Polymer Nanostructure

Encyclopedia of Nanoscience and Nanotechnology

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Polymer Nanostructures Liming Dai The University of Akron, Akron, Ohio, USA

CONTENTS 1. Introduction 2. Polymer Nanoparticles 3. Polymer Nanowires, Nanotubes, and Nanofibers 4. Polymer Nanofilms 5. Nanostructured Polymers with Special Architectures 6. Concluding Remarks Glossary References

1. INTRODUCTION As the term implies, polymer (or macromolecule) refers to a molecule of extraordinarily large size (typically >1000 in molecular weight). Although polymer molecules are large, they are made up of individual, repeating units termed monomers. The overall properties of a given polymeric material depend on: (i) the constituent monomer units and the way they are arranged in the macromolecule and (ii) the spatial arrangement of the constituent macromolecular chains and the nature of the intermolecular interactions that hold them together [1]. These interesting structure– property relationships provide a broad basis to allow the development of various polymeric materials (e.g., polymer fibers, films, powders) with different properties from the same macromolecules. Polymers have been widely used in all aspects of our daily life. After all, our clothes are made from synthetic fibers, car tires from rubbers, and computer chips from plastics. It is now difficult to imagine what our life would be like if there were no polymers. With the recent significant advances in nanoscience and nanotechnology, various nanostructured polymers have been devised for a wide range of advanced applications. Examples include the use of polymer nanoparticles as drug delivery devices, polymer nanofibers as conducting wires, and polymer thin films and periodically structured polymeric structures for optoelectronic devices. ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

Like most other nanomaterials [2, 3], nanostructured polymers could also possess interesting mechanical, electronic, optical, and even magnetic properties that are different from those of the bulk materials, depending on their size, shape, and composition. The development of nanostructured polymers has opened up novel fundamental and applied frontiers, which has attracted tremendous interest in recent years. This chapter provides an overview on the rapidly developing field of polymer nanostructures. We will first discuss the preparation of polymer nanoparticles, with an emphasis on their potential applications for controlled drug delivery. We will then describe the preparation of fiberlike polymers (i.e., nanowires, nanotubes, and nanofibers) of potential applications in electronic devices. Finally, we will examine the use of polymer thin films (nanofilms or nanosheets) in organic optoelectronic devices, along with the periodically structured polymers as photonic crystals.

2. POLYMER NANOPARTICLES Polymer nanoparticles are of special interest in medical applications. In particular, the recent development of polymer nanoparticles as effective drug delivery devices is revolutionizing the way in which medical treatments are performed. Encapsulating or incorporating drugs in polymer nanoparticles allows effective delivery of the drug molecules to the target site and their controlled release, leading to an increase in the therapeutic benefit and reduction of the side effects [4]. Significant advances in polymer science and biomedical engineering have facilitated the development of new synthetic approaches to polymer nanoparticles. Depending on the method of preparation, polymer nanospheres or nanocapsules can be obtained. These polymer nanoparticles, usually having a diameter ranging from 10 to 300 nm, can be prepared by various techniques, including in-situ polymerization, dispersion of preformed polymers, and self-assembling. The in vitro and in vivo performances of various polymer nanoparticles as drug delivery devices have recently been discussed in several excellent review articles [5, 6]. In what follows, we provide a status summary of the preparation of polymer nanoparticles. Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (763–790)

764 2.1. Polymer Nanospheres by Polymerization While conventional emulsion polymerization has been widely used to produce polymer particles with the size range of 0.1–1 m, miniemulsion and microemulsion polymerization methods have recently been developed for the preparation of polymer nanoparticles in the ranges of 50–200 and 15–50 nm, respectively [7, 8]. Using polymerizable amphiphilic PEO macromonomers [i.e., CH3 O– (CH2 CH2 O)n –(CH2 11 –OOCC(CH3  CH2 , n = 10 15, or 40; designated as C1–PEO–C11–MA-n] as stabilizers in dispersion/emulsion polymerizations, for example, Gan and co-workers have successfully prepared polymer [e.g., polystyrene (PS), poly(methyl methacrylate) (PMMA)] microlatexes with particle sizes ranging from 15 to 200 nm [9–11]. A large amount of surfactants, however, is normally required for these emulsion polymerization, though high solid-contents up to 40 wt% have been reported [12, 13]. The relatively high content of surfactant in the dispersions often limits their biomedical applications [14–16]. Recently, considerable efforts have been devoted to investigating ways to minimize the amount of surfactant used and to maximize the solid content. In this context, both solution and interfacial polymerization methods have been used to encapsulate drug molecules in polymer nanoparticles. For example, Couvreur and co-workers [16], synthesized poly(alkylcyanoacrylate) nanoparticles by polymerizing methyl or ethyl cyanoacrylate in aqueous acidic medium in the presence of polysorbate-20 as a surfactant. The polymerization follows an anionic mechanism and the schematic representation of the procedure used for the production of poly(alkylcyanoacrylate) nanoparticles is given in Figure 1. For controlled release purposes, drug molecules can be dissolved in the polymerization medium either before or after the polymerization reaction. The resulting suspension of polymer nanoparticles is then purified by ultracentrifugation or by resuspending the nanoparticles in an isotonic surfactant-free medium. More recently, Lowe and Temple [17] have used the interfacial polymerization technique originally proposed by Al-Khouri Fallouh, et al. [18] to prepare poly(isobutyl cyanoacrylate) nanoparticles for encapsulating calcitonin. In this case, isobutyl cyanoacrylate monomers and calcitonin were dissolved in an ethanol solution containing Mygliol@ 812 oil. The oil solution was then added dropwise into an aqueous solution of poloxamer 188 under stirring for the interfacial polymerization to take place at the surface of the Mygliol@ droplets. The resulting poly(isobutyl cyanoacrylate) nanoparticles were separated from other preparation additives by diafiltration [19]. Calcitonin was found to be incorporated into the poly(isobutyl cyanoacrylate) nanocapsules with an efficiency of 90%. Along with these efforts in preparing polymer nanoparticles via polymerization, nanoparticles with tailor-made surface properties (e.g., chemical, electrical, mechanical) by surface engineering have been reported [20]. For instance, Dokoutchaev and co-workers [21] have developed several methods to deposit metal colloids (e.g., Pt, Pd, Ru, Ag, Au) onto the surface of polymer micro- and nanoparticles.

Polymer Nanostructures Initiation Step – δ+ δ– HO + CH2 C

CN

CN – HO CH2 C CO2R

CO2R

CN

CN CN CH2 C – HO CH2 C n CO2R CO2R

H+

H n+j CO2R

HO CH2 C

Termination Step Monomer+Drug

Acidic medium in presence of Surfactant/stabilizer

Purification

Nanoparticle Surfactant Drug Cyanoacrylate monomer

Figure 1. Schematic representation of poly(alkylcyanoacrylate) nanoparticle production by anionic polymerization of alkylcyanoacrylate. Reprinted with permission from [16], P. Courvreur et al., J. Pharm. Pharmacol. 31, 331 (1979). © 1979, Kluwer/Academic Plenum.

Examples include the controlled hydrolysis of surface acetoxy groups to generate hydroxyl functionalities for specific adsorption silver and ruthenium nanoparticles [22], electrostatic deposition of Au colloids on positively charged polymer micro/nanoparticles [21, 23], and simple adsorption of Pd nanoparticles on poly(vinylpyridine) nanospheres [24]. As shown in Figure 2, these polymer micronanospheres (0.2– 3 m in diameter) functionalized with metal nanoparticles

Figure 2. (a) Scanning electron microscopy (SEM) image of poly(4vinylpyridine) microspheres and (b) transmission electron microscopy (TEM) micrographs showing the surface coverage of poly(4vinylpyridine) microspheres with colloidal palladium nanoparticles. Reprinted with permission from [24], S. Pathak et al., Chem Mater. 12, 1985 (2000). © 2000, American Chemical Society.

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(1–10 nm in diameter) possess a high ratio of surface area to volume and are promising for various potential applications, especially as efficient catalysts, chemical/electronic/optical sensors, active substrates for surface enhanced Raman scattering, and photocatalysts for solar energy conversion.

