B4. 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 ºC, ES > 250 ºC), or if they are principally unmoldable. Let us assume the first option. Then, we will further assume that the heat of fusion is double as high as for PE, i.e. 50 kJ/mol.

Using eqn (4) we want to know the number of moles for a truly dissolved PAni (EB) of a 1% concentration in DMSO by taking a solvent temperature of 500 K, and assuming G just slightly negative, but around Zero. Then (4) turns to be

(6) nP = (- H/RT - nS lnS) / lnP = (- 50,000/RT + 0.01) / (-4.6) = 2.62

Even with these very conservative and (for the "solution hypothesis") optimistic assumptions, a 1% concentrated solution of EB in DMSO would only be feasible (according to this estimation) if the molecular weight were 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 calculating for a 0.1 % concentration.

A polymer, which cannot be molded, cannot be dissolved [1] (the solvent cannot introduce the melt enthalpy, and we cannot heat the solvent above melting point, because it would boil earlier, and because PAni does not melt at all, hence the solvent cannot replace all of the interchain and intramolecular interactions by monomer unit / solvent interactions).

It is therefore not surprising that in the serie of (neutral, un-“doped”) digomeric n-thiophenes the heptamer has no melting point and is insoluble [2]. The m.p. increases steadily (linearly?) from ...ºC (for n = 2) to ...ºC (for n = 6).

Footnotes

[1] Poly-tetrafluorethylene, melting at 327 ºC, cannot be dissolved, as one cannot heat potential solvents high enough. But: PTFE is moldable, even though at high temperatures, whereas EB is not moldable up to 400 ºC!

[2] P. Bäuerle

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