University of Groningen

Thin transparent conducting films based on core-shell latexes Huijs, Franciscus Maria

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1 Introduction

1.1 Intrinsically Conducting Polymers

Since the discovery of conductivity in polymers in 1961 by Hatano and coworkers [1], the so-called Intrinsically Conducting Polymers (ICPs) have been investigated extensively. Hatano et al. discovered that a polyacetylene sample had a conductivity of the order of 10–5 S/cm. A very important result was obtained in 1977, when Shirakawa, MacDiarmid, Heeger and coworkers reported [2] that the conductivity of freestanding polyacetylene films increased up to 12 orders of magnitude upon exposure to halogen vapors. This can be attributed to the formation of free charge carriers in the polymer chain upon reduction or oxidation of the ICP and this process is called doping. The negatively charged ions that are incorporated in order to compensate for the positive charges in the polymer chain are referred to as counter anions or dopants. From the first moment it was realized that the applicability of polyacetylene is very limited because of its processing difficulty and the rapid decrease in conductivity upon exposure to air. Therefore other ICPs that are more environmentally stable and that can be polymerized in an (electro)chemical synthesis have been developed. Thus new ICPs were synthesized, such as polypyrrole (PPy) [3], polyaniline (PANI) [4], polythiophene [5], and polyphenylene vinylene [6] (Figure 1.1).

N N S n n n n H n H a b c d e Figure 1.1 Intrinsically Conducting Polymers: (a) polyacetylene, (b) polypyrrole, (c) polythiophene, (d) polyphenylene vinylene, and (e) polyaniline.

ICPs are conducting because the electrons are delocalized in the conjugated structure. This in turn brings insolubility and infusibility to the polymers,

1 Chapter 1 resulting in a poor processability. A number of general techniques have been developed in order to improve the processability of ICPs. These techniques include block copolymerization [7], increase of chain flexibility via incorporation of flexible centers in the main chain [8], side chain substitution [9], use of processable precursor polymers [10], and the formation of polymer blends [11]. For some applications, transparency of the conducting films is required. Producing such films from ICPs is not only complicated because of the difficult processability, but also because ICPs are usually heavily colored due to the conjugated system. Therefore, an extremely thin film or extremely strong dilution with non-colored polymers is needed. Our approach to obtaining such a strong dilution is based on the chemical polymerization of a very thin ICP shell around latex particles. Upon film formation of such a core–shell latex, the ICP shells touch and a conducting path is formed through the film. This chapter will give a short overview of the different routes described in literature to obtain transparent intrinsically conducting films, followed by the outline of the thesis. For the sake of clarity, first a short overview will be given of the mechanism of conductivity in ICPs and the methods used to describe the conductivity of ICPs.

1.2 Charge carriers in ICPs

Intrinsically conducting polymers can be prepared via chemical or electrochemical polymerization. In this reaction a conjugated monomer is polymerized and charge carriers are generated via doping. The doping process is an oxidation or reduction reaction in which electrons are transferred away from or to the polymer chain, respectively. The mechanism of charge transport in ICPs can be described as follows [12]. The general structure of ICPs is an alternating sequence of single and double bonds. In the prototype conjugated polymer, polyacetylene (Figure 1.1), there is no preferred sense of bond alternation. Most ICPs, however, possess a nondegenerate ground state with a preferred sense of bond alternation. Polypyrrole and polythiophene are two examples of nondegenerate ground state polymers that possess an aromatic configuration with long bonds between the rings and an aromatic structure within the ring. The other sense of bond alternation, the so-called quinoid configuration is characterized by shortened bonds between the rings and quinoid rings

2 Introduction

(Figure 1.2). The quinoid geometry can be considered as an excited state configuration of the aromatic structure.

H H N N N N H H aromatic structure

H H N N N N H H quinoid structure

Figure 1.2 Two senses of bond alternation for polypyrrole: aromatic and quinoid structure.

