New Strategies For The Synthesis Of Conducting Polymers

Conducting polymers are widely used in many fields such as energy storage,sensors,metal corrosion protection,electromagnetic shielding and controlled drug release due to their advantages of good conductivity,simple preparation process and high environmental stability.The physicochemical properties of conducting polymers are significantly related to their morphology and structure.Currently,it is still a hot topic to develop new strategies for the polymerization of conducting polymers with controllable morphology and structure to meet different application requirements.The morphology and structure of a conducting polymer is not only related to the structure of the monomer itself,but also to its preparation method and conditions.It should be also effective to tune the morphology and structure of a conducting polymer using multifunctional dopants with optical,electrical and pH response groups.As far as the electrochemical polymerization of a conducting polymer is concerned,the electrolyte has a great influence on the morphology and structure of the resulting polymer.As emerging green solvents,ionic liquids have been tried in recent years as solvent as well as supporting electrolyte for the electrochemical synthesis of conducting polymers.It would be a meaningful work to design and synthesize functionalized ionic liquids to simplify the system for the electrosynthesis of conducting polymers,control their morphology and structure,and therefore improve their electrochemical performance.Based on the above considerations,some attempts have been made to develop new strategies for the synthesis of conducting polymers and the results are briefly outlined in the following sections:1.Tuning the morphologies and electrical properties of polypyrrole by changing the steric configuration and the existing form of functionalized dopantThe steric configuration and the concentration of multi-functionalized dopant have significant effect on the morphology,structure and properties of polypyrrole.In this work,the azobenzene group-containing carboxylic acid salt azobenzene-4,4'-dicarboxylate disodium salt(ADDS)was selected as the dopant for the controlled synthesis of nanostructured polypyrrole(PPy).By controlling the cis-trans configuration of ADDS(via ultraviolet light irradiation)as well as the concentration of divalent ADDS anion(via the medium pH alteration),a series of ADDS-doped PPy hydrogels with different morphologies and electrical properties were prepared accordingly.Under weak alkaline conditions(pH=8.0),the morphology of the resulting PPy changes from nanofiber(nanobelt)to nanoflake and then to granular structure when the exposure time of ADDS aqueous solution to the UV-light irradiation was extended step by step from 0 to 15 min.This phenomenon is mainly due to the gradual decrease of the trans isomers in ADDS aqueous solution.Under visible light conditions,the morphology of the resulting PPy transforms from nanofiber(nanobelt)to granular structure when the pH of ADDS aqueous solution was varied from pH 8.0 to pH 3.0.This phenomenon is mainly due to the gradual decrease of the divalent doping anions(all trans isomers).Since trans ADDS isomers are co-planar linear molecules,and under weakly alkaline conditions,ADDS exists mainly in the form of divalent anion,the conductivity of PPy doped with all-trans ADDS(10.4 S·cm-1)is therefore higher than that of PPy doped with UV-light irradiated ADDS(some trans isomers are transformed to the corresponding cis isomers)(1.0 S·cm-1).It can be concluded that the morphologies and the electrical properties of ADDS-doped polypyrrole conducting hydrogels can be tuned just by varying the UV irradiation time and the medium pH,which results in the change of the steric configuration and the existing form of ADDS.The present new strategy is helpful for the controlled synthesis of conducting polymer hydrogels.2.Electrochemical polymerization of aniline in a protic ionic liquid with high proton activityElectrochemical polymerization of aniline in conventional protic ionic liquids(PILs)usually requires an exogenous proton source or the aniline monomer pre-protonation to obtain high quality polyaniline(PANI).In the present work,a proton functionalized ionic liquid pyrrolidinium hydrogenosulfate([Pyrr][HSO4]),which has high proton activity,has been tried for the first time as an electrolyte for the aniline electropolymerization.It is shown that in this medium the electropolymerization of aniline is facile and the resulting PANI has porous structure and high electrode specific capacitance.The electropolymerization in pyrrolidinium nitrate([Pyrr][NO3])as control experiment demonstrates that the high efficiency of the electropolymerization in[Pyrr][HSO4]is due to the high proton activity of HSO4-.The present work is of significance for simplifying pre-existing ionic liquid media for aniline electropolymerization and further improving the electrochemical performance of PANI obtained in ionic liquid media.3.Effects of substitutent on the electropolymerization of aniline in functionalized ionic liquidHere,the effect of the substituent on ortho position of aniline on the electropolymerization of aniline was investigated in functionalized ionic liquid[Pyrr][HSO4].The results show that the stronger the electron-donating effect of the substituent,the lower the initial oxidation potential of the monomer.In contrast,the stronger the electron-withdrawing effect of the substituent,the higher the initial oxidation potential of the monomer.This is caused by the electron-donating/withdrawing effect of the substituent adjacent to the amino group which affects the electron density of C atom opposite to the aniline amino group.For all the polymers,prepared under the same electropolymerization conditions,the electrochemical activity(capacitance performance)of the polyaniline with electron-donating group is higher than that with electron-withdrawing group.However,the electrochemical activity of the polymer from substituted aniline is lower than that from unsubstituted aniline whether the substituent is an electron-donating group or an electron-withdrawing group.Obviously,this result is due to the steric hindrance effect of the substituent.It is also observed that the morphology and structure of the as-prepared polymer depends on the substituent on the ortho position of aniline.For the same substituent,it is found that the morphology of the polymer prepared in the present ionic liquid medium is quite different from that prepared in aqueous medium.It follows that the medium is also a key factor affecting the synthesis of conducting polymers.4.Synthesis,characterization and electropolymerization of functionalized organic salt-anilinium saccharinate and electrochemically controlled release of saccharinate anionsIn this work,a novel functionalized organic salt-anilinium saccharinate([HANI][Sac])was synthesized by the ion exchange method,and its composition and properties were characterized by1H NMR,13C NMR,ESI/MS and TG-DSC techniques.[HANI][Sac]can be used as both monomer and supporting electrolyte for efficient electrosynthesis of polyaniline(PANI)in acetonitrile.The obtained PANI has hierarchical porous structure and its doping degree with saccharinate anion([Sac]-)is as high as 33.5%.The resulting[Sac]--doped-PANI(PANI-[Sac])can be used as an electrochemically controlled drug delivery system.The in vitro release kinetics of[Sac]-under different potential stimuli conditions showed that,at a given time,the release rate of[Sac]-and its release percentage(ratio of the amount released to that loaded)increase with the negative shift of the applied potential.The amount of[Sac]-loaded and/or released can also be regulated by varying the charge for PANI electropolymerization.The present work provides a new strategy for the facile construction of conducting polymer-based electrochemically controlled drug release system.