2.2. Dispersion of Preformed Polymers Several other related methods involving the dispersion of preformed polymers, such as those based on the use of emulsifying agents [25–28] and supercritical fluids [29–33], have also been used to prepare polymer nanoparticles.

2.2.1. Polymer Nanosphere by Emulsifying Dispersion In this case, a preformed polymer is dissolved in an organic solvent, which is then emulsified into an aqueous solution, with or without the presence of drug molecules, to make an oil (O) in water (W) emulsion (O/W emulsion) by using a surfactant/emulsifying agent [25, 26]. Upon the formation of a stable emulsion, the organic solvent is evaporated at room temperature by continuous stirring or by heating to produce the polymer nanoparticles. In order to control the size of the resultant nanoparticles, a modified version of the W/O method has been developed that uses both water-insoluble organic solvent and watersoluble solvent (e.g., acetone, methanol). In this case, the spontaneous diffusion of water-soluble solvent could cause an interfacial turbulence between the two phases to create smaller particles. Therefore, the increase in the concentration of water-soluble co-solvent was shown to cause a considerable decrease in the nanoparticle size [27].

2.2.2. Polymer Nanospheres by Supercritical Fluid Method These methods inevitably involve the use of organic solvents that are hazardous to the environment and physiological systems [28]. The need for environmentally safer encapsulation has been a driving force for the development of new methods for the preparation of polymer nanoparticles with desired physicochemical properties. In this regard, the supercritical fluid technique has been an attractive alternative for generating polymer nanoparticles of high purity without any trace amount of the organic solvent [29]. As detailed discussions on supercritical fluid technology are beyond the scope of this chapter, interested readers are referred to specialized reviews and monographs [31–33]. Briefly, in a typical supercritical fluid method the solute of interest is solubilized in a supercritical fluid. The solution is then expanded through a nozzle to reduce the solvent power of the supercritical fluid, resulting in the precipitation of the solute. Although the supercritical fluid method has advantages due to its solvent-free nature, further research breakthroughs are required before any commercial applications will be realized as most polymers exhibit little or no solubility in supercritical fluids.

2.3. Self-Assembling of Preformed Polymers Self-assembly involves the aggregation of molecules or macromolecules to thermodynamically stable structures that are held together by weak noncovalent interactions. These weak noncovalent interactions include hydrogen bonding, – stacking, electrostatic forces, van der Waals forces, and hydrophobic and hydrophilic interactions. Because of the fast dynamic and very specific noncovalent interactions involved, self-assembling processes are usually very fast. The resulting supramolecules could undergo spontaneous and continuous de-assembly and re-assembly processes under certain conditions due to the noncovalent nature. Supramolecular materials can, therefore, select their constituents in response to external stimuli or environmental factors and behave as adaptive materials. Although self-assembling (or complexition) of small molecules has for decades been an active research area, self-assembling of polymeric chains is a recent development. Various supramolecular aggregates with unusual structures [34–38], including shell–core and shell–hollow core, have been successfully prepared which are difficult to form by conventional chemical reactions.

2.3.1. Shell–Core Polymer Nanoparticles Self-assembling of diblock copolymers in a selective solvent, in which only one block is soluble, can form shell–core nanoparticles (Fig. 3). To demonstrate this concept, several groups have carried out detailed studies on various amphiphilic copolymers, including poly(ethylene oxide)-block-poly(propylene oxide), PEO-PPO, and poly(N -isopropylacrylamide)-blockpoly(ethylene oxide), PNIPAM-b-PEO [39–44]. It was demonstrated that the formation of shell–core architectures in the copolymer systems can be induced by changes in temperature, solvent composition, ionic strength, and/or pH. For example, self-assembled shell–core polymer nanoparticles consisting of PNIPAM core and PEO shell have been prepared in water above ∼32  C. While both PNIPAM and PEO blocks dissolve in water at room temperature, PNIPAM becomes hydrophobic and undergoes an “coil-to-globule” transition at temperatures higher than ∼32  C [45, 46]. By

More chains assembled

R

R

∆R

Figure 3. Schematic representation of a core–shell nanostructure formed by a self-assembly of coil–rod diblock copolymer chains in a selective solvent. Reprinted with permission from [39], G. Zhang et al., Acc. Chem. Res. 34, 249 (2001). © 2001, American Chemical Society.

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controlling the heating rate, Zhu and Napper [45] have prepared core–shell nanoparticles of a narrow size distribution from PNIPAM-b-PEO block copolymers. The core–shell formation from appropriate diblock copolymers has also been induced by certain chemical reactions. Of particular interest, Wu et al. [47] have used the soluble poly(4-methylstyrene-block-phenylvinylsulfoxide), PMS-b-PVSO, as the starting material in THF. Thermal elimination of phenylsulfenic acid converted the soluble and flexible PVSO block into an insoluble rodlike polyacetylene (PA) block [48, 49], leading to the formation of core–shell nanoparticles with the conducting PA core surrounded by the THF soluble PMS shell (Fig. 4). Nanoparticles with conducting polymers either as the core or shell have also been prepared by colloidal chemistry [50]. Although the block copolymer approach to the core–shell nanoparticles is an effective method and has recently been extended to include the use of triblock copolymers to form polymer nanoparticles with a multilayered shell structure [50], the general application of this method is largely limited by the rather delicated synthesis of diblock copolymers with well-defined chemical and physical structures. To eliminate the use of block copolymers, Qiu and Wu [51] investigated the temperature-induced self-assembling of PNIPAM and PEO graft copolymers. They have demonstrated that the size of nanoparticles formed by the PNIPAM chains grafted with short PEO chains is inversely proportional to the number of the PEO short chains on individual PNIPAM polymer backbones (Fig. 5a) [51]. By suppressing the interchain association, these authors have even prepared single-chain core–shell nanoparticles, as schematically shown in Figure 5b [52]. Apart from the formation of core–shell nanoparticles from pure grafted polymers, the self-assembling in mixture systems of a homopolymer and a graft copolymer has also been effectively exploited as an alternative “blockcopolymer-free” approach to polymer core–shell nanoparticles. Based on the homopolymer and graft polymer approach, Liu and co-workers [53] have recently reported an interesting approach to hollow nanospheres (i.e., shell– hollow core nanoparticles). These authors first prepared core–shell nanoparticles by self-assembling a polymer pair of poly( -carprolactone) (PCL) and a graft copolymer of methylacrylic acid and methyl methacrylate (MAF) in water. The resulting nanoparticles have a PCL core stabilized by the MAF shell with its short PCL branches anchored onto the core (Fig. 6a).

Temperature increase (a)

Temperature increase (b)

T < LCST

T > LCST

Figure 5. Schematic representation of the formation of (a) core–shell polymer nanoparticles from PNIPAM-g-PEO graft copolymers and (b) a single chain core–shell nanostructure through the coil-to-globule transition of the chain backbone of PNIPAM-g-PEO. Reprinted with permission from [52], X. Qiu and C. Wu, Phys. Rev. Lett. 80, 620 (1998). © 1998, American Chemical Society.

These authors then chemically cross-linked the miceller shell to form the so-called “shell-cross-linked knedel-like nanoparticles” [54], while they biodegraded the core with an enzyme to produce the hollow core (Fig. 6a). The morphologies of the nanoparticles before (A&B) and after (C&D) core degradation are shown by the TEM micrographs in Figure 6b. Careful examination of the nanoparticles shown in Figure 6b indicates that the particles have expanded significantly to from ca. 100 to 300 nm in diameter and a shell thickness from ca. 10 to 100 nm, suggesting a swelling effect associated with the core removing process. Closely related to this study, Blomberg and co-workers [55] have successfully prepared hollow polymeric nanocapsules through surface-initiated living free-radical polymerization. In particular, these authors first attached trichlorosilyl-substituted alkoxyamine initiating groups onto the surface silanol groups of silica nanoparticles. They then carried out living free-radical polymerization from the surface initiating groups, resulting in the formation of core– shell morphology (Fig. 7a). A

B

100nm

In water

50nm

Self-assembly

PMS

PVSO

PVSO

Polyacetylene (PA)

C

PCL

D

MAF Cross-linking

Degradation 300nm 200nm

(a)

Figure 4. Schematic representation of chemical reaction-induced selfassembly of PMS-b-PVSO diblock copolymer chains in solution upon heating. Reprinted with permission from [47], C. Wu et al., J. Am. Chem. Soc. 121, 1954 (1999). © 1999, American Chemical Society.