The degeneracy of the ground state has an important effect on the nature of the charged species that can be obtained via oxidative or reductive doping. Here, the oxidative doping will be considered as an example. Reductive doping can be described in a similar way. Oxidation of polyacetylene generates a cation radical. Because there is no preferred sense of bond alternation, the positive charge and the unpaired electron can move independently along the polymer chain, forming domain walls between the two identical parts of bond alternation. In solid state physics a charge associated with a boundary or domain wall is called a soliton, because it has the properties of a solitary wave which can move without deformation and dissipation [13]. In this view the unpaired electron can be considered as a neutral soliton, or an excitation of the system that separates two potential wells of the same energy. It is important to note that neutral and charged solitons are not localized on one carbon atom, but spread over several atoms. The bond alternation changes gradually, giving the soliton a finite width. Oxidation of a nondegenerate ground state polymer such as polypyrrole has a somewhat different result. The positive charge and the unpaired electron of the initially formed cation radical cannot move independently. The

3 Chapter 1 structural motif of the chain segment between the positive charge and the unpaired electron is that of a quinoid configuration, which is higher in energy and confines the charge and spin density to a single self-localized structural deformation that is mobile along the chain (Figure 1.3). In condensed-matter physics such a cation radical with an associated lattice deformation is called a polaron and carries a spin (S = ½).

H H H N N N N N N H H H

Electron acceptor

polaron H H H N + N . N N N N H H H

Electron acceptor

H H H N + N N N N + N H H H bipolaron

Figure 1.3 p-Type doping of polypyrrole introducing a polaron and a bipolaron on the p-conjugated backbone.

4 Introduction

Upon further oxidation of a nondegenerate polymer chain two things can happen. A second electron can be removed from a different segment of the polymer chain creating a new polaron, or the unpaired electron of the previously formed polaron is removed. The latter produces a spinless dication confined to a single lattice deformation on the chain, which solid- state physicists call a bipolaron. Bipolarons can also originate from an attractive interaction between two lattice deformations of two polarons in which their unpaired electrons form a bond on a doubly oxidized polymer chain (Figure 1.3). The latter also results in a single lattice deformation. Conduction by polarons or bipolarons is now generally considered to be the dominant mechanism of intrachain transport. Of course, interchain mechanisms such as hopping are necessary to explain the conductive behavior of bulk materials. Although several theories have been proposed to explain the hopping mechanism, the hopping mechanism of charge carriers between polymer chains is not well understood yet.

1.3 Measurement of conductivity

In literature a variety of units is used to describe the efficiency of charge transport in an ICP sample. In principle, there are two different quantities that can describe this efficiency. One can either talk about the conductivity or the resistivity of an ICP. The choice between these two is arbitrary because they are linked by definition, the conductivity is defined as the reciprocal resistivity. The unit of conductance, the reciprocal Ohm (O–1), is usually called Siemens (S). When referring to the resistivity of an ICP, often the volume resistivity or intrinsic resistivity ?v is used. The S.I. unit of the intrinsic resistivity is O m, but in literature this resistivity is usually given in O cm. The intrinsic resistivity is defined as the resistance between opposite faces of a unit cube. To characterize the current flow over a surface, the surface resistivity ?s is often used. The surface resistivity is often used to characterize current flow over a film surface and is defined as the resistance between opposite edges of an unit square. The surface resistivity is independent of the size of the square and its unit is simply the Ohm (O), but for the sake of clarity usually Ohm per square (O/?) is listed. The relation between the surface resistivity and the intrinsic resistivity is given in equation 1.1, where d is the layer thickness.

