INTELLIGENT POLYMER SYSTEMS Introduction Intelligent polymer systems possess the ability to sense, process information, and actuate responses. Energy is usually required to implement these functions and so energy conversion/storage capabilities are desirable if the system is to be truly autonomous. Ideally, these functions would be integrated at the molecular level. It has been clearly demonstrated over the past 20 years that inherently conducting polymers (ICPs) are capable of providing all of the above functions and as such they have a critical role to play in the development of intelligent polymer systems (see ELECTRICALLY ACTIVE POLYMERS). This article briefly reviews the properties of ICPs and their ability to function as sensors, processors, actuators, and energy conversion/storage systems. To illustrate the ability of ICPs to provide the range of functions required for intelligent polymer systems, we will draw on examples that utilize polypyrroles (1) or polythiophenes (2).

231 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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For polypyrroles and polythiophenes, n is usually ∼3–4 for optimal conductivity; ie, there is a positive charge on every third or fourth pyrrole or thiophene along the polymer chain, near which the dopant anion A − is electrostatically attached. Polyanilines are also of interest but they are less amenable to use in a wide range of environments because of the need to retain protonation to ensure conductivity. Consequently, most of the examples here will focus on polypyrrole and polythiophene. Each of these materials may be produced via either chemical or electrochemical oxidation of the appropriate monomer (1). For polypyrrole the electrodeposition process is described simplistically in equation 1.

(1) A dopant counterion (A − ) is incorporated during electrosynthesis to balance the charge on the polymer backbone. A wide range of dopants can be incorporated using this approach. Common chemical oxidants such as FeCl3 and (NH4 )2 S2 O8 may also be used and these provide the anion from the oxidant, as the dopant anion A − . In general, chemical oxidation provides ICPs as powders, while electrochemical synthesis leads to films. An important feature of ICPs is that they are amenable to oxidation/reduction processes that can be initiated at moderate potentials. For polypyrroles and polythiophenes, two oxidation states can be reversibly switched, as shown in equation 2 (Z = NH or S). The oxidized forms exhibit good electrical conductivity (σ = 1–100 S · cm − 1 ), while the reduced forms have very low conductivity (σ ∼ 10 − 8 S · cm − 1 ).

(2) If the dopant anion A − is small and mobile (eg, Cl− ) and the polymer has a high surface-area-to-volume ratio, then upon reduction the anion will be efficiently ejected from the polymer. However, if the dopant is large and immobile (eg, if A − is a polyelectrolyte such as polystyrene sulfonate) then an electrically induced cation-exchange process occurs, according to equation 3:

(3) where the cation (X+ ) is incorporated from the supporting electrolyte solution (2). The exact nature of the reduction process has a dramatic effect on the physical and

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chemical properties of the polymer. These changes are important in determining the sensing, information processing, and actuation capabilities of the systems as discussed below.

Sensing Nature has developed recognition systems that are able to discriminate on the basis of highly specific molecule–molecule interactions generating a unique signal. Alternatively, nature utilizes arrays of less specific sensors to collect information that is deciphered using pattern recognition processes carried out in the brain. Both approaches have also been pursued using ICPs in the development of synthetic sensors, as both chemical and physical sensors. As chemical sensors, specificity can be induced by using molecular recognition components from nature. For example, the ICP may be used as an immobilization platform for enzymes (3–5), antibodies (6–10), oligonucelotides (11–13), or even whole living cells (14,15). The bioactive component may be incorporated during the polymerization process and the ICP provides signal transduction and transmission capabilities (Figure 1a). The majority of enzyme-containing ICP sensors generate a signal because of the enzymatic generation of an electroactive product (eg, H2 O2 ) or the consumption of an electroactive product (eg, O2 ). Alternatively, the bioevent may trigger a change in pH of the analyte solution that alters the resistance of the polymer (16). The mechanism of signal generation with antibody-containing conducting polymer sensors appears to be associated

Fig. 1. (a) Polymeriztaion of polypyrrole onto an electrode using an oligonucleotide (A) as the dopant. The oligonucleotide is physically entrapped within the polymer during synthesis. (b) As a target oligonucleotide (T) passes across the surface of the oligonucleotide (A) doped polypyrrole electrode, a hybridization reaction occurs (indicated by the dashed lines). This results in a change in the current/potential response observed at the electrode.

