Both areas, the batteries and the LEDs, require the ICP (or undoped conjugated polymer, resp.) in a pure unblended form, as the active material. For batteries, the idea was to polymerize directly into the form of the later electrode. No processing research was thought to be necessary. This was the reason why most scientists active in the ICP arena did not realize the importance of basic research devoted to materials science and processing aspects.
With the end of the battery research the need for processing became more evident, as new ideas connected with LEDs emerged [1]. These demanded at least a kind of solvent based processing technique. Based on earlier work by Elsenbaumer et al. [2] a variety of “soluble” conductive polymers and solvents even for “doped” polyaniline have been proposed. This area will be discussed in more detail in part B.
Most other applications or potential applications require a raw material form which can be processed further to the end product form. In the meantime, our very early concepts [3] of preparing polymer blends between ICPs and various insulating polymeric matrices have been followed by several groups, although often using a somewhat different technical approach, however without changing the basic concept: blends are dispersions of the ICP in a matrix polymer, and conductivity above a certain critical volume concentration is possible only due to very complex self-organized structures [11, 14].
The technique most widely used in university laboratories is (or has been) the polymerization directly in the matrix polymer [4]. The disadvantage is that the monomer must be able to diffuse into the matrix, and so has the polymerization agent, too. Furthermore, there is no possibility to purify the resulting product, neither the resulting polymer, nor the blend. Finally, such blends are not processable afterwards without loosing conductivity.
Another option is to polymerize the ICP directly as a latex (in fact, S. Jasne from Polaroid had proposed this route in the mid 80s [5], finally without practical success), recently again proposed by DSM [6], or in a sterically stabilized colloidal form [7]. Both concepts are based on the idea, that an ICP polymer blend should be a dispersed system, but that one could get around the - beyond all question - very complicate dispersion task by starting with already colloidal particles. This is not basically wrong, however, one did not take into account (a) that conductivity in a blend is not only a question of the presence of a dispersed conductive phase, but also of its interfacial structure, (b) that the dispersed phase has to have the capability of self-organizing to flocculates which is only possible with a very specific mechanism [8], first described in [9], cf. also [22], (c) the restrictions that are caused by the colloidal matrix system to begin with for the usability in various polymer blends (most latexes are water born - which might be an advantage if water based blends are the only goal - hence most other water free matrix polymers are not accessible), (d) the problems of recovery of the latex or (even more) the sterically stabilized colloidal system before entering the next process step.
We therefore had decided very early, before the first proposals of this kind became public, not to follow this route, but to strictly polymerize and recover a dry (dispersible) ICP powder, even if the dispersion process itself proved to be the toughest technical and also scientific question of all in this whole context.
A variation of this concept is to disperse (other people would prefer to say: "dissolve") the ICP (PAni) in a solvent, preferably with the help of ultrasound, and to mix this with a dissolved matrix polymer. Such an approach was described in the early dispersion patents by us [5]. Other groups have confirmed the principal feasibility of this approach by either "counter-ion induced processability" [10], or by dispersing the neutral emeraldine base in a suitable solvent like NMP, and mix this dispersion with a solution of the matrix polymer (e.g. PVP, PMMA etc.), leading to blends, which had to be post-doped in order to show conductivity [11]. It should be noted, that in contrast to the opinion expressed by these authors, such blends are never to be considered as "compatible", as the phase size of PAni will be around 50 to 100 nm, not resolvable by light microscope (cf. also [12(b)], where the authors are showing comparable network structures of aggregated submicron particles, as we have shown earlier [22] for blends resulting from "dry" dispersion techniques.
Insofar, such blends do not principally differ from other ICP blends which would be considered as non-equilibrium 2-phase systems [12], with the conductive phase being the dispersed (and above the critical concentration: flocculated to a dissipative structure network) phase. This has also been supported by the principally equal conductivity and transport properties, as can be seen by comparing the results in [12(b)] and [18].
