Piezoelectric and Dielectric Properties of Aromatic Polyureas Synthesized by Vapor Deposition Polymerization

Four different kinds of aromatic polyurea were synthesized by vapor deposition of monomers. As a diisocyanate, [4,4 ′ -diphenyl methane diisocyanate] (MDI) was used. As a diamine, [4,4 ′ -diamino diphenyl methane] (MDA), [4,4 ′ -diamino-3,3 ′ -dimethyl diphenyl methane] (MeMDA), [4,4 ′ -diamino diphenyl ether] (ODA), and [4,4 ′ -diamino diphenyl sulfide] (SDA) were used. Poled thin films of P(MDA/MDI), P(MeMDA/MDI), P(ODA/MDI), and P(SDA/MDI) exhibited piezoelectric activity which remained up to almost 200° C. The stable value of the piezoelectric constant for P(ODA/MDI) was 26 mC/m 2 . The piezoelectric constant plotted against the evaporation temperature of MDI showed a maximum, where the stoichiometrically balanced composition of MDI and diamine monomer was obtained. The curve was sharp for P(MDA/MDI), but broad for P(ODA/MDI) and other polyureas. It was suggested that the high probability of production of balanced oligomers might be due to the longer time of residence of ODA molecules on the substrate.