(b)

Figure 6. (a) A schematic illustration of the processes of self-assembly, cross-linking, and degradation. (b) TEM images of MAF-2/PCL (1:1, w/w) nanoparticles before (A and B) and after (C and D) biodegradation of the PCL core. Reprinted with permission from [53], X. Liu et al., Angew. Chem. Int. Ed. 41, 2950 (2002). © 2002, Wiley–VCH.

767

Polymer Nanostructures

O N

O

X-link

Et3N

+ ON3

Crosslinked polymer silica nanoparticle

Polymer modified silica nanoparticle

HF

Hollow crosslinked polymer nanocapsule

(a)

removing the core via chemical degradation [56–59], though the “block-copolymer-free” approaches discussed may be regarded as a simpler and more economic approach. Given that the hollow polymer nanospheres can be used to encapsulate a large variety of guest molecules, including many catalysts and drugs, there has been increasing interest in the research and development of polymer hollow nanoparticles. Clearly, a promising potential for future research and application exists in this area.

(b)

Figure 7. Schematic representation of (a) the polymer-modified silica nanoparticles and (b) the formation of hollow, cross-linked polymer nanocapsules from the polymer-modified silica nanoparticles. Reprinted with permission from [55], S. Blomberg et al., J. Polym. Sci. A 40, 1309 (2002). © 2002, Wiley–VCH.

Thermal or chemical cross-linking of the polymeric shell, followed by removal of the silica core with HF led to stable hollow polymeric nanocapsule (Fig. 7b). Figure 8a and b shows field-emission scanning electron microscopic (FESEM) images for cross-linked poly(styrene-covinylbenzocyclobutene)-modified nanoparticles before and after removal of the silica core, respectively. Similar hollow nanospheres have been previously prepared through self-assembling block copolymers in selective solvents followed by cross-linking of the micellar shell and

3. POLYMER NANOWIRES, NANOTUBES, AND NANOFIBERS Just as polymer nanoparticles have broadened the potential applications of polymer materials, polymer wires, tubes, and fibers of diameters down to a nanometer scale have also been studied for a large variety of potential applications. Polymer nanowires, nanotubes, and nanofibers are useful as both “building blocks” for nanodevices and “connecting components” between the nanoscale entities and the macroscale world. In this regard, conducting polymer nanowires, nanotubes, and nanofibers are of particular interest as they have been shown to possess the processing advantages of plastics and the optoelectronic properties of inorganic semiconductors or metals. We provide an overview on the syntheses, properties, and potential applications of polymer nanowires, nanotubes, and nanofibers, with an emphasis on those based on conducting polymers.

3.1. Tip-Assisted Syntheses of Polymer Nanowires

(a)

(b)

Figure 8. FESEM images for cross-linked poly(styrene-co-vinylbenzocyclobutene)-modified nanoparticles before (a) and after (b) removal of the silica core. Reprinted with permission from [55], S. Blomberg et al., J. Polym. Sci. A 40, 1309 (2002). © 2002, Wiley–VCH.

The use of a scanning tunneling microscope (STM) and scanning electrochemical microscope (SECM) for generation and manipulation of polymer structures as small as a few nanometers has been well documented [49, 60]. The polymerization of pyrrole onto specific regions of graphite substrates at a submicrometer resolution was achieved by using the STM tip as an electrode. Polypyrrole strips with a linewidth of 50 m and length of 1 mm were produced by a SECM [61], as were micrometer-sized polypyrrole towers [62]. By spin-coating a solution of Nafion and anilinium sulfate onto a Pt electrode in a SECM unit, Wuu et al. [63] polymerized aniline into a micrometer scale structure. Borgwarth et al. [64] successfully prepared a 20 m wide polythiophene line by using the tip of a SECM as an electrode for region-specific oxidation of bromide into bromine, followed by the diffusion of the bromine into a conductive substrate covered with a thiophene derivative, to produce localized oxidative polymerization of thiophene monomers. Aiming for polymer structures at the nanometer scale, Nyffenegger and Penner [65] produced electrochemically active polyaniline particles with a size ranging from 10 to 60 nm in diameter and 1 to 20 nm in height by using the Pt tip of a scanning tunneling microscope as an electrode for the electropolymerization of aniline. Maynor and co-workers [66] used an electrochemical reaction at an atomic force microscope tip for region-specific deposition of conducting polymer nanowires with diameters in the range from 50 to 500 nm on semiconducting and insulating substrates.

768

a

16 8 0 0.1

0.0

0.2 0.3 Time (seconds)

Conductance

100 ms

b

250 ms

c 2.0 s

Time Figure 10. (a) Conductance of a polyaniline nanowire during an entire stretching process. The arrows point to the initial increase and followed by a stepwise decrease in the conductance. (b) A zoom in of the stepwise decrease. For clarity, curves (a) and (b) are shifted upward by 2 and 3 divisions, respectively. The substrate and tip potentials were held at 0.45 and 0.5 V, respectively. Reprinted with permission from [68], H. X. He et al., Appl. Phys. Lett. 78, 811 (2001). © 2001, American Institute of Physics.

10

a

5

b

0.40 V 0.45 V 0.50 V 0.55 V

0

WE2 "Source"

I (nA)

Substrate

WE1 "Drain"

–0.2

c

4

CE

0

Etip

Figure 9. Schematic drawing of the experimental setup. The electrochemical potential of the nanowire was controlled with respect to a reference electrode (RE) in the electrolyte. A counter-electrode was used as in a standard electrochemical setup. In comparison to a field effect transistor, the RE, WE1, and WE2 electrodes are analogous to the gate, source, and drain electrodes. Reprinted with permission from [68], H. X. He et al., Appl. Phys. Lett. 78, 811 (2001). © 2001, American Institute of Physics.

0.4 V

–0.2

–0.2

0.1

Polyaniline Nano

0.25 V

–2

Conducting Polymer Nanowire

0

0

0.1

Vbias (V)

0.2

0.2

Vbias (V)

d

0.20 V

2

–0.2

xxxxxxxxx

"Gate" 6

Tip

0.30 V 0.35 V 0.40 V

–10

Tip

Stretching

0.5

6×10–8 S

a

–5

RE

0.4

b

I (nA)

Bumm et al. [67] demonstrated the use of a STM tip to probe electrical properties of individual conjugated conducting molecules (“molecular wires”) dispersed into a selfassembled monolayer film of nonconducting alkanethiolate molecules. In a closely related but separate study, He and co-workers [68] have electrochemically deposited a conducting polyaniline nanowire bridge between a STM tip and a gold electrode by region-selectively coating the STM tip with an insulation layer so that only a few nm2 at the tip end was exposed for localized growth of the polyaniline bridge (Fig. 9) [68]. Upon stretching the polymer nanowire by moving the STM tip away from the Au electrode, these authors observed a stepwise decrease in the conductance (Fig. 10), resembling that reported for metallic nanowires [69, 70]. The initial increase in the conductance seen in Figure 10a is attributed to the stretching-induced alignment of the polymer chains in the nanowire, since aligned conducting polymers have been demonstrated to show higher conductivities. The observed smaller conductance step height (30

[83, 84] [97]

nanofibers

1–2 m

[98]

