Influence of Preparation Conditions on Decay of Second-Harmonic Generation in Polyurea Thin Films Prepared by Vapor Deposition Polymerization

Abstract

Aromatic polyurea (PU) films were fabricated by vapor deposition polymerization using 4,4′-diphenylmethane diisocyanate and 4,4′-diaminodiphenylmethane. To investigate the influence of preparation conditions upon decay of optical second harmonic generation (SHG) after poling, two kinds of films were deposited: PU(No. 1) involving less residual isocyanate groups and PU(No. 2) with a smaller molecular weight and more residual isocyanate groups. PU(No. 2) showed significant decay, while PU(No. 1) did not. The results of IR spectrometry and differential scanning calorimetry indicated that the formation of urea bonds during poling of as-deposited films was due to reaction of the isocyanate groups with amino groups in PU(No. 1) and with water in PU(No. 2). We ascribed the SHG decay in PU(No. 2) to free volume formation by elimination of or plasticizing by CO2 generated during the above reaction, and/or to the smaller molecular weight.