Solution-Cast Emeraldine Base

Doping and dedoping of polyaniline membranes with aqueous HCl solution The emereddine base powder was dissolved in NMP (8 wt %). The emeraldine base solution in NMP was cast onto a glass plate and the solvent was removed under 120T for 3 hours. The as-cast membrane was, then, immersed into a 4M HCl solution for 24 hours to give a fully doped membrane. The fully doped membrane was completely dedoped by immersion into IM NH4OH solution for 48 hours. The dedoped membrane was subsequently redoped with 0.0150, 0.0175, 0.0200 ( 0.0002), and 1 M HCl solutions. Each membrane was dried under vacuum for 48 hours at room temperature. 

The most popular method of processing is based on limited evidence that the reduced or de-doped form is soluble. It is widely applied to PANi based on extensive experimental studies by MacDiarmid and his group. In brief, a solution of the de-doped and neutral form of the CP (the emeraldine base) in an organic solvent is used to coat a substrate and the cast film is chemically doped after drying. 

Polyaniline (PAn) is most amenable to solution processing. The emeraldine base (EB) form of PAn is soluble in selected solvents such as methyl pyrrolidinone1 or strong acids.2 3 More recently, it has been discovered that solubility of the doped form can be induced by the use of appropriate surfactant-like molecules as dopants.4 Camphorsulfonic acid (HCSA) and dodecylbenzenesulfonic acid have proved particularly useful in this regard. Once solubilized, these PAn s can be cast into sheets or blended with other conventional polymer structures. For example, the pres-... 

By the mid-80s it was clear to most researchers that success on the conductivity side had taken its toll on polymer processability. Attention turned back to restoring the solubility and mechanical properties of the polymer. Polyaniline received the most attention initially. The nonconductive emeraldine base form is soluble in A-methylpyrrolidone [28] and films can be cast. Subsequent doping with a protonic acid from aqueous solution, or in situ with a photo-acid generator [45], is necessary to achieve conductivity. Polyaniline is also soluble in sulfuric acid, not the most convenient of solvents. Nevertheless it proved possible to spin fibers [46], cast films and extmde sheets of conductive polyaniline sulfate, but the laboratory experiments did not make the transition into large-scale manufacmring. 

It can be conveniently prepared as an analytically pure, off-white powder, by the reduction of emeraldine base,28,29 the most commonly used reducing agents being phenyl hydrazine or hydrazine.7 Similarly to emeraldine base, free-standing films can be cast from NMP solutions.7 The surfaces of the particles in the powdei or of the film, are oxidized relatively slowly by air, turning blue however 13c NMR studies show that the bulk of the polymer is still in the leucoemeraldine oxidation state. 

The absorption spectra of polyaniline films cast from sulfuric acid are strongly dependent on the molecular weight (viscosity). Figure 2 shows spectra of three thin films spin-cast from sulfuric acid solution, and subsequently treated by 0.5M HCl solution to achieve full protonation. To study the effect of molecular weight on the absorption spectrum, we used emeraldine base samples fractionated from the same preparation batch (polymerization and compensation) as described in the previous section. This procedure prevents any uncertainty in the viscosity value, and avoids slight variations in the preparation procedure. 

As cast emeraldine films (obtained from a solution of emeraldine base in A-methyl pyrrolidone NMP) are diamagnetic due to the absence of a charge carrier, unlike this film on doping shows paramagnetism. The total magnetic susceptibility of this film on doping shows paramagnetism and this is due to temperature-independent Pauli susceptibility and temperature-dependent Curie susceptibility. 

As an example, solutions of emeraldine base and CSA in m-cresol, or DBSA in xylene (4) can be cast or spin-cast onto various substrates. The propeities of the material will then depend on the structural parameter of the polymer but also on the experimental details. 

---