Esters of 5-sulfo-i-phthalic acid as new dopants improving the solution processibility of polyaniline: spectroscopic, structural and transport properties of the doped polymer

Three esters of 5-sulfo-i-phthalic acid, namely di(n-amyl) (ASIP), di(2-butoxyethyl) (BESIP) and di(2-butoxy-2-ethoxyethyl) (BEESIP) 5-sulfo-i-phthalates, were synthesized and tested as polyaniline doping agents with the goal of improving its solution processibility. Polyaniline protonated with these acidic esters is soluble in chloroform, diethyl ketone, hexafluoro-2-propanol, m-cresol and dichloroacetic acid. Solutions of PANI(BESIP)0.5 in dichloroacetic acid (DCAA) are especially interesting because their spectra are strongly concentration-dependent, showing the change in polymer conformation upon the dilution. The spectra of diluted solutions exhibit features characteristic of the conformation that favors localized charge carriers, whereas with increasing polymer concentration, features ascribed to delocalized charge carriers begin to dominate the spectrum. Diffractograms of PANI(BESIP)0.5 and PANI(BEESIP)0.5 films cast from DCAA, in addition to a broad amorphous halo, show a sharp Bragg reflection at low angles (2θ=2–4°), which is indicative of self-organized supramolecular structures previously observed for other polyelectrolyte like systems with amphiphilic counterions. The highest conductivity (85 S/cm at RT) was obtained for PANI(BESIP)0.5.