"Electrically Active Polymers". - Wiley Online Library

ble and processable high molecular weight PPVs The most widely used ..... trical conductivity is limited by the disorder-induced localization of the ..... EAP-based actuators have been explored for use in artificial muscle fibers (AMFs) ...... 61, 2009. (1996). 424. P. Annadurai, A. K. Mallick, and D. K. Tripathy, J. Appl. Polym. Sci.
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ELASTOMERS, THERMOPLASTIC

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ELECTRICALLY ACTIVE POLYMERS Introduction The discovery that polyacetylene doped with iodine exhibited electrical conductivity many orders of magnitude higher than neutral polyacetylene was made in 1976 by Hideki Shirakawa, Alan MacDiarmid, and Alan Heeger; they received the 2000 Nobel Prize in Chemistry (1,2) for their work in this area. This discovery triggered the development of a new multidisciplinary field known as “synthetic metals” (3). The field of synthetic metals is at the crossroads of chemistry, physics, materials science and engineering, and biomedical engineering. This discovery marked what has been described as the “fourth generation of polymeric materials” (4). Since the early pioneering work of Shirakawa, MacDiarmid, and Heeger, numerous additional developments and discoveries in conductive polymers have occurred. As a result, the field of plastic electronics and photonics has matured to offer the promise of a wide range of novel applications exploiting the unique chemical, physical, and electrical properties of conductive polymers. In order to begin describing electrically conductive polymers, several definitions of conductive polymers must be presented. There are four major classes of conducting polymers: filled polymers, ionically conducting polymers, charge-transfer polymers, and electrically conducting polymers (ECPs). Filled polymers are intrinsically nonconductive polymers loaded with conductive fillers such as carbon black, graphite fiber, metal particles, or metal oxide particles (5–9). Filled polymers have the longest history and broadest application in electronic devices. These materials have been used since the 1930s in the prevention of corona discharge and later in advanced printed circuitries. The extensive use of these materials lies in their ease of processing, wide range of electrical properties, and relatively low cost. However, these materials are inhomogeneous, Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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containing three distinct phases: polymer, filler, and interface. This inhomogeneity results in a lack of reproducibility, a heavy process dependency, and a steep percolation threshold in conductivity (see CONDUCTIVE POLYMER COMPOSITES). Ionically conducting polymers, also called ionomers (qv) or polyelectrolytes (qv), have been known for more than 30 years; several excellent reviews are available (10–13). These materials have found a wide range of potential commercial applications including rechargeable batteries (14,15), fuel cells (16,17), and polymer light-emitting devices (18). Ionic polymers are highly processable and can be synthesized using a wide variety of approaches. The major drawback to these materials is their sensitivity to moisture and humidity. Charge-transfer materials were first investigated during the 1950s with the discovery of electrical conductivity on the order of 103 S/cm (19) in molecular charge-transfer complexes. In the 1960s a similar observation was made with iodine-doped poly(vinyl carbazole) (PVK) (20,21). The addition of an oxidant such as SbCl5 and/or tri-(p-bromophenyl)ammonium hexachloroantimonate) to a charge-transport polymer (PVK) has produced materials that are known as semiconducting charge-transfer polymers (22). By varying the oxidant and chargetransport polymer combinations, various semiconducting polymers have been produced (23,24). The conductivity of these materials can be readily tuned by adjusting the concentrations of the charge transport group and oxidant. These materials offer many advantages as a result of their wide range of conductivities, high stability, excellent wear resistance, and high dielectric strength. However, the major drawback to these materials has been their low conductivities. The highest conductivities of polymeric charge-transfer materials have been only on the order of 10 − 5 S/cm (25). The final materials to be discussed are intrinsically conducting/electrically conducting polymers (ICPs/ECPs) and/or electrically active polymers, which will be the focus of this review. While these terms have been used interchangeably, ICPs/ECPs are by definition conductive; electrically active polymers include ICPs/ECPs as well as conjugated but poorly conductive polymers. Throughout this article, the term electrically active polymers (EAPs) will be used to describe conjugated polymers, regardless of conductivity. EAPs are composed of conjugated polymer chains with delocalized π-electrons along the polymer backbone. EAPs are either insulating or semiconducting in the undoped or neutral form. These neutral polymers are converted to the electrically conductive or doped forms by oxidation or reduction reactions, which form delocalized charge carriers. To maintain charge balance in such systems, an oppositely charged counterion is required. The conductivity of these materials is electronic in nature; there is no concurrent ion migration occurring in the polymer solid state, and the doping processes are reversible. These redox doping processes can be achieved by chemical and/or electrochemical methods. Thus, the conductivities of EAPs range from those of insulators (104 S/cm) (Table 1) (26,27). There are many excellent reviews of EAPs that the reader is encouraged to examine that chronicle the historical achievements in this field of chemistry since 1976 (17,28–42). This article will review the synthesis, properties, and applications of redox-dopable electrically conducting polymers. Several specific examples will be used to illustrate general concepts and principles.

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Table 1. Comparison of Electrical Conductivities of Insulators, Semiconductors, Metals, and EAPsa Materials Ag, Cu, Pt Fe

Bismuth InSb Germanium Silicon Silicon bromide Glass DNA Diamond Nylon Sulfur Teflon/quartz a Refs.

Conductivity (logσ ), S/cm 6 5

4 1 −2 −5 −8 −10 −12 −14 −14 −16 −18

EAPs d-Polypyrrole,b d-polythiophene,b md-poly(para-phenylene),b and d-polyacetylene d-Polyaniline d-Poly(phenylene sulfide)b trans-Polyacetylene trans-Polyacetylene cis-Polyacetylene Polyaniline Polythiophene, polypyrrole

Polystyrene

Classification Metal Metal

Metal Semiconductor Semiconductor Semiconductor Insulator Insulator Insulator Insulator Insulator Insulator Insulator

26–28. material.

b Doped

Synthesis of Electrically Active Polymers The number of new conducting polymer structures that are reported in the open literature continues to grow at nearly 500 articles per month. This incredible amount of research devoted to the study of EAPs attests to their unique scientific properties and their potential widespread use in commercial applications. In order to fully exploit their potential applications one must begin with their synthesis and properties. The synthetic routes that have been developed for the preparation of conjugated polymers are diverse. This diversity is driven in part by the desire to develop substitutes for metallic conductors and semiconductors, but other properties unique to EAPs have also attracted intense interest. The effort to synthesize materials with tailor-made properties has been and continues to be pursued by hundreds of research groups. These researchers are seeking to develop EAP materials and improve their electrical, mechanical, optical, processing, and thermal properties. New EAP device concepts are being used to incorporate the most promising EAP material solutions. Currently, there are several classes of EAPs that have been shown to exhibit conductivities of metals and semiconductors in the doped state. They are the polyacetylenes, poly(para-phenylene)s, polyheterocycles, poly(phenylene vinylene)s, polyanilines, and conjugated ladder polymers. Conducting polymers are made by both chemical and electrochemical polymerization. In the neutral (uncharged) state, EAPs are insulators or semiconductors. Oxidized (p-doped) EAPs have had electrons removed from the backbone, resulting in delocalized polymer cation formation. Reduced (n-doped) EAPs have had electrons added to the backbone, resulting in polymer anion formation.

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The most stable (and therefore the most intensely studied) EAPs are the oxidized, cationic salts of highly conjugated polymers, although stability varies widely with chemical composition. Some cationic salts are quite stable in air (40), while others are highly reactive. The cationic salts (or p-doped polymers) are obtained by oxidation (chemical or electrochemical) of neutral polymers or of monomers. Chemical oxidation in this context refers to the removal of electrons using a chemical oxidant such as ferric chloride (43), whereas electrochemical oxidation occurs under an applied potential in the presence of an electrolyte (44). Anionic salts (n-doped polymers) can also be prepared from highly conjugated polymers by electrochemical reduction or treatment with reagents such as sodium naphthalide. However, the anionic salts, while conducting, are typically less stable than their cationic counterparts because they can react with air and water (45). Stability of both cationic and anionic salts of EAPs varies significantly with chemical composition. Electropolymerization offers several advantageous features. Simple monomer oxidation (or more rarely, reduction) results in rapid synthesis of the doped, conducting form of the polymer adhered to the electrode surface, thus simplifying isolation and purification. Film thickness is easy to control, and patterned electrodes can be used for precise placement of the polymer. No catalyst is necessary, and aqueous polymerization is possible with a wide variety of monomers, so electropolymerization can be an environmentally friendly process. Disadvantages of electropolymerization include the relatively small scale of the reactions and the need for expensive equipment; additionally, some monomers do not polymerize electrochemically. Polyacetylenes. Polyacetylene, one of the most studied of the EAPs, possesses the simplest EAP structure, consisting of alternating single and double carbon–carbon bonds, in either a cisoid or transoid configuration. (Fig. 1). The transoid configuration is thermodynamically favored, but cis–trans isomerization is a reversible process. The first reported synthesis of polyacetylene was by Natta and co-workers in 1958 (46). For many years this polymer remained a material of limited interest to organic/polymer chemists and theoreticians because of its poor processability (47,48). However, Ito and co-workers discovered a synthetic route for making high quality flexible films of polyacetylene in 1974 (49). This discovery and the subsequent discovery that the electrical conductivity of doped polyacetylene could

CH2

n

cis form

trans form H

H

H

H H

H

n

H H

H H

H H

H H

H H

Fig. 1. Structure of polyacetylene.

n

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ELECTRICALLY ACTIVE POLYMERS

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be increased from the semiconductor form to the metallic form ushered in the current field of EAPs (1,50,51). This synthetic process is known as the Shirakawa technique. The technique involves the polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler–Natta initiator system (eq. 1), which consists of a combination of Ti(O-n-C4 H9 )4 and (C2 H5 )3 Al. The initiator is soluble in organic solvents and is highly active for acetylene. The process produces mechanically strong, free-standing films of polyacetylene. The reagents are commercially available, allowing for low cost production. H n

H

C

C

H

Al(C2H5)3 TiO(C4H9)4 H

n

(1)

