Reversible and Irreversible Light-Induced Reactions Initiated BY SPEEK/PVA Polymer Systems

Abstract

Illumination of sulfonated poly(ether etherketone), SPEEK, and polyvinyl alcohol, PVA, in the presence of thionine (Th) results in systems exhibiting reversible redox processes. Efficient reduction of thionine took place when SPEEK/PVA/Th films were exposed to 350 nm photons. This photoreduction process involved radicals of SPEEK (sensitizer), which led to the formation of semithionine (thionine radical) as an intermediate, and finally leucothionine. Thionine was initially photoreduced (first 40 s – 50 s of illumination) via a zero-order rate law with a rate constant of k = 1 x 10-5 Ms-1. The reaction seemed to follow dispersive kinetics after 40 s - 50 s of photolysis. Reformation of thionine was very slow and took 30-45 days for completion, but addition of distilled water (3 ml) to illuminated films accelerated the reaction which then proceeded via first-order kinetics with k = 2.37 s-1. Unlike films, the Th photoreduction in SPEEK/PVA degassed solutions followed zero-order kinetics throughout the decay with k = 2 x 10-6 Ms-1. In the presence of air, Th reformation followed zero-order kinetics with a rate constant much lower than that of the reduction process (k = 3.3 x 10-8 Ms-1). Th reduction and reoxidation were reversible in both SPEEK/PVA/Th films and solutions for more than 8 consecutive illumination cycles. Efficient reduction of CCl4 and CHCl3 took place upon exposure to 350 nm photons that containing air saturated HCO2H/HCO2− buffers and swollen SPEEK/PVA films. The highest quantum yields for the photoreduction of above halomethanes were recorded when 0.36 M formate buffers served as H atom donors at pH = 7.3 that contained swollen 17 wt% SPEEK and 83 wt% PVA films (average thickness, 50 m). Both photoreductions consisted of two steps; slow initial step (induction period) followed by a linear step in which concentration of chloride ions increased rapidly compared to the initial step. Efficiency for the reduction of CCl4 is 100 times higher compared to the efficiency for the photoreduction of CHCl3. The occurrence of post-irradiation formation of Cl- during the reduction of CCl4/ CHCl3 in the dark confirmed that the photoreductions proceeded via a chain process.