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 dispersion s of the emeraldine salt, which are attractive starting material for the pr eparation of conducting polymer blends. The as-synthesized polyaniline (PAN I)-DBSA and its dedoped and redoped forms, studied by means of C-13, N-15,a nd Li-7 solid-state NMR and wide-angle X-ray diffraction (WAXD), reveal unu sual 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 incomple te and identifies residual bipolaron states. Also, the dielectric propertie s of the NH4OH dedoped material deviate significantly from those of fully d eprotonated polyaniline and resemble those of a conducting one. Dedoping wi th LiOH removes a large part of the dopant acid and leads to complete depro tonation. Rotating-frame relaxation measurements indicate that the dedoped forms obtained from polyaniline-DBSA exhibit single-component behavior, imp lying that DBSA is molecularly mixed within the polyaniline. 15N cross-pola rization magic-angle spinning NMR spectra of the dedoped samples resolve tw o distinct imine-amine chemical species, which are formed during this parti cular 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 counterion s at sites from which the acid anions are not removed by deprotonation. The REDOR experiments unambiguously show that the Li+ cations reside preferent ially 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 incorpo ration of the DBSA in the polyaniline structure.