2ND-HARMONIC GENERATION AND RELAXATION IN POLYUREA THIN-FILMS PREPARED BY VAPOR-DEPOSITION POLYMERIZATION

Aromatic polyurea thin (PU) films were fabricated by means of coevapor ation of 4,4'-diphenylmethane diisocyanate (MDI) and either 4,4'-diami no diphenyl methane (DDM) or 4,4'-diamino diphenyl ether (DDE). For th e two PU films, second-harmonic generation (SHG) caused by corona poli ng and the subsequent isothermal decay were investigated. The second-o rder nonlinear coefficient d33 was estimated to be 5.3 and 6.3 pm/V fo r PU(DDM) and PU(DDE), respectively, and proved to exhibit almost no d ecay with time up to 2000 h. It was also found that annealing prior to the poling process caused no additional increase of SHG. This result was interpreted in terms of increase in the packing density of molecul es, leading to suppression of the molecular orientation. PU films cont aining excess residual isocyanate groups showed a large SHG decay to a bout 60% of the initial value within 10 min. It was concluded that the residual isocyanate groups in as-deposited PU films greatly affects t he behavior of SHG relaxation.