Photoresponsive behavior of SPEEK/Cellulose films for oxygen sensing.

Abstract

Illumination of sulfonated poly(ether etherketone) (SPEEK) and cellulose films by 350nm photons resulted in an efficient redox system. The illuminated films resulted in radicals of SPEEK (SPEEKH•) which in the presence of air underwent radical oxidation that lasted minutes to hours depending on the cellulose polymers utilized and relative humidity (R.H.). Different cellulose polymers such as carboxymethylcellulose (NaCMC) and hydroxyethyl cellulose (2-HEC) have similar structures, however, changes in functional groups within the polymer matrix can have a major impact. When illuminated 2-HEC films SPEEKH• oxidation occurred via first-order kinetics with a rate constant of k = 6 x 10-2 min-1 at 20% R.H. to 2.5 x 10-1 min-1 at 55% R.H., while CMC oxidation occurred at k = 1.5 x 10-2 min-1 at 20% relative humidity to 6.7 x 10-2 min-1 at 55% R.H. The effect of R.H. on the radical oxidation rate is a result of the polymer matrix expansion in the presence of water. Consecutive illumination produced light-absorbing transients (LAT) which resulted in the failure of 2-HEC films by the 7th illumination. NaCMC films were only slightly affected by LAT after 10 illuminations. Efficient reduction of methyl viologen (MV2+) took place within SPEEK/cellulose films upon exposure to 350 nm photons. This photoreduction utilized radicals of SPEEKH• which led to the formation of methyl viologen radical (MV+). The resulting redox system yielded higher visible detection by the eye. Illuminated 2-HEC films MV+ oxidation occurred via first-order kinetics with a rate constant of k = 4.6 x 10-2 min-1 at 20% R.H. to 1.7 x 10-1 min-1 at 47% R.H., while CMC oxidation occurred with a rate constant of k = 2.2 x 10-1 min-1 at 20% relative humidity to 1 x 10-1 min-1 at 55% R.H. The MV2+ reduction and reoxidation were successful in both 2-HEC and NaCMC film systems; however, 2-HEC was affected by LAT upon consecutive illuminations. Reduction of MV2+ within argon-sparged SPEEK/cellulose solutions and swollen films with HCO2H/HCO2− was attempted with exposure to 350 nm photons. Ar sparged Solutions showed no reduction products while swollen films showed a reduction of MV2+. The reoxidation of MV+ was observed in the absence of air and showed an increase in the rate of decay upon subsequent illuminations. The half-life of each illumination was 1st t1/2 = 16 min, 2nd t1/2 = 11.2 min, and 3rd t1/2 = 4.6 min which followed second-order kinetic and then progressed to zero-order kinetics. This was the result of a LAT product being produced in excess during each illumination.