Pressure-dependent dielectric properties in a polyurethane elastomer

The dielectric responses of polyurethane films were investigated in the pressure range from atmospheric to 20 kpsi and in the frequency range from 0.05 Hz to 4 KHz at −50, 0, 29, 50, and 80 ◦ C, where T g is close to −15 ◦ C (α-relaxation) and where the glass temperature of the chain extender is about 80 ◦ C (I-relaxation). When a higher pressure was applied to the material, a lower dielectric constant (k’ ) was obtained owing to the suppression of polarization motions by the external pressure. However, k’ increased with rising pressure at temperatures above 50 ◦ C and at high frequencies due to the predominant thermal effect expanding the film outwards, where a dispersive α-relaxation occurred. Both α- and I-relaxations followed the pressure-frequency superposition principle with a different shifting factor, a ( p ), where the α-relaxation showed a faster migration of the relaxation time with changing pressure and a ( p ) values larger than those of I-relaxation. The two relaxations observed at 29 ◦ C demonstrated linear relations between 1/k’ and pressure and between ln a ( p ) and pressure, regardless of the film thickness.