5 Chapter 1

r r = v (1.1) s d

Thus the surface resistivity is not a material property, but only a property of a specific specimen. Therefore it is necessary to list the value of the surface resistivity always combined with the thickness of the film or a property that is related to the film thickness, like the transparency. The main problem in accurate measurement of a low resistivity is one of contact resistance between the measurement electrodes and the specimen. A method to deal with contact resistance is to use a so-called 4-point-probe method. In case the contact resistance is of the same order of magnitude as the input resistance of the voltmeter, contact resistance may be reduced by depositing the metal electrodes directly onto the surface either by vacuum deposition or by using a conducting (silver) paint. An illustration of the 4- point-probe method is given in Figure 1.4.

I

?V

x

Figure 1.4 Circuit diagram for a 4-point-probe resistivity measurement [14].

The intrinsic resistivity of the specimen can be calculated using equation 1.2.

I A r = (1.2) v DV x

6 Introduction where I is the current (A), A is the cross-sectional area (m2), ?V is the potential drop across the two inner electrodes, and x is the separation between the two inner electrodes. For practical, reproducible measurements of the intrinsic resistivity, special setups have been developed for the 4-point-probe method. One of the most practical setups is shown in Figure 1.5. ?V I I

d

x1 x2 x3

Figure 1.5 Schematic presentation of an intrinsic resistivity measurement with a 4-point-probe.

Provided that (i) the thickness d of the specimen is much smaller than the distances x1,2,3 between the electrodes, (ii) the electrodes have an equispaced distance (x1 = x2 = x3), (iii) the specimen is placed on a non-conducting surface, (iv) the contact-diameter of the electrodes is small compared to the distance between the electrodes, and (v) the distance between the electrodes and the specimen-boundary is large compared to the distance between the electrodes, the intrinsic conductivity of the material can be calculated using equation 1.3 [15].

DV ´p ´ d r = (1.3) v I ´ln 2

7 Chapter 1

1.4 Transparent intrinsically conducting films

In recent years, there has been a lot of research interest in the production of transparent conducting polymer films. Depending on the level of transparency and conductivity, such films can be useful as, e.g. antistatic films, electromagnetic shielding layers or transparent conducting electrodes in liquid crystalline displays or polymer light emitting diodes (LEDs). For antistatic applications, the surface resistivity of the film should be between 105 and 109 O/?. The desired transparency depends on the application. In the photographic industry a transparency of 98% is needed, whereas for conducting floors the main requirement is that flours in different colors can be made. In the latter case not the transparency is the main item, but the possibility to obtain different colors by blending etc without losing the antistatic properties. Nowadays, ionically conducting materials, such as quaternary ammonium-salts like tetraethylammonium- perfluoroctylsulfonate [16], and conductive fillers, such as carbon black [17], are often used for such applications. However, these materials have the disadvantage of, respectively, a conductivity dependency on the relative humidity and the strong (black) color of the material. For applications that require transparency with a higher conductivity, inorganic indium tin oxide (ITO) is the most frequently used material. ITO combines a high transparency for visible light (90%) with a low electrical resistivity (20 O/?). The disadvantage of this material, however, is that it is quite brittle and difficult to process [18]. In general, one of the important properties of polymers is their relatively good processability and therefore ICPs may be interesting to substitute ITO. Unfortunately, due to their conjugated nature, ICPs are not as easily processable as most polymers and they are usually strongly colored. Therefore, special approaches have been developed to make the ICPs processable and to obtain transparent conducting materials and films. In general, two different approaches to obtain transparent intrinsically conducting films can be distinguished. First, it is possible to try to modify the polymer, thus influencing the intrinsic transparency of the material. Second, one can try to make extreme dilutions of the ICP with a transparent polymer. The challenge in this last approach is to make these dilutions without losing any of the conducting properties of the original ICP.