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with the modification of cation movement into and out of the polymer upon oxidation/reduction in the presence of the antigen (17,18). In the case of complementary oligonucleotide binding, only limited examples use direct electrochemical signal transduction (19,20), and here the mechanism of signal generation is not clear (Figure 1b). The selective detection of metal ions has also been achieved by the covalent attachment of molecular recognition moieties to the ICP backbone. The usual approach has been to synthesize a monomer or dimer containing the appropriate recognition group and this is subsequently polymerized (21). Swager (22) have prepared polythiophenes containing crown ethers and calixarenes covalently bound to the bithiophene repeat units, which exhibit controllable selectivity toward Li+ , Na+ , and K+ ions. An alternative route to appropriately functionalized ICPs is the use of sulfonated species containing the desired molecular recognition/receptor site as the dopant anion for the conducting polymer chains. For example, calixarenecontaining polypyrroles (23) and polyanilines (24) have recently been prepared via the use of sulfonated calixarenes as dopant anions. Similarly, the incorporation of metal complexing agents such as sulfonated 8-hydroxyquinoline as dopants in polypyrroles provides a simple route to metal ion-selective ICPs (25). ICPs have also been assembled as microsensing arrays with a view to collecting less specific data subsequently deciphered using pattern recognition software. The so-called electronic noses are based on this principle (26–30). A range of conducting polymers with differing molecular selectivity respond (by changes in electronic resistance of the polymer) to a complex mixture to produce a unique pattern of responses. This approach has been used to differentiate beers (26), to detect microorganisms (28,29), for wastewater management (31,32), and for wine characterization (33). Stuetz and co-workers (31) demonstrated the ability of a nonspecific sensor array (consisting of 12 conducting polymers) to detect changes in the organic content of a wastewater sample. These researchers were able to draw a comparison between the odor profiles of sewage liquids and corresponding BOD, GOD, and TOC measurements using this sensor array. Using a nonspecific conducting polymer sensor array, Bourgeois and co-workers (32) developed an on-line continuous measurement system that monitored changes in water and wastewater quality. By analyzing the headspace gas from sparged samples, which flowed over the sensor array, they were able to successfully monitor samples both in a laboratory and in field setting. Guadarrama and co-workers (33) used the response of a conducting polymer sensor array to characterize varieties of Spanish wine. Using static and dynamic headspace sampling combined with statistical analysis utilizing pattern recognition techniques, they were able to characterize these wines by considering the sensitivity of the polymer films to moisture and ethanol content. The array approach has also been developed for amperometric sensing when used in solution. Change in amperometric responses in the presence of different ions is used as the signal transduction method. This has been used by us to discriminate between simple ions (34,35) and even proteins (36). The approach used is similar to the “electronic nose” in that none of the sensing elements is specific; however, each polymer has a different selectivity series, giving rise to a unique pattern of responses for any given protein.

