Polymer deposition from supercritical solutions for sensing applications

Polymer surfaces are important in many applications including chemical sens ing, corrosion protection, lubrication, and medicine. The growing demand fo r surfaces with specific and improved properties has catalyzed the developm ent 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 th e enormous solubility change that occurs in a rapidly expanding supercritic al solution in order to form precipitates with narrow and tunable size dist ributions. 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 includi ng temperature, pressure, solute concentration, and nozzle geometry. Micros pheres of high molecular weight poly(dimethylsiloxane) were deposited onto the sensing surface of a microfabricated transducer using the RESS techniqu e. The siloxanes are excellent candidates for chemical sensing applications because of their affinity to particular organic vapors and other advantage ous physical properties including low glass transition temperature, low cry stallinity, and the potential for chemical modification for enhanced select ivity. The miniature chemical sensor was tested upon exposure to hexane vap or and exhibits a fast, reversible response.