A new material processing approach yielding a thermally curable, optically active second-order material, based on a polyurethane derived from poly[(phenyl isocyanate)-co-formaldehyde] and 4-[[N-(2-hydroxyethyl)-N-methylamino]phenyl]-4- [(6-hydroxyhexyl)sulfonyl]azobenzene is presented. In this procedure a liquid polymer, poly[(phenyl isocyanate)-co-formaldehyde], containing one isocyanate group on each of the phenyl rings was selected in order to increase both the efficiency and the uniformity of the cross-linking reaction between polymer chains during the thermal curing process. The cross-linking reaction carried out for a period of 30 min at 160 degrees C produces a rigid polymeric structure with a glass transition temperature of 142 degrees C. The presented approach has been applied for preparation of high optical quality films for nonlinear optical (NLO) applications which, when corona poled and thermally cured, reveal a strong optically nonlinear second-order activity with a macroscopic second-order susceptibility chi((2)) of 3 x 10(-7) esu. The NLO activity of this material was monitored at 100 degrees C for a period of 1200 h without showing detectable changes in the chi((2)) value.