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Actuating ICPs are capable of undergoing transformations at the molecular level through to the macro-molecular level. The former can be used to build responsive molecule release systems, while the latter forms the basis of artificial muscles. A change in oxidation state of the polymer (see equations 2 and 3) is accompanied by dramatic changes in chemical and physical properties. Chemical changes have been studied at the molecular level using a technique known as inverse chromatography both in solution (37,38) and in gas phase (39,40). These studies revealed that, at least when the dopant A − is small and mobile, reduction decreases the anion-exchange capacity of the polymer and increases the hydrophobicity of the polymer backbone. The dopants can be chosen so that the release process has the desired effect on the chemical composition of the immediate environment. For example, Miller described the triggered release of glutamate (41) and salicylate (42) among other compounds. The movement of ions has also been tracked using electrochemical quartz crystal microbalance techniques (43–45). As ions move in and out of the polymer matrix, upon oxidation and reduction, a resonant frequency shift is observed for the polymer coated quartz crystal. The magnitude of the frequency shift is indicative of the amount of ions moving either in or out of the polymer. The direction of ion flow has been shown to be dependent upon the nature of the dopant bound within the polymer. Small spherical-like anions are known to move easily in and out of the polymer matrix. When the polymer is oxidized the backbone becomes cationic (p-type) and anions move into the polymer, effectively doping the polymer. Upon reduction of the polymer backbone, the cationic characteristics of the polymer are lost and the anions are expelled under the negative electric field of the reduction potential. Ion flux can be reversed by the incorporation of large anion species into the polymer during the growth phase, such as dodecylbenzene sulfonate (46,47) or polyelectrolytes such as polystyrene sulfonate (48). Upon reduction of the polymer the bulky anion cannot be expelled while the polymer backbone looses its net cationic charge, resulting in the bulk material having an anionic character. The result of this process is that cation species are incorporated into the polymer to achieve electrostatic balance. The incorporation of either anions or cations during redox cycling results in polymer swelling. This principle has been exploited in applications such as polymer actuators. These incorporation/exclusion events at the molecular level also result in changes in the overall volume (dimensions) of the polymer (49). It was these volume changes that led Baughman and colleagues to the concept of electromechanical actuators (artificial muscles) based on conducting polymers. Massoumi and Entezami (50,51) utilized conducting polymer bilayers to demonstrate that active cations and electrolyte cations (ie, sulfosalicylic acid and 2-ethylhexyl phosphate) can be transported in to and out (controlled release) of the bilayer outer film. Pyo and Reynolds (52,53) reported the selective releasing of ions, such as adenosine 5 -triphosphate and heparin respectively, from conducting polymer bilayer structures. Piro and co-workers (54) successfully incorporated oligonucleotides into conducting polymer films using a two-step polymerization process. The release of these trapped oligonucleotides was also investigated, where the

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release exhibited a three-step profile: a “burst” period during the first 5 min, followed by an intermediate and a very slow release stage lasting up to several days. The uptake and release of dopamine were investigated with a quinone-containing conducting polymer film (55). This study found that dopamine, once incorporated into the polymer, could be released by applying an oxidative potential. Pernaut and Reynolds (56) studied the controlled release of a polypyyrole membrane “loaded” up with adenosine triphosphate (ATP). Electrochemical triggering allowed ATP to be delivered with a variety of release profiles and adjustable rates. Controlled-release devices that utilize the unique properties of conducting polymer membranes can also be configured. It has been shown that the transport properties of ICPs are dependent on the oxidation state of the membrane. This has been demonstrated both in solution (57) and for transport of volatiles (58). Extraordinary selectivity factors have been reported for the separation of some volatiles; for example, selectivity factors of 3590 for H2 /N2 , 30 for O2 /N2 , and 336 for CO2 /CH4 were reported (59). With membranes operational in solution, the controlled transport of simple ions (60), metal ions (61,62), small organic molecules (63), and even proteins (64,65) has been demonstrated. The transitions that occur within the conducting polymers also result in dramatic changes in physical properties. For example, upon reduction the materials become more transparent (66), making them useful in electrochromic devices (67,68). By producing simple laminated membrane structures (Figure 2) containing the ICP, force generation occurs upon oxidation/reduction of the active polymers, and movement follows. A number of detailed studies on the effect of polymer composition, supporting electrolyte and rate of stimulation on the forces generated, have been carried out (69–72). In recent studies it has been shown that the electrolyte used for ICP-based devices including artificial muscles is critical (73). Subsequent to this, it was discovered that a new class of electrolytes, ionic liquids, have characteristics that are particularly useful for ICPs (74,75). Ionic liquids (ILs) have received a great deal of attention in recent years (see Ionic Liquids, Polymerization in). ILs have unique solvent properties and low volatility that provide advantages over conventional electrolyte systems with a wide range of application in green chemistry. Attention has been mainly directed toward modifying ILs to manipulate the solvent and electrical properties. Properties that are unique to ionic liquids include tunable hydrophobicity (76) and

Fig. 2. Cross section of a polypyrrole double-sided PVDF [poly(vinylidene fluoride)] electromechanical actuator, with the pores of PVDF filled with electrolyte.

