Poling of thin films of aromatic polyurea during polymerization and crystallization

Piezoelectricity and pyroelectricity were observed in poled films of aromatic polyurea less than 1 μm thick, which were synthesized by concurrent deposition of a diamine monomer and a diisocyanate monomer in vacuum. The films are poled by applying an electric field of 100 MV/m at 200°C. X-ray diffraction measurements show that crystallization takes place at about 160°C. The infrared absorption due to C=O stretch vibration in the isocyanate groups at the chain ends disappears after heat treatment. It is concluded that the alignment of urea dipoles takes place during polymerization and crystallization. Stable residual polarization produces the piezoelectric and pyroelectric activities persistent up to 200°C