Electron spin resonance studies in the doped polyaniline PANI-AMPSA: Evidence for local ordering from linewidth features

X-band electron spin resonance (ESR) of polyaniline (PANI) doped with 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPSA), at conductivity values of 90, 4, and 0.2 S/cm, has been investigated from 300 to 5 K. Free-standing films (30-40 mu m thick) of PANI were prepared by solution casting method. The spectra consist of two signals, one is broad (similar to 60 G) and the other is narrow (similar to 10 G), at nearly the same resonance field. The temperature variation of ESR susceptibility indicates a stronger Pauli component in the broad line signal, as compared to the narrow signal, at all values of conductivity. Both linewidths are independent of the presence of O-2. The temperature variation of linewidths is explained in terms of the quasi-two-dimensional (Q2D) model. These results suggest that in some regions the doped PANI chains have self-assembled Q2D structure, and the presence of a significant Pauli susceptibility indicates that the charge carriers are highly delocalized in these Q2D regions.