LB films

1–5

[132]

coaxial nanowires

100–200

197

n

PVDF (CF2CH2)n

Poly(phenylene oxide) On

PMMA CH3 (CH2-C)n COOCH3

PTFE (CF2CF2)n

Polyethylene (CH2CH2)n

Poly(4-vinylpyridine) (CH2-CH)n N

Polyacrylonitrile H (CH2-C)n CN

Polyaniline/PEO mixtures H N

/

H N

N

N

x

(CH2CH2O)n

l-x n

Poly(3-alkythiophene) R S

n

Poly(meta-phenylene isophthalamide)/aluminum (HN / Al

NHOC

CO)n

778

Polymer Nanostructures a

(a)

Core-shell latex Dispersion

A

O N C

N m

CH CH2

H

n

PPOm PSn Drying Rod B

3D particle assembly

Coil Good solvent for coil

b

Annealing Polymer block nanocomposite

(a)

(b)

Figure 21. (a) Schematic representation of the approach for producing nanostructured polymers with 3D ordered structure. (A) Synthesis of the core–shell polymer nanoparticles with hard functional cores and soft inert shells; (B) self-assembling of the polymer nanoparticles into a 3D compact; and (C) heat treatment of the compact to induce melting of the soft shells to form polymer nanocomposite with nanostructured spheres. (b) Laser confocal fluorescent microscopy images of the surface and the bulk morphology of the 3D nanostructured polymer films. Top: Composite morphorlogy at the surface of the sample; inset shows the same structure with the width of the area being 3.3 m. Bottom: Polymer structure in the layer located 30 mm below the surface. Scale bar is 10 m. Reprinted with permission from [186], E. Kumacheva, et al., Adv. Mater. 11, 231 (1999). © 1999, Wiley–VCH.

images of the surface and the bulk morphology of the 3D nanostructured polymer films thus prepared are given in Figure 21b, which shows a highly ordered structure in the z-direction as well as in the lateral domains. These highly ordered nanostructured polymers with 3D periodic structures have been demonstrated to be promising media for data storage. On the other hand, Jenekhe and Chen [187] have prepared multidimensional ordered mesoporous polymer materials by self-assembling hollow spherical micelles made by self-organized rod–coil diblock copolymers in a selective solvent. Figure 22a schematically shows the approach used with the morphology of the resulting materials given in Figure 22b. Although micropores were observed in this particular case (Figure 22b), the diameter, periodicity, and wall thickness of the ordered arrays of spherical holes were demonstrated to depend on the copolymer molecular weight and composition, indicating the possibilities of forming porous polymer films with nanostructured holes for a variety of applications ranging from size/shape-selective separation to photonic modulation.

5.2. Coaxial Polymer Nanowires and Nanofibers 5.2 The formation of ordered structures from metal nanowires and carbon nanotubes has been an active research area for some years. Various nano- and microfabrication techniques have been developed. Examples include the use of electric and magnetic fields for aligning suspended metallic nanowires [188–190], the combination of fluidic alignment with patterned surface structures (e.g., microchannels)

Solvent evaporation

(b) A

B

10 µm 5

C D. P. and h (µm)

C

D

4 3

p D

2 h

1 0

0

500 1000 1500 2000 2500

Repeat units of PS

Figure 22. (a) Schematic illustration of its hierarchical self-assembly into ordered microporous materials. (b) Polarized optical and (B and C) scanning electron micrographs of microporous micellar films obtained from a diblock solution by solution casting on a glass slide (A) and an aluminum substrate (B and C). (D) The molecular weight dependence of geometric parameters of the pores: the diameter, D; minimum wall thickness, h; center-to-center hole periodicity, ph = p − D. Reprinted with permission from [187], S. A. Jenekhe and X. L. Chen, Science 283, 372 (1999). © 1999, American Association for the Advancement of Science.

for nanowire patterning, [191, 192] and the lithiographic patterning of aligned carbon nanotubes [193]. The effective fabrication of ordered polymer nanowires or nanofibers is a more recent development. While a few aligned polymer nanowires prepared by synthesizing them at a scanning microscope tip or by using a template were described (Section 3), the possibility of forming aligned electrospun nanofibers by using a rotating cathode collector has recently been reported [194, 195]. Nonconducting electrospun polymer fibers have also been used as “templates” for coating with appropriate conducting polymers and for deposition of a metal layer from solution or vapor [196]. MacDiarmid and co-workers [96] reported the uniform coating of an electrospun polyacrylonitrile nanofiber with a layer of conducting polypyrrole (20 to 25 nm thick) by immersing the

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Polymer Nanostructures

electrospun fibers in an aqueous solution of polymerizing polypyrrole. These authors also prepared gold coated polyacrylonitrile nanofibers through treatment of the electrospun fibers with a solution of AuS2 O3 and ascorbic acid. Reneker and co-workers [197] coated electrospun poly(meta-phenylene isophthalamide) nanofibers with subnanometer thick coatings of various other materials (e.g., carbon, Cu, and Al) by chemical and physical vapor deposition. They also prepared nanotubes consisting of pure aluminum or mixtures of aluminum and aluminum oxide by coating the electrospun poly(meta-phenylene isophthalamide) nanofiber with an aluminum layer, followed by selectively removing the polymer nanofiber core via solvent dissolution or thermal degradation [197]. During thermal degradation of the poly(meta-phenylene isophthalamide) nanofiber cores, the aluminum coated layer was subjected to a limited degree of oxidation, which produced nanotubes of mixed aluminum and aluminum oxide. The aluminum coated layers did not oxidize when the template fiber core was removed by dissolution. Figure 23A(a– c) shows TEM images of aluminum-coated poly(metaphenylene isophthalamide) fibers with the coating thickness increasing from ca. 10 to 100 nm. As seen in Figure 23B(a– c), aluminum nanotubes remained after dissolution of the fiber cores of poly(meta-phenylene isophthalamide) with N ,N -dimethylacetamide solvent. The electron diffraction pattern associated with the aluminum nanotubes shown

in Figure 23B(a) is given in Figure 23B(d), in which all the d-spacings are characteristic of the aluminum crystal unit cell. Another interesting area closely related to the fabrication of polymer nanowires and nanofibers is the synthesis of coaxial nanowires of polymers and carbon nanotubes. We have recently prepared large-scale aligned carbon nanotubes perpendicular to the substrate surface by pyrolysis of iron (II) phthalocyanine (FeC32 N8 H16 , designated as FePc) under Ar/H2 at 800–1100  C and developed a novel approach for chemical modification of aligned carbon nanotubes [198]. Radio-frequency glow-discharge plasma treatment activated the surface of the nanotubes for subsequent reactions characteristic of the plasma-induced surface groups. We then successfully grafted polysaccharide chains onto the plasmaactivated aligned carbon nanotubes through Schiff-base formation, followed by reductive stabilization of the Schiff-base linkage with sodium cyanoborohydride (Fig. 24). The resulting amino-dextran grafted nanotube film showed zero air/water contact angles. The (acetaldehyde) plasma treated carbon nanotube film gave relatively low advancing (90 ), sessile (78 ), and receding (45 ) air/water contact angles compared to the advancing (155 ), sessile (146 ), and receding (122 ) angles for an untreated sheet of aligned carbon nanotubes. The glucose units within the surface-grafted amino-dextran chains (Fig. 24) can be further converted into dialdehyde moieties by periodate oxidation [199], thereby providing a method for creating multilayer coaxial structures.

Acetaldehyde Plasma

Aligned Carbon Nanotubes

(a)

(b)

(c)

(A)

Plasma Polymer Coating

O O CHO

Carbon Nanotube

OH

(

+H2O

HO

(a)

(b)

(c)

(B) Figure 23. (A) (a–c) TEM images of aluminum-coated poly(metaphenylene isophthalamide) electrospun nanofibers with the coating thickness increasing from about 10 nm in (a) to nearly 100 nm in (c). (B) (a–c) TEM images of Al nanotubes prepared by solvent dissolution of the poly(meta-phenylene isophthalamide) core. (d) Electron diffraction pattern of nanotubes associated with the aluminum nanotubes shown in Figure 24B(a). Reprinted with permission from [197], W. Liu et al., J. Mater. Res. 17, 3206 (2002). © 2002, Materials Research Society.

