Induced doping by sodium ion in poly(m-aminophenol) through the functional groups

Abstract

Poly(m-aminophenol) (PmAP) was synthesized by the oxidative polymerization of m-aminophenol in sodium hydroxide medium using ammonium persulfate oxidant at room temperature. The synthesized polymer showed very good solution processability as it was well soluble in aqueous sodium hydroxide, dimethylsulfoxide (DMSO), dymethylformamide (DMF), etc. A free-standing film was cast from thermal evaporation of DMSO solution of the synthesized PmAP. The film was then doped with aqueous sodium hydroxide and methanol mixture by solution doping technique at room temperature. The doping conditions were standardized in terms of the DC-conductivity of the doped film. The doped PmAP was characterized by ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, Electron dispersion spectroscopy, X-ray diffraction spectroscopy, elemental analysis by atomic absorption spectroscopy, thermogravimetric analysis and DC-electrical conductivity. The DC-electrical conductivity of PmAP film was increased to 2.34 × 10⁻⁵ S/cm from <10⁻¹² S/cm due to sodium ion doping. From all the above characterizations it was confirmed that the sodium ions were not the reason for the conduction. The incorporated sodium cation in the polymer through free –OH groups of the polymer chain was induced the electron cloud of the polymer and so the polymer became conducting.

Introduction

The conductivity of conjugated organic polymers, which are either electrical insulators or semiconductors, increases by several orders of magnitude due to ‘doping’ [1], [2]. The concept of doping in conjugated polymers is unique in its process, type, and working mechanism [2]. A relatively small quantity of a chemical substance, called dopant changes the electrical conductivity of the non-conducting or very low conducting polymer to a semi-conducting polymer. Though the term ‘doping’ is employed to correlate the similarities with that in the inorganic semiconductor, doping in conducting polymers is somewhat different [3]. In conjugated polymers generally redox doping [4] are used, i.e., electron donation to a π-conjugated system (n-type) or electron withdrawal from a π-conjugated system (p-type). Polyaniline and its derivatives have attracted much technological interest as useful conducting polymers in recent years because of easy synthesizability, better stability, good processability and wide application [5]. In case of polyaniline and its derivatives the redox [5], [6] as well as protonic acid doping are applicable for that purpose. Various organic [7], [8], [9], [10] or inorganic acids [10], [11] are used as protonic acid dopant in conducting polyaniline and its derivatives. In situ protonic acid doping occurs if polyaniline or its derivatives are synthesized in acid medium. Again in doped form the polymers are not generally soluble in organic solvents [12]. In such cases polymers are dedoped first with base to make the polymers soluble and then further doped with acid to increase the conductivity. The conductivity of polyaniline and its derivatives is closely related to pH of the working medium [13]. All the researchers reported that polyaniline and its derivatives in basic pH medium are non-conducting in nature.

The poly(m-aminophenol) (PmAP) was synthesized in a basic medium to increase the processability avoiding an extra acid dedoping step [14]. A well free-standing film was possible from the dimethylsulfoxide (DMSO) solution of the polymer. Then the polymer film was doped with aqueous sodium hydroxide and methanol mixture by solution doping technique. In this work we have optimized the doping conditions and also characterized the doped films. From the characterization we have tried to explain the doping effect of sodium ion on the PmAP through the free –OH functional groups.