Poly(o-methoxyaniline): Synthesis, Doping and Composites

Abstract

Intrinsically conducting polymer (ICP) composites, especially ICP/thermoplastic polymer composites have recently received great attention because of their potential synergistic benefits and promising applications in a variety of fields. Yet the fabrication of ICP/thermoplastic composites remains a challenge because of the limited compatibility between the ICP component and the thermoplastic matrix. The focus of this thesis is to develop novel ICP/thermoplastic composite materials which can be processed through melt blending at relatively low temperature and maintain certain level of electrical conductivity and mechanical properties which are essential for field applications. Polyaniline (PANI) is one of the most extensively studied ICPs and a variety of PANI based composites have been developed. However the low processability of PANI, together with its limited compatibility with thermoplastics generally leads to composite materials with weak mechanical properties. On the other hand, poly(o-methoxyaniline) (POMA) possesses better compatibility with thermoplastics yet up to now no study regarding melt processing of POMA/thermoplastic matrix composites have been reported. Therefore POMA was synthesized and incorporated with other components to prepare novel composite materials in this thesis. Chemical oxidative polymerization was adopted throughout this thesis for the synthesis of most ICP samples, because of its high yield, low cost and scale-up ability. Doping is essential for the electrical conductivity of POMA; in addition dopant exerts significant effect on structure, morphology and physical, chemical and functional properties of POMA. A variety of inorganic acidic dopants, including methane sulfonic acid (MSA), hydrochloric acid (HCl) and para-toluene sulfonic acid (TSA) were used to protonate POMA and compared with POMA synthesized in the absence of any additional dopant. The samples were then thoroughly characterized in terms of structure, morphology, electrical conductivity, thermal stability and functional properties including anti-oxidant and anti-microbial activities. Based on the properties of characterized POMA samples, especially conductivity and thermal stability that are critical for thermal processing, TSA was located as the suitable inorganic dopant to synthesize POMA that can be melt blended with a thermoplastic matrix. Recently polymeric materials have been reported to behave as dopants in ICP synthesis, with the benefits that external polymeric material could induce better processability and possibly higher thermal stability. Lignosulfonate (LGS) in particular has been adopted by different research groups to synthesize LGS doped PANI. Since POMA possesses similar physical and chemical properties with pristine PANI, in this thesis POMA was for the first time chemically polymerized in the presence of LGS, with the objectives to elucidate the reaction mechanism and to clarify the actual role acted by LGS in the reaction. In addition, the effect of different LGS contents on the properties of the final product is also evaluated. Results suggest that instead of acting as a dopant or stabilizer as reported in literature, LGS behaves as a particulate adsorbent while POMA serves as particulate adsorbate. Research in ICP copolymers, especially copolymer synthesized from PANI and its derivatives, have gained attention because of the possible synergistic combination of the high electrical conductivity provided by PANI and the processability induced from the derivatives. A successfully synthesized copolymer could benefit later stage work of melt blending, as copolymers are expected to exhibit higher electrical conductivity than homopolymers of aniline derivatives. However in most of the relevant literature copolymer formation was taken for granted when chemical oxidative polymerization was carried out without any further kinetic manipulation while the possibility of composite material formation was largely overlooked. In this thesis chemical oxidative copolymerization of aniline and o-methoxyaniline was attempted, with the objective to first understand the reaction mechanism and to distinguish formation of copolymer or composite. In addition the effect of different corresponding monomer ratios (aniline/o-methoxyaniline) on the final product was quantitatively evaluated. Obtained results suggest that a composite material mainly composed of corresponding homopolymers of PANI and POMA, instead of a uniform copolymer, was obtained when chemical oxidative polymerization was performed without external kinetic manipulation. Based on all the characterization results, TSA doped POMA (POMA-TSA) was selected as the ICP component for the preparation of a novel composite material. On the other hand, ethylene vinyl acetate copolymer (EVA) was selected as the thermoplastic matrix mainly owing to its low processing temperature and superior flexibility. POMA-EVA composite was for the first time prepared through melt blending. Compared to the ICP component loading, only very a small amount of attention has been paid to the effect of matrix polarity, even though the matrix polarity has been reported to significantly affect the physical and chemical properties of composite materials. Therefore in this thesis the effect of matrix polarity on the properties of the POMA-EVA composite is also investigated for the first time.