8 Introduction

1.4.1 Pure ICP systems

In the first approach, the transmittance of light through the ICP is maximized. The transmittance of a material, defined as the ratio between the light intensity through the medium and the intensity before the medium, is determined by reflection, absorption, and scattering [19]. If absorption and scattering are negligible with respect to reflection, the material is called transparent. In an opaque material, the practical transmittance is very low because of a high scattering power [19]. The reflection of ICPs depends on the wavelength of the light. For metallic ICPs, the plasma frequency is at approximately 1 eV (1237 nm) [20, 21]. Below the plasma frequency (higher wavelength) the reflectance is high, above this frequency the reflectance is low. Therefore, ICPs are semitransparent in the visible part of the spectrum, but exhibit high reflection (and thus look “shiny”) in the infrared. Light scattering takes place if regions with different refractive indices are present in the material. The amount of scattered light depends on the size of the regions and the wavelength of the scattering light. The wavelength at which the absorption of an ICP is maximum, depends on the bandgap of the polymer. The bandgap is the difference in energy between the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e. the energy required to transfer an electron from the top of the valence band to the bottom of the conduction band. The relation between the energy bandgap (E) and the wavelength (?) is given in equation 1.4, where c is the speed of light (3.0 ´ 108 m/s), 1 eV equals 1.6 ´ 10–19 J and h is the Planck constant (6.6 ´ 10–34 Js).

h *c E = (1.4) l

A lot of research has been done on tuning the width of the bandgap. For most of ICPs, the p-p* absorption of the neutral polymer is in the order of 2-3 eV [22]. Upon increasing the conductivity by doping, the p-p* absorption band decreases in favor of two new electronic transitions located at lower energy levels [23]. These transitions result in an absorption in the visible and near IR region and thus in coloration of the polymer. At higher doping levels, the ICP usually starts to deteriorate. There are, however, examples of ICPs that have an intrinsically low bandgap. Well-known examples are alkoxy-substituted poly(thienylene vinylenes) [18, 24, 25] and alkoxy-substituted polythiophenes [26], and fused ring-systems like

9 Chapter 1 polyisothionaphtalene [27, 28] and poly(dithieno[3,4-b:3’,4’-d]thiophene) [29]. These materials indeed become highly transparent upon doping. Disadvantages of these materials, however, are the rather low conductivity and stability. Polyaniline (PANI) and poly(ethylene dioxythiophene) (PEDT) have a much higher conductivity and stability, combined with a low absorption. It is postulated that this is due to the fact that the doping process in these materials is different, involving no shifts in absorption peaks and leading to low-lying near IR absorption bands at less than 0.6 eV [18]. The low energy of this near IR band leads to a low absorption in the visible region. The high conductivity is attributed to the more regular structure of the doped polymers. A further advantage of both PANI and PEDT is that these ICPs can be made processable. Doped PANI can be made soluble in common organic solvents by a proper choice of the dopant-solvent combination [30, 31, 32], thus enabling the formation of transparent conducting films by simple methods such as spincoating and dipcoating. Films with a transparency over 95% and a resistance of 6400 O/? have been reported [31]. An 0.5 wt% aqueous solution of PEDT doped with poly(styrene sulfonate) (PSS) has been developed by Bayer AG [33]. From this solution, antistatic films with an optical density less than 0.01 can be deposited on polymeric substrates [34, 35]. The stability of the conductivity in PEDT is very good, much better than the stability of other polythiophenes or polypyrrole [36, 37], but the reactivity of the monomer is lower than that of pyrrole [36]. Other kinds of doped polyalkoxythiophenes soluble in common organic solvents such as acetonitril, tertrahydrofuran (THF) and toluene have been reported, but the conductivity of these ICPs is rather low [38]. Interesting, however, is that the wavelength at maximum absorption for the doped polyalkoxythiophenes depends on the length of the alkoxy chain. A longer chain results in a shift to the infra-red region (longer wavelength and broadening of the absorption peak). It has also been reported that the bandgap of polythiophenes can be lowered by attaching electron donating substituents to the thiophene ring [39]. Steric interaction of the side groups of adjacent thiophene rings, on the other hand, force the conjugated backbone to twist and increase the bandgap [39]. Recently it has been shown that several substituted p-conjugated polymers become highly transparent if they are extremely highly doped (>50%) [12, 40]. Unfortunately, the stability of such highly doped ICPs is low.

