Reversible Redox-responsive Gel-sol Transition Switched By Electrochemistry And Electrogelation In Poly(acrylic Acid) System Containing Fe(â…¢)

The main purpose of this thesis is to prepare homogeneous hydrogel of Poly (acrylic acid) which was crosslinked by Fe (III), investigate the specific stimuli-responsive performance and gelation of the PAA system. The main contents and results of the work are as follow:In our previous work, we have achieved a redox-responsive gel-sol/sol-gel transition in aqueous poly(acrylic acid) (PAA) solutions triggered by light radiation and oxidation, where the ferric ion (Fe⁺³) was photoreduced to the ferrous ion (Fe⁺²) in the presence of citric acid (CA) and the ferrous ion was restored to the ferric ion by oxygen exposure. The gel of PAA chains was cross-linked by the trivalent cation Fe⁺³ due to the stronger binding affinity to the carboxyl groups, which was observed with the change in the UV-Vis spectrum of the solution. This finding suggests a novel method to switch the physical gelation in polyelectrolyte solutions reversibly by changing the valence of counterions. In the present work, we have applied the electrochemical redox of Fe²⁺/Fe³⁺ to trigger the gelation and dissolution of PAA(Mn=210000,Mw/Mn =1.2, C=25wt%) in aqueous solutions. To our knowledge, no paper has concerned the reversible polymer physical gelation controlled by electrochemical redox reaction of counterions up to now.The PAA solution is converted into a homogeneous hydrogel when the ferrous ions are electro-oxidized to ferric ions and this gel is recovered to solution again by electro-reduction of the ferric ions to ferrous ions. This electrical sol-gel transition is completely reversible by simply change the electrode polarity. In principle, this transition can be repeated limitless as long as the alternate electrical field is applied. The properties of the Fe (III)/Citrate-PAA hydrogel are affected by many factors. The mechanical property of Fe (III)/Citrate-PAA is increasing with increased the concentrations of PAA and molar ratio of Fe (III) to Citrate.Here, the homogeneous hydrogel of PAA was also prepared by adding the FeCl2 solution into the PAA21 directly without chelated with citrate acid.Used polarization curves and cyclic voltammetry scan tracking the process for PAA gelation under different electrochemical system and determine the influenced factors of the gel-sol transition; we defined the mole radio'f'between the iron ionic and the carboxyl in PAA as a structure parameter which controls the gelation process.