A third variation was proposed by the Finnish company Neste Oy which aimed at preparing a "melt processable polyaniline" [13]. It still remains a matter of debate where the melt behavior they observed resulted from, but it was evident, that the resulting blend again is a 2-phase system with network structures formed by the dispersed PAni phase.
Up to now, none of these alternative approaches (for a review, cf. [17b]) found any practical application, and they do not seem to offer an advantage over the dispersion concept favored by us.
Also only very little work has been devoted outside of our laboratory to the fundamentals of a blend technology, as there are dispersion, interfacial phenomena, conductivity breakthrough at the critical concentration, electron transport phenomena in blends and others. It is not the purpose of this article to review these aspects. Here, it should be sufficient to summarize: the basis of any successful ICP (PAni) blend with an other (insulating and moldable or otherwise processable) polymers is a dispersion of ICP (here: PAni, which is present as the dispersed phase) and a complex dissipative structure formation under non-equilibrium thermodynamical conditions [14].
It might be surprising that dispersion generated polymer blends with PAni are showing a typical metallic behavior [18, 24], some are even exhibiting an increasing conductivity with decreasing temperature (for the first 50 - 70 K below room temperature) [18] in contrast to the raw PAni used, and at any temperature a several orders higher conductivity compared to the raw PAni (measured as pressed plate). But our dispersion theory is able to explain this phenomenon [16a, e.g. p. 566/567].
Many proposals have been made, where ICP, mainly PAni, blends could be used, even some of them of visionary and creative character, like roofs coated with photovoltaic cells, wallpaper with electrical heating capability, heated textiles, dust-filters and many more [15]a. Often the expectation, that such blends would have properties superior to those of carbon black filled ones, in conductivity, in mechanical and in color aspects, guided the vision. Whereby PAni blends can deliver actually somewhat higher conductivity values (up to 50 S/cm as best value for laboratory samples [16], 5 S/cm for technical scale [17]) compared to those of carbon black compounds (best values around 0.5 S/cm [18]), the other presumed advantages are not there: neither are mechanical or processing properties, nor the electrochemical stability under applied voltage and current (like for heating devices), or the color aspects of PAni blend any better than with carbon black compounds.
There is also often a misunderstanding in the ICSM community insofar, as if carbon black compounds were a relatively bad compromise. This is not the case, as many highly performing compounds have been developed and are in commercial use since many years [19].
Both systems, however, the PAni blends and the carbon black compounds, are based on the same structure principle: the conductive phase being the dispersed phase, which suddenly self-organizes into complex networks above the critical concentration [20]. This is the reason for all properties, including mechanical or rheological, and also abrasion. But most of the poorer products have been replaced by products based on subtle and successful developmental work [21]. As a consequence, the market does not ask for replacements of carbon black compounds, which provide more or less comparable properties, and this at a higher price [21].
A new material, like ICPs, and blends based on them, will only find a way to those markets, where either a drastic cost reduction (which is not to be expected) or new useful properties or new useful combinations of properties can be offered. PAni and its blends are new in the following aspects:
Based on these properties specific for PAni, applications are actually on their way to the market or may be realized in further developments. A review by J. Miller [23] is still relatively accurate except for technical and market progress in corrosion protection, transparent coating and printed circuit board production applications (see below and [26, 35]).
[1] other product concepts like electrolytic capacitors, as have been realized with TNCQ salts [1, ref 30], are often approached by polymerizing polypyrrole or polyaniline directly on the substrate; such a mostly unreproducible and “dirty” procedure can be replaced by using solvent born systems (dispersions)
[2] R. L. Elsenbaumer, K. Jen, R. Obodi, Polym. Master. Sci. Eng. 53, 79 (1985)
R. L. Elsenbaumer, K. Jen, R. Obodi, Synthetic Metals 15, 169 (1986)
[3] B. Wessling, US-PS. 4,935,164, EPC-0168620, Patent-No. 85107027.6 (priority 1984), Zipperling Kessler & Co.
[4] cf. as one of the earliest examples, polyacetylene in LDPE: M. Galvin, G. Wnek, J. Polym. Sci., Polym. Chem. Ed. 21 (1983) 2727;
[5] S. Jasne, EP appl. 022992 (priority: Dez. '85, US 811 281), Polaroid Corp.
