Polyimide-polyurethane/urea block copolymers for highly sensitive humidity sensor with low hysteresis

Polyimide (PI)-polyurethane-urea (PU) block copolymers (PI-PU75/25, PI-PU50/50, and PI-PU25/75) were prepared by reaction between anhydride-terminated poly(amic acid) prepolymers with various number-average degree of polymerization X_n = 73/49/25) and isocyanate-terminated urethane-urea prepolymers with various X_n (11/21/31) to obtain high performance capacitive humidity sensors. Pure PI and PU were also prepared to compare with PI-PU copolymers. This study examined the effect of PU content on the water absorption %/water vapor transmission rate, thermal and mechanical properties and sensing properties of PI-PU block copolymers. The thermal stability and mechanical properties of the copolymer decreased markedly with increasing PU content. The sensitivity of sensor increased sharply with increasing PU content from 0 to 25 wt %, and then increased a little. The hysteresis of sensor decreased sharply with increasing PU content up to 50 wt %, and then decreased a little. These results demonstrate the apparent upside of using two copolymers (PI-PU75/25 and PI-PU50/50) compared to using pure PI, in terms of sensor performance.