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
greek symbols.
Everybody
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
Conductive Polymer / Solvent Systems: Solutions or Dispersions?
2. Thermodynamical considerations
2.1 Solutions
2. 1. 1 Basic solution thermodynamics
There is another major difference between solutions and dispersions, their
thermodynamical status. Solutions are systems which have found a point of
thermodynamic equilibrium lower than the energy of the pure components, and can
be described with equilibrium thermo-dynamics[5]. Many solution processes are
exothermic (
H
< 0), all of them are exergon:
The driving force for solvation is the increase of entropy. If H is
positive, but its value smaller than - TS, then the polymer will be soluble. If
H is > 0 and > |- TS|, the polymer is insoluble.
For real dissolution (and not e.g. partial "swelling") every monomer unit must be completely solvated (= surrounded by solvent molecules). This means, that the macromolecule takes up as many solvent molecules as is necessary for solvation. Conventional and "easily" soluble polymers take up a volume of solvents by a factor of 10 (low molecular weight polymers) to around 1000 (high Mw) of their own volume!
In other words: a macromolecule coil in solution consists by 90 to 99.9 % of solvent molecules! This is the reason, why coil diameter of well over 100 nm can be found in polymer solutions.
From here (coil volume and mass of macromolecule) one can derive the critical concentration above which the whole solvent is incorporated into macromolecules. It can be understood from looking at these values, why polymer solutions of 5% (low molecular weight) and down to 0.1% (high MW) are just saturated. Concentration values in the range of 10, 20% or even higher cannot be achieved with polymers in solution.
Such systems (true polymer solutions with a molecule size of 10 to > 100 nm and solvent incorporated) would be called "molecular colloids", as the colloidal particle is identical with one (completely solvated!) molecule. In this paper, I will not call such systems "colloidal", but "solutions" in order to differentiate from those systems in which the colloidal particle (independent of whether it is made up from one single or from many molecules) does not contain solvents at all or not in the amount for complete solvation of all monomer units).
For understanding and discussing Shacklette's contribution [3] to the topic
of this article we have to go back to define, what the "solubility parameter"
means. For transferring a liquid into the gas phase we need to invest half of
the interaction energy 
between the molecules per molecule, or NL times per
Mol. This energy (the negative inner molar vaporization enthalpy)
defined per molar volume is just the so-called "cohesion energy density
e":
the "solubility parameter 
" is defined as the square root of this cohesion
energy density:
When you are now going to dissolve a chemical species A, you have to disrupt an interaction between the species molecules A and A', A'' and so forth, and to replace it by an interaction between A and the solvent molecules. For each disrupted A - A' interaction a new interaction A - S will be generated.
In case of polymers, all intramolecular interactions (hence those between the monomer units) and all intermolecular interactions have to be replaced by polymer (= monomer unit!)/solvent interactions, otherwise one did not dissolve the polymer.
The use of a solubility parameter in characterizing an interaction between a species is only in accordance with its definition, if all monomer unit/monomer unit interactions have been replaced by monomer unit/solvent interactions. And only with the assumption, that the molar volume of the solvent and the molar volume of the monomer unit are of comparable size, one can work with solubility parameters when describing polymer solutions. Then one can derive [5]:
The general approach to use the difference of the solubility parameter as a measure for the interaction between the solvent and the material to be dissolved (in comparison to the interactions between the molecules of the solvent, and the material, resp.) is only productive, if the basic assumptions for the definition of the solubility parameter and the a.m. equation are fulfilled, i.e., the solvent molecules have replaced all interactions between all the polymer monomer units.
In the case of polymers the maximum solubility parameter difference for effective solvents experimentally found is around 3 (J/m3)1/2.
It has to be taken into account, that the parameter is "3-dimensional" and consists of dispersion, dipole and hydrogen bonding contributions. Therefore, the conditions for solubility are even more constrained, as the difference has to be made for all 3 contributions.
The experimental determination of solubility parameters for polymers is generally made either by measuring it for lower molecular weight analogues, or by measuring the swelling degree of comparable cross-linked polymers. In the latter case, one plots the swelling degree versus the solubility parameter of the solvent used. The solubility parameter of the solvent with the maximum swelling effect is then equivalent with the one of the polymer.
It has also to be considered, that for the dissolution of (partially) crystalline polymers one has to invest the melting enthalpy, which is not considered in the solubility parameter theory. Crystalline polymers are therefore often soluble only above their melting point in the solvent having the right solubility parameter.
