SYNTHESIS, SURFACE MODIFICATIONS AND CHARACTERIZATION OF POLYANILINE

Abstract

Polyaniline (PAN) salts were prepared via oxidative chemical polymerization. The PAN bases were obtained by treatment of the PAN salts with a base. Both PAN salt and base powders were characterized by x-ray photoelectron spectroscopy (XPS), and were found to be in the emeraldine (EM) oxidation state. The protonation of PAN powders by H2SO4 has been studied by XPS. The amount of acid incorporated can be effectively varied by varying the synthesis conditions. At a sulphate/monomer ratio of > 0.5, both the HSO4- anion and absorbed molecular H2SO4 can be quantitatively differentiated in the S(2p) core-level spectrum. The N(1s) core-level spectrum indicates that the amine units of the EM oxidation state of PAN are also susceptible to protonation in the presence of excess H2SO4. Sulphonated and self-doped EM, leucoemeraldine (LM) and deprotonated polypyrrole (DP-PPY, consisting of 25% oxidized imine-like nitrogens) powders have been studied by XPS. In the case of EM and DP-PPY, self-protonation occurs at the imine units. The structure of the sulphonated LM is similar to that of the sulphonated and self-protonated EM. In DP-PPY, XPS results suggest sulphonation and protonation occur on separate pyrrolylium units. The surface structures of pristine LM, EM and HCl protonated EM films have been studied by static secondary ion mass spectrometry (SIMS) under low “damage” conditions. The positive ion spectra of ion fragments up to 100 Dalton reveal the hydrocarbon structures of the three polymers. The negative ion spectrum of the protonated EM-HCl film show the presence of the halogen dopant. The mass counts ratios are also useful for the logical deduction of structure from the fragmentation pattern. Surface structures and redox states of pristine and protonated EM films have been studied by angle-dependent XPS. The first protonation-deprotonation cycle can increase the intrinsic oxidation state (imine/amine ratio) of EM to nearly that of pernigraniline. The outermost surface of the EM film generally has a significantly lower intrinsic oxidation state than the bulk. In contrast to protonation by volatile acids, the outermost surface of EM film protonated by a non-volatile acid can exhibit a significantly higher protonation level and almost all the amine nitrogens, in addition to the imine nitrogens, are susceptible to protonation. The pristine, O3 -pretreated and once-cycled (acid-base) EM films exhibit distinctive differences in behaviour towards surface modifications by near-UV light induced graft copolymerization with acrylamide (AAm), acrylic acid (AAc) and the sodium salt of 4-styrenesulfonic acid (NaSS) . The structures and chemical compositions at each copolymer interface are studied by angle-dependent XPS. The surface morphology of the films after grafting are also observed by scanning electron microscopy (SEM). Surface grafting with the three polymers leads to more hydrophilic EM film. Furthermore, grafting with the AAc and Nass readily gives rise to a self-doped or self-protonated EM surface structure. A substantial proportion of the grafted protonic acid groups at the surface remains free for further modification and functionalization.