We demonstrate thermoreversible gelation of a conductive polymer, i.e., rubber-like melt processible electrically conducting compounds. Combination of viscoelastic and electrical conductivity measurements suggests network formation in the gel state and gel melting at elevated temperatures. The gels have been prepared by dissolving polyaniline in dodecylbenzenesulfonic acid (DBSA) using formic acid as a processing medium which was removed at the end. Importantly, without formic acid, reversible gelation and particle-free materials were not achieved even at the resolution of optical microscopy. For T < Tgel the materials behave elastically in compression experiments, the storage and loss moduli do not depend much on frequency, and the electrical conductivity is primarily electronic, probably due to high chain-to-chain hopping conductivity. For T > Tgel the onset of liquid-like flow is detected using modified ball drop method by dynamic mechanical analysis, the dynamic moduli become strongly frequency dependent, and the electrical conductivity drops orders of magnitude to the value corresponding to the ionic conductivity of DBSA, suggesting that the chains are not in direct contact. The physical crosslinks are probably localized mesomorphic domains which allow melting.

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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