Sensors for liquids based on conductive immiscible polymer blends

Abstract

The present paper discusses the application of conductive immiscible polymer blends as sensor materials for detection of organic liquid solvents and of their vapors. Immiscible polymer blends of high impact polystyrene (HIPS), ethylene vinyl acetate copolymer (EVA) and carbon black (CB), and compounds of EVA/CB have been used to produce a series of electrically conductive filaments by a capillary rheometer process. In these immiscible blends, HIPS serves as a matrix and EVA as the semi-crystalline dispersed phase. The enhancement of conductivity in these blends is due to the attraction of CB to EVA, giving rise to conductive networks. The dc electrical resistivity of extruded filaments, produced at different shear levels, is found to be sensitive to various organic liquid solvents. The shear rate, at which the filaments are produced, has an important effect on the HIPS/EVA/CB filament's sensitivity. The compositions studied were close to the double-percolation structure believed to perform best as sensor materials. The HIPS/EVA interface seems to play an important role in the sensing process. In some cases, liquid contact/drying cycling of filaments indicates stabilization of the sensitivity change, making the sensing process reversible. Liquid transport principles are an important basis for interpretation of the sensing behavior of immiscible blend-based filaments in contact with liquids.

Introduction

The present report addresses a new concept of liquid sensing materials based on electrically conductive immiscible polymer blends. The concept's feasibility has been proven; nevertheless, establishment of the exact mechanisms involved is still under investigation. Understanding of some of the research findings is still open for future research efforts.

In recent years, new classes of polymeric materials have been developed and studied for applications as sensors in the chemical and biomedical fields [1], [2], [3], [4], [5]. These materials primarily work on the basis of their change in electrical properties, which are modulated upon interaction with a variety of chemicals. Sensors of this kind have potential applications, such as a solvent leak detector in detection of organic/inorganic gases or liquids (such as hydrocarbons, chlorinated solvents, gasoline, ammonia, and nitrogen dioxide) [6].

There are few studies reported in the literature on conductive single polymer systems as sensing materials. One paper on conductive miscible polymer blends [7] and just one (the present authors) on conductive immiscible polymer blends have been reported in the literature [8]. Some reports describe thin films as the sensing materials, usually based on an array of carbon black (CB) polymer compounds, to detect chemicals in the gaseous state [9], [10]. CB-based sensors have been studied as humidity, temperature and pressure sensors, and not much as organic liquids or gas sensors [11]. Some patents discuss the design of sensors for leak detection of organic solvents by conductive polymer compounds [12], [13], [14], [15]. Electrically conductive conjugated polymer is another important class of sensing materials. These doped conductive polymers also function as sensors on the basis of the change of their electrical properties upon exposure to the sensing environment. These doped polymers however, are not very stable, and their long-term stability is of prime concern for practical applications. Sensors of these classes have been primarily used for the detection of gases and vapors [16], [17].

In general, good sensors should show a high rate of detection, possess reversible and reproducible responses, be easily fabricated with a simple, compact and economical design, and be stable and resisting corrosion and weathering.

The present work emphasis is on immiscible polymer blends of high impact polystyrene (HIPS) as the matrix and ethylene vinyl acetate copolymer (EVA) as the dispersed phase, studied as extruded filaments, containing electrically conductive CB, by a capillary rheometer process. In the present immiscible HIPS/EVA blends, the conductive CB particles are preferentially located within the dispersed polar EVA particles and along the interface at the higher CB concentrations. Enhancement of conductivity, in these blends, is thus due to the selective localization of CB particles, giving rise to segregated conductive networks [18], [19], [20]. These immiscible polymer blends were designed to have a double-percolation structure, associated with a low CB concentration. The dc electrical resistivity of the extruded filaments, at different shear levels, is found to be sensitive to various organic liquids. Thus HIPS/EVA/CB conductive filaments close to their percolation concentration may show a resistivity increase upon contact with a given liquid due to partial destruction of the delicate conductive network by selective swelling effects, and or liquid diffusion into the interface, thereby providing feasible means of sensing the presence of a given liquid.