Polymer substrates for optical chemical sensing

Abstract

Membranes for chemical sensing based on changes in light reflectance from amine-modified, porous, rubber-toughened poly(vinylbenzyl chloride-co-divinyl benzene) were prepared by bulk free radical polymerization. Xylene and dodecane were used as porogenic solvents and solutions were prepolymerized to viscous solutions prior to forming membranes. The crosslinked membranes are turbid and scatter light when unswollen. They swell in response to increases in hydrogen ion concentration, causing the membrane to reflect less light. Turbidity is ascribed mainly to scattering by water-filled macropores within the amine-functionalized membranes. Swelling decreases the refractive index of porous, hydrated polymers with 2% crosslinking by approximately 0.04 units, bringing it closer to the refractive index of water, which decreases scattering. The effects of four variables on the turbidity, swelling and optical change of these membranes indicates that turbidity is affected by a number of interacting effects. Increases in rubber content, total porogen volume and the dodecane fraction of the porogen increase turbidity while crosslinking decreases turbidity. The rubber is a styrene/ethylene-butylene/styrene triblock copolymer, Kraton G-1652, that is believed to act as a surfactant between the growing polymer chains and porogenic solvents to form more large macropores that scatter light. Crosslinker and dodecane decrease the optical response by limiting swelling. Kraton increases the response by helping form more scattering centers. Membranes can be formed without prepolymerization if dodecane is excluded from the formulation. Membranes cured by photoinitiated polymerization indicate that cure temperature influences the morphology and performance of membranes. Membranes cured at 85\sp\circC with 2% crosslinker have the best response. Membranes consisting of swellable polymer microspheres in poly(hydroxyethyl methacrylate) were prepared. Swelling decreases the refractive index of the microparticles and the membranes scatter less light. The microparticles were prepared by dispersion polymerization of vinylbenzyl chloride, aminated, cast into a hydrogel monomer and cured. The membranes are prepared quickly and with great precision. The turbidity change is similar to earlier membranes and conveniently controlled by the number of microspheres in the hydrogel. The response time of micron diameter particles in poly(HEMA) is on the order of an hour and limited by diffusion through the hydrogel and into the microparticles