Metal–polymer composite sensors for volatile organic compounds: Part 2. Stand alone chemi-resistors

In Part 1 flow through chemi-resistors based on a novel, granular, metal–elastomer composite subject to mechanical compression were described. Despite the composite having a high metal particle loading no conductive chains of particles are formed and it is intrinsically insulating. In compression the composite becomes conductive, sample resistance falling from >1012 Ω to <0.01 Ω. Use as a chemi-resistor required the composite to be compressed to set an initial resistance. Casting from polymer solutions containing dispersed granules has been explored as an alternative method for producing chemi-resistors. The shrinkage of the soluble polymer during solidification brings the granules into contact and sets an initial resistance. On exposure to volatile organic compounds (VOCs) swelling of the surrounding polymer and the elastomer in the granules results in an increase in resistance. Chemi-resistors were fabricated with different soluble polymers and substrates. Factors affecting sensor performance, e.g. film stability were investigated. Selected examples were exposed to a wide range of concentrations of tetrahydrofuran (THF) vapour and differences in response at low and high vapour concentrations, similar to that reported in Part 1, observed. The temporal response at different THF concentrations was determined. This data together with quartz crystal microbalance measurements was used to investigate possible mechanisms.