This paper has been submitted at Nov 4th, 1996, for publication and has finally
appeared as part of a broader article in "Handbook of nanostructured Materials and
Nanotechology", E. Nalwa, ed., Academic press 1999, Vol. 5.
Dr. Bernhard Wessling
Zipperling Kessler / Ormecon Chemie
D-22949 Ammersbek
A Contribution to a Fundamental Approach in this Debate
Conductive polymers (Organic Metals) have been considered from the beginning of their history in the late 70s as intractable and insoluble. It was an important goal both in basic research as in application oriented materials science to develop techniques by which they can be processed. The use of solvents was one of the options. As early as 1983/84 we happened to create the first clear dispersions of polyacetylene, polypyrrole and polyaniline, with and without conventional polymeric binders being present.[1]
Since 1986, there was no International Conference on Science and Technology of Synthetic Metals in which the question "Solution or Dispersion?" didn't raise exciting discussions. Most of the scientists are supporting the position[2] according to which the clear (colored) mixtures of (organic) solvents with intrinsically conductive polymers (ICP) are considered as "solutions". Only a few are supporting our position[1b] which asks for calling them "dispersions". Actually the debate concerning polyaniline/solvent systems has attracted growing interest.
The main problem is, however, that most of the colleges still do not seem to care about the differences. Often, a discussion is ended with the statement: "Does it matter at all to distinguish between `solutions' and `dispersions', they look the same, and they behave the same, why do we care?" L. Shacklette wrote[3]: "This level of testing does not distinguish true solutions from those which are simply fine particle dispersions. In either case, however, it is necessary for the `solvent' to possess solubility characteristics (parameters) which are a close match to those of the polymer."
This statement is summarizing the deep misunderstanding that we find in the field towards the fundamentally differing interactions between solvents and dispersed particles or solvated molecules, resp., and the basic difference in properties of dispersions and solutions.
In this paper, I am trying to review the existing knowledge about solvent-ICP systems and to comment on it from the theoretical standpoint of polymer solution as well from colloidal science standpoint of view. Thermodynamical aspects will be treated more extensively as they are still not enough considered in the ICP field. And we have to correct a basically wrong judgement: colloidal systems are often disregarded as "simple" (see the a.m. citation [3]), as if "dispersions" were a lower or simpler class of systems in relation to solutions ("you have only a simple dispersion, but this here is a true solution ...").
It is obvious, that at least this attitude is inappropriate for a scientific approach. And it should not raise too many emotions when one would finally find out that the systems are in fact real solutions, or in fact they are real dispersions. Therefore, I will try - although my position is known - to discuss this question scientifically without unreasonable prejudices, as a heuristic contribution to better understanding and later optimization of such systems.
The goal of these considerations would be to answer the question: "Is it more senseful to assume that such systems are solutions, or that they are dispersions? Which one of these assumptions is explaining more (or all) of the empirically known facts, and which one would be more powerful to predict results of new experiments?"