Abstract The paper deals with electromechanical and dielectric properties of polyurethanes (PU) block-copolymers. Most of the works published in the literature only consider electrostriction at room temperature at a given frequency. In this work, it is shown that electrostrictive coefficient ME is divided by 3-10 at increasing frequency over 3 decades of frequency, depending on the ratio of hard to soft segments in PU. Thus it is important to analyze the energy conversion efficiency by investigating the dielectric and viscoelastic properties. This work deals with the study of dielectric properties of 3 PU with different fractions of hard segments. Three relaxation phenomena (β, α and conduction) were investigated for each PU in the temperature-frequency range studied here, in order to optimize the copolymer composition in view of their best efficiency as actuators or mechanical energy harvesting devices. © 2015 Elsevier B.V. All rights reserved.