INFLUENCE OF POLING FIELD ON POLYMERIZATION IN POLYUREA THIN-FILMS FOR 2ND-HARMONIC GENERATION PREPARED BY VAPOR-DEPOSITION POLYMERIZATION

Using 4, 4'-diphenylmethane diisocyanate and 1,4-diamino-2-nitrobenzen e (DNB), an aromatic polyurea thin film (PU(DNB)) was fabricated by va por deposition polymerization. In PU(DNB); although the enhanced optic al nonlinearity due to the introduction of the NO2 groups was achieved , a drastic decay of second-harmonic intensity was observed immediatel y after the poling field was turned off at 200 degrees C. The structur al analysis revealed that the polymerization proceeded during heat tre atment, but not during the poling process. This was attributed to the reduction of the reactivity of DNB monomers under the poling field due to possible interaction between the NO2 group (electron acceptor) and the NH2 group (electron donor) through the pi-electron system.