Highly Cross-Linked Polyurethane with Enhanced Stability of Second-Order Nonlinear Optical Properties

A new material processing approach yielding a thermally curable, optically active second-order material, based on a polyurethane derived from polyKphenyl 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 °C produces a rigid polymeric structure with a glass transition temperature of 142 °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 χ⁽²⁾ of 3 × 10^-7 esu. The NLO activity of this material was monitored at 100 °C for a period of 1200 h without showing detectable changes in the χ⁽²⁾ value.