1.4.2 Composite systems

10 Introduction

A different, more macroscopic, approach for the preparation of transparent conducting ICP films is to minimize the absorbance of visible light by minimizing the amount of ICP in the film. In order to achieve this, blends of ICPs with other, non-conducting but processable and transparent, polymers and composite materials of ICPs and other polymers have been developed. Preparation methods include mechanical mixing, casting of a solution containing the components of the blend or polymerization of one polymer into or onto the other. The latter method can be achieved either chemically or electrochemically, producing blends or interpenetrating networks. A very important property of composite systems is the percolation threshold. In order to transfer charges, a continuous connection of the ICP has to be present in the material. If the fraction of ICP is beneath a certain value, the connecting path is interrupted and the conducting properties are lost. The minimum amount of ICP necessary to maintain conductivity in the composite system, is usually called the percolation threshold. One of the first attempts to obtain composite systems of ICPs and other polymers was by Galvin and Wnek [41]. They impregnated low density polyethylene (LDPE) films with a Ziegler-Natta catalyst and subsequently exposed this film to acetylene gas at 100–110 °C. Conductivities of ca. 10 S/cm were measured after doping the flexible film with iodine. This method has the advantage that the host polymer matrix is hardly changed during the preparation of the conducting composite and therefore preserves its desired mechanical properties. A further advantage was that the decay of the conductivity of the composite film was much less than that of pure polyacetylene, indicating that a further effect of the LDPE is to protect the overoxidation of the ICP. The method was soon improved, allowing the preparation of more stable conducting composites like PPy/polystyrene using chemical polymerization [11]. Composite films with about 10 wt% PPy were prepared having a conductivity of about 50 S/cm and mechanical properties similar to pure polystyrene. Since strong dilution of the ICP was possible without losing much of the conductivity, this method was very promising for the production of transparent, conducting ICP films. Such transparent films were reported to be formed when a poly(vinyl alcohol) film containing ferric chloride with PET as a substrate was exposed to pyrrole vapor [42]. By sorbing the pyrrole monomer in a PET film, followed by chemical polymerization using FeCl3, films possessing high transmittance (70–85%) and conductivity (0.1 S/cm) could be produced [43]. Using a similar procedure, it is possible to produce transparent, conducting polymer films with other ICPs like PANI [44]. In a slightly different procedure, various non-conducting substrates have been made 11 Chapter 1 conducting by placing them in a chemical polymerization of pyrrole or aniline [45, 46]. Because the ICP chains are not soluble in water, they are deposited on the non-conducting substrate, thus forming a thin conducting coating. About 85% transparent films of PPy/acrylic with a resistance of about 500 kOhm were produced [45]. The major disadvantage of this method is that the ICP is formed in its final form and that no further processing is possible. Therefore efforts have been made to develop composite systems from which the conducting film can be formed subsequently. Cooper et al. [47] have prepared mixtures of a poly(methyl methacrylate)/poly(butyl acrylate) (PMMA/PBA) latex with preformed PANI and PPy particles. From these water-based systems, films