CH

O

OH

+

HO

O

OH O

O

–H2O

O

OH

N

NH2 / OH Amino-dextran (23NH2/chain) = Amino-dextran backbone)

NaBH3CN

CH2

O

NH O

Figure 24. Scheme showing the grafted polysaccharide chains onto plasma activated aligned carbon nanotubes through Schiff-base formation, followed by reductive stabilization of the Schiff-base linkage with sodium cyanoborohydride. Reprinted with permission from [198], Q. Chen et al., J. Phys. Chem. B 105, 618 (2001). © 2001, American Physical Society.

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Polymer Nanostructures

In addition to the chemical grafting of polymer chains onto the carbon nanotube surface, we recently used the aligned carbon nanotubes as nanoelectrodes for making novel conducting coaxial nanowires by electrochemically depositing a concentric layer of an appropriate conducting polymer uniformly onto each of the aligned nanotubes to form the aligned conducting polymer coated carbon nanotube coaxial nanowires (CP-CNT) [200]. The SEM image for these CP-CNT coaxial nanowires given in Figure 25b shows the same features as the aligned nanotube array of Figure 25a, but the tubes had a larger diameter due to the presence of the newly electropolymerized polypyrrole coating. The presence of the conducting polymer layer was also clearly evident in TEM images [200] (Fig. 25c). The aligned CP-CNT coaxial nanowires were demonstrated to show much stronger redox responses than the conventional conducting polymer electrode [200]. The coaxial structure allows the nanotube framework to provide mechanical stability [201, 202] and efficient thermal and electrical contact with the conducting polymer layer [203, 204]. The large interfacial surface area per unit mass obtained for the nanotube-supported conducting polymer layer is potentially useful in many optoelectronic applications, for example in sensors, organic light-emitting diodes, and photovoltaic cells where the charge injection and separation are strongly limited by the interfacial area available in more conventional devices [49]. To demonstrate the potential sensing applications for the CP-CNT coaxial nanowires, we have also immobilized glucose oxidase (GOx  onto the aligned carbon nanotube substrate by electropolymerization of pyrrole in the presence of glucose oxidase [205]. The glucose oxidase-containing polypyrrole-carbon nanotube coaxial nanowires were used to monitor the concentration change of hydrogen peroxide (H2 O2 ) generated from the glucose oxidation reaction by measuring the increase in the electro-oxidation current at the oxidative potential of H2 O2 (i.e., the amperometric

(a)

(b)

100 nm

200 nm

(c)

Figure 25. (a) Aligned nanotubes, (b) CP-CNT coaxial nanowires, and (c) TEM images of the CP-CNT coaxial nanowires. Reprinted with permission from [200], M. Gao et al., Angew. Chem. Int. Ed. 39, 3664 (2000). © 2000, Wiley–VCH.

method) [205]. The amperiometric response was found to be much higher than that of more conventional flat electrodes coated with glucose oxidase-containing polypyrrole films under the same conditions. The CP-CNT nanowire sensors were also demonstrated to be highly selective for glucose with their amperiometric responses being almost unchanged even in the presence of some interference species including ascorbic acid, urea, and d-fructose. Therefore, the CP-CNT nanowires could be used for making new glucose sensors with a high sensitivity, selectivity, and reliability, which is clearly an area in which future work would be of value.

5.3. Multilayered Polymer Nanofilms Polymer-based multilayer thin films have recently received significant attention due to their potential use as photonic crystals for manipulation and control of light. Indeed, certain multilayer polymer nanostructures with the phaseseparated lamellar domains of different refactive indices have been used as 1D optical reflectors for reflecting a band of frequencies for both TE and TM polarized light regardless of angle of incidence [206, 207]. Apart from the multilayer film formation by polymer phase separation to be discussed, the construction of multilayer polymer nanofilms has also been achieved by the LB technique (Section 4.3) and a layer-by-layer self-assembly process developed by Ferreira and Rubner [208]. The principle of the layer-by-layer process lies in the alternate spontaneous adsorption of oppositely charged polymers from dilute solutions. This technique has been successfully used to generate multilayer thin films comprised of sequentially adsorbed layers of partially doped polyaniline and a polyanion (e.g., sulphonated polystyrene), leading to conductivities comparable to those obtained with spin-cast films (0.5–1.0 S/cm) after having been doped with strong acids such as HCl and methanesulfonic acid [209]. Based on the layer-by-layer adsorption process, Rubner et al. [210– 212] have also constructed multilayer LEDs from PPV and poly(styrene sulfonic acid), (SPS), or poly(methacrylic acid) (PMA) and found that the type of polyanion used has a significant effect on the performance of the LED devices fabricated with Al and ITO as electrodes. In particular, LEDs fabricated from PMA/PPV multilayers were found to exhibit luminance levels in the range of 20–60 cd/m2 with a thickness dependent turn-on voltage and rectification ratios greater than 105 , whereas the SPS/PPV counterparts showed nearly symmetric I–V curves with a thickness independent turn-on voltage and much lower luminance levels. The observed difference in the device performance can be attributed to a doping effect associated with the sulfonic acid groups in SPS. Furthermore, these authors have recently extended the layer-by-layer adsorption process to include the hydrogen-bonding interactions between the polyaniline and poly(vinylpyrrolidone), poly(vinyl alcohol), poly(acrylamide), or poly(ethylene oxide) [213]. By using preordered/prepatterned substrates, the layer-by-layer absorption process should, in principle, lead to the construction of oriented/patterned conjugated polymers. Another interesting area closely related to the polymer– polymer multilayer structure is the intercalation of polymer chains into the layered structures of clay nanoparticles,

781

Polymer Nanostructures

λmax [nm]

600

Normalised PL Intensity

leading to the formation of organic–inorganic hybrid multilayer films [214]. Organic–inorganic hybrid composites constitute a new class of materials, possessing properties characteristic of both constituent components with potential synergetic effects [215–217]. Owing to their unusual properties, organic–inorganic hybrid nanocomposites have attracted increased attention in recent years. Consequently, various polymers including poly(ethylene oxides) [218, 219], poly(olefins) [220], polyimide [221], polypyrrole [222], and polyaniline [223, 224] have been incorporated into clay nanoparticles through either a solution or a melt intercalation process [225, 226]. The most widely used clay is montmorillonite, which consists of an alumina octahedral and two silica tetrahedral sheets stacking into the layered structure with a gallery gap of ca. 1 nm between the layers. The galleries are normally occupied by cations (e.g., Na+ , Ca2+ , Mg2+ ), which can be easily replaced through alkylammonium ion exchange reaction to form the so-called organoclay. The alkylammonium ions in the organoclay layered structure facilitate the intercalation of polymer chains. At high intercalation levels, the clay may exfoliate into their nanoscale building blocks to disperse uniformly in the polymer matrices, forming exfoliated polymer–clay nanocomposites. The organic–inorganic hybrid nanocomposites thus prepared have been demonstrated to show improved environmental stability, mechanical strength, and lower permeability for gases with respect to corresponding pure polymers [227–229]. On the other hand, certain electroluminescent organic– inorganic hybrid materials have also been prepared by sol– gel chemistry [230]. In this context, we have recently intercalated conjugated conducting and light-emitting polymers into clay nanoparticles at an intercalation level below the critical value required for the clay exfolation to explore the conformational effects on their optoelectronic properties. As is well known, the band gap energies of conjugated polymers, and hence the related optoelectronic properties, depend strongly on their chain conformations [71]. Therefore, the conformational changes of light-emitting polymer backbones could be exploited as an alternative approach to color tuning, in addition to chemical modification of the polymer structure, in polymer light-emitting diodes. On this basis, we intercalated light-emitting poly[1,4-(2,5-bis(1,4,7,10tetraoxaundecyl)phenylenevinylene)] (EO3 -PPV), into clay nanoparticles for light-emitting measurements [231]. The intercalation process is experimentally revealed by a steady increase in the height of the clay galleries as determined by X-ray diffraction [231]. The photoluminescent (PL) spectra of the composites at several different intercalation levels are given in Figure 26. As can be seen, the PL emission shows a redshift with increasing intercalation level. Included in the inset of Figure 26 is the dependence of the PL peak position (!max ) on the polymer content, which shows that the !max continuously increases with increasing polymer content to a limiting value of ca. 610 nm, characteristic of EO3 -PPV, at ca. 20% (w/w), then remains unchanged despite further intercalation. These features could be attributed to an intercalation-induced conformational transition from a “compact coil” to “expanded coil,” which should lead to an increase in the effective con-

