Characterization of Plasticizer-Polymer Coatings for the Detection of Benzene in Water Using Sh-Saw Devices

Jude K. Coompson, B.S. Marquette University, 2014 Benzene is a constituent component of crude oil that has been classified as a carcinogen by the EPA with a maximum contamination level (MCL) of 5ppb in drinking water. However, of the aromatic compounds, benzene has one of the lowest polymer-water partition coefficients using commercially available polymers as sensor coatings, resulting in poor limits of detection. This work investigates new coating materials based on polymer/plasticizer mixtures coated onto a shear horizontal surface acoustic wave (SH-SAW) sensor to detect benzene in water. There are many polymers which are unavailable for use as a sensing polymer due to their glassy nature. The use of plasticizers allows the polymer properties to be modified to give a more sensitive polymer by reducing the glass transition temperature, Tg, and increasing the free volume creating a more rubbery polymer which will absorb benzene. Three polymers, polystyrene (PS), poly (ethyl acrylate) and poly (methyl acrylate) were chosen to be plasticized with dioctyl phthalate (DOP). Polystyrene, which also possesses benzene rings, was chosen as its glass transition temperature is 100°C making it glassy. PEA was chosen because it has previously been used as a sensing polymer for benzene and has a Tg of -21°C. PMA was chosen because it has a Tg of 9°C and has previously been shown to have a low sensitivity to benzene. Dioctyl phthalate was chosen as the plasticizer because it possesses a benzene ring and had previously been used as a plasticizer in industry and research for polystyrene and acrylate polymers. The plasticizer-polymer mixtures are spin coated on a lithium tantalate (LiTaO3) SH-SAW dual delay-line device at various thicknesses. Each coating was exposed to multiple concentrations of benzene and frequency shifts were measured. Plasticization led to increased sensitivity for all polymers to benzene.