Functional composite coatings containing conducting polymers

Sammanfattning: Organic coatings are widely used to lower the corrosion rate of metallic structures. However, penetration of water, oxygen and corrosive ions through pores present in the coating results in corrosion initiation and propagation once these species reach the metal substrate. Considering the need for systems that offer active protection with self-healing functionality, composite coatings containing polyaniline (PANI) conducting polymer are proposed in this study. In the first phase of my work, PANI was synthesized by various methods and characterized. The rapid mixing synthesis method was chosen for the rest of this study, providing PANI with high electrical conductivity, molecular structure of emeraldine salt, and morphology of spherical nanoparticles. PANIs doped with phosphoric and methane sulfonic acid revealed hydrophilic nature, and I showed that by incorporating a long-chain alkylphosphonic acid a hydrophobic PANI could be prepared. The second phase of my project was dedicated to making homogenous dispersions of PANI in a UV-curable resin based on polyester acrylate (PEA). Interfacial energy studies revealed the highest affinity of PEA to PANI doped with phosphoric acid (PANI-PA), and no attractive or long-range repulsive forces were measured between the PANI-PA surfaces in PEA.This is ideal for making conductive composites as, along withno aggregation tendency, the PANI-PA particles might come close enough to form an electrically connected network. Highly stable PEA/PANI-PA dispersions were prepared by pretreatment of PANI-PA in acetone followed by mixing in PEA in small portions under pearl-milling. The third phase of my project dealt with kinetics of the free radical polymerization that was utilized to cure the PEA/PANI-PA mixture. UV-vis absorption studies suggested a maximum allowed PANI-PA content of around 4 wt.% in order not to affect the UV curing behavior in the UV-C region. Real-time FTIR spectroscopy studies, using a laboratory UV source, revealed longer initial retardation of the photocuring and lower rates of crosslinking reactions for dispersions containing PANI-PA of higher than 3 wt.%. The presence of PANI-PA also made the formulations more sensitive to changes in UV light intensity and oxygen inhibition during UV curing. Nevertheless, curing of the dispersions with high PANI-PA content, of up to 10 wt.%, was demonstrated to be possible at either low UV light intensities provided the oxygen replenishment into the system was prevented, or by increasing the UV light intensity to very high levels. In the last phase of my project, the PEA and PEA/PANI-PA coatings, cured under high intensity UV lamps, were characterized. SEM analysis showed small PANI-PA particles to be closely packed within the matrix, and the electrical conductivity of the composite films was measured to be in the range of semiconductors. This suggested the presence of a connected network of PANI-PA, as confirmed by investigations of mechanical and electrical variations at the nanoscale by PeakForce TUNA AFM. The data revealed the presence of a PEA-rich layer at the composite-air interface, and a much higher population of the conductive network within the polymer matrix. High current signal was correlated with a high elastic modulus, consistent with the level measured for PANI-PA, and current-voltage studies on the conductive network showed non-Ohmic characteristics. Finally, the long-term protective property of the coatings was characterized by OCP and impedance measurements. Short-term barrier-type corrosion protection provided by the insulating PEA coating was turned into a long-term and active protection by addition of as little as 1 wt.% PANI-PA. A large and stable ennoblement was induced by the coatings containing PANI-PA of up to 3 wt.%. Higher content of PANI-PA led to poorer protection, probably due to the hydrophilicity of PANI-PA facilitating water transport in the coating and the presence of potentially weaker spots in the film. An iron oxide layer was found to fully cover the metal surface beneath the coatings containing PANI-PA after final failure observed by electrochemical testing.