could be prepared with a conductivity of about 10–2 S/cm with only about 5 wt% PANI or about 20 wt% PPy. These percolation thresholds have been confirmed in later studies with other solid matrix polymers [48, 49]. The difference in the percolation threshold is attributed to the difference in shape of the ICP particles; the PANI particles are needle shaped and the PPy particles have a more globular appearance. Cooper et al. reported that the films essentially retain the mechanically properties of PMMA/PBA films up to ICP particle concentrations of ~20 wt%. At higher concentrations the films become brittle and easily torn [47]. Very low percolation thresholds have been obtained using preformed ICP particles and solutions of matrix polymers [50, 51]. Using very small-sized PANI particles (± 20 nm), composite films with poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) have been prepared with a calculated percolation threshold of 4 ´ 10-4 volume fraction of PANI [50]. From these systems, 50% transparent freestanding films with a conductivity of about 10–2 S/cm have been prepared with only 2½ wt% PANI [51]. Zipperling has commercialized a PANI/acrylic blend in xylene and toluene under the name Incoblend™. They claim to achieve 80% transparent films with a surface resistivity of 103–105 O/? [52]. A different approach in obtaining conducting composite systems with low percolation threshold involves mixing solutions of an ICP and of a matrix polymer. Upon film formation of such a mixed solution, phase separation should take place leading to a conducting ICP path through an insulating matrix. Soon after the development of the soluble PANI with the camphorsulfonic acid (CSA) counter anion by Cao et al. [30, 53], such solution processable blends of PANI with different host polymers have been reported. Solution cast PANI/CSA/PMMA films from meta-cresol having a surface resistance of about 400 O/? and a transparency over 80% in the visible range have been prepared [54]. Electrical conductivities of the order 12 Introduction of 10 S/cm were obtained in blends containing only about 5 wt% PANI. Similar results could be obtained with blends of PANI in a variety of other amorphous polymers; for example polystyrene, polyvinylacetate, amorphous nylon etc [54]. Such transparent, conducting blends can be used for instance as electrodes in polymer light emitting diodes [55] or in polymer transistors [56]. It was found that PANI forms fibrillar networks in the polymeric matrix, resulting in the low percolation threshold in these materials [57]. Comparable results have been obtained with other counter anions [58] and in different solvents, like xylene [31] and hexafluoro-2- propanol [59]. A disadvantage of these solution-processable blends is the use of organic solvents. Because of safety and environmental reasons, there is a demand for water-based systems. The aqueous solution of PEDT/PSS has the disadvantage that it is an extremely diluted solution (0.5 wt% ICP). Evaporation of the water is energy consuming and it limits the practical applicability for other applications than antistatic foils. As stated above, water-based systems of dispersions have been developed, but the percolation threshold was rather high [47]. A decrease of the percolation threshold might be obtained if core–shell latex particles with a conductive shell are used instead of pure ICP particles. Such core–shell particles may be formed if the polymerization of pyrrole is carried out in the presence of conventional latex particles, as was shown by Yassar et al [60]. DSM uses a somewhat similar approach and has commercialized a product-line under the name ConQuest® [61].

