Polyaniline-dodecylbenzene sulfonic acid polymerized from aqueous medium: A solid state NMR characterization

The characterization of the product of a novel polymerization procedure of aniline in aqueous medium in the presence of dodecylbenzene sulfonic acid (DBSA) as a dopant is reported. This polymerization yields stable dispersions of the emeraldine salt, which are attractive starting material for the preparation of conducting polymer blends. The as-synthesized polyaniline (PANI)-DBSA and its dedoped and redoped forms, studied by means of C-13, N-15,and Li-7 solid-state NMR and wide-angle X-ray diffraction (WAXD), reveal unusual properties. NMR and weight loss measurements show that a large portion of the dopant acid is retained upon standard dedoping by NH4OH. While this dedoped material is an insulator, NMR shows that deprotonation is incomplete and identifies residual bipolaron states. Also, the dielectric properties of the NH4OH dedoped material deviate significantly from those of fully deprotonated polyaniline and resemble those of a conducting one. Dedoping with LiOH removes a large part of the dopant acid and leads to complete deprotonation. Rotating-frame relaxation measurements indicate that the dedoped forms obtained from polyaniline-DBSA exhibit single-component behavior, implying that DBSA is molecularly mixed within the polyaniline. 15N cross-polarization magic-angle spinning NMR spectra of the dedoped samples resolve two distinct imine-amine chemical species, which are formed during this particular polymerization procedure. One of the two imine-amine chemical species binds the dopant acid much stronger than the other. This is confirmed by N-15{Li-7} rotational-echo double-resonance (REDOR) NMR experiments on LiOH-dedoped samples. Li+ cations are shown to substitute for H+'s as counterions at sites from which the acid anions are not removed by deprotonation. The REDOR experiments unambiguously show that the Li+ cations reside preferentially next to the imine species with the stronger DBSA binding, and within a distance of ca. 4 Angstrom. WAXD indicates that the doped as-synthesized and dedoped powders are highly amorphous. This is attributed to the incorporation of the DBSA in the polyaniline structure.