Influence of poling field on polymerization in polyurea thin films by vapor deposition polymerization

Using 4,4´-diphenylmethane diisocyanate (MDI) and p-diamino nitro benzene (DNB), polyurea (PU(DNB)) films were fabricated by vapor deposition polymerization for second-order nonlinear optical application. In PU(DNB), although the enhanced optical nonlinearity due to the introduction of the NO₂ group was achieved, a drastic decay of SH intensity was observed at 200°C. The structural analysis revealed that the polymerization proceeded during the heat treatment but not during the poling process. This is attributed to the hindrance to the polymerization due to the reduction of reactivity of the NH₂ group under the poling field