1.5 Outline of the thesis

In our research, we have developed a method for the preparation of transparent conducting polymer films based on core–shell latexes with an ICP shell. The advantages of our system is that it (i) is water-based, (ii) has a low percolation threshold of the ICP (~0.25 wt%), (iii) is easily processable, and (iv) reasonable solid contents of the dispersion can be used. Furthermore, all chemicals that are required for the preparation of the conducting core–shell latexes are commercially available, not very toxic, and rather cheap. In chapter 2 the preparation of the conducting core–shell latexes is described. Chemical polymerization of an ICP in the presence of a latex results in the formation of an ICP shell around the latex particles. The optimum in the ICP/binder ratio, as well as the optimum reaction 13 Chapter 1 circumstances are discussed. It is shown that very thin and smooth shells of the ICP can be synthesized around the core latex and that the conductivity of these conducting core–shell latexes is of the same order of magnitude as the conductivity of the pure ICP. For obtaining homogeneous films, the stability of the latex is very important. We have developed several stabilization methods and the pros and cons of these methods are also described in this chapter.

In the subsequent chapters the film formation of the conducting core–shell latexes is discussed. In order to obtain transparent, mechanically rigid films, the latex particles have to deform considerably and this deformation is hindered by the presence of the ICP shell. The thickness of the ICP shell is a major factor controlling the rate of film formation, as is shown in chapter 3. Even though the shell thickness is extremely small compared to the diameter of the core latex particles (some nanometers compared to about 700 nm), the deformation of the particles is hindered considerably. The film formation process is investigated using transparency measurements, atomic force microscopy, and transmission electron microscopy. With these techniques the deformation of the particles in the body of the film as well as at the film surface can be monitored. In chapter 4 the influence of the annealing temperature on the latex film formation is discussed. It is shown that, even with a fraction of ICP of only 1 wt%, the annealing temperature has to be far above the Tg of the core polymer in order to achieve film formation within reasonable time scales. The fusion of the conducting latex particles during film formation is studied using the Non-Radiative Energy Transfer technique (NRET). In the NRET technique the interdiffusion of polymer chains across the latex particle boundaries is studied. The interdiffusion of chains from adjacent particles is important for the mechanical properties of the latex film. From an application-oriented point of view, film formation at elevated temperatures is not favorable since it can destroy the ICP, it is energy demanding and it limits the freedom of choice in substrates. Therefore conducting latex systems that are film forming at room temperature have been developed. In paint industry, coalescing aids are frequently used to facilitate the latex film formation process. Coalescing aids are organic solvents that lower the Tg of the latex binder polymer. After formation of the film, coalescing aids evaporate and the film regains the, high, Tg of the binder polymer. We have employed coalescing aids to achieve film formation of our conducting core–shell latex particles and the results are shown in chapter 5. 14 Introduction

A different approach to obtain transparent conducting polymer films at room temperature is the use of core polymers with a glass transition temperature below room temperature. Latex particles having a poly(butyl methacrylate) core, a low-Tg acrylic inner shell, and a thin polypyrrole outer shell have been prepared and the influence of the nature and Tg of the inner acrylic polymer is investigated. Also blends of conducting core–shell latexes and non-conducting, low-Tg, latexes have been prepared and the optimum ratio between the conducting and non-conducting latexes is determined. The results of these investigations are also given in chapter 5.

The resistance of the transparent latex films and the influence of variables like temperature and nature of the counter anions is described in chapter 6. It is shown that films with a resistance below 1 MO/? (so well in the antistatic region) can be prepared using conducting core–shell latexes. The film resistance is quite stable, even at 120 °C in air the film remains antistatic for about 16 hours. The stability of the film resistance can be further enhanced by the incorporation of more stable counter anions than the chloride that is incorporated during the synthesis of the conducting shell. The optimum in the nature and the amount of the counter anions, as well as the optimal procedure to incorporate the counter anions is discussed in chapter 6.

1.6 References

1 Hatano M., Kambara S., Okamoto S., J. Polym. Sci., 51 (1961) S26 2 Shirakawa H., Louis E.J., MacDiarmid A.G., Chiang C.K., Heeger A.J., J. Chem. Soc. Chem. Commun., (1977) 578; Chiang C.K., Fincher C.R., Park Y.W., Heeger A.J., Shirakawa H., Louis E.J., Gau S.C., MacDiarmid A.G., Phys. Rev. Lett., 39 (1977) 1098; Chiang C.K., Druy M.A., Gau S.C., Heeger A.J., Louis E.J., MacDiarmid A.G., Park Y.W., Shirakawa H., J. Am. Chem. Soc., 100 (1978) 1013 3 Diaz A.F., Kanazawa K.K., Gardini G.P., J. Chem. Soc. Chem. Commun., (1979) 635 4 Diaz A.F., Logan J.A., J. Electroanal. Chem., 111 (1980) 111 5 Tourillon G., Garnier F., J. Electroanal. Chem., 135 (1982) 173 6 Capistran J.D., Gagnon D.R., Antoun S., Lenz R.W., Karasz F.E., Polym. Prepr., 25 (1984) 282 7 Aldissi M., Hou M., Farrell J., Synth. Met., 17 (1987) 229 8 Aldissi M., Polym. Plast. Technol. Eng., 26 (1987) 45

15 Chapter 1

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