Figure 4(a)]. Furthermore,

SEM observations on the same sample further confirmed

the cellular network structure, as shown in Figure 4(h) and

(i). In contrast, no assembled structures but only self-

standing films, where the CNTs were uniformly dispersed

and embedded in the polymer phase, were formed if a

multipore lacey carbon TEM sample grid (Agar S166-4

Lacey Carbon 400 Mesh Cu) was used as the surface

[Figure 4(d)]. This result further proves that the formation

of assembled structures is related to the movement of

nanotubes. The multipore surface restricts such movement,

and hence no self-assembly occurs. For the assembled

networks, the fine nanostructures of nets could be clearly

observed at a higher magnification; the nets are constructed

of one tube to several tens of functionalized nanotubes.

Interestingly, the nanotubes are aligned and straightened,

as shown in Figure 4(b) and (c). This is likely due to

the aggregation of nanotubes caused by the strong shrink-

age of the polymer phase during the assembly process. The

straightening effect was proven by high-voltage TEM

measurements. Two aligned and one straightened

nanotube are shown in Figure 4(e) and (f). A HR TEM

image shows the core-shell structure of PMMA-

coated MWNT [Figure 4(g)]. The results of TEM character-

izations are in full agreement with the assembly results

and mechanism mentioned above. Such an extension

effect was also observed in the case of PEO-grafted

SWNTs.[10]

Conclusion

Supramolecular self-assembly of polymer-functionalized

CNTs into suprastructures on surfaces was discovered. The

CNT-based core-shell hard-soft nanohybrids could assemble

into basketworks, cellular networks, and needles or fibers

depending on the solution concentration. Polymer phase

separation and aggregation during the solvent evaporation

provides the driving force, and the rigid nanotubes support

and stabilize the assembled structures. Because the polymer-

coated nanotubes are not really traditional ‘‘molecules,’’ but

core-shell type materials, the self-assembly phenomenon

presented here can be named as ‘‘suprastructured self-

assembly’’ as well. The porous structures can be used as

templates to fabricate CNT-reinforced cellular foams and

films of metals, metal oxides, ceramics, and so forth by the

conventional calcination methodology. This finding would

open the door to self-assembled suprastructures and offer a

versatile strategy to create novel materials and structures.

Acknowledgements: This work was financially supported bythe National Natural Science Foundation of China (no. 50473010and 20304007), the Program for NewCentury Excellent Talents inUniversity, the Foundation for the Author of National ExcellentDoctoral Dissertation of China (No. 200527) Fok Ying TungEducation Foundation (no. 91013) and Rising-Star ProgramFoundation of Shanghai (no. 03QB14028). The Alexander vonHumboldt Foundation (Germany) for granting the research fellowis specially acknowledged. I thank Professor Sir Harold W. Kroto(The Florida State University), ProfessorAdi Eisenberg, ProfessorAxel Muller (Bayreuth University), Dr. Yi Zheng Jin (University ofSurrey), and Dr. Hao Kong for helpful contributions.

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Supramolecular Self-Assembly of Polymer-Functionalized Carbon Nanotubes on Surfaces

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