The influence of composition on the properties of pH-swellable polymers for chemical sensors

Aminated polystyrene swells at low pH due to electrostatic repulsion between charged sites on the polymer. The effects of polymer formulation on the rate and degree of swelling and on the penetration modulus have been evaluated for porous aminated polystyrene beads prepared by suspension polymerization. The following parameters were varied in a 3x3x3x2 factorial experiment: 1) percent crosslinking, 2) percent porogenic solvent, 3) composition of porogenic solvent and 4) level of Kraton G1652, a styrene-ethylene, butylene-styrene triblock copolymer, added to toughen the polymer. The degree of swelling decreases at higher levels of crosslinking. Swelling is greater when the polymer morphology is continuous rather than consisting of connected small spheres. The percentage of xylene in the porogenic solvent affects the degree of swelling for underivatized beads in toluene but not for derivatized beads in acid. The rate of swelling of underivatized beads in toluene appears to involve toluene entering the pore space of the bead by capillary action. However, rates of swelling of derivatized bead in acid appear to be governed by the rate at which protons move through the bulk of the polymer rather than involving transport through pore space. The penetration moduli are small for all beads and depend on the degree of crosslinking, the amount of pore space in the bead and the polymer morphology. (C) 1997 Elsevier Science S.A.