Dependence of Piezoelectric and Pyroelectric Activities of Aromatic Polyurea Thin Films on Monomer Composition Ratio

Thin films of aromatic polyurea were prepared by simultaneous deposition of 4,4'-diamino diphenylmethane (MDA) and 4,4'-diphenylmethane diisocyanate (MDI) on a polyimide film substrate. By changing the evaporation temperature of the monomers, the monomer molar ratio of MDA to MDI of the deposited film was varied to yield MDA-rich, balanced, and MDI-rich films. After poling, the balanced films exhibited the maximum values of pyroelectric and piezoelectric constants, which were stable up to 200-degrees-C. Elemental analysis confirmed that oligomers consisting of about five monomers were present in as-deposited films. In the balanced films, an exothermic peak appeared at about 110-degrees-C in the first run of DSC measurements and the IR absorption peak due to isocyanate groups disappeared after heat treatment. These results suggest that during the poling process polymerization of oligomers, which forms polar urea bonds, takes place at about 110-degrees-C under a high electric field. The large residual polarization thus produced leads to high pyroelectric and piezoelectric activities.