Synthesis, Charicterization And Structure-property Relationship Of Poly(2,3-dimethylaniline) And Its Composites In Fenton System

As one of ring-substituted PANI derivatives, poly(2,3-dimethylaniline)(P3) hasbetter anticorrosive property than PANI. Two asymmetrically substituted electrondonating group(-CH3) on the aromatic ring in P3, which effectively reducing thepolymer chain stiffness and interaction between molecular chains, can improve thesolubility and dispersibility in organic solvents, and therefore solve the dispersiondifficulty of the PANI in anticorrosive coatings. For the existence of the electrondonative group, on the other hand, can also help improving the anti-corrosionperformance of materials, so the anti-corrosion performance of the P3is better than thatof PANI, and it is expected to replace PANI of becoming a new kind of anticorrosivematerial. However, P3and its composites are normally synthesized by chemicaloxidative polymerization of their monomers using ammonium persulfate (APS) asoxidant, whose decomposite is toxic to environment and further increases the cost of P3.Hence, a replacement of APS is becoming more and more important for promoting P3and its composites’ industrialization.Hydrogen peroxide (H2O2) is an environmental friendly oxidant. In the presence ofFe2+ion, a powerful oxidant, hydroxyl radical (·OH), is generated from thedecomposion of H2O2,which is likely to make2,3-dimethylaniline generate morephenazine structure units in the early stage of the polymerization, then a flake-likestructure is form through self-assembly. The flake-like structure is beneficial to enhancebarrier effect for P3in the coatings, and thus improve the anti-corrosion performance.Therefore, the H2O2/Fe2+system was used firstly for the synthesis of P3and itscomposites in hydrochloric acid solution. The main research contents and results are asfollows:①With H2O2/Fe2+catalytic oxidation system as oxidant, P3was prepared byin-situ chemical oxidation polymerization in hydrochloric acid solution. Themorphology, structure, crystallization, thermal stability, electrochemical properties andcorrosion resistance of the product have been characterized by scanning electronmicroscopy (SEM)、infrared spectra (FTIR)、ultraviolet-visible spectra (UV-vis)、X-raydiffraction (XRD)、thermogravimetric analysis (TGA)、cyclic voltammograms (CV) andTafel polarization technique, and compared with the traditional APS oxidationpolymerization product. The results showed that P3synthesized with H2O2/Fe2+system is prone to be flake-like structure, while the one synthesized with APS system is proneto be fibrous structure. P3synthesized with H2O2/Fe2+system contains morephenazine-like structure, and the product has better thermal stability and corrosionresistance, but with poor crystalline and electrochemical activity. The variation of opencircuit potential (OCP) during the polymerization, as well as the yields and viscosityshowed that synthesis of P3using H2O2/Fe2+system needed shorter reaction time andgot higher degree of oxidation and smaller molecular weight but lower yields. Inaddition, the formation mechanism of the flake-like P3was discussed. The relativelyflat molecular structure of phenazine-like segments could act as templates to formflake-like structures through π-π stacking during the polymerization. The two kinds ofP3were added into epoxy resin (EP) to mix uniformly. Then P3-A/EP(APS as oxidant)and P3-H/EP(H2O2/Fe2+as oxidant) coatings were obtained, respectively. Theanticorrosive properties of P3/EP coatings were studied by open circuit potential, Tafelpolarization curves and salt spray test. The experiment results showed that P3-H/EPcoating has better dispersibility than that of P3-A/EP coating in the n-butanol/cyclohexanone mixed solvent. Moreover, the open circuit potential, corrosion potential,polarization resistance and anticorrosive efficiency were higher than the P3-A/EPcoating before and after soaking in3.5%NaCl solution, and the corrosion current waslow. After30days of salt fog test, scratches of P3-A/EP coating appeared bubblingphenomena, while the scratches of P3-H/EP coating just turned up slight rust. Therefore,P3-H/EP coating showed better anticorrosion performance than P3-A/EP coating.②On the basis of intercalation modification of sodium montmorillonite (MMT)with imidazoline quaternary ammonium salt (ODD), ODD-MMT/P3composite wasprepared through in-situ polymerization of2,3-dimethylaniline. FTIR spectroscopy andXRD results showed that the ODD has entered into the layer of MMT, and adopted theway of paraffin arrangement; According to the results of TGA analysis, ion exchangecapacity of MMT was45.60mmol/100g after the ODD intercalation. TheODD-MMT/P3composite had lower peak current in CV curve and higher corrosionpotential. ODD-MMT/P3/EP coating showed a higher anticorrosion performance thanP3and MMT/P3during the accelerated corrosion test in3.5%NaCl solution.③In order to improve the conductivity of P3, with the method of in situ graftpolymerization, which modification with a layer of PANI in P3surface, P3/PANIcomposite was prepared. The changes in absorption peak on3434cm-1、2930cm-1、2850cm-1、1640cm-1、800cm-1of FTIR showed that conductive PANI was successfully grafted and coated on the P3. Moreover, due to the introduction of PANI, thecrystallinity, electrochemical activity and electrical conductivity of P3/PANI compositewere improved. This composite also had high dissolving capacity and corrosionresistance compared with pure PANI, which could widen the application field of P3. Inaddition, geometry optimization of octamer of aniline and2,3-dimethylaniline werecarried out using GAUSSIAN03software in order to analyze the solubility in organicsolvents of P3and PANI from theoretical calculation point of view. The results ofshowed that the bond angle and the length of the molecular chain of P3were smallerthan that of PANI, suggesting that distortion degree of P3molecular was higher thanthat of PANI, which is favorable for solvent entering. Moreover, the dipole moment ofP3was larger than of PANI, demonstrating once again that P3is more easily soluble inpolar solvent.