MECHANICALLY ROBUST AMINE DERIVATIZED POLYSTYRENE FOR PH SENSING BASED ON POLYMER SWELLING

Beads that change size as a function of pH have been prepared by suspe nsion polymerization of vinylbenzyl chloride crosslinked with divinyl benzene followed by reaction with pure diethanolamine. Toluene is adde d to induce porosity, and Kraton G1652, a styrene-ethylene,butylene-st yrene triblock copolymer, is included to increase mechanical robustnes s. The optimum formulation includes 48% toluene by volume and 2% (w/w) Kraton G1652. As the pH decreases, protonation of the amine puts a ch arge on the amine, causing the polymer to swell due to electrostatic r epulsion between charged sites on the polymer. After the polymer is co nditioned by exposure to acid, the ratio of the diameter in acid to th e diameter in base is as high as 1.6 at an ionic strength of 0.10 M. T he diameter ratio decreases with increasing ionic strength as predicte d by theory. The diameter changes continuously from pH 6.0 to 8.0. Thi s means that immobilization shifts the pK(a) of the amine. The polymer beads undergo many swelling and shrinking cycles without degrading me chanically but are softer than desired for use in a pH sensor based on polymer swelling.