Humidity-Induced Volume-Expansion of Polyimide Films

Thin polyimide layers were investigated to determine their volume expansion as a result of humidity absorption and desorption, respectively. Furthermore, we used the experiments in order to clarify the physical and chemical processes of polyimide films in wet environment as well as their humido-mechanical properties. It could be shown that the humidity-induced material extension is fairly linear in a wide range of humidity. The observed material behaviour strongly depends on the conditions of the polyimide fabrication process. Analogous to the well-known temperature extension coefficient, a humidity extension value of about 60 to 90 ppm/%RH was observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bimetal elements influenced by temperature.