Polymer Deposition from Supercritical Solutions for Sensing Applications

Polymer surfaces are important in many applications including chemical sensing, corrosion protection, lubrication, and medicine. The growing demand for surfaces with specific and improved properties has catalyzed the development of new methods of polymer synthesis and processing that provide control of surface properties at the micro- and nanometer scales. Rapid expansion of supercritical solutions (RESS) is a technique that takes advantage of the enormous solubility change that occurs in a rapidly expanding supercritical solution in order to form precipitates with narrow and tunable size distributions. We have developed and tested a new RESS apparatus, incorporating a 340 cm 3 extraction vessel and a capillary nozzle. The system provides the ability to independently control important expansion parameters including temperature, pressure, solute concentration, and nozzle geometry. Microspheres of high molecular weight poly(dimethylsiloxane) were deposited onto the sensing surface of a microfabricated transducer using the RESS technique. The siloxanes are excellent candidates for chemical sensing applications because of their affinity to particular organic vapors and other advantageous physical properties including low glass transition temperature, low crystallinity, and the potential for chemical modification for enhanced selectivity. The miniature chemical sensor was tested upon exposure to hexane vapor and exhibits a fast, reversible response.