DEPENDENCE OF PIEZOELECTRIC AND PYROELECTRIC ACTIVITIES OF AROMATIC POLYUREA THIN-FILMS ON MONOMER COMPOSITION RATIO

Thin films of aromatic polyurea were prepared by simultaneous depositi on of 4,4'-diamino diphenylmethane (MDA) and 4,4'-diphenylmethane diis ocyanate (MDI) on a polyimide film substrate. By changing the evaporat ion 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 va lues of pyroelectric and piezoelectric constants, which were stable up to 200-degrees-C. Elemental analysis confirmed that oligomers consist ing 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 is ocyanate groups disappeared after heat treatment. These results sugges t that during the poling process polymerization of oligomers, which fo rms 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.