580

560

500

550

0

60 20 40 EO3-PPV in composite [wt%]

600 650 700 Wavelength [nm]

750

800

Figure 26. PL emissions from the EO3 -PPV intercalated clay nanoparticles at different intercalation levels. Reprinted with permission from [231], B. Winkler et al., J. Mater. Lett. 18, 1539 (1999), © 1999, Plenum.

jugation length, and hence a concomitant decrease in the bandgap energy. Our results from a single layer LED device based on the EO3 -PPV intercalated nanoparticles showed an EL spectrum similar to the PL emission. As expected, the similarity between the EL and PL emissions suggested that the same singlet excitons were generated upon both the PL and EL excitation. Closely related to the preparation of layered nanocomposites consisting of polymer thin films interspersed with clay sheets, Matsumoto and co-workers [232] have moved a step further forward by showing novel polymer sandwiches with different fillings. In so doing, these authors first created layered structures consisting of layers of muconate anions (i.e., reactive dienes with carboxylate group at either end) sandwiched between layers of alkylammonium cations (Fig. 27). They then polymerized the layered muconate anions into molecule-thick polymer sheets by exposure to ultraviolet light. Upon exposure of the “synthetic clay” to acid, the alklammonium cations were removed to free the uncharged polymer sheets. The released polymer sheets can be “glued” together again by one of the many alkylammonium ions for a large variety of potential applications, ranging from molecular recognition to chemical purification.

5.4. Nanostructured Polymers by Phase Separation and Nanopatterning Apart from the discussed isolated polymer nanostructures (e.g., nanoparticles, nanofibers, and nanofilms) and their assemblies, nanostructured polymer domains consisting of an intrinsic part of the polymer morphology (hence cannot be separated from other features of the polymer material) have also been prepared through, for example, polymer phase separation and nanopatterning. Like the self-assembling technique discussed, polymer phase separation plays an important role in fabrication of nanostructured polymers, as it provides the opportunity for nanoscale structuring that otherwise is difficult by lithographic techniques [233, 234]. The conditions necessary for microphase separation in immiscible polymer mixtures depend on their molecular architectures, nature of

782

Polymer Nanostructures

a

b

Light d1 (polymer)

Acid

d c

d2 (polymer)

Free polymer sheets

Figure 27. Schematic representation showing how to build molecular sandwiches with different fillings. Reprinted with permission from [232], A. Matsumoto et al., Nature 405, 328 (2000). © 2000, Macmillan Magazines Ltd.

monomers, compositions, and molecular weights [235, 236]. By varying polymer structures and phase separation conditions, at least seven well-defined microdomain structures with sizes typically in the range of a few tens of nanometers have been observed in polymer materials (Fig. 28). Among these well-defined microdomain structures observed in phase separated polymer systems, cylindrical microdomains formed by block copolymers are of particular interest as the selective removal of either the minor or major component can transform thin polymer films into an array of nanopores or nanoscopic posts. The resultant polymer nanostructures could be used as membranes for molecular separation and/or as templates for preparing electronic and magnetic nanostructured materials.

The phase separation of block copolymers has been the subject of intensive study over the past several decades [233–236]. It has recently been demonstrated that cylindrical microdomains in certain phase-separated block copolymer thin films can be oriented normal to the substrate surface by either controlling interfacial interactions between the copolymer and substrate with anchored random copolymers [237, 238] or by applying an external electric field normal to the surface [239–241]. Figure 29A and B shows atomic force microscope (AFM) images for a phase-separated asymmetric polystyrene and poly(methyl methacrylate) (PSb-PMMA, ca. 30% volume fraction of PMMA) block copolymer thin film (40 nm thick) spin-cast onto Si substrates precoated with a random copolymer of PS and PMMA having a PS fraction of 0.60 [242]. Cylindrical microdomains oriented in the direction perpendicular to the substrate are clearly evident, especially in the phase image of Figure 29B. The corresponding AFM height and phase images for the same PS-b-PMMA film after selectively removing the PMMA component by deep UV radiation and dissolution in a selective solvent (i.e., acetic acid for PMMA) are given in Figure 29C and D, respectively. Both show an ordered array of circular nanoholes perpendicularly oriented in the polymer film. By adding PMMA homopolymers with appropriate molecular weights into the PS-b-PMMA matrices, Jeong et al. [243] were able to tune the size of the cylindrical microdomains (and hence the circular holes) formed in the block copolymer films without perturbing their spatial order and orientation. They found that pore diameters either larger or smaller than those achievable with the pure copolymer could be obtained by selective removal of the homopolymer and corresponding block of the copolymer or the homopolymer only. By so doing, both 6 and 22 nm diameter pores have been prepared from the same block copolymer film. A

1.00

3.0 nm

0.75

1.5 nm

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Modulated Lamellae

Figure 28. Experimentally observed nanostructure form by phase separation of diblock copolymers. Reprinted with permission from [236], T. P. Lodge and M. Muthukumar, J. Phys. Chem. 100, 13275 (1996). © 1996, American Chemical Society.

0.25

0.50

0.75

0 1.00 µm

D 0

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Figure 29. AFM image for a PS-b-PMMA block copolymer thin film on a neutral substrate after annealing at 170  C in tapping mode before [(A) shows the height image and (B) the phase image] and after [(C) shows the height image and (D) the phase image] removal of the PMMA cylinders. Holes at the original locations of the cylinders are clearly seen. Reprinted with permission from [242], T. Thurn-Albrecht et al., Adv. Mater. 12, 787 (2000). © 2000, Wiley–VCH.

783

Polymer Nanostructures

More interestingly, these authors have recently used the phase-separated PS-b-PMMA nanoporous films and nanoscopic posts as templates for producing metal nanodots and nanoporous metal films, respectively, by evaporation metal (e.g., Cr, Au) thin films on these polymer nanostructures, followed by selective removal of the polymer templates [244]. Based on polymer phase separation, Berggren et al. [245] have also demonstrated the formation of submicrometersized domains having a range of compositions and emission characteristics by self-organization (i.e., phase separation) in polymer blends consisting substituted polythiophenes with different bandgaps. As a result, a voltage-controlled multiple-color emission was achieved, suggesting an attractive way for making future polymeric electroluminescent color screens. Like the soluble substituted polythiophenes, polyacetylene can be solubilized by making copolymers with polyisoprene, polystyrene, or polybutadiene via the so-called anionic to Ziegler–Natta route [49]. The polyisoprene– polyacetylene (PI–PA) diblock copolymers thus prepared present an unusual situation where the high flexibility and solubility associated with polyisoprene chains and the stiffness and conductivity characteristic of insoluble conjugated polyacetylene chains are combined into one macromolecule. Due to the large difference in physical properties between these two blocks, the copolymers of PI–PA are expected to show phase separation in solution and domain structure in the solid state [246, 247]. Indeed, we have recently found that I2 doping of the PI–PA copolymer films leads to the formation of a pseudo interpenetrating polymer network (PIPN) [247]. The aggregated structures (especially the PIPN network) play an important role in regulating the electrical properties of the PI–PA copolymers [247], and the doping-induced phase separation process may have potential implications for making various multiphase conducting polymers with different ordered structures ranging from the nanometer to micrometer scale. Furthermore, the chain segregation has also been either experimentally observed or theoretically predicted for block copolymers confined between two constituent homopolymer layers [248], two foreign walls [249–252], or end attached at the liquid/solid interface [253, 254], suggesting a considerable scope for patterning of polymer chains through polymer phase separation. To mention but a few examples, Zhao and co-workers [255] have experimentally demonstrated nanopattern formation by tethered PS-b-PMMA brushes upon treatment with selective solvents, while a number of other groups have reported the patterning of polymer brushes by various lithographic techniques [256]. As is well known, photolithographic processing has been widely used for the pattern formation in semiconducting industries (e.g., for delineating the circuit elements in todays’s large-scale integrated devices) for many years with an ultimate resolution at the submicrometer scale. Recent work on photolithographic patterning of conducting polymers and carbon nanotubes [100, 257] has demonstrated that photolithographic techniques, coupled with appropriate chemical reactions, could provide effective means for the micro/nanopattern formation of various organic electronic materials [100, 257, 258]. For instance, in 1988, Thakur

