Viscoelastic and electrical transitions in gelation of electrically conducting polyaniline

In an effort to study viscoelastic (vector) percolation and conductivity (scalar) percolation within a single system, we demonstrate a conducting polymer/solvent system, which upon gelation undergoes a viscoelastic transition, followed by a separate conductivity transition. Polyaniline doped by camphorsulfonic acid, i.e., PANI(CSA)(0.5), shows three regimes in mixtures with m-cresol at a narrow concentration window of ca. 6.5-7.5 wt %. At small aging times, a viscous fluid (sol) is encountered with poor conductivity. Upon gelation, the system enters a state with elastic behavior but with poor conductivity. Further aging leads to conductivity percolation with ca. 4 orders of magnitude increase of conductivity. The scaling exponents for gelation are t approximate to 2.0-2.5 for the equilibrium modulus G(0) similar to E\epsilon\(t) and s approximate to 1.0-1.2 for the zero shear rate viscosity eta(0) similar to \epsilon\(-s) where epsilon = (t - t(gel))/t(gel). The present observations may also be relevant in applications to understand the homogeneity of solid films upon solvent evaporation: The early "trapped" junction zones in gelation can cause the previously observed granular structures in solid films, if the junction zones grow during the solvent evaporation. This may limit the available conductivity levels in solid films.