Proton NMR study on the molecular motion and conformation of polyaniline doped with HCl

Summary

Proton NMR spectra and spin-lattice relaxation time (T₁) for polyaniline doped with HCl were measured at 90MHz over a temperature range of 110–430K. A narrow line with a width of about 1–2KHz superimposed on broad NMR peak was observed above 210K, and it was tentatively assigned to the protons in dopant and absorbed water. The analysis of linewidths and second moments for the broad lines reveal two distinct relaxation processes with activation energies of 1.9 and 7.8Kcal./mole,respectively. The former may be associated with the motion of the dopant and water absorbed and the latter with the rotation of benzoid ring about C-N bond axis. The second moment data suggest that the twist angle between the adjacent rings decreases with rising the doping ratio. Proton NMR relaxation rate was fitted with a theoretical equation containing the contributions from nuclear dipolar interaction and from electron-proton interaction, R_d and R_p, respectively.R_d is negligible below room temperature and R_p has little change above 100K. The total relaxation rate is dominated by R_p near room temperature.