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Fig. 3. Typical ionic liquid ions.

choice of anion (77). ILs generally have excellent thermal stability (78) and low volatility (79) and, in some instances, offer unique solubility properties as a result of following different dissolution rules with respect to common organic solvents (80). The minimal vapor pressure they exhibit also leads to good environmentally stability. ILs also offer wide electrochemical potential windows, which can be manipulated by choice of the anion and cation system (81,82). There are potentially thousands of IL systems yet to be discovered based on the combination of organic cation structures with the myriad of anions that are potentially available. Figure 3 shows a number of cation systems that have been reported with anions such as PF6 − , BF4 − , NO3 − , Cl − , CF3 SO3 − , and trifluoromethanesulfonimide, to list but a few. Given the intimate interaction of ICPs with the electrolyte, especially during oxidation/reduction of the polymer the chemical nature of the ionic liquid is also important. The versatility of ionic liquids in this regard has proven extremely useful. For example, the use of imidazolium cations during oxidation/reduction of PPy PF6 used as electromechanical actuators has been shown to not only give markedly improved stability (increasing cycles length from hundreds to hundreds of thousands of cycles) but also increased performance. The latter feature arises from the inclusion of the imidazolium cation during reduction, a process that apparently preserves the inherent modulus of the materials (83). Another interesting form of these electrolytes involves their use as solid electrolytes. By performing polymerization in the ionic liquid a solid can be produced (84). A highly stable, easily handled actuator can be constructed using this electrolyte.

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All of the above responses (actuators) are initiated by creating an appropriate electrical potential that causes the polymer to change form. This is usually achieved by imposition of a potential from an external source.

Electrochromic Devices Electrochromic devices (ECDs) have been extensively researched as potential large area electrochromic windows. The technology is in direct competition with liquid crystal display devices. ECDs have a distinct advantage over the LCD in that the device color remains even after the driving potential has been removed (68) (see Electrochromic Polymers). The use of ICPs in electrochromic devices has been discussed and reviewed in a number of recent articles (85–88). The observed color changes of these devices are ideally reversible and can undergo thousands of cycles without degradation. Device scale-up is strongly dependent upon the electrochemical behavior of the polymer in the electrolyte, which has a strong influence upon the redox reversibility of the polymer. Solvent-based electrolytes, although shown to be effective in the laboratory, suffer from component evaporation, often leading to device failure. Sealing the unit to prevent solvent leakage has also proven to be a challenge. This issue has been addressed by the adoption of solid polymer electrolyte systems. More recently, the application of nonvolatile ionic liquids (74–91) to electrochromic devices has produced devices capable of millions of cycles, with little loss of performance over the device lifetime (92). Electrochromic effects in conducting polymers are achieved by varying the band gap (HOMO − LUMO) of the π –π ∗ transition and is associated with π -bond conjugation. Band gap, and therefore color, is influenced by the type of monomer repeat unit, the polymer conjugation length, and, ultimately, the level of polymer oxidation. Copolymerization of different monomers has also been used as a method of modifying these band gaps. Polypyrroles have a color shift from blueviolet (670 nm) to yellow-green (420 nm) when oxidized and reduced respectively. Polyanilines have three redox states that are yellow when fully reduced to the leucoemeraldine state (300–400 nm), green in the mid-oxidation emeraldine salt state (400 and >750 nm), and blue when fully oxidized as the pernigraniline salt (600 nm). Polythiophene is blue when oxidized (730 nm) and red when reduced (470 nm) (93). The challenge that remains for conducting polymer electrochromic applications is clearly in the domain of device scale-up. The application of these materials to large area devices, such as windows, requires uniform real-time switching of the polymer oxidation state to access different color regions. Although this has been demonstrated for small-scale devices, it is difficult at larger scales.