The films obtained from these early studies were insoluble in all solvents and infusible, so structural characterization was not possible. The films had conductivities about 0.5 and 38 S/cm when doped with bromine and iodine, respectively (1,2,51), and 560 S/cm when doped with AsF5 (52). Both the conducting and the insulating forms of these films are unstable in air. There have been numerous attempts to improve the conductivities of doped polyacetylene films and the synthetic methods leading to higher quality films. There are now several routes by which high quality polyacetylene can be prepared. Shirakawa has recently published an excellent review of synthetic approaches to polyacetylene (53). There are a wide variety of Ziegler–Natta, Luttenger, and metathesis catalysts that are very effective in the polymerization of acetylene; however, some of these catalysts produce undesireable by-products during the polymerization. Careful selection of the catalyst is important to produce good yields of high quality polyacetylene. Heterogenous catalysts are almost always employed to allow easy removal of the residual catalyst from the polymer. The polymerization of acetylene with Luttinger catalysts was first carried out in the 1960s (47,54). The Luttinger catalyst consists of a hydridic reducing agent such as sodium borohydride and a salt or complex of a group VIII metal such as nickel chloride. This synthetic technique produces high molecular weight polyacetylene with no byproduct formation. It is easier to handle experimentally than the Ziegler–Natta or metathesis processes; hydrophilic solvents can be employed. In contrast, the Ziegler–Natta and metathesis processes require vigorously dehydrated hydrocarbon reagents and the rigorous exclusion of air and moisture. The catalytic activity of the Luttinger catalysts is much lower than that of the Ziegler– Natta catalysts, and high quality films of polyacetylene are typically difficult to obtain. However, Lieser and co-workers prepared high quality thin films of cisrich polyacetylenes that showed no difference in morphology from those obtained using a Ziegler–Natta catalyst (55). The polymerization of acetylene using metathesis catalysts was carried out by Aldissi and co-workers (56). Uniform films were prepared using a concentrated solution of a soluble catalyst in toluene. Polyacetylene has also been produced using Rh and Re catalysts (57,58). The chlorine-bridged Rh(I) complexes such as [Rh(COD)Cl]2 and Rh(NBD)Cl]2 where COD is cycloocta-1,5-diene and NBD

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ELECTRICALLY ACTIVE POLYMERS

93

is bicyclo[2.2.1]hepta-2,5-diene with sodium ethoxide as cocatalyst produce polyacetylene as solid flakes. These catalysts are similar to the Luttinger catalyst in that they are stable in air and their catalytic ability is not inhibited by the presence of oxygen or moisture. Acetylene has also been polymerized by electrochemical methods (59), on the surface of metal oxides such as rutile (TiO2 ) (60) and γ -alumina (γ -Al2 O3 ) (61), and in the presence of a catalytic amount of AsF5 at −75 to −198◦ C (62). Several miscellanous techniques have also been employed to polymerize acetylene, such as irradiation with γ -rays from a 60 Co source (63) and high pressure (64). Each of the above techniques has resulted in high quality polyacetylene films. The major drawback for these techniques has been the intractable nature of polyacetylene, resulting in difficulties in purification, characterization, and processing. To eliminate these processability problems, it is necessary to employ a precursor route such as the widely studied Durham method to polyacetylene (65,66). In this process, an acetone-soluble precursor polymer is formed; processing occurs in this phase of the synthesis before conversion to the intractable polyacetylene. This approach is shown in equation 2.

F3C

CF3

Ziegler-Natta catalyst



F3C C C CF3 +

F3C

CF3

∆ vacuum

F 3C

CF3

+ n

n

1

2

3

Polyacetylene

4

(2) Monomer synthesis is first accomplished by the thermal reaction of hexafluorobut-2-yne (1) with 1,3,5,7-cyclooctatetraene (COT) (2) to yield 7,8-bis(trifluoromethyl)tricyclo-[4.2.2.02,5 ]deca-3,7,9-triene (3). Ring-opening metathesis polymerization of 3 using a catalyst such as WCl6 –(C2 H5 )4 Sn (W/Sn = 1:2) gives a cis/trans mixture of polyacetylene after thermal treatment of (4). The classical Ziegler–Natta catalyst such as TiCl4 -(C2 H5 )3 Al(Ti/Al = 1:2) gives the precursor polymer (1). Low temperature (ca 60◦ C) thermal elimination gives predominantly cis-polyacetylene, while higher temperatures yield the predominantly trans polymer. The major problem with the Durham method and other precursor routes is the neccessity of eliminating a large molecule during the thermal process. An alternative precursor method developed to eliminate this problem is the ring-opening metathesis polymerization (ROMP) of the highly reactive and commercially available monomer benzvalene (5) (67). The reaction is shown in equation 3.

ROMP 5

(RO)2W(N(2,6-(i-Pr)2Ph)CHC(Me)3 RT or −20°C where R = t-butyl or hexafluoro-t-butyl

n

isomerization HgCl2 or HgBr2 in THF

n

trans-Polyacetylene

(3) A method has been developed recently to eliminate the thermal process for producing polyacetylene films. This technique is a liquid-crystal polymerization

94

ELECTRICALLY ACTIVE POLYMERS

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R C

C

n

(CH2)m R′

Fig. 2. Functional modifications of polyacetylene. Where R = H, alkyl, aryl; R = alkyl, aryl, ester, ether, amine, thienyl, cyano, vinyl, ethynyl, alkyl ammonium; m = 0, 2.

method, which allows oriented films to be prepared directly through an anisotropic reaction field (68–70). The field is produced by macroscopically orientated nematic liquid crystals, which are used as the polymerization solvent for a Ziegler–Natta catalyst. This technique yields highly oriented films in the trans configuration. This process produces polyacetylene with good conductivity (104 s/cm) without the need for thermal treatment and stretching, which causes defects and breaks fibrils in films. Although most of the previous approaches have been directed at the synthesis of polyacetylene, much additional work has gone into the synthesis of derivatized polyacetylene (see ACETYLENIC POLYMERS, SUBSTITUTED). Incorporation of side groups, either before or after polymerization, is commonly used to improve solubility (71) and impart functionality. Functional groups such as ester, ether, amine, thienyl, cyano, vinyl, ethynyl (72,73), aryl (74), and ionic substituents such as pyridinium (75), alkyl sulfonates (76), and alkyl ammoniums (76) also yield soluble polymers (Fig. 2). In most cases, solubility increases are accompanied by decreases in conductivity due to loss of conjugation resulting from unfavorable steric interactions (75). Liquid crystalline groups are incorporated to increase order in the polymer, with alignment resulting in improved conductivity, reduced band gaps, and interesting photoresponsive properties (77,78). Poly(para-phenylene). Poly(para-phenylene) (PPP) can be synthesized by both direct (79) and indirect methods (80). A comprehensive review of these synthetic routes has been presented by Schluter (81). The direct approach consists of using monomers that contain a phenylene group that becomes the repeat unit of the final polymer. In the indirect approach, a precursor polymer is synthesized first, and the PPP is obtained after thermal treatment. Both approaches have serious drawbacks. In the direct approach, regioirregularities often result from the coupling reactions, as do low molecular weights, cross-linking, and other side reactions. In the indirect approach, structural irregularities contained in the precursor polymer are transplanted into the final product, and there are very few suitable precursor polymers available for PPP synthesis. The direct approach has been demonstrated by Yamamoto (82,83); pdibromobenzene was combined in a Grignard-type coupling reaction with one equivalent of magnesium in the presence of a transition-metal catalyst in THF to give a para-linked low molecular weight PPP (eq. 4). Linear poly(pphenylene) and poly(m-phenylene) have also been obtained through a reductive coupling of p-dihalobenzene by electrolysis or with chemical reductants, although these materials remained insoluble and could not be completely characterized (84,85).

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ELECTRICALLY ACTIVE POLYMERS

95

[Ni] Br

Br

Mg

(4)

n

Several alternative direct methods have been employed to increase the molecular weight and processability of PPP. The key is the incorporation of solublizing substituents, such as long alkyl or alkoxy chains. Mullen reported the synthesis of relatively high molecular weight PPP derivatives by preparing alkylated polytetrahydropyrenes from the corresponding dibromo monomers using Ni/Mg catalysts (86). Highly soluble PPP derivatives have been obtained by oxidative copolymerization of alkoxy-substituted benzenes (87) and by polymerization of a charge-transfer (CT) complex between the monomer N,N-dimethylaniline and tetrachloro-p-quinone (88). Perhaps the most successful direct approach to soluble PPPs is that pioneered by Rehahn and co-workers, first published in 1988 (89). In this method, aryl bromides are coupled with aryl boronic acids (or boronic esters) with palladium catalysts under Suzuki cross-coupling conditions (eq. 5). When the bromo and boronic acid groups are both on the same phenylene ring (6), the result is a homopolymer as shown in equation 5; copolymers can be prepared through reactions of dibromophenylenes with bisboronic acid substituted phenylenes. Boronic acids are typically much more stable than the corresponding Grignard compounds, allowing for purification of the intermediates and resulting in higher molecular weight polymers. This approach is now commonly used for the synthesis of soluble PPPs; an excellent review of the Suzuki polycondensation has recently been published (90).

R Br

Br

1) BuLi 2) B(OCH3)3 3)

R

H+

R

R Br

B(OH)2

[Pd]

R

R 6

n

(5)

The indirect or precursor method for the preparation of PPPs was poineered by Marvel in the 1950s (91,92). Although Marvel’s pioneering work produced low molecular weight precursor polymer, ICI researcher Ballard was successful in developing a monomer based on cyclohexadiene (93). The reaction sequence is shown in equation 6. The reaction starts with monomer 7, which is a derivative of 5,6-dihydroxycyclohexa-1,3-diene and is polymerized using radical initiators to high molecular weight polymer precursors (8). The precursors are soluble in common organic solvents, and films and spun fibers can be produced. Thermal treatment results in elimination of the side groups to yield fully aromatic PPP, which is insoluble and intractible. An industrial process based on this chemistry was found to be impratical because the synthesis was unscaleable.

96

ELECTRICALLY ACTIVE POLYMERS

RO

Ziegler−Natta catalysts or



cationic initiators

−2 ROH

OR 7

Vol. 6

RO

OR

n

n

8

(6)

However, Ballard used a biomediated synthesis to circumvent these obstacles. The microorganism Pseudomonous putida 11767 oxidizes benzene within the bacterial cell (79). The dioxygenase E1 with the assistance of the protonated cocatalyst nicotinamide adenine dinucleotide (NADH) reacts with oxygen to give 5,6-dihydroxycyclohexa-1,3-diene. The organism was genetically modified in order to eliminate the aromatization step of 5,6-dihydroxycyclohexa-1,3-diene. The mild conditions of this process were found to be scaleable and are used in an industrial process. The resultant PPP has limited regio and stereospecific control during polymerization and is therefore not entirely composed of 1,4-linkages; approximately 15% 1,2-linkages result from this process. Polyheterocycles. Heterocyclic monomers constitute a third class of monomers that can be polymerized to form fully conjugated polymers; the most common of these monomers, shown in equation 7, are pyrrole (X = NH), thiophene (X = S), and furan (X = O). They can be doped to give electrical conductivity when polymerization occurs in the 2,5-positions. These monomers can be polymerized both by electrochemical and chemical methods. The polyheterocycles have received considerable attention for their electron-rich nature, which leads to materials that are easily oxidized and therefore more stable in the oxidized state. Additionally, the increased structural complexity of polyheterocycles relative to polyacetylenes makes structural modifications possible for improved processability.

