AMINATED POLYSTYRENE MEMBRANES FOR A FIBER OPTIC PH SENSOR-BASED ON REFLECTANCE CHANGES ACCOMPANYING POLYMER SWELLING

We have prepared toughened, porous, aminated polystyrene membranes tha t undergo an increase in reflectance as the pH increases from 6.8 to 8 .0. Vinylbenzyl chloride (VBC) is copolymerized with divinylbenzene (D VB) in the presence of a toughening agent, Kraton G1652, a styrene-eth ylene, butylene-styrene triblock copolymer, and a porogenic solvent, x ylene/dodecane. The optimum formulation for sensing is 2% DVB (mel DVB /mol VBC), 2% Kraton (g Kraton/g VBC) and 40% (v/v) 2:1 xylene: dodeca ne. Benzoyl peroxide is used as the initiator. The components are part ially polymerized at 85 degrees C to a viscosity of 600-800 centipoise . The polymerization is then stopped by reducing the temperature. A dr op of the partially polymerized solution is confined between two micro scope slides and the polymerization reaction is completed. The resulti ng membrane is then swollen in 1,4-dioxan and reacted with diethanolam ine. These membranes have been incorporated into a pH sensor based on changes in reflected intensity measured through a bifurcated bundle of twenty unbuffered 50/55 core/cladding glass-on-glass optical fibers w ith numerical apertures of 0.57. The resulting sensor is stable and re quires inexpensive optical components, a red-emitting LED as the sourc e and a silicon photodiode as the detector.