Information Processing The use of inorganic, silicon-based materials in the development of complex circuitry that is capable of processing vast amounts of information is a well-developed

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industrial technology. The concept of plastic electronics has only attracted considerable interest in recent years. Polymers are increasingly being utilized as new materials for electronics and optics. Necessary elements for this imperative are of course the functional properties but with the added advantage of polymer processability and flexibility in structural design (94). In fact, replacement of traditional inorganic semiconductors by organic molecules, polymeric or even biological materials have recently been termed as molecular electronics (ME) (95). Rambidi (96) identified one of the main directions in the development of devices for molecular information processing to be based on the concept of distributed molecular (chemical) information processing media. It has been proposed that the electrical conduction in ICPs occurs via nonlinear (or topographic) defects (solitons/polarons) generated either during synthesis or as a result of doping. Solitons and polarons have recently been shown to have implications in the technical development of molecular electronic devices (97). To realize molecular electronics, one of the key issues is how to fabricate the ordered structures in a planned manner since various properties of organic molecules are of anisotropic nature, and consequently the functions of molecular assemblies depend strongly on the way molecules are arranged in them (97). Much research has been done on the molecular engineering of these assembly processes, namely Langmuir–Blodgett film (98,99) and self-assembly techniques (100,101). Pal and co-workers (102) used the Langmuir–Blodgett technique to develop polymeric light-emitting diodes (LEDs) (see LANGMUIR-BLODGETT FILMS). Both dc and ac LEDs were fabricated with a precise thickness ranging from a few molecular layers to tens of layers. In dc LEDs, they showed that as few as three layers of active polymer can yield the same luminance as the thicker ones. Zhang and co-workers (103) fabricated binary DNA/surfactant-modified arrays on a heterogeneous patterned surface through a controlled condensation and dewetting process using the SAM (self-assembled monolayers) technique. Studies on the formation of DNA SAMs at surfaces were driven by an inherent interest in understanding different aspects of this molecule and its importance in biomimetic materials science and molecular electronics. ICPs have been studied for their application to such molecular electronic devices as diodes (104,105), field effect transistors (FETs) (106,107), and light emitting diodes (LEDs) (108,109) (Figure 4). Rectifying junctions are the basic element of many electronic components. Since their discovery, ICP rectifying junctions such as p–n junction and Schottky junction have been studied (97). Chen and Fang (111) found that the acid doping of free-standing polyaniline film can cause a higher rectifying behavior and photovoltaic conversion. It has been shown that increased diode performance is observed when the device is fabricated from regioregular polymers having a short alkyl substituent and/or by the presence of electron-withdrawing groups (97). The performance of ICP-based FETs are quite encouraging and recent use of these devices in logic circuits or active matrix emissive displays has geared up the research in this direction immensely (112,113). In conventional semiconductor devices, “field effect” has been used to improve performance (114). Control of current passing through a “gate electrode” using the field-effect phenomenon opens the possibility of a transistor action without requiring the existence of p–n junctions,

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Fig. 4. Schematic of a polymer LED device configuration (110).

which in turn improves the device characteristics (97). Koezuka and co-workers (106) were the first to use this phenomenon to fabricate ICP-based FETs. ICPs have been demonstrated to exhibit light emission in the entire visible spectrum, quantum efficiency over 4%, fast response time, brightness of 106 cd · m − 2 , and device lifetime more than 2 months (97). Heeger (115) reported that the operating life of polymer LEDs, with an initial brightness of 400–500 cd · m − 2 at room temperature, approached several thousand hours. This equates to operating lifetimes in excess of 104 h at display brightness. The current interest in exploring the luminescent properties of ICPs is for flat-panel displays used in information processing devices, such as laptop computers, cellular telephones, small handheld devices, large panel displays, and notebook computers (97). The main advantages of these ICP materials over conventional luminescent materials are the tuning of wavelength emitted by chemical modification, low operating voltages, flexibility, easy processing, low cost, possibility of making large area device, and output color in whole visible spectrum (97).

