Mechanically robust amine derivatized polystyrene for pH sensing based on polymer swelling

Abstract

Beads that change size as a function of pH have been prepared by suspension polymerization of vinylbenzyl chloride crosslinked with divinyl benzene followed by reaction with pure diethanolamine. Toluene is added to induced porosity, and Kraton G1652, a styrene-ethylene,butylene-styrene triblock copolymer, is included to increase mechanical robustness. The optimum formulation includes 48% toluene by volume and 2% (w/w) Kraton G1652. As the pH decreases, protonation of the amine puts a charge on the amine, causing the polymer to swell due to electrostatic repulsion between charged sites on the polymer. After the polymer is conditioned by exposure to acid, the ratio of the diameter in acid to the diameter in base is as high as 1.6 at an ionic strength of 0.10 M. The diameter ratio decreases with increasing ionic strength as predicted by theory. The diameter changes continuously from pH 6.0 to 8.0. This means that immobilization shifts the p K a of the amine. The polymer beads undergo many swelling and shrinking cycles without degrading mechanically but are softer than desired for use in a pH sensor based on polymer swelling.