X

X n

where X = O, NH, S

(7)

By far the most commonly exploited polymerization of heterocycles is the oxidative polymerization, which can be carried out using chemical or electrochemical oxidation conditions. Chemical oxidative polymerization is advantageous in that the reactions are fast and simple, using relatively mild conditions (94), and polymers could presumably be mass-produced at a reasonable cost (95). Oxidation potentials depend upon the electron density of the monomers; the more electronrich a monomer is, the easier it is to oxidize. The oxidative polymerization of thiophene is shown in Figure 3; this mechanism is equally applicable to other heterocycles. The mechanism is thought to involve a one-electron oxidation of the monomer to form a resonance-stabilized radical cation. This can couple with a molecule of starting material to form a radical cation dimer, which loses another electron to form the dicationic dimer, or the radical cation can couple with another radical cation to form a dicationic dimer. The dicationic dimer then loses two protons to form the neutral dimer, and the entire process is repeated to form polymer. The fundamental polymerization mechanism is the same for both chemical and

Vol. 6

ELECTRICALLY ACTIVE POLYMERS •

−e− H



H

S +

H

H



+

H

S

+

H

S

H

S

97

H

H

radical−monomer coupling

S +

H



H

S +

H S H

S +



H

H

H −e−

H



S +

H

+

H

+ H S

radical−radical coupling



S +

H

H

S +

H

H −2H+

polymer

S H

H

S

Fig. 3. Mechanism of oxidative polymerization of thiophene.

electrochemical polymerization, although there are additional factors (diffusion, scan rate, electrode characteristics, etc) that must be considered for electrochemical polymerization. Among the problems associated with oxidative polymerization is the abundance of possible side reactions. Oxidation of the simple, unsubstituted heterocycles results in poorly soluble, poorly processable materials, so structural modifications are typically used to improve polymer properties. Often, the strong oxidation conditions required to effect polymerization cause overoxidation and decomposition. While coupling at a carbon adjacent to the heteroatom is favored, as it is the most electron-rich site, coupling is also possible at other unsubstituted carbons, and irregular polymer backbones can be formed as shown in Figure 4 for oxidative polymerization of thiophene. Coupling of heterocycles occurs predominantly through the 2- and 5-positions (α,α-coupling), but some coupling also occurs at the 3- and 4-positions to give a small amount of α,β and β,β-couplings. The mixture of couplings induces larger twist angles, reducing conjugation and yielding poor electronic properties (96). Multiple couplings on the same heterocyclic molecule can also occur, resulting in cross-linked polymers. Coupling at β-positions during polymerization can be avoided by blocking the 3- and 4-positions of the heterocycle. If symmetrical substitution is used, problems associated with regioirregularity (see below) can be avoided. For instance, in the polymerization of 3,4-ethylenedioxythiophene (EDOT) shown below (Fig. 5), the ethylenedioxy bridge effectively prevents β-coupling, thereby yielding a linear polymer. Polypyrroles. The first reported polymerization of pyrrole was in 1916 (97). Polypyrrole was prepared by the chemical oxidation of pyrrole using hydrogen peroxide. An amorphous black powder known as “pyrrole black” was obtained, which

98

ELECTRICALLY ACTIVE POLYMERS

Vol. 6





+



S

S +

S +

α,α-coupling: + •

S +

H

S S

+



S

−2H+

S

+

H

S

β,β-coupling: +

+

H





S

S

S

−2H+ H

S +

S

S

+

α,β-coupling: +

S

S

H

+





+

+ S

S −2H+

S

S

H

Fig. 4. Coupling pathways during oxidative polymerization of thiophene.

O

O

O

O



O

O



+



S

S +

S +

Only α,α-coupling is possible: O

O

O

+





S + O

H

S S + O

O

+

O

S

H O

O

−2H+

S S

O O

O

Fig. 5. Coupling pathways during oxidative polymerization of EDOT.

was insoluble in common organic solvents. One of the advantages of polypyrrole (PPy) is the low oxidation potential of pyrrole relative to other EAP precursors, facilitating oxidative polymerization; the half-wave oxidation potentials (in acetonitrile/0.5 M NaClO4 ) of benzene, thiophene, and pyrrole are 2.08, 1.60, and

Vol. 6

ELECTRICALLY ACTIVE POLYMERS

99

0.76 V vs Ag/Ag+ , respectively (44,98). Numerous oxidizing reagents have been used to polymerize pyrrole, including hydrogen peroxide in acetic acid, lead dioxide, ferric chloride, nitrous acid, and ozone (99,100). The chemical oxidative polymerization of pyrrole results in low conductivities in the range of 10 − 10 to 10 − 11 S/cm, although conductivities of 10 − 5 S/cm have been reported with iodine doping (101). The low conductivities obtained from acid and peroxide initiators have been attributed (102) to the high degree of saturation of the pyrrole ring due to either oxygen incorporation and/or hydrogen saturation. The use of oxidative transition-metal ions such as ferric salts (eg FeCl3 , Fe(NO3 )3 , Fe(ClO4 )3 , FeBr3 ) at low temperatures in both aqueous and organic solvents has improved the conductivities of PPy from 10 − 5 to 200 S/cm (103–106). The polymers that were obtained are infusible and insoluble powders. Approaches to produce processable PPy materials include the synthesis and polymerization of pyrrole compounds containing solubilizing groups in the 3-position (107) or attached to the nitrogen (108) as well as the use of soluble precursor molecules (109,110). Although these efforts result in improved processability of the polypyrroles, there is a concurrent loss of conductivity. The chemical polymerization of pyrrole remains the simplest route to PPy, but electrochemical polymerization is the most important and versatile method for producing PPy films of high conductivity (111). There are numerous examples of PPy doped with a wide variety of anions that can be electrochemically polymerized on a variety of electrode materials in both aqueous and organic solvents (112–114). Polythiophenes. Preparations of polythiophene (PT) include chemical oxidative couplings of thiophene (95), the cross-coupling of Grignard reagents of dihalothiophenes (96,115), and electrochemical polymerization (95,116). The PT powders and films that are produced either chemically or electrochemically are insoluble and infusible. In order to better characterize and process PTs, synthetic methods were developed to incorporate functional groups (such as n-alkyl groups) on the 3-position to improve processibility without significantly sacrificing conductivity. The synthesis of poly(3-alkylthiophene)s (P3ATs) that are chemically stable, soluble, processable, and moderately to highly conductive has been achieved by several research groups. Early work on the synthesis of P3ATs utilized the Kumada cross-coupling reaction (115), oxidative polymerization using FeCl3 (116) or Cu powder with a catalytic amount of PdCl2 in pyridine (118). These methods produce irregular homopolymers and copolymers of P3ATs with reasonable molecular weights and low to moderate conductivities. Unsymmetrical monomers such as 3-alkylthiophenes are said to have a “head” (ie the 2-position of the thiophene ring) and a “tail” (ie the 5-position of the thiophene ring). Standard homopolymerization of 3-alkylthiophenes yields P3ATs that have “head-to-head” (2,2 , or H-H), “head-to-tail” (2,5 , or H-T), and “tail-to-tail” (5,5 , or T-T) bonds. Polymers having this type of bonding are said to be regioirregular, while polymers possessing purely H-T bonds are regioregular (Fig. 6). Regioirregular P3ATs are poorly conjugated, because the mixture of linkages leads to unfavorable interaction of side chains, resulting in a sterically driven twist of the polymer backbone (118). Polymer morphology is also affected by regiochemistry; regioregular polymers are typically crystalline, while regioirregular polymers are amorphous (119,120). Regioregularity problems can also be

100

ELECTRICALLY ACTIVE POLYMERS

Vol. 6

R

R S

S S T(ail)

R H-T

H(ead) S

S

R H-T H-T Regioirregular P3AT

T-T

R

R

R S

S

H-T

R H-T

H-T

S

S R H-T

S

R S

S

R S

S R H-H

R

R S

S

R R H-T H-T

H-T

S R H-T

H-T

Regioregular P3AT

Fig. 6. Regiochemistry in poly(3-alkylthiophene).

avoided by using symmetrical monomers; extended conjugation monomers such as alkyl-substituted terthiophenes (121) and mixed heterocycle extended conjugation monomers (122,123) have become commonplace (124). These extended conjugation monomers also exhibit lower oxidation potentials, and they can be designed to tailor properties for specific applications. Several groups have demonstrated regiospecific synthetic processes for obtaining regioregular P3ATs. In the now-standard method pioneered by McCullough and Lowe (125), a monomer (2,5-dibromo-3-alkylthiophene) is reacted with lithium diisopropylamide at cryogenic temperatures, followed by transmetallation with MgBr2 ·Et2 O. Polymerization is then accomplished using Kumada crosscoupling methods with catalytic amounts of NiCl2 (dppp) to give P3ATs with 98– 100% H-T coupling in moderate to high yields (eq. 8) (126–128).

R R

R

MgBr2ⴢ OEt2

LDA/THF S

Br

Li

S

Br

low temp.

R

R NiCl2(dppp) BrMg

S

Br

S

S

S R 98−100% H-T coupling

n

(8) Since this early work by McCullough and others, improvements have been made in the synthesis of analytically pure highly regioregular H-T P3ATs without cryogenic temperatures. The Grignard metathasis (GRIM) polymerization (eq. 9) (129,130) is quick, easy, and cost-effective. The synthesis is as follows: a 2,5-dibromo-3-alkylthiophene is treated with one equivalent of an alkyl or vinyl Grignard reagent, followed by polymerization at room temperature with a catalytic amount of NiCl2 (dppp).

Vol. 6

ELECTRICALLY ACTIVE POLYMERS R

Br

S

R

R′MgX, THF RT or reflux Br

R

S

Br

NiCl2(dppp)

+ R′Br

+ XMg

101

Br

S

1h

MgX

R

R S S

S

n

R

(9) An alternative synthetic approach to the preparation of regioregular P3ATs was demonstrated by Chen and Rieke (131,132). Their method utilizes the highly reactive “Rieke zinc” (Zn∗), which is added to a solution of 2,5-dibromo3-alkylthiophene at cryogenic temperatures. The metal reacts quantitatively to form a mixture of isomers: 2-bromo-3-alkyl-5-(bromozincio)thiophene and 2(bromozincio)-3-alkyl-5-bromo-thiophene. The ratio between these two isomers depends on reaction temperature and the steric influence of the alkyl groups. The addition of a nickel cross-coupling catalyst leads to the formation of regioregular P3ATs (regardless of the isomer ratio), and regiorandom P3ATs are formed with the addition of a palladium cross-coupling catalyst (133). Several additional homo-, co-, and terpolymers based on P3ATs have been synthesized recently. Monomers that incorporate two or three thiophenes such as the 4,4-dialkyl-2,2 -bithiophenes or 3,3-dialkyl-2,2 -bithiophenes with various substituent groups such as alkyl, oxyalkyl, β-aryl have been prepared by chemical and electrochemical methods (134–137). The introduction of electron-donating substituents such as ether moieties lowers the polymer band gap (the energy difference between the valence and conduction bands of the polymer; for a more thorough discussion on band gap, please see under Transport Theory) by raising the energy of the valence electrons (HOMO destabilization) along the conjugated chain (138,139). These alkoxy substituents also decrease the oxidation potential of both the monomers and their polymers, allowing milder oxidative polymerization conditions, which results in fewer side reactions. Polymers based on 3,4-disubstituted thiophene derivatives such as EDOT (Fig. 5) have recently been synthesized by chemical and electrochemical methods. Poly(3,4-ethylenedioxythiophene) (PEDOT) (140–142) has a band gap of 1.5–1.6 eV as compared to thiophene (2.0–2.1 eV); poly(3,4-dimethoxythiophene) has an electrochemical band gap of 1.37 eV (143,144). Polyfurans. Relative to polythiophenes and polypyrroles, little has been reported on polyfurans (PFu); this is likely a result of the high oxidation potential of furan (≥1.7 V vs Ag/Ag+ ), which results in side reactions during polymerization (145). While chemical polymerization of furan has been reported (146), the majority of PFu publications focus on electrochemically prepared PFu. PFu was first reported by Tourillon and Garner (147), who used an electrochemical technique similar to that used by Diaz and co-workers to polymerize pyrrole (148). This PFu had reported conductivities from 10 − 5 to 10 − 3 S/cm. However, these early reports on the synthesis of PFu were incorrect because of inconsistent chemical and spectroscopic properties of the polymer obtained (149).