Energy Conversion/Storage The intelligent polymer system must also be capable of converting energy from a natural source, such as sunlight, and storing it until required. ICPs that are capable of carrying out these functions have potential as a replacement for Si photovoltaic cell technology as well as energy storage in rechargeable battery systems. Polymer Photovoltaics. The potential of making lightweight, low cost, and flexible solar cells has driven growing interest in device development. It has been demonstrated that ICPs are capable of functioning as the active layer in photovoltaic devices (116–120). Upon irradiation of the ICP electron–hole pairs (excitons) are generated. If these excitons are transported to an interface where they can be split before recombination occurs, then electrical energy is generated. Both Schottky devices (121) and photoelectrochemical cell (122–124) configurations have been used. Photovoltaic responses were obtained early on from a variety of ICPs in single-polymer Schottky devices with quantum efficiencies up to

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Table 1. Summary of Photovoltaic Performance of Some Conducting Polymers Polymer system Polyacetylene (PA) Polymethylthiophene Polypyrrole (PP) Polyaniline (PAn) Polythiophene

Device

Voc,a V

Isc,b mA/cm2

FF,c

Y,d %

n-Si/p-PA Al/PMT/Au PP/n-Si Al/PAn/ITO Photoelectrochemical cell

0.53 0.23 0.29 0.43 0.41

18.18 0.16 (x10 − 3 ) 9.0 — 0.35 (×10 − 3 )

0.32 0.30 0.46 0.70 —

4.3 — 1.2 0.8e 0.6f

a Voc:

open circuit voltage. short circuit current. c FF: fill factor. d Y: engineering conversion efficiency. e Power conversion efficiency. f Monochromatic photon to current efficiency. b Isc:

1% (125–127); the best results have been obtained using interpenetrating blends of functionalized polyphenylenevinylenes (128,129) and/or polythiophenes (130) with quantum efficiencies up to 29% and energy conversion efficiencies up to 2%. The addition of dyes (131–133) and electron acceptors (134–136) to such devices has also increased the overall power conversion efficiency to around 3%. Performance parameters for some typical conducting polymers are listed in Table 1. Although the performance of these materials is significantly low than what has been achieved for traditional silicon-based technologies, the production of these devices are still highly attractive as device manufacture can be undertaken without the need for a clean room, the relatively low cost with respect to silicon, technology, and the potential for large-scale flexible cells. Energy Storage. A number of strategies have been adopted for the utilization of conducting polymers as rechargeable battery devices. Many studies have looked at the potential of replacing either the anode or cathode electrode with a conducting polymer, more typically the cathode, as tradition metal anodes have significantly higher energy densities, gravimetrically, than do polymers (137). A more interesting possibility is the development of all polymer batteries where an electroactive polymer comprises both the ahde and cathode supported on inert conducting supports such as graphite and platinum. Killian and co-workers were the first group to report the assembly of an all-polymer battery structure (138) utilizing both p- and n-dopable polypyrrole with a specific charge capacity of 22 mAh · g − 1 at a cell potential of 0.4 V. The cells showed no loss in capacity when cycled 100 times. The observed cell potential was clearly limited by the n-doping ability of the cathode. More recently, an all-polymer battery based on derivatized polythiophenes supported on graphite-coated supports was described (139). In this instance, polythiophene functioned more effectively in the n-doping region and provided an improved cell discharge voltage of 2.4 V and capacities of 9.5–1.5 mAh · g − 1 . A recent approach is the development of a polymer/polymer battery based on polyaniline anode and poly-1-naphthol cathode (140). This device has been reported with an impressive cell voltage of 1.4 V, a specific capacity of 150 Ah g − 1 , and a loss of 15% of cell capacity after 100 cycles. Other attempts at fabricating battery systems have been made (141) with polyacetylene, polypyrrole, and polyaniline (142),

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where the batteries were shown to compare favorably on a gravimetric basis. On a volumetric basis, more traditional inorganic insertion compound electrodes were found to outperform these systems. The application of conducting polymers as either a cathode or anode combined with more traditional inorganic or metallic electrodes has been extensively reviewed by others (137,139).