reported that the conductivity of nonconjugated cis-1,4polyisoprene (i.e., natural rubber) could be increased by about 10 orders of magnitude upon “doping” with iodine [259]. In view of the ease with which conducting rubber can be made and the potential challenge of Thakur’s claim to modern theory of conducting polymers, Thakur’s work received complimentary comments from several scientific journals [260–262]. Meanwhile, various other polymers with isolated (nonconjugated) double bonds, such as trans-1,4polyisoprene, 1,4-poly-(2,3-dimethylbutadiene), and trans1,4-polybutadiene, have also been found to become dark in color and conductive when “doped” with iodine in the solid state. Surprisingly, however, cis-1,4-polybutadiene does not change color or become conductive upon “I2 doping” under the same conditions [260–262]. We were the first to demonstrate that “I2 doping” of 1,4-polyisoprene produces conjugated sequences of unsaturated double bonds in the polyisoprene backbone through polar addition of I2 into the isolated double bonds in the polymer chain, followed by HI elimination [263]. The reaction mechanism for the I2 -induced conjugation is shown in Figure 30, and the conjugated sequences thus produced have been shown to be responsible for the high conductivity of the “I2 -doped” polyisoprene. The reactions shown in Figure 30 have been confirmed by optical and other spectroscopic measurements reported by several groups, including ours [263–267]. In our further investigation of conducting polydienes [264], we have found that cis- and trans-1,4-polybutadiene exhibit different behavior toward the reactions of Figure 30. For cis-1,4-polybutadiene the reaction sequence given in Fig. 30 terminated, at room temperature, at product (II), whereas for the trans-isomer the reaction sequence proceeded toward product (III) and/or (IV) thus leading to the R1 CH2 C

R1 C CH2

CH2

C

R1 C

CH2

CH2 C

C

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R2

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I C

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–HI (2) R1 CH2 C I

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CH

C

R1 C R2

CH

CH2 C H

(IV) Pale-yellow, orange or dark brown

+ I2

R2

R1 , R2 refer to either H or CH 3 group.

Figure 30. Reactions of polydiene with iodine, leading to the formation of conjugated sequences. Reprinted with permission from [263], L. Dai and J. W. White, Polymer 32, 2120 (1991). © 1991, Elsevier Science.

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Polymer Nanostructures

formation of conjugated sequences which confer electrical conductivity. Spectroscopic measurements and molecular orbital calculations demonstrated that it was an unfavorable combination of electronic and steric interactions within the iodinated cis-1,4-polybutadiene backbone (product II) that inhibited the elimination of hydrogen iodide through the E-2 elimination mechanism [reaction (2) of Fig. 30] at room temperature and hence halted the formation of conjugated sequences in the case of cis-1,4-polybutadiene [264]. However, cis-1,4-polybutadiene can be photoisomerized into the trans-isomer [268], and the isomerized material is then amenable to “I2 doping.” This discovery provides means for photolithographic generation of conducting patterns from cis-1,4-polybutadiene films through region-specific photoisomerization of the cis-isomer into its trans-counterpart and subsequent I2 -induced conjugation of the photoisomerized trans-1,4-polybutadiene chains [269]. The conducting patterns thus formed are colored and show strong fluorescence emission, which enables visualization of the conducting polymer regions. An example of the conducting patterns thus generated is shown in Figure 31a. It is a close replication of the photomask structure, and conducting wires on a micrometer scale are clearly evident. A corresponding fluorescence microscopic image of the conducting pattern is given in Figure 31b. It shows the same features as the optical micrograph (Fig. 31a), but with inverse intensities in the image. The dark regions characteristic of the “I2 -doped” trans-1,4-polybutadiene in Figure 31a gave rise to bright fluorescence emission in Figure 31b, consistent with the fluorescence emission originating from the conjugated structures [270, 271]. The dark regions in Figure 31b represent nonfluorescent components associated with the cis-isomer. The resolution obtained in this study is limited by the light wavelength and photomask structure, which can, in principle, be improved for generation of polymer patterns with a submicron resolution. More recently, we have also demonstrated that lithographic techniques could be applied to the pattern formation of carbon nanotubes. Although carbon nanotubes synthesized by most of the common techniques often exist in a randomly entangled state, [272, 273] aligned carbon nanotubes have been prepared either by postsynthesis

fabrication or by synthesis-induced alignment [100]. As briefly mentioned in Section 5.2, we, among others [274– 278], have prepared large-scale aligned carbon nanotube arrays perpendicular to the substrate surface by pyrolysis of iron(II) phthalocyanine onto the pristine quartz glass plates [279]. Subsequently, we have also developed microfabrication methods for patterning the aligned carbon nanotubes with a submicrometer resolution [280]. Our method allows not only the preparation of micropatterns and substrate-free films of the perpendicularly aligned nanotubes but also their transfer onto various substrates, including those which would otherwise not be suitable for nanotube growth at high temperatures (e.g., polymer films) [279]. In one study, we have developed a novel method for photolithographic generation of the perpendicularly aligned carbon nanotube arrays with resolutions down to a micrometer scale [281]. Figure 32a shows the steps of the photolithographic process. In practice, we first photolithographically patterned a positive photoresist film of diazonaphthoquinone (DNQ)modified cresol novolak onto a quartz substrate. Upon UV irradiation through a photomask, the DNQ-novolak photoresist film in the exposed regions was rendered soluble in an aqueous solution of sodium hydroxide due to photogeneration of the hydrophilic indenecarboxylic acid groups from the hydrophobic DNQ via a photochemical Wolff rearrangement [282]. We then carried out the pyrolysis of FePc, leading to region-specific growth of the aligned carbon nanotubes in the UV exposed regions (Fig. 32b). In this case, the photolithographically patterned photoresist film, after an appropriate carbonization process, acts as a shadow mask for the patterned growth of the aligned nanotubes. This method is fully compatible with existing photolithographic processes [283]. Furthermore, we have also used the micro-contact printing (CP) and micromolding techniques [284] to prepare micropatterns of carbon nanotubes aligned in a direction normal to the substrate surface [285]. While the CP process involves the region-specific transfer of self-assembling monolayers (SAMs) of alkylsiloxane onto a quartz substrate and subsequent adsorption of polymer chains in the SAMfree regions (Fig. 33a), the micromolding method allows the formation of polymer patterns through solvent evaporation from a precoated thin layer of polymer solution confined between a quartz plate and a polydimethylsiloxane (PDMS) UV Photomask photoresist layer quartz plate development

carbonization

pyrolysis of FePc

(a)

(b)

Figure 31. (a) Optical microscopy image of a pattern obtained by “I2 doping” of the photoisomerized trans-1,4-polybutadiene regions in the iodinated cis-1,4-polybutadiene matrix. The dark areas are regions of “I2 -doped” polybutadiene, and the width of the white rectangles at the bottom part of the picture is 18 m. (b) Fluorescence micrograph of the conducting pattern. Reprinted with permission from [269], L. Dai et al., Macromolecules 29, 282 (1996). © 1996, American Chemical Society.

aligned nanotubes

(a)

(b)

Figure 32. (a) Schematic representation of the micropattern formation of aligned carbon nanotubes by photolithographic process. (b) Typical SEM micrographs of patterned films of aligned nanotubes prepared by the pyrolysis of FePc onto a photolithographically prepatterned quartz substrate. Reprinted with permission from [281], Y. Yang et al., J. Am. Chem. Soc. 121, 10832 (1999). © 1999, American Chemical Society.