102

ELECTRICALLY ACTIVE POLYMERS

Vol. 6

The high voltages required for electropolymerization (1.8–2.5 V vs SCE) of furan result in irreversible oxidation of the polymer, and the polymer contains significant hydrogenated units (150). A polyfuran structure was reported by Zotti and co-workers (151) by the cathodic reduction of 2,5-dibromofuran in acetonitrile using a Ni2+ catalyst. The PFu was obtained in trace amounts, with a significant content of the undoped, insulating form, which resulted in incomplete characterization of the material. Polyfuran films doped with ClO4 − were obtained potentiostatically at several deposition potentials (152) and characterized by cyclic voltammetry, which showed highly complex voltammograms, indicative of a poorly defined polymer system. Free-standing films of PFu were obtained by the electrochemical polymerization of furan at 1.2 V vs Ag/AgCl in a binary solvent system (153); the resultant PFu showed well-defined cyclic voltammetry, moderate conductivity when doped with I2 (10 − 2 S/cm), and good mechanical properties. Regioregular and regiorandom poly-3-alkylfurans have also been reported (154). Conductivity was found to be highly dependent on degree of regioregularity, reaching 10 − 2 S/cm in the 95% head-to-tail (H-T) polymer but only 10 − 7 S/cm in the 75% H-T polymer. To avoid the problems associated with furan’s high oxidation potential, extended conjugation monomers incorporating furan have been prepared (145,155). A PFu prepared from terfuran showed a conductivity of 10 − 3 S/cm when doped with CF3 SO3 − (155). The PFu was highly susceptible to acid/base effects, which led to ring opening and a decrease in the conjugation length. Poly(phenylene vinylene)s. The earliest reported synthesis of a Poly(pphenylenevinylene) (qv) was accomplished by McDonald and Campbell in 1960 (156). They produced a PPV oligomer by a repetitive Wittig-type coupling of an aromatic bisphosphonium salt and a bisaldehyde. Several routes have been investigated for producing PPVs, such as coupling reactions of dichlorobenzene in liquid ammonia (157), reactions of tetrabromo-p-xylene with two equivalents of methylithium (158), and McMurray deoxygenative coupling of an aromatic dialdehyde (159). In all these cases PPVs were produced that were either insoluble or sparingly soluble and poorly processable. Precursor routes to PPVs can be easy and efficient, and they produce soluble and processable high molecular weight PPVs The most widely used soluble precursor method is the Wessling and Zimmerman route eq. 10. This process involves the synthesis of the bis-sulfonium salt of 1,4-bis(chloromethyl)benzene, followed by sodium hydroxide elimination and polymerization at 0◦ C to give an aqueous solution of a precursor polymer. The soluble precursor is processed into films, foams, and/or fibers and converted to PPV by thermal elimination (160–163). The heating occurs under vacuum at temperatures ranging from 120◦ C for bromide salts to 200◦ C for chloride and fluoride salts to 300◦ C for acetate salts (164). Normally, a large variety of substituents can be tolerated on the aromatic ring, such as aromatic (165), alkoxy (166), silyl (167), halogen (168), sulfur (169), and amino groups (170), but electron-poor substitutents such as nitro or cyano groups polymerize with extreme difficulty (171). The major disadvantages of this method are the instability of the intermediate polymers and the stench of the sulfonium salts.

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ELECTRICALLY ACTIVE POLYMERS + S − Cl S − + Cl

base

Cl− + S

103

base or ∆

n

n

poly(phenylene vinylene) (PPV)

(10) There are several alternate routes for preparing PPV from soluble precursors. The Gilch route (172) uses a dichloro-p-xylene that is polymerized with potassium tert-butoxide in organic solvents. The alkoxy route (173) uses a methoxysubstituted polymer that is soluble in common organic solvents and is more stable than the sulfonium precursor. Polymerization is accomplished by removal of the alkoxy group by acid catalysis, followed by heating to produce PPV derivatives. A third route utilizes the Heck coupling reaction, which is compatible with a wide variety of functional groups (174,175), allows regiospecific synthesis (176), and yields polymers with molecular weights ca 104 g/mol (174,177). The Heck polymerization involves coupling of vinyl groups with bromides or iodides, either in an AA/BB copolymerization (as in the reaction of ethylene or a divinylbenzene with a dibromobenzene) or in an AB homopolymerization (typically the reaction of a 1-bromo-4-vinylbenzene), in the presence of a palladium catalyst (174). All of these alternate routes allow a large range of substituted organic-soluble PPV derivatives to be synthesized, but none of the methods has a distinct advantage over Wessling and Zimmerman’s sulfonium precursor method. The most studied solution-processable PPV is poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylene vinylene (MEH-PPV), commonly prepared via the Gilch method (178). Careful choice of PPV substitutents allows for tuning of the optical band gap, enabling the use of these polymers in electrooptical applications. Several heteroaromatic ring structures have also been incorporated into poly(arylene vinylene) structures using the same soluble-precursor polymer methods used for PPV. Poly(thienylene vinylene) (179) and poly(furylene vinylene) (180) have been prepared. Metalloporphyrins have also been incorporated into poly(arylene vinylene)s (175) to investigate photorefractive properties. Aromatic poly(azomethines) have also been prepared from a solution condensation of an aromatic diamine with aromatic dialdehydes (181), but these materials are less conjugated than their PPV counterparts. Various copolymers of phenylene vinylenes with phenylene, thienylene, and furylene moieties have also been studied (182). Polyanilines. Polyaniline was made nearly 140 years ago in 1862 by H. Letheby (183). At that time, polyaniline was known as “aniline black.” This material was prepared by the oxidation of aniline under mild conditions. The material found use in dyes and printing. Initial preparations of polyaniline (PANI) led to insoluble materials that had poor thermal processability and solvent solubility. Only recently has the structure of PANI been fully determined. This was accomplished by using model compounds and polymers for definitive structural analysis. Poly(pphenylene amineimine) (PPAI) was synthesized directly to demonstrate that PANI is purely para-linked (184–186).

104

ELECTRICALLY ACTIVE POLYMERS H N

N H

H N N H

H N

n

N H

Leucoemeraldine (neutral form)

N N

N N n

Emeraldine base (intermediate form) H

H N

N N n

Pernigraniline (quinoid-like form)

Vol. 6

N H

A-

N + + N A-

H Emeraldine salt (conducting form)

n

Fig. 7. Oxidation states of polyaniline.

PANI is commonly prepared by chemical polymerization of aniline using (NH4 )2 S2 O8 in hydrogen chloride solution (187) and/or by electrochemical polymerization (188). PANI can exist in four different oxidation states: leucoemeraldine (neutral form), pernigraniline (quinoid-like form), emeraldine base (intermediate form), and emeraldine salt (conductive form), as shown in Figure 7. PANI is prepared as the emeraldine salt and then treated with base to yield the emeraldine base form, which is soluble in common organic solvents. After film casting or other processing, the emeraldine base can be treated with HCl to regenerate the conductive emeraldine salt form of PANI. Methods have been developed to improve solubility and processability by introducing various alkyl, alkoxy, aryl, sulfonyl, and amino groups onto the PANI backbone (189). However, the presence of these substituents dramatically reduces yields of polymer, polymer molecular weight, and conductivity. Novel water-soluble PANI has been recently synthesized using horseradish peroxidase catalyzed by oxidative free-radical coupling of 2,5-diaminobenzenesulfonate (190). Unlike standard PANI preparations, the resultant polymer is fully sulfonated upon treatment with fuming sulfuric acid, and it shows strong pH dependance of absorption and other physical properties. Conjugated Ladder Polymers. Since the 1930s double-stranded, laddertype polymers have been prepared in a multistep process with limited success of cyclization (191,192). Other routes have also been explored such as those for poly(acrylonitrile) (193,194), poly(1,2-butadiene), poly(3,4-isoprene) (195), or poly(butadiyne)s (196). These materials were found to be poorly soluble and unworkable, with a considerable number of defects in the structure (incomplete cyclization, cross-linking, radical sites). The first successful synthesis of a ladder polymer with a completely defined structure was accomplished in 1991 by Sherf and Mullen (197). The first step was the AA/BB-type polycondensation of an aromatic diboronic acid with a substituted 2,5-dibromo-1,4-dibenzoylbenzene to give a single-stranded precursor PPP-type polymer, followed by cyclization to the ladder structure (Fig. 8). Several other examples exist that have resulted in ladder-type structures. These include angular polyacene (198,199), Diels– Alder polyaddition of AB-type diene–dienophiles (200), AA/BB-type Diels–Alder polyaddition of a bisdiene and a bisdienophile (201), thienylene units (202),

Vol. 6

ELECTRICALLY ACTIVE POLYMERS O

O R′

R

105

R

R′

Pd(PPh3)4/toluene B(OH)2 + Br

(HO)2B

Br R

R′

R

n

R′

O

O

R′ R

R′

OH

H

H

R

R′

BF3ⴢ Et2O/CH2Cl2

LAH/toluene n

R

HO

R R′

n

R′

H

Fig. 8. Formation of ladder structures. Where R = n-C6 H13 ; R = −1,4-C6 H4 -n-C10 H21 .