Polymer Processing All of the foregoing applications of ICPs have significant commercial potential. However, in most cases, exploitation has been limited because of the lack of convenient polymer processing and device fabrication protocols. For example, polypyrroles exhibit the desirable properties mentioned above; however, polymerization usually results in the formation of an insoluble, infusible material not amenable to subsequent fabrication. Conducting polythiophene salts are similarly intractable. Several approaches have recently been employed to overcome this problem of intractability. Solubility has been induced for polypyrroles by attaching alkyl (143,144) or alkyl sulfonate (145,146) groups to produce 3-substituted pyrrole monomer prior to polymerization. This results in markedly enhanced solubility in organic or aqueous medias respectively. Polythiophenes can also be rendered either organicsolvent-soluble (147) or water-soluble (148) using these derivitization approaches. These improvements in solubility achieved above via ring substitution generally result in significant loss in electrical conductivity of the final polymer. Consequently, formation of colloidal dispersions is an attractive alternative route to solution processing in water, as this allows for post-synthesis handling while retaining reasonable conductivity. Conducting polymer colloids can be produced by chemical (149) or electrochemical (150,151) oxidation of monomer in the presence of a steric stabilizer. Colloids produced electrochemically are formed by intercepting the polymer deposition on the electrode surface utilizing hydrodynamic control. This is facilitated by the presence of a steric stabilizer in solution. The electrochemical approach is advantageous in that the polymer properties can be altered by accurate control of the oxidation potential during polymerization. This technique also allows a wide range of dopants to be incorporated into the polymer to give different properties. For example, proteins can be incorporated into conducting polymers while retaining their biological integrity (151). Emulsion polymerization is the most appropriate method for producing submicrometer (nano) particles owing to its versatility and relative simplicity. Formation of ICP nanodispersions consisting of highly conducting nanoparticles dispersed in either organic or aqueous solution has been achieved using polyaniline (152), polypyrrole (153), and regioregular polythiophene (154). Jang and Oh (155) used thermodynamically stable microemulsion micelles as nanoreactors to synthesize, at low temperatures, polypyrrole nanoparticles with dimensions of several nanometers. Deng and co-workers (156), using a core–shell approach, synthesized magnetic and conducting Fe3 O4 –polypyrrole nanoparticles (30–40 nm in diameter) in the presence of Fe3 O4 magnetic fluid (core material) in aqueous solution containing sodium dodecylbenzenesulfonate as a surfactant and dopant.

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The successful polymerization of polythiophene nanoparticles has been plagued by the insolubility of most thiophene monomers, and their subsequent polymer. Although advances have been made in solubilizing the monomer, in particular the utilization of surfactants, polymerization normally results in either precipitated powders or deposition of solid films. In recent times the substituted thiophene, poly(3,4-ethylenedioxythiophene) (PEDOT) has received much attention owing to its excellent environment stability. Baytron (PEDOT/PSS), a polythiophene dispersion commercially available from Bayer Corp., is widely used in electrochromic and antistatic device fabrication (157). Advances in substituted thiophene monomer synthesis are now providing a means of polymerizing polythiophenes that are stable as nanoparticle solutions. Edder and Frechet (158) recently synthesized a series of bridged oligothiophenes where their novel molecular architecture is expected to improve their utility as electroactive surfactants for semiconducting nanoparticles or organic electronics.

