A Contribution to Solving this Debate
In the previous part it was shown, what a productive role the “dispersion concept” is playing both for basic scientific questions (thermodynamics, self-organization of dissipative structures, mesoscopic metallic conductivity etc.) as for actually emerging commercial applications.
Whereas the dispersion in polymer blend matrix (like coatings, paints, thermoplastic polymers) has been widely accepted and repeated in the meantime, and whereas this field is also theoretically much better understood than 10 or 20 years ago [14, 16], pure solvent / ICP systems (like DMSO/PAni or m-cresol/ PAni) are far less understood.
The use of solvents for processing purposes was one of the options, because they are insoluble and intractable. As early as 1983/84 we happened to create the first clear dispersions of polyacetylene, polypyrrole and polyaniline [1], with and without conventional polymeric binders being present, starting from dry, completely polymerized powders. Other groups have chosen the route to polymerize polypyrrole or polyaniline in colloidal form, either in sterically stabilized micelles [2], or as graft copolymers on latex particles [7, 8].
Many research groups have tried to prepare “solutions”, by changing the “dopants”, by copolymerization, by side chain modification.
From both practical and theoretical aspects it is of crucial importance to differentiate between true solutions and dispersions, and to find out whether solutions are principally feasible or impossible at all. From the standpoint of the “dispersion theory”, it is still a matter of uncertainty, if the non-equilibrium theory [14, 16] can be generalized also for pure ICP / solvent systems as outlined in [44b]. Or, from the standpoint of the “solution paradigm”, it would still be necessary to prove the existence of true solutions and to elaborate an ICP solution theory. It would especially be very interesting - if true solutions really can exist - to recrystallize an ICP (preferably a homopolymer like polyaniline) from solution and to prepare single crystals with significantly enlarged coherence length.
It is not the goal of this paper to discuss by which techniques dispersions of conductive polymers in solvents or water could be prepared. We want to discuss, if there can also exist systems made from solvents and conductive polymers (like polyaniline) which are not dispersions, but “true solutions”. Most of the scientists are supporting such a position [3] which says that clear (colored) mixtures of (organic) solvents with intrinsically conductive polymers (ICP) are "true solutions". It is, in contrast, my position [44b] that also those are to be considered as "dispersions".
It will be shown, that it is necessary to differentiate between “solutions” and “dispersions”, not only because they are fundamentally different systems, but also because the scientific and technical conclusions to be drawn from experiments with them are in opposite directions. This statement is not widely accepted, and “dispersion” and “solution” are often taken as just different words for more or less the same kind of system. An example for this misunderstanding was expressed like this [4]: "This level of testing does not distinguish true solutions from those which are simply fine particle dispersions. In either case, it is necessary for the `solvent' to possess solubility characteristics (parameters) which are a close match to those of the polymer." This conclusion is not correct, as will be shown later.
In this part B [5], some new arguments will be developed based on thermodynamical considerations for the statement, that all (transparently colored) conductive polymer / solvent systems are dispersions, and true solutions of conductive polymers cannot exist.
[1] a) at that time preferably using ultrasound: B. Wessling,
DE 34 22 316 and corresponding international patents (Zipperling Kessler & Co.)
b) Adv. Mater. 5 (4), 300 - 305 (1993)
[2]for a review see M. Aldissi, S. Armes, Progress in Organic Coatings 19, 21 - 58 (1991)
[3] the first reports of "soluble" and
"moldable" conductive polymers were published by 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)
[4]L. Shacklette, C. Han, “Solubility and dispersion characteristics of Polyaniline", Mat. Res. Soc: Symp. Proc. 328, 157 - 166 (1994), a paper in which the authors tried to determine the solubility parameter of polyaniline, emeraldine base and salt (EB, ES)
[5]an extensive and more tutorial version of this part is available in the World Wide Web under http://www.ormecon.de/