Piezoelectric thickness resonance of aromatic polyurea films prepared by vapor deposition polymerization

Abstract By vapor deposition of a diisocyanate monomer (MDI) and a diamine monomer (MDA or MeMDA) onto a polyimide substrate, thin two-layer films of aromatic polyurea, P(MDA/MDI) or P(MeMDA/MDI), and polyimide were prepared. The polyurea layers acquired piezoelectric activity after poling treatment. The electric admittance of the films was measured as a function of frequency. Piezoelectric resonance and antiresonance were observed at the frequency of thickness mode resonance of the polyurea layer and at side-band frequencies corresponding to the harmonic thickness resonance frequencies of the polyimide layer. Theoretical analysis yielded the electromechanical coupling factor kt = 12%, the piezoelectric constant e 33 = 60 mC/m2, and the elastic constant C 33 = 6.87 GN/m2 for P(MDA/MDI). MDI: 4,4'-diphenyl methane diisocyanate, MDA: 4,4'-diamino diphenyl methane MeMDA: 4,4'-diamino-3,3'-dimethyl diphenyl-methane.