Study On The Electropolymerization Mechanism Of Melamine In Nonaqueous Solvents And Electrochemical Analysis

Conductive polymers prepared by electropolymerization have unique advantages in regulating the thickness of polymeric films,doping ions,charge transport,and ion channels.Conditions such as electrochemical parameters and solvent types can directly change the polymerization process and reaction mechanism.Aqueous solutions are often the first choice,but there are two inherent problems:the electrochemical window is narrow,and the oxygen evolution or hydrogen evolution reaction often affects the main polymerization reaction;some monomers are insoluble or unstable in water,and not suitable in aqueous solutions.Adopting nonaqueous solvent is an important way to solve the above problems.On the one hand,it is beneficial for the inert monomers to generate free radicals to achieve polymerization at high potentials.On the other hand,the prepared conductive polymer films exhibit specific morphological structures,excellent physical and chemical properties,and are more diverse applications than those prepared in aqueous solutions.As a new green solvent to replace traditional organic solvents,deep eutectic solvents（DES）have received extensive attention in the fields of polymer synthesis,metal electrodeposition,nanomaterial preparation,and sensing technology development due to their economical and environmental friendliness.At present,the electropolymerization conditions of conductive polymers in non-aqueous media are not fully explored,and most of the research in the DES system is realized in the hydrous ternary system.The repeatability of the preparation of some conductive polymers is poor,which originated from the unclear mechanism of electropolymerization in aqueous solvent.Polymelamine（p Mel）,as a new type of conductive polymer with redox electroactivity,needs to be further explored for its preparation in non-aqueous solvents.At present,the electrochemical polymerization of p Mel is basically limited to aqueous media,and traditionally only carried out in a strongly acidic environment.In recent studies,it was found that p Mel can be synthesized in acidic,neutral and alkaline aqueous solutions containing trace amounts of Cl^-or Br^-.The active chlorine and active bromine electrogenerated in situ play a key role for the catalyzing polymerization.In this paper,the electropolymerization mechanism of p Mel was studied in detail in a series of non-aqueous media by combining electrochemical techniques,quartz crystal microbalance（EQCM）and atomic force microscopy（AFM）,and then p Mel modified electrode was further applied to electrochemical detection.The main research contents of this paper are as follows:（1）The electrochemical behavior and electropolymerization results of Mel in three DES（referred to as Ethaline,Glyceline,and Reline）were re-discussed and analyzed,which composed of choline chloride and ethylene glycol,glycerol,and propionic acid in the literature.The experimental results of in-situ EQCM,cyclic voltammetry（CV）inspection,and AFM morphology jointly deny the conclusions of the existing literature for electropolymerization of Mel.Further experiments found that the electropolymerization of Mel can be realized in a ternary DES including Ethaline and a small amount of water.The mass change of the electrode surface during the CV process was monitored in situ by EQCM.Under optimized condition,it was found that at least 10%v/v H₂O are required for polymerization to occur in the system.The morphology of the p Mel film was characterized by AFM,and a new condition was proposed to realize p Mel electropolymerization in DES.（2）Based on the mechanism research in aqueous solution,the electropolymerization mechanism in nonaqueous solvent by active halogen catalysis is further proposed.By replacing choline chloride with choline acetate,electropolymerization of Mel was carried out in non-aqueous system and chlorine-free DES,and it was proved that the most important factors is the addition of chloride ions and the active chlorine generated by in-situ electrochemical oxidation in non-aqueous system.However,too much choline chloride will higher the polymerization,and only tiny amount is needed in nonaqueous systems.Therefore,it is confirmed that the electropolymerization of Mel in anhydrous DES can be realized only when the concentration of Mel monomer is properly matched with the active chlorine.Extending this conclusions to the bromine-containing non-aqueous system,the introduction of a trace amount of bromine into ethylene glycol can also achieve Mel polymerization.The EQCM data indicated that the amount of p Mel gradually increased with the increase of the number of polymerization cycles in the bromine/chlorine-containing non-aqueous system.（3）The p Mel-modified glassy carbon electrode was prepared by electropolymerization,and its redox activity can be used to realize the electrocatalytic oxidation of H₂S at different p H and further electrochemical detection of H₂S.Electrochemical cleaning of the electrode can be realized by multiple CV scans to convert S into high-valence ions under acidic conditions The detection for H₂S has good selectivity,anti-interference ability,good linear relationship in the concentration range of 20～500μM with R²=0.9899,and a detection limit with 2.8μM.