The specific interactions between sulphonic acid protonated polyaniline (PANI) and solvents are here studied both by the semiempirical AM1 method and experimentally. Phenolic solvents are shown to have a relatively large interaction with the sulphonate anions of the counterions and with the amines in PANI. In addition, a properly functionalized counterion may form cyclic associations provided that there is a steric match between the molecules concerned. This concept is called molecular recognition and it is a novel concept in the context of PANI. For example, the carbonyl group in (±)-10-camphor sulphonic acid (CSA) can form a hydrogen bond to the hydroxyl group of m-cresol, whereby the phenyl ring becomes coplanar with one of the PANI rings thus enabling enhanced van der Waals interaction. This additional specific interaction agrees with our observed increased solubility with CSA doped PANI in m-cresol, compared to its solubility in dimethyl sulphoxide or chloroform, or to tosylene sulphonic acid doped PANI in m-cresol. The above cyclic associations are suggested for dilute solutions and for the amorphous domains of solid films. In the latter case, they provide a potential mechanism to yield planar conformation in the crystalline domains: during the evaporation of m-cresol, stacking to crystal structure may twist the rings due to the planar m-cresol molecules on top of PANI rings. This is in agreement with the observed high conductivity. The present results indicate that computational methods combined with the concept of molecular recognition may open new possibilities to tailor the rigidities and solubilities of macromolecules.

Due to its semirigid nature, electrically conductive polyaniline (PANI) has long been regarded as an intractable material, i.e. infusible and poorly soluble in organic compounds. Among the rare exceptions is camphorsulfonic acid (CSA) doped PANI, which exhibits good solubility in m-cresol, whereas for other sulfonic acid dopants (e.g. dodecylbenzenesulfonic acid (DBSA)) the solubility in common solvents is poor. We report exceptionally high solubility of fully DBSA and CSA protonated PANI in a crystalline compound, 1,3-dihydroxybenzene, i.e. resorcinol. Up to 20−30 wt % of PANI(DBSA)0.5 and PANI(CSA)0.5 can be dissolved in resorcinol at 200−220 °C to form particle-free films as observed by optical microscopy. High PANI complex concentrations require high temperatures for dissolution, suggesting UCST behavior with a high critical temperature. Optical microscopy, calorimetry, and X-ray analysis suggest that the solution initially is amorphous. With time, crystallinity develops within the sample, due to partial phase separation of resorcinol while part of it remains miscible. Calculations show that a resorcinol molecule is able to simultaneously form two hydrogen bonds and one phenyl/phenyl interaction with the PANI/sulfonic acid complex, because of their steric match. The conditions required to achieve such matching interactions, i.e. molecular recognition, are discussed. The concept can be extended to find a large category of novel solvents for electrically conductive PANI to yield soluble and fusible complexes.

Polyaniline (PANI) is known to dissolve in strong acids, such as sulphonic acids. PANI, in its electrically conductive form, is generally regarded to be poorly soluble in low‐acidic solvents and to be infusible, closely resembling fully aromatic rigid rod polymers. We show that “less” acidic solvents and plasticizers can be found based on phenyl‐phenyl interactions in combination with hydrogen bonding. The requirement is that the interactions are strong enough and, importantly, sterically match the complementary moieties of the sulphonic acid doped PANI. Dihydroxybenzenes and bisphenols are examples of such low‐acidic compounds. This type of molecular recognition allows solution and melt processibility of PANI doped by generic sulphonic acid, such as methanesulphonic acid or alkylbenzenesulphonic acid. Molecular recognition is also offered as an explanation for the previously observed high solubility of camphorsulphonic acid (CSA) doped PANI in phenols.

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