This paper has been submitted at Nov 4th, 1996, for publication in a
scientific journal and is now subject to the reviewing process.
You may ask for a hardcopy with the complete manuscript, which contains all figures and all
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is invited to comment to this paper. I will consider every argument and
comment back.
This is the 3rd revised version.
Dr. Bernhard Wessling
Zipperling Kessler / Ormecon Chemie
D-22949 Ammersbek
5. What experimental evidence do we have for the "dispersion hypothesis"?
There is no surprise for the reader that I am claiming: there are many experimental hints supporting this hypothesis, especially many more than for the contrary. But I would like to point out again that the purpose of this discussion should be to learn more about conductive polymers and organic metals, about polyaniline and the others. The presentation of those experimental results or observations which had led me to promote the "dispersion hypothesis" should be understood by those preferring the other options to make new and improved experiments to finally find out the "truth".
5.1 Own experimental studies
The following experimental results have been found by us or in cooperation with others:
5.1.1 Particle size (morphological)
We have determined the particle size of PAni in different states, as pure material or in dispersion, and here in thermoplastic polymers, paints (coatings), pure (organic) liquid dispersions and thin coating prepared therefrom[36]. We found
- around 100 nm in thermoplastic polymers, with a tendency to smaller particles with better dispersion, which leads to lower critical volume concentration[37] and higher saturation conductivity
- around 50 nm in paints, connected with even lower critical volume concentration
- a fine structure in TEM pictures of such systems, pointing to smaller primary particles as the building units of the particles being found in polymeric dispersing media[38]
- particles with about 10 nm size in pure (organic) solvent dispersions (by membrane filtration and photon correlation spectroscopy), see fig 7
- about 100 nm (secondary) particles in pure layers of PAni deposited from such dispersions, see fig 8, consisting of
- about 10 nm primary particles, a fine structure which we resolved by STM, see fig 9.
It is most important for our debate, that the particles' diameter is not at all different from the state in solvent systems (liquid dispersions) as it can be found in solid state / solid dispersions, where the 10 nm particles are the building blocks of the secondary particles.
May be, the reader would agree, that our results from photon correlation spectroscopy are showing the correct particle size range: how can such particles consist of 90% solvents (or even more) as they would have to if they were solvated (cf. section 2. 1. 1.). And how could we understand, that we find (using microwave absorption of polyaniline blends) a primary particle size of 9.6 nm (with a metallic core of 8 nm), which is the same size as we find in the solvent system? How else should we conclude rather than assuming, that the particle (size) has not (principally) been changed by the interaction with the solvent.
We call this a dispersion process, the particles have just been wetted by it, and the result is a "dispersion", a sol.
5. 1. 2 Particle size (electronically)
During our research together with Nimtz and Pelster [7] we found that the metallic core is a (spherical) body of 8 nm diameter, surrounded by an (amorphous) non-metallic shell which is less conductive, of about 0.8 nm thickness, so that an overall diameter of 9.6 nm results. This is in surprisingly good accordance with all conclusions we had drawn earlier from various particle size determinations.
From 5. 1. 1. and 5. 1. 2. it should be convincing that we are obviously dealing with a material consisting of 10 nm particles as primary building unit, which cannot be altered principally by actions of solvents or other dispersing media, which do not change shape and size during interaction with solvents, and which are also the basic unit for the metallic (although quantum-size limited) conductivity.
5. 1. 3 Viscosity and gelation
We had determined the viscosity of pure PAni dispersions in an organic solvent system and had found a good correlation with a relation valid for dispersions, see fig 10.1. However, one could also have concluded that deviations are not far from a behaviour of true solutions, even though we showed that these systems had 10 nm particles as dispersed phase. Additionally, we had reported a sudden increase of viscosity to infinity at a certain critical point, when our dispersion exceeded a certain concentration (at 2.4 %, in other experiments around 0.5 %).
Other authors also observed this behaviour, but often at much higher concentrations (which we assign to a greater particle size), which they interpreted as "cross-linking".
But why should PAni cross-links suddenly (within seconds!) just above 0.5 %, whereby we can store a 0.5 % or less concentrated dispersion "forever"? Gelation of dispersion is the result of a process of structure formation, induced by long-range interparticle attraction forces. The particles are forming long chains (filaments) of aggregating particles (instead of big particles precipitating) which penetrate the whole solvent volume and - as soon as it form a complete network - the system cannot flow any more.
