INFLUENCE OF PREPARATION CONDITIONS ON DECAY OF 2ND-HARMONIC GENERATION IN POLYUREA THIN-FILMS PREPARED BY VAPOR-DEPOSITION POLYMERIZATION

Aromatic polyurea (PU) films were fabricated by vapor deposition polym erization using 4,4'-diphenylmethane diisocyanate and 3,4'-diaminodiph enylmethane. To investigate the influence of preparation conditions up on decay of optical second harmonic generation (SHG) after poling, two kinds of films were deposited: PU(No. 1) involving less residual isoc yanate groups and PU(No. 2) with a smaller molecular weight and more r esidual isocyanate groups. PU(No. 2) showed significant decay, while P U(No. 1) did not. The results of IR spectrometry and differential scan ning calorimetry indicated that the formation of urea bonds during pol ing 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 ascrib ed 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.