and polybenzimidazobenzophenanthroline (BBL) (203). All of these materials are structurally defined materials, but their drawbacks include poor solubility, poor thermal processability, and generally lack of workability. Alternative Synthetic Approaches. Most EAPs are prepared via chemical or electrochemical oxidative polymerization, precursor routes, or coupling chemistry. However, several alternative approaches have recently been gaining attention. Template-assisted polymerization is used to prepare specific conducting polymer architectures for use in a variety of applications (204). Polyacetylene has been prepared in the presence of liquid crystalline solvents for subsequent alignment (68–70). Polyaniline has been synthesized in the presence of DNA (205) to form helical PANI with interesting electrochemistry that is useful as a biosensor. Fibrillar morphologies can be produced by polymerization in channelized templates (such as microfiltration membranes); the resulting polymers show improved conductivity over analogous polymer films and were suggested for use in polymer light-emitting diodes (206,207). A variety of polymers have been grown around self-assembled colloidal templates (see Photonics section, below); the template can subsequently be dissolved away from the conducting polymers, producing opal-like EAP structures with the aim of improved charge transport for battery and capacitor applications. (208,209) While not strictly a template method, EAPs have also been prepared on the surface of a wide variety of particles and fibers (210,211), typically via chemical oxidative polymerization; the resulting composites/nanocomposites have found commercial application in static-dissipating paints, cloths, and carpets. One synthetic approach gaining popularity for its ecologically friendly nature and its potential biological applications is that of biomediated polymerization. Enzymatic oxidants, such as horseradish peroxidase (212,213), are commonly used for chemical oxidative polymerization. Additionally, bacteria can be used to effect chemical reactions, as in the Ballard route to PPP described above (79). Plasma polymerization is a solvent-free process that can be used to rapidly deposit thin polymer films onto a wide variety of substrates at room temperature (214,215). Monomer vapor is exposed to an electric field, and the resultant reactive fragments recombine to form polymer films. This technique is used for a wide

106

ELECTRICALLY ACTIVE POLYMERS

Vol. 6

variety of polymer types including EAPs, and properties of the resultant polymers are often quite different from those prepared using more traditional techniques. Reaction conditions, including monomer flow rate, plasma power, reactor pressure, deposition time, and gas-phase composition, can be varied to tailor the properties of the resultant polymer. Other benefits of this technique include the environmentally friendly aspect of the solvent-free process, the ease of controlling film thickness, and the pinhole-free nature of the films. Plasma polymerization has been used to prepare polythiophene (216), polypyrrole (217), and polyaniline (218,219). While mechanical properties are typically better than those of traditionally prepared polymers (219), relatively poor conductivities are common in plasma-polymerized EAPs, possibly as a result of structural defects. Conducting Polymer Blends, Composites, and Colloids. EAPs are in most cases prepared in the form of intractable films, gels, or powders that are insoluble in common organic solvents. Incorporation of solubilizing functional groups is but one technique available for chemists, materials scientists, and engineers to improve the processability of conductive polymers. Over the past decade research has focused on improving the processing characteristics of EAPs through the use of polymer blends, composites, and colloids (220). Blends and composites of polymers offer certain combinations of desired properties that cannot be obtained from the individual components. EAP blends and composites offer a unique combination of good mechanical properties of an insulating host combined with the electrical conductivity of the guest. The conductive component’s concentration threshold for the onset of electrical conductivity can be reduced to volume fractions well below that required for classical percolation (221). The critical factor in these formulations is the amount of EAP used; it must be enough to give a continuous conductive network (222). Blends and composites of EAPs are formed via simple, electrochemical, or diffusive mixing, by chemical treatment, by light-induced conversion and by counterion-induced processability. Numerous examples exist that describe the various methods used to form blends and composites using conductive polymers (221,223). EAP colloids typically consist of a colloidal suspension of EAP particles or EAP-coated particles (latex, metal, silica, etc) in an aqueous medium; the dimensions of the dispersed particles are in the 1–1000-nm range. These materials are normally prepared via dispersion polymerization (224,225). While colloidal dispersions of EAPs are more processable than their noncolloidal EAP counterparts, the dispersions do not readily form films because of the high glass-transition temperatures (T g s) of the EAPs, and applications are limited (226). A more promising approach involves coating particles (polystyrene, polyurethane, etc) with EAPs to form colloidal composites and nanocomposites. Much lower concentrations (ca 5–20%) of EAPs are used, so that the majority of the material is composed of low T g , film-forming material. Conductivities of EAP-coated particles range from 10 − 5 to 101 S/cm (227–229), and these materials are currently being marketed as water-borne anticorrosion and antistatic coatings.

Transport Theory Conjugation and Conduction. As conjugation length increases, as with the progression from ethylene to 1,3-butadiene to 1,3,5,7,9,11-dodecahexane to

Vol. 6

ELECTRICALLY ACTIVE POLYMERS

107

H2C CH2 n Colorless gas

}

Black powder

E1 = 7.4 eV

}E

2

}

E3

}

Eg = 1.4 eV

E1 > E2 > E3 > Eg

Fig. 9. Effect of conjugation on molecular energetics.

polyacetylene (Fig. 9), the degenerate orbitals spread to form nondegenerate energy bands, and the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases. This energy difference is a simple π to π ∗ transition in simple molecules and a band gap (Eg ) in case of polymers (44). It can be determined for a neutral species from the onset of absorption in the UV–vis spectrum. Much research has focused on the development of low Eg materials for electrochromics applications (230). Eg can be lowered by functional group modification to raise the energy of the HOMO, but as a result the neutral polymers are very easily oxidized, and stability can be an issue. Just as increasing conjugation length from ethylene to polyacetylene decreases the π to π ∗ transition, it also lowers oxidation potential. This is due to the high HOMO and increased resonance stabilization of the radical formed in the process. Similarly, other EAPs have lower oxidation potentials than their monomeric counterparts. Increasing conjugation in monomers also lowers oxidation potentials (124); the oxidation potential of thiophene (2.07 V vs SCE) is much higher than that of terthiophene (1.05 V vs SCE), which is higher than that of polythiophene (0.7 V vs SCE) (231). Extended conjugation monomers have therefore received considerable interest, because lower oxidation potentials lead to fewer side reactions and fewer defects (232,233). Among the defects minimized through the use of extended conjugation monomers are those caused by α,β-coupling, since several α,α-linkages are predetermined in these materials. The conjugated nature of neutral EAPs allows them to have two electronically extreme states (eqs. 11 and 12). Unlike all other EAPs, the two resonance forms in trans-polyacetylene are degenerate, ie their ground states are thermodynamically equivalent (234). Other organic electroactive polymers possess nondegenerate ground states (235); for example, poly(p-phenylene) has both benzenoid and quinoid configurations (eqs. 11 and 12), of which the latter is the higher energy configuration (44), and so makes less of a contribution. Addition of either donors or acceptors in the process known as doping results in charge transfer and

108

ELECTRICALLY ACTIVE POLYMERS

Vol. 6

the formation of charge carriers along the polymer chain, yielding the conductive form of EAPs (235). Several comprehensive treatments of charge carriers in EAPs can be found in the literature (38,236–238).

n

n

(11)

n

n

(12) As a result of the degenerate ground states of trans-polyacetylene, charges formed during doping readily separate (because there is no increase in distortion energy.) The charge carrier, called a soliton, is relatively stable as a result of the degeneracy, and the energy required to create and move such a defect is relatively small (235). Calculations suggest that solitons are delocalized over 15 carbon atoms (239). The most widely accepted mechanism of conduction in nondegenerate EAPs, illustrated in Figure 10 for polythiophene, was proposed simultaneously by Bredas and co-workers (240) and Bishop and co-workers (241) in 1981. This mechanism involves a one-electron oxidation to form a radical cation, which is called a polaron. This radical cation is resonance-stabilized over several rings. Removal of a second electron gives a dicationic species with no unpaired electrons, called a bipolaron. The two ions in a bipolaron need to be isolated from each other to minimize unfavorable interactions; recent work suggests that at least five rings are needed to stabilize a dicationic species (242,243). The conversion between neutral, polaronic, and bipolaronic species is reversible, using either chemical or electrochemical means to oxidize or reduce the polymer.

S

S S

S S

Neutral

−e−

S

S S +

S

S S +



S

−e−

S

S

S

+e− S

S

S

+e−

S

Polaron

S S

S S +

S

Bipolaron

Fig. 10. Mechanism of conduction in nondegenerate EAPs.

Vol. 6

ELECTRICALLY ACTIVE POLYMERS

109

H N

H N N H

N H

n

Leucoemeraldine/neutral

e−

−2 +2 A− H N +.

H N +. N H

N H

n

Polysemiquinone radical/polaron charge redistribution H

H N A− + N

N H

N + A−

H

Emeraldine salt/bipolaron

+2 H+A−

H N N H

n

N N n

Emeraldine base/partially oxidized

Fig. 11. Chemical doping of polyaniline.

PANI and other ECPs possessing strongly basic functionalities undergo a more complex chemical doping mechanism (187,188) (Fig. 11). While the leucoemeraldine form can undergo oxidative doping, leading to radical cation (polaron) formation, the emeraldine base form of PANI can also undergo protonation by acid–base chemistry to yield the emeraldine salt (bipolaron) form. Internal charge redistribution in the emeraldine salt also yields the polaron form. Doping Processes. Reversible doping can be achieved by both chemical and electrochemical methods. The control of electrical conductivity of a conductive polymer from insulator to metal can be accomplished through the doping process; conductivity increases with an increase in the doping level (194–196). Several techniques exist for doping conjugated polymers (38). These techniques are as follows: (1) chemical doping by charge transfer, (2) electrochemical doping, (3) doping of PANI by acid–base chemistry (discussed above), (4) photodoping, and (5) charge injection at the metal–semiconducting polymer interface. Chemical doping involves charge-transfer redox chemistry. Oxidative doping (p-type, electron accepting) with dopants such as I2 , AsF5 , O2 , and FeCl3 is

110

ELECTRICALLY ACTIVE POLYMERS

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commonly used, while reductive doping (n-type, electron donating) using dopants such as sodium naphthalide (244) or alkali metal vapors (245) is less common because of the reactivity of n-doped materials. This technique is simple and efficient, but control of the degree of doping is limited. High levels are easily achieved, whereas intermediate levels are not usually obtained. Electrochemical doping refers to the supply by an electrode of the necessary redox potential to the conducting polymer (197). Ions diffuse in and out of the polymer structure from the nearby electrolyte, compensating for electronic charge formation along the polymer backbone. The doping level is controlled by the voltage between the conducting polymer and the counterelectrode. Thus, precise control of doping levels can be achieved. Photodoping is a rapid process in which light is used to generate ions in solid polymer films (38). When an appropriate wavelength of light is absorbed to create an excited state, a close contact ion pair can be formed. No external counterions are present to stabilize these two mobile charge carriers. A transient change in the optical properties of the polymer occurs upon charge separation; resultant applications may include nonlinear optics and waveguides. In the absence of an electric field, the charges recombine in either a radiative (ie photoluminescence) or nonradiative process. In an electric field, these charges migrate to opposite electrodes, generating current; applications include solid-state photovoltaic cells. Doping can also be accomplished by charge injection at a metal– semiconducting polymer interface. This type of doping, found in light-emitting diodes, occurs when electrons and holes are injected into the π and π∗ bands of an EAP. In this method, the polymer is not doped in the chemical or electrochemical sense, because there are no counterions present.

