Second-Harmonic Generation and Relaxation in Polyurea Thin Films Prepared by Vapor Deposition Polymerization

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Abstract
Aromatic polyurea thin (PU) films were fabricated by means of coevaporation of 4,4'-diphenylmethane diiso-cyanate (MDI) and either 4,4'-diamino diphenyl methane (DDM) or 4,4'-diamino diphenyl ether (DDE). For the two PU films, second-harmonic generation (SHG) caused by corona poling and the subsequent isothermal decay were investigated. The second-order nonlinear coefficient d 33 was estimated to be 5.3 and 6.3 pm/V for PU(DDM) and PU(DDE), respectively, and proved to exhibit almost no decay 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 molecules, leading to suppression of the molecular orientation. PU films containing excess residual isocyanate groups showed a large SHG decay to about 60% of the initial value within 10 min. It was concluded that the residual isocyanate groups in as-deposited PU films greatly affects the behavior of SHG relaxation.