Temperature-Induced Transitions in Doped Polyaniline:  Correlation between Glass Transition, Thermochromism and Electrical Transport

Doping (protonation) of polyaniline (PANI) in the oxidation state of emeraldine with diesters of 4-sulfophthalic acid containing flexible alkyl or alkoxy substituents leads to flexible, partially crystalline polymeric conductors which show supramolecular organization of layered-type. Diesters doped PANI exhibits two glass transition temperatures T g1 and T g2 corresponding to the freezing of the movement of the dopant substituents and of the movements of polymer-dopant anion association, respectively. A strong thermochromic effect in the UV−vis−NIR spectra is observed at sub-Tg1 temperatures indicating increasing charge carriers localization. This thermochromic effect can be correlated with the σdc= f(T) relationship which unequivocally shows that at low temperatures (T < 100 K) the sample is in the insulating state which is another manifestation of the increasing localization of charge carriers.