Properties While many properties of EAPs are also found in other materials, EAPs exhibit unique combinations of properties that have led to use in a variety of applications. Perhaps the most promising feature of EAPs is the switchable nature of many of the properties. The ability to reversibly switch EAPs between insulating and conducting, opaque and transparent, absorptive and emissive, or expanded and contracted drives much of the current EAP research. Stability. In most cases, stability of EAPs is gauged in terms of changes in electrical conductivity. Thus, doped polyacetylene, while highly conductive in an inert environment, loses conductivity when exposed to air and is therefore considered unstable (51). Polypyrroles and polyanilines, on the other hand, are counted among the most stable EAPs, while polythiophenes exhibit intermediate stabilities. Environmental and thermal effects on polypyrrole and poly(3-alkylthiophene)s have been studied extensively (246,247); increased oxygen exposure and increased temperature frequently have detrimental effects on conductivity. Mechanical Properties. Normally the mechanical properties of conductive polymers are very poor (211) because of the inherent nature of EAPs, which are normally intractable and poorly processable. Poor mechanical properties result from a lack of flexible linkages in the polymer backbone as well as from

Vol. 6

ELECTRICALLY ACTIVE POLYMERS

111

low molecular weights. PANI is one exception that can be prepared as highly oriented crystalline fibers (248). The incorporation of EAPs into non-conductive polymer matrixes such as poly(vinyl chloride) (PVC) or poly(vinyl alcohol) (PVA) yields blend, composite, or laminate EAPs with unique combinations of electrical conductivity and processability (249). Such materials are discussed in detail elsewhere in this chapter. Processability is also improved by incorporating solublizing moieties pendant to the conjugated polymer chain as discussed previously; this often results in improved molecular weights, because polymer molecular weight is frequently solubility-limited. Optical Properties. The optical properties of EAPs are a direct consequence of the electronic structure, which is determined by the chemical structure of the polymer. The optical properties of EAPs result from the energy difference between the valence and conduction bands in the conjugated polymer; this energy difference is called the band gap (Eg ). EAP band gaps range from 0.5 to 3.0 eV (230,250), and typical absorption coefficients are on the order of 105 cm − 1 (251). The absorption characteristics of EAPs can be controlled by tailoring molecular structure; common methods include incorporation of electron-donating and electron-withdrawing groups (138,252–254) as well as alterations in conjugation length (usually by introduction of steric constraints). Because conjugation in neutral and doped EAPs is very different, changes in optical properties commonly occur during the redox process, leading to a variety of applications. These properties are exploited in the development of electrochromic and light-emitting devices, where materials have been produced spanning the entire spectrum. Electrical Properties/Electrical Conductivities of EAPs. While conductivity in EAPs is typically at least one order of magnitude lower than that of the best metals, the ability to switch EAPs from conducting to insulating and the ease of processability of many EAPs have led to some unique applications. The electrical conductivity, σ , of a conjugated polymer is equal to the inverse of its specific resistivity, ρ, which is a measure of the ability of the conjugated polymer to conduct an electrical charge. Electrical conductivity is determined by measuring the resistance, R, to charge transport through a known volume, where L is the length over which resistance is being measured and A is the cross-sectional area through which the current passes: σ = 1/ρ = L/(R·A) Typically, electrical conductivity of EAPs is measured in units of S/cm. Both two- and four-point probe techniques are used to measure σ in films and pressed pellets (255). Conductivities as high as 105 S/cm have been reported for several EAPs. Values vary depending on synthetic procedures, fabrication techniques, and measurement methods. EAPs are typically only partially crystalline (256–258); amorphous EAPs have been reported (259–261). Therefore, disorder plays a significant role in their electrical conductivities, and the conductivity arises from the metal–insulator transition (262). As an example, PA with the highest conductivity reported so far shows temperature dependence of conductivity (263). Thus, electrical conductivity is limited by the disorder-induced localization of the electrons rather than by the intrinsic conductivity of the delocalized charges on the polymer backbone. Table 2 shows conductivities for several well-known EAPs (26–28).

112

ELECTRICALLY ACTIVE POLYMERS

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Table 2. Electrical Conductivities of EAPs EAP Poly(acetylene) (PA) Poly(pyrrole) (PPy) Poly(thiophene) (PT) Poly(aniline) (PANI) Poly(p-phenylene) (PPP) Poly(phenylene vinylene) (PPV)

Conductivity, S/cm 103 –105 102 –103 102 10–102 102 –103 103 –104

Applications Light-Emitting Diodes (LEDs)/Light-Emitting Electrochemical Cells (LECs). Electroluminescence (EL) is the emission of light from a material when it is excited by an electric current. The discovery of EL in neutral (undoped, nonconductive) conjugated polymers, specifically PPVs, by Burroughs and co-workers in 1990 (264) led to extensive research into materials development and polymer light-emitting diode (PLED) research. EAPs compete with and may eventually replace current display technologies such as those based on inorganic materials and liquid crystalline displays (LCDs). Inorganic EL materials have been used in display applications for years; light-emitting diodes (qv) based on these materials have been commercially available since the 1960s (265). PPV is a yellow fluorescent EAP with emission maxima at 551 nm (2.25 eV) and 520 nm (2.4 eV), in the yellow–green region of the visible spectrum (266). Soluble PPVs have been prepared by the precursor method, allowing for solution processing by spin coating onto a substrate (267). PPVs and other EAPs offer unique advantages over their inorganic LED counterparts. Advantages include ease of processing on large-area substrates, good tensile properties of the chain-extended polymer (allowing the polymer to survive harsh conditions during device operation), and the ability to fine-tune the electronic and optical properties of EAPs through functionalization. The simplest PLED is a thin film of PPV sandwiched between two electrodes. The anode (hole-injecting electrode) is a semitransparent, high work-function material such as indium tin oxide (ITO), whereas the cathode (electron-injecting electrode) is a low work-function metal such as Al, Ca, or Mg (Fig. 12). Under an applied voltage in an LED, oppositely charged carriers (electron and holes) are injected into the emissive layers from the opposing contacts and are swept through the device by the high electric field. The efficiency for EL devices is measured by the number of photons emitted per electron injected. This number is called the internal efficiency and is based on the assumption that all light generated is received by the viewer. The external efficiency is a factor of 2n2 smaller than the internal efficiency (where n is the refractive index). Typical external efficiencies for LED devices are between 0.1 and 5% (268); normal luminous efficiencies for LEDs range from 1 to 10 lm/W (269,270), and quantum efficiencies are now greater than 10% (269). Such a single-layer device is called an EL diode because it shows nonlinear I–V characteristics typical of diodes.

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Al, Ca, or Mg cathode

Light-emitting polymer (eg PPV)

ITO anode

Glass or polymer substrate

Fig. 12. Polymer LED.

The most studied of the PPV compounds are those containing at least one solubilizing alkoxy group (271–273); poly[(2-methoxy-5-(2-ethylhexyl)oxy-1,4phenylene]vinylene (MEH-PPV) is most commonly used (274). The ability to finetune the color to produce red, green, and blue PLEDs has been demonstrated by appropriate functionalization of polymers (275–279), copolymerization (280–282), and blending (283–285). Several representative electroluminescent polymers covering the visible spectrum are shown in Table 3 (286–290). In single-layer PLED devices the EL occurs mainly in the vicinity of the cathode because of the better mobility of the positive charges in conjugated polymers. The major drawback is the negative impact on luminance efficiency. To overcome this problem, two-layer devices have been investigated (291,292) in which the cathode and the EL polymer are separated by an electron-conducting/hole-blocking layer such as a polyoxadiazole (293). Multilayer devices made from several EL polymers have also been developed to emit white light (294,295). The enormous progress in developing materials with high purity, processibility (ink-jet techniques), and good thermal and oxidative stability has allowed commercialization of such devices by Epson, Philips, Toshiba, and UNIAX (296). Highly pure, undoped materials have been the primary focus for LEDs; doped polymers in such devices lead to a dramatic decrease in luminescence and device performance (297). Light-emitting electrochemical cells (LECs) have received considerable interest because of the involvement of ionic species in the luminescent layer. LECs exhibit many unique features different from conventional LEDs (298). The principal idea behind the operation of an LEC is electrochemical doping to combine Table 3. Light-Emitting Polymers Covering the Visible Spectrum Polymer

Color (λmax ), nm

MEH-PPV CN-PPV PT PPV PPP

Red–orange (610) Red (710) Red (662) Green (550) Blue (459)

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electrons and holes (forming a pn junction) in situ. An electrolyte provides the necessary ionic conductivity, allowing dopant ions to mix with the EAP. Under an applied electric field the electrons and holes interact to form excited species, which undergo radiative decay to emit light (299). Numerous examples of LECs are found in the literature (300,301), but slow response times (normally a second or longer) have limited the ability of LECs to compete with their LED counterparts (302–304). Electrochromism. One of the most promising applications for EAPs is in electrochromic devices. Electrochromism, a reversible, reproducible spectral change that is produced electrochemically, results from changes in conjugation during the redox process. Until recently, the majority of research in this area has focused on inorganics, organometallics, and molecular organics (305–309), but developments in the area of EAPs have shifted the focus in recent years (310–313). EAPs are desirable substitutes for other types of electrochromic materials because EAPs typically switch much faster, they make good films, and their structures can be tailored to provide the desired color changes. Most work in electrochromics has focused on the ultraviolet and visible spectra; IR and RF electrochromics have also received attention. Visible Electrochromics. Most EAPs exhibit UV–vis electrochromism, but not all exhibit significant optical contrast or switching stability for use in electrochromic devices. Polypyrrole switches reversibly between yellow in the neutral state and blue–black in the oxidized state, but contrast is poor in all but the thinnest films, and practical applications are limited (311). Polythiophene switches reversibly between red (neutral) and blue (oxidized), (313) while polyaniline switches very rapidly between four color states (yellow/green/blue/black). PANI switching stability is very good (>106 cycles) when switching is limited to the yellow–green transition, although this somewhat limits its potential applications (311). With goals of additional color states, increased optical contrast, faster switching, and higher stability, recent research has looked at modifications of basic EAP structures. Incorporation of electron-donating substituents as in the PEDOTbased systems mentioned earlier lowers oxidation potentials and decreases band gaps while increasing the stability of oxidized polymers (314), and further incorporation of alkyl substituents tunes color and improves contrast without (in some cases) detracting from switching speeds (315). Carbazoles have been incorporated, providing multiple color transitions, fast switching, reasonable contrast, and high switching stability (316,317). Visible electrochromic applications for EAPs include smart windows, displays, automatically dimming rearview car mirrors (already commercially available), protective eyewear, and optical storage. Infrared Electrochromics. While applications for visible electrochromics are widespread with many commercial possibilities, infrared (3–17 µm) electrochromics applications are limited. Disordered transition-metal oxides have been pursued because of their broad polaron absorption (318), but slow response times and processing difficulties have limited applications. As with most electrochromics applications, EAPs are attractive for IR electrochromics because of their rapid response times and ease of processing. Research to date in EAPs for IR electrochromics has focused on polyaniline-based (319–321) and poly(3,4-alkylenedioxythiophene)-based (322,323) systems. The

