Synthesis and nonlinear optical properties of polyurethanes containing nitro-substituted 1,3,4-oxadiazole chromophores

NLO active nitro-substituted oxadiazole chromophores were prepared and condensed with tolylene-2,4-diisocyanate (TDI) and 4,4′-methylenedi(phenyl isocyanate) (MDI) to yield a series of polyurethanes. The resulting polyurethanes exhibited an excellent solubility in many common organic solvents, suggesting that these polyurethanes offer good processability. Molecular structural characterization of these polyurethanes was achieved by FT-IR, UV–vis, 1H NMR and analytical technique. The weight-average molecular weights of polyurethanes determined by gel permeation chromatography (GPC) were in range of 21,000–23,000 (Mw/Mn = 1.87–2.00). Thermal behavior of polyurethanes was investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The glass transition temperatures of the polyurethanes were in the range of 155–170 °C. The second harmonic generation (SHG) coefficients (d33) of the poled polyurethanes were in the range of 38.93–48.64 pm/V at 532 nm. High thermal endurance of poled dipoles was observed for all the polyurethanes. Further, it was found that all the developed polyurethanes were stable up to 140 °C, signifying the acceptability for nonlinear optical devices.