Dielectric Relaxation in Polyurea Thin Films Prepared by Vapor Deposition Polymerization

Polyurea thin films with the thickness of 500 nm were prepared by the vapor deposition of 4,4′-diamino diphenylmethane (MDA) and 4,4′-diphenylmethane diisocyanate (MDI) on a polyimide film substrate. The dielectric constants observed at 10 Hz for poled MDA-rich and MDI-rich films were about 10 and 3.5, respectively, and almost independent of temperature up to 200°C. The dielectric constant at 10 Hz for a poled balanced film was 4 at room temperature and increased with increasing temperature. A large dielectric relaxation was observed for the poled balanced film at the temperature range from 152 to 192°C and the frequency range from 10-2 to 102 Hz. The estimated relaxation strength was about 700. It was presumed that polar domains consisting of about 20 monomers were formed by the poling process, in which urea bonds were aligned in parallel and linked by hydrogen bonds.