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poly(3,4-alkylenedioxythiophene)-based devices exhibit improved contrast ratios over the PANI-based devices and good switching stability. Microwave/Radio Frequency Electrochromics. In radio direction and ranging (radar) systems, electromagnetic waves are bounced off a target and collected by a receiver. The receiver analyzes the signal and determines the range, direction, and speed of the target. RF electrochromics function by absorbing the electromagnetic waves rather than reflecting them back to the receiver. RF electrochromics find use in commercial electromagnetic interference (EMI) shielding, and in law enforcement and military radar systems. EMI shielding materials (addressed below) are typically used in the conductive state only. A number of studies on the microwave properties of EAPs have been published (324–328). EAP-coated textiles (329–331) and devices (332,333) have been explored for use in radio frequency applications. Photovoltaics. The use of EAPs in the construction of plastic photovoltaic cells is another example of the versatility of these materials (334–336). Photovoltaic cells are constructed in a similar manner to single-layer PLEDs, but the photovoltaic cell transforms radiant energy into electricity (337,338). The most basic device construction consists of a conjugated polymer sandwiched between two working electrodes, usually Al and ITO. In devices this simple, the photoinduced charge generation is inefficient (339). To improve these devices it is necessary to have an electron-accepting molecule in close proximity to the undoped conjugated polymer, which acts as an electron donor upon photoexcitation; a charge separation takes place because of photoinduced electron transfer. This process produces a stable charge storage configuration in which the polaron produced is highly delocalized and mobile. One of the best electron-accepting materials for this process is the fullerene molecule (C60 ). The construction of photovoltaic cells using a combination of semiconducting polymer as the donor and molecular electron acceptor such as fullerene has been studied (340–344). Devices based on fullerenes and composite films of semiconducting polymer are known as “bulk-heterojunction” polymer photovoltaic cells. These devices have shown fast photoinduced charge transfer at the donor–acceptor interface. This results in a metastable charge-separated state. The interpenetrating network of both components consists of a large interfacial area and results in efficient charge generation. Photovoltaic power conversion efficiencies (ηe ) of 2.5% under AM 1.5 illumination have been reported (345). Organic Field Effect Transistors/Molecular Electronics. Organic field effect transistors (OFETS) and molecular electronics are two areas of intensive research for both academic and industrial institutions (346). The extensive research on these devices stems from their ability to be processed at low temperatures and from their compatibility with plastic substrates. Several applications of OFETs have been proposed, including smart cards (347), active matrix displays (348), and logic circuits (349). Both polymeric and oligomeric materials have been studied for use in OFETs because the properties can be tailored to vary the HOMO–LUMO gap; physical characteristics such as mechanical flexibility and processability are also advantageous (350). Typical OFET construction is similar to that of inorganic FETs, which are a critical component of computer chips. The OFET device consists of electrodes (drain, gate, and source), a dielectric layer, and a semiconducting layer (Fig. 13).

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Gate electrode

Drain electrode Source electrode Gate dielectric

Organic semiconductor

Substrate

Fig. 13. OFET device.

The current flow between the drain and source electrodes is modulated by the applied gate voltage. When there is no voltage between the source and gate electrodes, the FET is in an insulating state. The device is activated by application of an electric field, resulting in charge carrier formation. These charge carriers can then move between source and drain; the charge carrier concentration can be controlled by small variations in the gate voltage. A crucial parameter in OFETs is the On/Off ratio, which is the ratio of conductivities with the gate voltage switched on and off (351). Highly purified and ordered layers of oligomers typically have charge carrier mobilities of between 0.1 and 0.2 cm2 /(V · s) (352), sufficient for most commercial applications. Molecular electronics is an emerging field that proposes to replace wire, transistor, and other solid-state silicon components with conjugated, organic-based molecules of precise length and constitution (353). The main disadvantage of these systems is the difficulty of inducing three-dimensional order to allow for rapid charge transport (354,355). This fact currently limits their use as replacements in logic circuits and computing systems. Nonlinear Optical Applications. Second-order nonlinear (NLO) materials based on EAPs have shown promise for use in the photonics industry; extensive research has been conducted in this area over the past several decades (356,357). Devices based on poly(diacetylene) have been used to demonstrate alloptical switching at 1.6 µm (358). The NLO process occurs when an electromagnetic field interacts with a medium. When the medium is subjected to an electric field E(0) and an optical field E(ω), the nonlinear effect arises from field-induced modulation of the refractive index. This is known as the second-order NLO effect or the linear electrooptic (Pockels) effect. An NLO chromophore acts as the active molecular component that gives rise to the NLO property in π -conjugated polymers. These polymers typically consist of donor–acceptor (D-A) groups attached to a conjugated polymer backbone for increased charge transfer through π-electron

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π

117

A

Fig. 14. Donor–acceptor groups attached to a conjugated polymer backbone.

delocalization (Fig. 14). The π-bridge system is very important in construction of the dye molecule; acetylenic and vinylic groups have been used extensively to construct such molecules (359,360). Nonlinear optical materials are recognized as the key element for future photonic applications. Photonics, a technology that uses photons rather than electrons to carry information, offers several advantages over electronics. Advantages include larger bandwidths, faster response times, and less noise from extrinsic electromagnetic fields (361). Several approaches using EAPs for photonics applications have been reported (362). Biomedical Applications. Electronic properties of EAPs are often very sensitive to small perturbations caused by exposure to chemical/biochemical analytes. This high degree of sensitivity and the variety of properties that are affected result in systems far superior to the conventional small molecule devices known as chemosensors (363). Surface charge characteristics are fundamental to cellular– biomaterial interactions (364); thus, the ability to control charge density in EAPs has led to promising biomedical applications in the areas of chemical/biological sensors, drug delivery, and for monitoring dynamic cell behavior. Applications of EAPs in tissue engineering and nerve regeneration are also promising, although little work has been published to date (365). Most of the biomedical applications of EAPs have focused on polypyrrole because of its proven stability in biological systems. Chemical/Biological Sensors. EAPs are unique materials for coupling analyte–receptor interactions and nonspecific interactions into an observable response (366). Several EAP properties, including absorption and emission spectra as well as redox activity, are exploited for use in chemical and biological sensors. The band gap of EAPs yields efficient absorption or emission at the band edge, giving rise to a strong luminescence of the EAP. Fluorescence is widely used because of its high sensitivity (367), and because this method offers diverse transduction schemes based on changes in intensity, energy transfer, wavelength, and lifetime (368,369). Many sensors rely on the redox activity of EAPs; exposure to many analytes results in oxidation-state changes. These changes are manifested as changes in conductivity, which can easily be monitored. For a review of polymerbased chemical sensors, see reference 370. Numerous examples are found in the literature of analyte detection using EAP systems. Arrays of EAPs are used in biomolecular recognition schemes, resulting in devices known as “electronic noses” and “electronic tongues” (371– 373) that mimic the mammalian sensor receptors. EAPs functionalized to contain polyalkyl ether and crown ether moieties (374–378) or polymerized in the presence of polyelectrolyte dopants (379) have been used to detect a variety of ions (Li+ , Na+ , K+ , Ba2+ , Mg2+ , Cs+ , Zn2+ , Cu+ , NR4 + ) (see Fig. 15). EAPs have also been prepared with enantioselectivity toward chiral dopant ions (380,381) and functionalized for molecular recognition (382).

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O

O

O O O

S

n

S

O O

O O

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O

O

n+ O M

O

O

S

OR

S

O S

O

O

O

O

O

S

O

O

n

O

O

O

RO n

S

O

O

O

O

n

H3C

O(CH2)5OCH2

N

N M

S

N

S

S S

N

O

O

O O

O

O

O

O

+ PVP−

H N

O

n

n

PVP = polyvinylphosphate

S

S S

n

O

O

O

O

O

O

Fig. 15. EAPs functionalized with polyalkyl ether and crown ether moieties.

EAPs have proved to be effective at detection of small quantities of biomolecules, specifically in cancer detection and real-time monitoring of bloodborne analytes such as glucose and nucleic acids (383–389). These sensors employ EAPs containing natural biomolecular recognition units sometimes referred to as “biosensors.” These biosensor moieties can be ligands, single strands of DNA, enzymes, antibodies, or synthetic proteins that can detect whole cells or individual proteins (390–392). Drug Delivery. One very promising application for EAPs is in the area of drug delivery. This method takes advantage of the swelling and deswelling that occur in EAPs during the redox process. Bioactive molecules such as dopamine (393–395) and ATP (396–398) can be reversibly trapped in the polymer during polymerization or polymer oxidation and then released during reduction of the polymer. Such systems could conceivably be used in conjunction with biosensors as electroresponsive membranes, delivering small quantities of medicines as needed. Dynamic Cell Behavior. While not as well-explored as sensors and drug delivery systems, dynamic cell behavior can also be investigated using EAPs.

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Cell motility, motion, and proliferation can be monitored. For example, DNAfunctionalized EAPs can be used to monitor genetic mutations (399). Direct detection of DNA hybridization has been accomplished with a functionalized polypyrrole (400). Membranes for Gas Separation and Ion Exchange. As a result of the volume changes that occur as part of the redox process, EAPs undergo reversible changes in porosity. This interesting effect has led to studies of EAP membranes for separation of gases and liquids. Water permeability of PANI and PPy is much higher in the doped state than in the neutral state (401). Gas separation has been demonstrated with EAPs (402–404), and ion permselectivity has also been observed (405). Batteries/Supercapacitors. Electrochemical charge storage systems (batteries and supercapacitors) are currently employed in a wide variety of applications (14); there is a constant demand for performance improvements and reduced environmental impact. Redox-active polymers are promising materials for use in batteries and supercapacitors because of their high gravimetric and volumetric charge capacities, fast charge/discharge rates, robustness, environmentally friendly nature, and lower costs relative to noble metal oxides (406–408). Battery designs using electroactive polymers feature the EAP as the cathode, which is separated from an anode (such as Li, Na, Mg, and Zn) by an electrolyte. Normally, high specific energies of up to 3.5 V can be obtained (409). EAPs that have been used in rechargeable batteries include polyacetylene-, PANI-, PPy-, PT-, and PPP-based materials (410). The current drawback to full utilization of EAPs for rechargeable batteries is the rate limitations associated with low ionic mobilities of the polymers as well as the electrolytes (14). Electroactive polymers have also been investigated for use in supercapacitors. These devices provide a higher power density than batteries at a lower operating voltage. Supercapacitors have a higher energy density than traditional capacitors because of the high capacitance of the electrode materials (411). Target applications are as computer memory backup, peak power for electric vehicles, and burst power for military platforms. Ruthenium oxide, the most promising inorganic supercapacitor material, exhibits capacitance values up to 720 F/g (412). EAP-based supercapacitors, which are expected to be less expensive than those based on metal oxides, exhibit capacitance values up to 350 F/g (413); further research is expected to increase this value considerably. Most EAP-based supercapacitors combine one or two p-dopable polymers (type I and type II supercapacitors, respectively); devices based on polypyrrole (414), polythiophenes (413,415), polyaniline (416), and poly(3,4-ethylenedioxythiophene) (417,418) have been investigated. Type III supercapacitors, those combining a p-dopable and an n-dopable polymer, could theoretically yield higher energy and power densities and operate at higher voltages than type I or type II supercapacitors (419). However, type III supercapacitors are currently plagued with long-term stability issues related to instability of the n-doped polymers.