Device Fabrication For the final device, the functional properties of ICPs must be integrated within a host structure that provides the mechanical/physical properties required. ICPs have been assembled inside a number of host polymers including polyacrylonitrile, polyvinyl alcohol, and others (159). A number of attempts have been made to form composites with improved processability and mechanical properties while maintaining the inherent properties of the conducting polymer (160). Conducting polyaniline blends and composites are prepared mostly via the chemical oxidation route, although electrochemical synthesis is also employed in some cases (161,162). Using the chemical approach in situ polymerization of monomer in the presence of a host polymeric matrix has been reported (163–165). Polyaniline– poly(ethylene terephthalate) (163) and polyaniline–polystyrene (164) composites have been prepared using this approach. Emulsion polymerization in heterogeneous systems (166) has been used to prepare processable conductive composites of polyaniline–poly(alkyl methacrylate). Polyaniline composites have also been prepared via dispersion polymerization (167–169). In such studies colloidal dispersions of electrically conductive polyaniline particles have been prepared using vinyl methyl ether (168) or methyl cellulose (170) stabilizers. ICPs have also been assembled inside hydrogels, retaining both the electronic properties of ICPs and the water adsorption properties of gels (171,172). Conducting polymers have also been prepared as coatings on both natural and synthetic fibers and fabrics. For example, silk and wool (173,174) or nylon (175) have been coated. Electrospinning (176,177) is another method for the production of ICP composite fibers and nanofibers. This simple approach is based on the electrostatic fiber spinning of composite fibers of polyaniline with polyethylene oxide, polystyrene, and polyacrylonitrile. These fibers are formed when a high electric field (5–14 kV) is placed between the tip of a metallic anodic spinning needle loaded with the dissolved polymer solution (0.5–4 wt% PAn and 2–4 wt% host polymer) and an opposing cathode plate separated by 20 cm. The presence of the high electric field results in the surface tension of the polymer-loaded solution

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at the needle tip to be exceeded, expelling a polymer fiber from the surface toward the opposing cathodic plate. The transit time from anode tip to the cathode plate is accompanied by a desolvation and drying process in part assisted by the electrostatic charges placed upon the solvent molecules which undergo electrostatic repulsion processes. The resulting nanofiber composite is reported to have lengths in the meter range collected as an interwoven mesh with large surface to volume ratios (∼103 m2 · g − 1 ). Fiber dimensions of less than 100 nm have been routinely produced by this technique. More recently (176) fibers of PAn have been directly spun from a 20 wt% solution of polyaniline (Versicon, JGB Enterprises, Inc., Liverpool, NY) in 98% sulfuric acid at 5 kV. The ability to provide more processable ICPs, as described above, enables new approaches to device fabrication, including ink-jet printing (178) and screen printing (179). Photolithography has also been used to produce ICP patterns (180–182), while spin coating has been used to produce thin, even films (183). Screen printing is a simple and environmentally friendly way to produce electronic circuitry and make interconnections (184). Bao and Lovinger (185) used screen printing to develop organic transistors with high field-effect mobilities (approx 0.015–0.045 cm2 · V − 1 · s − 1 ) using regioregular poly(3-hexylthiophene). Garnier and co-workers (186) have demonstrated the usefulness of printing in organic FET fabrication, while Gustafsson (187) showed it was possible to print polyaniline to form the gate electrode. While screen printing is a highly efficient and relatively simple way to fabricate intricate electronics and circuits, this technique suffers from low resolution; in most instances, screen printing is used to pattern the parts of the circuits that do not demand high resolution (188). Because of the ability to print, typically at higher resolution, ink-jet printing has emerged as an attractive patterning technique for conjugated polymers in areas such as LEDs (189). Patterning of organic molecules by ink-jet printing can be achieved either by printing a solution of the molecule (in organic solvent) or by printing dispersions of the molecule in water or organic solvents. Magdassi and Ben Moshe (190) have demonstrated the concept of direct patterning of waterinsoluble organic molecules in the form of nanoparticles contained in a microemulsion droplet, which are converted into organic nanoparticles upon impact with the substrate surface due to evaporation of the volatile solvent. Rogers and co-workers used microcontact printing techniques to fabricate various electronic components, such as transistors (191) and logic gates (192) from conducting polymer dispersions and conducting polymer composites. While these printing techniques mentioned above offer higher resolution than conventional screen printing techniques, these still suffer from resolution limitations for some circuit designs. Sirringhaus and co-workers (193) developed a method to print high-resolution all-polymer transistor circuits utilizing hydrophobic surface patterning which confines the spreading of water-based conducting polymer ink droplets upon ink-jet printing.

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G. G. WALLACE P. C. INNIS S. E. MOULTON University of Wollongong