Semiconducting Polymers - American Chemical Society

Chapter 12

Polyanilines: Progress in Processing and Applications Vaman G. Kulkarni

Downloaded by PRINCETON UNIV on September 26, 2013 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch012

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Despite its ease o f synthesis, facile chemistry and novel doping by simple treatment with protonic acids, industrial, real-world applications o f polyaniline and other conducting polymers are limited by processability and electrical decay during formulation and use. Over the last several years significant advances have been made i n processing o f Intrinsically Conductive Polymers (ICP's). I C P coatings and blends have been tested for application in electrostatic discharge ( E S D ) applications and corrosion protection and are ready for commercial applications. This article w i l l discuss progress in processing and applications o f polyaniline coatings and melt processable blends with special emphasis on processing by dispersion techniques.

Background Doped polyaniline (conductive) is a green powdery material with a bulk conductivity o f 1-10 S / c m In this form it is insoluble i n most solvents and infusible, therefore, it has been traditionally categorized as intractable polymer. The neutral or emeraldine base form o f polyaniline is a coppery bronze powder with a conductivity o f less than 10" S/cm. Polyaniline is an unique I C P i n that its conductivity can be reversibly controlled chemically and electrochemically. The emeraldine base is soluble in a wide variety o f amine solvents such as N methylpyrrolidone ( N M P ) , N, N' dimethylacetamide, dimethylformamide and N,N'-dimethylpropylene urea ( D M P U ) . However, even in these solvents the solubility is limited and solutions have limited stability. 10

Polyanilines have been processed from solutions o f neutral polyaniline i n N M P , D M P U and others for some time[l,2]. Significant strides have been made i n making fibers form these solutions. Nonetheless, the technique suffers from the disadvantage that processed articles are non-conductive and need to be doped i n a secondary step. The technique is not suited for preparation o f coatings on a commercial scale. Processability o f polyaniline i n the doped form is more attractive as it removes the subsequent doping step. Functionalized protonic acids such as camphor sulfonic acid, preferably in the presence o f m-cresol and dodecyl

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©1999 American Chemical Society

In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


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benzene sulfonic acid which dope polyaniline and simultaneously render it soluble in common organic solvents [3,4] provide such an alternative. Highly transparent and conductive coatings have been reported by this technique. However, application o f these coatings is limited to very few substrates that are resistant to attack by m-cresol, such as P E T and glass. A further disadvantage may be seen i n the fact that some o f the m-cresol remains i n the film and potential toxicological problems arise both during process and later use. Polyaniline doped with dodecyl benzene sulfonic acid offers processability from xylene and may be better suited for industrial applications. However, the maximum solubility is l o w (0.5% w/w[5]). Dodecyl benzene sulfonic acid has also been shown to plasticize polyaniline and form melt processable complexes[6]. M o r e recently, an emulsion polymerization process for preparation o f an organically soluble polyaniline has been reported [7]. This process uses dinonylnaphthalene sulfonic acid as dopant and the reaction is carried out in 2-butoxyethanol. The polyaniline/2-butoxy ethanol solution is readily soluble i n xylene and other selective organic solvents for further processing. The technique represents a major advancement i n solution processing o f polyaniline, however limitations can be seen i n its solubility i n select organic solvents and environmental friendly formulations. Processing of Polyanilines Using Dispersion Techniques Processing o f polyaniline or other intrinsically conductive polymers (ICPs) by dispersing the doped conductive polymer in a processable non-conductive matrix offers a direct and practical route. Polyaniline is prepared i n the conductive form by oxidative polymerization o f aniline in the presence o f a protonic acid. Conversion to the neutral form for processing requires dedoping o f the conductive polymer using an alkaline solution - a secondary step. While other processing techniques rely on a specific dopant, solvent or a particular form o f polyaniline, dispersion techniques offer the freedom to choose the matrix into which the conductive polymer is dispersed. This key feature provides the ability to tailor performance properties o f the resulting coating or blend. N o other processing technique offers this flexibility to prepare conductive materials based on polyaniline or other ICPs. The technique is suitable for preparing coatings and melt processable blends. A detailed discussion on understanding o f processing o f intrinsically conductive polymers using dispersion techniques has been reported earlier[8,9]. In this technique, the intractable doped polyaniline is dispersed in the solution or melt o f a non-conductive processable matrix. The resulting processable dispersion is ready for use without further modification. K e y advantages o f dispersion techniques for processing polyaniline are: • • • • •

Direct and practical technique Processable in the doped form Independent o f the chemistry Solution and melt processable Tuned conductive coatings



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•

Control over performance properties o f the coating/blend

•

Enhanced environmental stability especially in thin films

C o n d u c t i v e Coatings/Solution Processing. Conductive coatings can be prepared by dispersing doped polyaniline in a solution o f a non-conductive film forming resin i n common organic solvents [10-12]. Coated substrates and films are prepared by applying the dispersion and flashing off the solvent. Coatings can also be prepared by dispersing polyaniline in polymerizable monomers, oligomers and polymer precursors. Such formulations are environmentally friendly, since there is no volatile organic content ( V O C ) and 100% o f the formulation is converted to film. F i l m formation is brought about by heat or U V / E B radiation curing. The powdery conductive polymer, is broken down to primary particles o f the conductive polymer, in the range o f 200 to 300 nm and encased i n the polymer matrix during the dispersion process. Coatings thus prepared exhibit good transparency and a characteristic green color. Due to the excellent dispersion quality, the coatings are often indistinguishable from true solutions by visual observation. The conductivity o f the coating composition depends on processing, concentration o f polyaniline and the percolation behavior o f polyaniline i n the film forming matrix. While polyaniline offers the electrical properties, other properties such as transparency, abrasion resistance and flexibility can be tailored by proper choice o f the film forming matrix. Figure 1 shows typical transmission spectra o f polyaniline coating prepared by dispersion o f polyaniline.

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40 H 250

1 750

1

1

1250 1750 Wavelength (nm)

1 2250

Figure 1. Spectra o f polyaniline coating prepared by dispersion of polyaniline i n polymethyl methacrylate Figure 2 shows a typical percolation curve for polyaniline i n a film forming matrix. The resistivity o f the system remains unchanged until a critical volume fraction o f the polyaniline is reached, at which point there is a sudden very large decrease i n the resistivity o f the system. W i t h further addition o f the polyaniline,


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there is only a marginal change in the resistivity of the system and the resistivity o f the system is saturated. The critical volume fraction o f the conductive filler for the sudden onset o f conductivity is referred to as the percolation threshold. Typically this occurs with a change o f conductivity o f 10' to 10" S/cm within a change o f volume fraction o f conductive phase o f 0.5 to 3 volume percent. B y controlling the composition and the amount o f polyaniline, the electrical properties could be tailored over a range o f 100 to 10 ohms/square. 12

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Semiconducting Polymers - American Chemical Society

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