Conductive Coatings for Electromagnetic Shielding and StaticDissipating Applications. In addition to the RF electrochromics research discussed above, EAPs are also used as conductive coatings on textiles for radarabsorbing materials (RAMs) (420). Polypyrrole and PANI are both examples of EAPs used in composite formulations as RAMs (420,422).

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Electromagnetic interference (EMI) shielding has become increasingly important because of the combination of increasing number of electrical systems (vehicles, computers, etc) and the increasing use of microwaves in advanced navigation and domestic appliances. Traditional EMI shielding has involved incorporation of conductive fillers (metal particles, conducting carbon black, or graphite fibers) into plastic housing materials (423,424); poor mechanical properties (328) and poor reproducibility of performance characteristics are the major drawbacks of these composite systems (324). EAPs do not necessarily suffer from these problems and have therefore been investigated as replacements for these filled materials (326), although mechanical instability of many EAPs of interest has led to the use of EAPs blended with non-conductive polymers (324,328,425). The need for improved static control or electrostatic discharge materials poses a challenge for materials scientists and engineers (426). The protection of sensitive electronic devices requires an environment in which unwanted electrical charges, voltages, and electric fields are dissipated. The bulk of electronic packaging requires an insulating material that is clear, durable, and inexpensive. Insulating plastics are widely used as structural materials for the packaging industry, and static charge often builds up; the accumulated charge must be dissipated as rapidly as it is formed, and so conductive additives are required. Static-dissipating materials should have resistivities between 106 and 109 /sq, and they should be transparent and durable even under extreme conditions (427). The cheapest and most-often-employed materials have been polymer blends or composites containing carbon black as filler (428,429). However, high loading levels of conductive fillers are often required, leading to significant decreases in durability and transparency. In these cases, the use of EAPs, either as coatings or as fillers, is becoming more common (426). EAPs are currently used as additives and melt processed with conventional resins such as polyethylene to form static-control articles such as films and trays (430). EAP coatings are also used in the production of static-dissipating carpets (431) and fabrics (432). Recently, EAP coatings have been shown to dissipate static in energetic materials more effectively than carbon black additives, providing a much-needed safety feature for the handling and storage of energetic materials (433). Actuators. Inorganic piezoelectric and electrostrictive materials have shown a major technological role in converting electrical energy to mechanical energy in actuators. However, electroactive polymers offer several unique advantages over inorganic materials, including large dimensional changes, high stress generation, high work capacity per cycle, ease of fabrication, and low cost (434). Numerous electroactive polymers, including polythiophenes, polypyrrole, poly(p-phenylene vinylene)s, poly(p-phenylene)s, and polyaniline, have shown significant dimensional changes upon doping and dedoping (435–440). These dimensional changes result from changes in the volume required to accommodate anions, cations, and/or cointercalating solvent species. Volume changes ranging from 0.1 to 10% are normally observed with doping (441). This unique behavior has been exploited for applications including controlled release (discussed above) and electrochemically driven actuators.

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EAP-based actuators have been explored for use in artificial muscle fibers (AMFs) (442–444) and microelectromechanical systems (MEMS) (437). Non-conductive polymeric gel systems have been used for several decades as actuator devices (445–447); but with low elastic modulus and low yield strength, applications of gel-based actuators are very limited. Gel-based actuators offer one significant advantage over EAPs in that their volume changes can exceed a factor of a thousand (448). Corrosion Inhibition. EAPs have been investigated over the past decade as corrosion-inhibiting materials (449). The most extensively studied of the EAPs for corrosion prevention has been PANI (450–452). Researchers at Los Alamos National Laboratory (LANL) and the NASA Kennedy Space Center demonstrated that doped PANI coatings inhibited corrosion of carbon steel (451). Their tests were conducted in a 3.5 wt% NaCl/0.1 M HCl environment using ca 0.005-cm-thick films of PANI doped with p-toluenesulfonic acid on carbon steel; the PANI was covered with an epoxy topcoat. The PANI/epoxy coating performed significantly better than the epoxy topcoat alone. These initial results were used by researchers at LANL–NASA to develop EAP coatings to resist the corrosive effects of acid vapor generated during space shuttle launches. Their approach was based on earlier work suggesting that the interfacial contact between the metal and a doped EAP would generate an electric field that would restrict the flow of electrons from the metal to an outside oxidizing species, thus preventing and/or reducing corrosion (453). There have been numerous reports in the literature regarding the use of EAPs in retarding corrosion on steel alloys (454,455). There is still debate on whether the doped or undoped form of PANI provides the best corrosion protection; several reports have shown that the undoped form of PANI can perform as well as or better than the doped form of PANI (456,457). While there is general agreement that PANI performs well in retarding corrosion on carbon steel, the mechanism for this process is still under investigation. Several hypotheses are suggested for corrosion protection using EAPs: (1) PANI contributes to the formation of an electric field at the metal surface, restricting the flow of electrons from metal to oxidant; (2) PANI forms a dense, strongly adhering, low porosity film similar to a barrier coating; and (3) PANI causes the formation of protective layers of metal oxides on a metal surface. While PANI has been extensively studied for corrosion control, its major drawback is its pH dependence. It works very well in acidic environments because of the formation of the conductive emeraldine salt, but at higher pH (>7) the non-conductive emeraldine base is formed, and the material no longer provides adequate protection against corrosion. Thus, use in a marine environment (pH ∼ 8) is not possible. For marine applications, pH-stable EAPs have been investigated. Doublestranded PANI has been coated onto Al alloys (458); immersion tests in simulated seawater showed improved corrosion resistance as compared to control samples (epoxy coating) (459). Recently, poly(bis-dialkylamino)phenylene vinylene) (BAMPPV) was shown to adhere to Al alloy in an immersion test using simulated seawater (pH ∼ 8) to retard corrosion (460,461). Quantitative evidence was obtained to show corrosion inhibition as compared to noncoated Al alloys.

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Summary The reversible addition or removal of electrons from the backbones of electrically active polymers is accompanied by significant changes in their physical and chemical properties. The ability to reversibly switch EAPs between insulating and conducting, opaque and transparent, absorptive and emissive, and expanded and contracted drives much of the current research. EAP properties are strongly dependent on polymer structure; the ability to tailor these properties through thoughtful synthetic methods facilitates the development of new devices to meet the increasing demands of today’s society. Industry has already achieved success with commercial devices that utilize EAPs. Attempts to increase stability and to better understand structure–property relationships are crucial to the development of materials for new and improved applications.

BIBLIOGRAPHY “Electrically Conductive Polymers” in EPSE 2nd ed., Vol. 5, pp. 462–507, by Jane E. Frommer and Ronald R. Chance, Allied-Signal Corporation. 1. H. Shirakawa, E. Louis, A. MacDiarmid, and A. Heeger, Chem. Commun. 578 (1977). 2. A. G. MacDiarmid and A. J. Heeger, Synth. Met. 1, 1013 (1978). 3. J. L. Bredas, S. R. Marder, and W. R. Salaneck, Macromolecules 35(4), 1 (2002). 4. B. Ranby, in W. R. Salaneck, I. Lundstrom, and B. Ranby, eds., Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structures, Oxford University Press, Oxford, 1993, Chapt. “3”. 5. R. H. Norman, Conductive Rubbers and Plastics, Elsevier, Amsterdam, 1970. 6. E. K. Sichel, ed., Carbon Black Polymer Composites, Marcel Dekker, Inc., New York, 1982. 7. S. K. Bhattacharya, Metal-Filled Polymers: Properties and Applications, Marcel Dekker, Inc., New York, 1986. 8. W. M. Wright and G. W. Woodham, in J. M. Margolis, ed., Conductive Polymers and Plastics, Chapman and Hall, New York, 1989. 9. K.-M. Jager, D. H. McQueen, I. A. Tchmutin, N. G. Ryvkina, and M. Kluppel, J. Phys. D: Appl. Phys. 34, 2699 (2001). 10. A. Eisenberg, Ion-Conducting Polymers, Academic Press, Inc., New York, 1977. 11. A. Eisenberg and J.-S. Kim, Introduction to Ionomers, John Wiley & Sons, Inc., New York, 1998. 12. J. R. MacCallum and C. A. Vincents, eds., Polyelectrolytes, Elsevier, New York, 1989. 13. J. Owen, in C. Booth and C. Price, eds., Comprehensive Polymer Science: The Synthesis, Characterization, Reactions, and Applications of Polymers, Vol. 2, Pergamon Press, Oxford, 1989, Chapt. “21”. 14. P. Novak, K. Muller, K. S. V. Santhanam, and O. Haas, Chem. Rev. 97, 207 (1997). 15. F. B. Dias, L. Plomp, and J. B. J. Veldhuis, J. Power Sources 88, 169 (2000). 16. C. Heitner-Wirguin, J. Membr. Sci. 120, 1 (1996). 17. G. Inzelt, M. Pineri, J. W. Schultze, and M. A. Vorotyntsev, Electrochim. Acta 45, 2403 (2000). 18. A. J. Heeger and J. Long Jr. Opt. Phot. News p. 24, August 1996. 19. D. O. Cowan and F. M. Wiygul, Chem. Eng. News, 28, July 21, 1986. 20. H. Akamatu, H. Inokutchi, and Y. Matsunaga, Nature 173, 168 (1954). 21. A. H. Hermann and A. Rembaum, J. Polym. Sci., C 109 (1967).

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PETER ZARRAS JENNIFER IRVIN Naval Air Warfare Center Weapons Division (NAWCWD)

Vol. 6

ENVIRONMENTALLY DEGRADABLE PLASTICS

ELECTRONIC PACKAGING.

See Volume 2.

ELECTROOPTICAL APPLICATIONS. EMULSION POLYMERIZATION.

See Volume 2.

See HETEROPHASE POLYMERIZATION.

ENGINEERING THERMOPLASTICS, OVERVIEW. See Volume 2.

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