Piezoelectric properties of thin films of aliphatic polyurea 79 synthesized by vapor deposition polymerization

Thin films of polyurea 79 were prepared by the deposition of monomers, 1,7-diaminoheptane and 1, 9-diisocyanatononane, onto a substrate cooled below 0°C. The x-ray diffraction patterns, infrared spectra, elastic, dielectric and piezoelectric measurements were carried out at various temperatures. Similar to polyurea 9, three elastic relaxations were observed at 110°C (α), -50°C (β) and -150°C (γ) for polyurea 79. The temperature dependence of the intensity and wavenumber of IR absorption peaks suggested that the hydrogen-bonded sheets of polymers are oriented parallel to the surface of the films. The thermal dissociation of hydrogen bonds as well as the melting of polyurea 79 takes place at a temperature lower than polyurea 9, indicating the less ordered alignment of urea dipoles in polyurea 79. The piezoelectric constant for polyurea 79 decreased above 40°C, whereas that for polyurea 9 increased above 40°C and decreased only above 130°C. The residual polarization in polyurea is caused by the field-induced alignment of urea dipoles, which is stabilized by hydrogen bonds between them. This stabilization effect is greater for polyurea 9 than for polyurea 79 because of the difference of their molecular structures.