[6] M. van Doorn, L. Bremer, Proc. ICSM '96, Salt Lake City, to be published
[7] a) A.Björklund, B. Liedberg, J. Chem. Soc, Chem. Commun. (1986) 1293
b) S. Armes, B. Vincent, J. Chem. Soc, Chem. Commun. (1987) 288
[8] the delamination of the adsorbed matrix polymer monolayer and the formation of a joint layer surrounding all flocculating particles in the complex network structure
[9] B. Wessling, UK Patent Application GB 2214511, (1/1989) Zipperling Kessler & Co.
[10] (a) Y. Cao, P. Smith, A. Heeger, Synth. Met. 48, 91 (1992)
(b) M. Reghu, C. Yoon, C. Yang, D. Moses, P. Smith, A. Heeger, Phys. Rev. B 50 (19), 13 931 (1994)
[11] cf. as one of the more recent examples: W. Stockton, M. Rubner, Mat. Res. Soc. Symp. Proc. 328, 257 (1994)
[12] B. Wessling, Synthetic Metals 45 (1991), p. 119-149
B. Wessling, Z. Phys. Chem. 191 (1995), p. 119-135
[13] "Counter-ion induced processability of polyaniline: Conducting melt processable polymer blends" O. Ikkalaa, J. Laakso, K. Väkiparta, H. Ruohonen, H. Järvinen, T. Taka, P. Passiniemi, J. Österholm, Y. Cao, A. Andreatta, P. Smith, A. Heeger; ICSM 1994 (Seoul), Proc. Synth. Met.; it was introduced at the international plastics show "K 95" in Duesseldorf as "Neste Conductive Polymer NCP" (as a trial product, not available in commercial scale) and shortly later withdrawn
[14] for an overview, see [2], for the thermodynamic theory itself [14], for detailed discussions, cf.
(a) B. Wessling in: “Handbook of Organic Conductive Molecules and Polymers”, Vol. 3, ed. By H.S. Nalwa 1997, John Wiley and Sons
(b) B. Wessling in: “Handbook of Conducting Polymers”, 2nd Edition (T. Skotheim, R. L. Elsenbaumer, J. R. Reynolds, eds; to be published by M Dekker
[15] a) E. Geničs, in: “Intrinsically Conductive Polymers - an Emerging Technology”, M. Aldissi (ed.), Kluwer Acad. Publ. 1993, p. 75
b)E. Geničs (a review on processing techniques for conductive polymers) New Journal of Chemistry 15 (5), 373 - 377 (1991)
[16] a) C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, C.-J. Liu, B. Wessling: "Conductivity and Thermopower
of Blends of Polyaniline with Insulating Polymers (PETG and PMMA)", Sol. State Commun., Vol. 97, No. 3
(1996), pp. 235-238
b) A.B. Kaiser, C.-J. Liu, P.W. Gilberd, B. Chapman, N.T. Kemp, B. Wessling, A.C. Partridge, W.T.
Smith, J.S. Shapiro: "Comparison of electronic transport in polyaniline blends, polyaniline and polypyrrole";
lecture at the ICSM '96 in Salt Lake City
[17] Zipperling Kessler, technical data sheet INCOBLEND PVC
[18] Zipperling Kessler, technical data sheet product 400..., now produced and sold by Clariant Masterbatch GmbH for the commercial use as electrode material in the zinc-bromine battery of PowerCell, Austria
[19] cf [16b], section 11
[20] B. Wessling, Polymer Science and Engineering 31 (16) 1991, 1200-1206
[21] Neste Oy offered PP and PE blends with PAni as a replacement for carbon black compounds, in 1995, and stopped the program in 1996
[22] R. Pelster, G. Nimtz, B. Wessling: "Fully protonated polyaniline: Hopping transport on a mesoscopic scale", Physical Review B, 49 (1994) cf. also [7]
[23] J. Miller, Adv. Mater. 5 (8), 587ff and 671ff (1993)
