EFFECT OF SORBED WATER ON THE DIELECTRIC-PROPERTIES OF ACETYLENE-TERMINATED POLYIMIDE RESINS AND THEIR APPLICATION TO A HUMIDITY SENSOR

A cured acetylene-terminated polyisoimide oligomer was used to prepare a capacitive-type humidity sensor. Sorption behavior of water on the cured film was examined by measuring the dielectric constant and water content. The amount of sorbed water was small and the sorbed water did not form clusters. This is due to the hydrophobic nature and rigid structure of the film. The sorption behavior reflects its excellent sensor properties, i.e., low hysteresis, low temperature coefficient, durability against acetone vapor, and long-term stability in a high temperature and humid atmosphere. Among the polyimides prepared by curing the oligomers with different chain lengths, the most suitable composition for a reliable capacitive-type humidity sensor was API-3-API-1.