The dissolution of high-molecular-weight polyaniline emeraldine base in N-methyl-2-pyrrolidinone containing secondary amines: Thermodynamics and characterization

The addition of secondary amines to polar aprotic solvents (e.g. n-methyl-2pyrrolidinone (NMP)) significantly enhances the solubility of polyaniline in its emeraldine base (EB) form. The hydrogen bond formation between the amine proton of 2-methylaziridine and the lone electron pair of imine nitrogens in the EB molecules limits interactions between polymer chains which leads to enhanced solubilities and dramatically prolonged gelation times. Thus, highly concentrated high molecular weight EB solutions (>$\sim$20 wt%) without gelling for >24 hours can now be obtained.; Thermodynamic studies of EB/NMP/amine solutions suggest that the large increase in the entropy of mixing is the predominant factor in decreasing the free energy of mixing of these solutions.; Spectroscopic characterization of aged EB/NMP solutions containing varying amine/EB molar ratios, and of the resulting films and the HCl doped films reveals that amine basicity and molecular size determine the reactivity of the EB polymer and the structure of the resulting polymer films. Pyrrolidine, which is the strongest base (pK_b = 2.72), degrades EB significantly, regardless of the concentration used or the age of the solution. 2-Methyl-aziridine with relatively weak basicity (pK_b = 5.73), does not degrade the polymer appreciably, as long as the 2-methyl-aziridine/EB molar ratio is kept to less than 10. The reactivity of heptamethyleneimine with medium basicity (pK_b = 3.72) to EB is between those of 2-methyl-aziridine and pyrrolidine. The conductive and electrochemical studies of the resulting EB films give consistent results with those from spectroscopic studies.; Rheological properties of concentrated EB/NW/2-methyl-aziridine ($\sim$20 wt%) solutions were investigated by means of rotational viscometry as functions of 2-methylaziridine/EB molar ratio, EB concentration, time, shear rate, and temperature. By using rheokinetic methodologies, the kinetic data, including rate constant, activation energy, equilibrium constant and the free energy of EB·2-methyl-aziridine complexation, were determined. Additionally, such systems have been studied in the dilute regime by measuring the intrinsic viscosity of solutions as a function of varying 2-methylaziridine/EB molar ratios and temperatures. It is found that intrinsic viscosity, and thus the interaction between solvated polymer chains, decreases with increasing 2-methylaziridine/EB molar ratios.