Piezoelectric and Ferroelectric Properties of Polyurea Thin Films Copolymerized by Vapor Deposition Polymerization of Aliphatic and Aromatic Monomers

Abstract

Thin films of polyurea were prepared by vapor deposition polymerization of 1,9-diaminononane and 4,4′-diphenyl-methane-diisocyanate. The films were mechanically tenacious and easily separated from the substrate surface, similar to aliphatic polyurea films. Piezoelectric stress constant e31 ( =4 mC/m2) and pyroelectric constant p3 (=8 µ C/m2 K) were obtained for poled films. Ferroelectric hysteresis was also observed. Annealing the deposited film under an electric field decreased the ionic conductivity as well as the residual polarization of the D–E hysteresis curve. The oriented urea dipoles gave rise to piezoelectricity but the ionic polarization did not. Three relaxation processes in the temperature dependence of elastic constant c1 and dielectric constant ε3 were observed at -150° C (γ), -50° C (β), and 130° C (α), similar to those for aliphatic polyureas. The piezoelectric constant e31 remained unchanged until about 100° C, where depolarization took place due to α relaxation.