785

Polymer Nanostructures DNQ-nanolak photoresist quartz glass piece

PDMS solution of CH, (CH2lnSlCl2 in hearne

quartz exp

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DNO-nanolak photoresist solution SAM polymer pattern

carbonization

carbonization

pyrolysis of FePc

pyrolysis of FePe

processed aligned nanotubes

(a)

(b)

(c)

Figure 33. Schematic illustration of the procedure for fabricating patterns of aligned carbon nanotubes by (a) microcontact printing and (b) solvent assisted micromolding. (c) A typical SEM image of aligned nanotube pattern prepared by the pyrolysis of FePc onto the photoresist prepatterned quartz via micromolding technique. Reprinted with permission from [285], S. Huang et al., J. Phys. Chem. B 104, 2193 (2000). © 2000, American Chemical Society.

elastomer mold (Fig. 33b). The DNQ-novolak photoresist patterns formed in both cases were then carbonized into carbon black for region-specific growth of the aligned nanotubes in the polymer-free regions by pyrolysis of iron(II) phthalocyanine under Ar/H2 atmosphere at 800–1100  C (Fig. 33c), as is the case for the aforementioned photolithographic patterning. The spatial resolution is limited by the resolution of the mask used and submicrometer features were obtained. Subsequently, we found that 3D micropatterns of wellaligned carbon nanotubes can also be prepared by pyrolysis of FePc onto appropriate photopatterned substrates [286]. In this particular case, the photopatterning was achieved by photolithographic cross-linking of a chemically amplified photoresist layer spin-cast on a quartz plate or a Si wafer, coupled with a solution development to generate a negative photoresist pattern. Owing to an appropriate surface characteristic, the patterned photoresist layer was found in this case to support the aligned carbon nanotube growth by pyrolysis of FePc, as were the photoresist-free substrate surfaces. The difference in chemical nature between the surface areas covered and uncovered by the photoresist film, how-

ever, caused a region-specific growth of the nanotubes with different tubular lengths and packing densities (Fig. 34). As can be seen from this discussion, micropatterns of aligned nanotubes with resolutions down to a submicrometer scale, suitable for fabrication of various electronic and photonic devices, have been prepared by lithographic techniques. These lithographic patterning methods, coupled with the ease with which polymer chains can be chemically and/or electrochemically attached onto the carbon nanotube wall (Section 5.2), should allow the formation of the nanostructured polymer-nanotube coaxial nanowires in a patterned fashion, attractive for constructing various on-tube optoelectronic devices and sensor arrays. On the other hand, three-dimensional polymer nanopatterns have also been prepared by some nonconventional lithographic techniques. Notably, a mask-induced selfassembling (MISA) process has been applied to the nanopattern formation in polymer films [287–290]. In this process, a mask with protruded patterns is placed at a certain distance above the top surface of a polymer melt (e.g., PMMA, PS), in the presence or absence of an external field (e.g., thermal or electrical field), allowing attraction of an excess amount of polymer chains to the area below the mask protrusions. Subsequent solidification of the polymer melt thus leads to the formation of polymer patterns with a lateral dimension identical to that of the mask protrusions. Using the MISA patterning technique, 3D polymer patterns with feature size down to nanometer scales can be prepared [288]. Figure 35A shows schematically a typical procedure for the MISA patterning. An AFM image of polymer nanopatterns produced by the MISA technique is given in Figure 35B, which shows well-defined 3D nanopatterned structures. As can be seen, the MISA method is a straightforward and feasible technique for fabricating polymer patterns with submicrometer/nanometer features. Continued research efforts in this embryonic field could give birth to a flourishing area of nanopatterning technologies.

Figure 34. Typical SEM micrographs of the 3D aligned carbon nanotube micropattern. Reprinted with permission from [286], Q. Chen and L. Dai, J. Nanosci. Nanotechnol. 1, 43 (2001). © 2001, American Scientific Publishers.

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Polymer Nanostructures POLYMER Si SUBSTRATE

(a)

SPACER

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(b)

350 nm

MASK

Si SUBSTRATE

5 µm

(c)

(A)

(B)

Figure 35. (A) Schematic of MISA: (a) a thin polymer film cast on a flat silicon substrate, (b) a mask with protruding patterns placed a distance above the polymer film, and (c) the polymer film self-constructs into a mesa under a mask protrusion. The mesa has a lateral dimension identical to that of the mask protrusion, a height equal to the distance between the mask and the substrate. (B) AFM image of the PMMA pillars formed by MISA. Each PMMA pillar is formed under each dot protrusion on the mask. Reprinted with permission from [288], S. Y. Chou et al., Appl. Phys. Lett. 75, 1004 (1999). © 1999, American Institute of Physics.

6. CONCLUDING REMARKS An overview of the recent progress in research and development of nanostructured polymers was presented. As can be seen, nanostructured polymers, including nanoparticles, nanowires, nanotubes, nanofibers, nanofilms, and their derivatives, possess interesting biomedical, electronic, optical, and mechanical properties for use in many potential applications. In order to realize their commercial applications, nanopatterning of polymer materials and selfassembling of nanostructured polymers have also been developed for effective incorporation of polymer nanostructures into nanodevices, and proper connections of the nanoscale structures and devices to the macroscale world. With the promising approaches already reviewed in this chapter, and more to be developed, practical applications of nanostructured polymers are imminent.

ABBREVIATIONS AFM Atomic force microscopy. CP-CNT Conducting polymer-carbon nanotube. DNQ Diazonaphthoquinone. EL Electroluminescence. EO3 -PPV Poly[1,4-(2,5-bis(1,4,7,10tetraoxaundecyl)phenylenevinylene)]. ESR Electron spin resonance. FePc Iron(II) phthalocyanine. FTIR Fourier transform infrared. GOX Glucose oxidase. HCSA Camphorsulfonic acid. ITO Indium tin oxide. LB Langmuir–Blodgett. LED Light-emitting diode. MAF Graft copolymer of methylacrylic acid and methyl methacrylate.

MISA Mask-induced self-assembling. NR Neutron reflectivity. PAN Polyacrylonitrile. PANI Polyaniline. PCL Poly( -carprolactone). PEO Poly(ethylene oxide). PI-PA Polyisoprene–polyacetylene diblock copolymer. PIPN Pseudo interpenetrating polymer network. PL Photoluminescence. PMA Poly(methylacrylic acid). PMMA Poly(methyl methacrylate). PMS Poly(4-methylstyrene). PNIPAM Poly(N -isopropylacrylamide). PPV Poly(p-phenylene vinylene). PS Polystyrene. PTFE Polytetrafluoroethylene. PVDF Poly(vinylidene fluoride). PVP Polyvinylpyridine. PVSO Poly(phenylvinylsulfoxide). SECM Scanning electrochemical microscope. SEM Scanning electron microscope. SFA Surface force apparatus. SPS Poly(styrene sulfonic acid). STM Scanning tunneling microscope. TEM Transmission electron microscope.
CP Microcontact printing. UV-vis Ultraviolet-visible.

GLOSSARY Carbon nanotube An elongated tubular structure consisting of carbon hexagons arranged in a concentric manner with a diameter at the nanometer scale. Electroluminescence The emission of light by a substance upon the excitation by an applied voltage. Fluorescence The emission of light by a substance immediately after the absorption of energy from light of usually shorter wavelength. Nanostructure Structural entities with at least one dimension down to a nanometer scale. Polymer A chain-like molecule of large size in which the atoms are held together by covalent bonds. Polymer brush An assembly of end-anchored polymer chains at the liquid-solid interface in which each of the constituent polymer chains adopts an extended linear chain conformation. Self-assembling The spontaneous generation of welldefined supramolecular architectures through non-covalent aggregation of macromolecular or low molecular weight compounds. Template synthesis The production of materials with controllable structures by synthesizing them within the pores of a template membrane.

Polymer Nanostructures

ACKNOWLEDGMENT I thank my colleagues and many others who made contributions to the work that is reviewed in this chapter.

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