From eqn (1) it can be understood, that if the melt enthalpy, which is part
of H, is too high, the (negative) entropic term cannot over-compensate the
(positive) enthalpic term - this specific polymer would be insoluble.
These considerations are clarifying that the solubility parameter concept is only applicable in case of interactions of the solvent with all polymer segments. We have therefore to conclude that there is a drastic difference if we are dealing with "solutions" or "dispersions". The latter does not involve a polymer/solvent interaction at the monomer unit level, but between surfaces: the polymer particle surface interacts with the solvent surface.
Therefore, in such cases, their interaction cannot be described with solubility parameters. Solubility parameters cannot be derived from wetting and dispersion interactions.
2. 1. 2. Semi-quantitative approach
It will be helpful to approach our discussion topic with some at least semi-quantitative figures[6]. It is well-known, that the surface tension of polymers is significantly higher (and therefore their solubility significantly lower) than that for the corresponding monomers or oligomeric analogues, e.g.:
| monomer/analogue |  / |
polymer | / |
| hexane | 18.4 / 14.9 | PE | 28 - 32 / 17.5 |
| styrene | 25.5 / 16.8 | PS | 30 - 36 / 19.5 - 22 |
| caprolactam | * / 26.0 | PA 6.6 | 41 / 28 - 31 |
| diethylphthalat | 37.5/20.5 | PET | 43 / ca 30 |
| aniline | 43 / 21.1 | PAni EB | 69.4 ? / ? |
* not available
The reasons can be found in the entropic term of eqn (1), for which Flory and Huggins developed their theory, leading to eqn (6)
(with S and P being the solvent and the polymer, resp.)
It is easy to see, that for a low molecular weight compound, like hexane,
the term TS will reach rather high values, as there are many moles for a given
concentration 
P. (The value at 250 K is about 115 J/Mol). So, hexane is easily
soluble in decalin. But for PE with a Mw of 10.000 (only a wax) TS will only
reach about 33 J/Mol even at 400 K (where one tries to dissolve PE in hot
decalin).
But in contrast to hexane, we also have to consider the melt enthalpy, which for PE is around 250 J/g. Then, eqn (1) turns for a 1% solution of PE in one mole of decalin:
a rather high positive value: PE is not soluble in decalin, unless we heat the solvent just above the melt temperature of PE, whereby we introduce the melt energy from outside, not by solvation, not by solvent-polymer interactions. The entropic driving force of -33.2 J/g is rather low, from where we understand that the dissolution is going on slowly.
Increasing the Mw further will reduce this term even more.
The reason for a higher solubility parameter and hence a much lower solubility of polymers is the drastically reduced entropy contribution to the free energy of dissolution.
Note: Crystalline polymers can only be dissolved if either the entropic term is higher than the melt enthalpy (minus solvation enthalpy) or if dissolution can be performed above the melting point of the crystallites.
Note: The surface tension of crystalline polymers increases with increasing crystallinity. PE, when crystallized on various surfaces like highly oriented graphite or NaCl, may reach a surface tension of up to 60 mN/m!
2. 1. 3 Estimation for Polyaniline
PAni, neither the base (EB), nor the salt (ES), can be molten. We do not know whether their melting point is above decomposition (EB > 400 °C, ES > 250 °C), or if they are not moldable at all. Let us be careful and assume the first option. Then, we will further assume that the heat of fusion is only double as high as for PE, i.e. 50 kJ/mol.
We want to know using eqn (1) the number of moles for a truly dissolved PAni
(EB) of a 1% concentration in DMSO by taking a solvent temperature of 500 K, a
G just slightly negative, but around Zero. Then (1) turns to be
Even with these very careful and (for the "solution hypothesis") optimistic assumptions, a 1% concentrated solution of EB in DMSO would only be feasible (according to this estimation) when the molecular weight is so low that 1% is equal to 2,62 Mol. This would be equivalent to a molecular weight of 0.25, i.e.: EB is not soluble! This does not change very much with trying a 0.1 % concentration.
This means: a polymer, which cannot be molded, of which the hypothetical heat of fusion is 50 kJ/Mol, cannot be dissolved (because the solvent cannot introduce the melt enthalpy, and we cannot heat the solvent above melting point, first, because it would boil earlier, second, because EB does not melt at all). The same is with ES.
Also poly-tetrafluorethylene, melting at 327 °C, cannot be dissolved, as one cannot heat potential solvents high enough. But: PTFE is moldable, even though at high temperature, whereas EB is not moldable up to 400 °C!
Note: if EB and ES are truly unmoldable, then they are also truly insoluble.
