Synthesis of thermally stable new polyurethanes containing nitro-substituted 1,3,4-oxadiazole chromophores for second order nonlinear optical applications

Push–pull type nonlinear optical chromophores containing nitro-substituted oxadiazoles were newly synthesized and successfully condensed with tolylene-2,4-diisocyanate, 4,4′-methylenedi(phenyl isocyanate) and 3,3′-dimethoxy-4,4′-biphenylenediisocynate to yield a series of polyurethanes. The resulting chromophores and their corresponding polyurethanes were systematically characterized by FTIR, 1H NMR, UV–vis and elemental analyzer. Thermal behaviors of these polyurethanes were investigated using differential scanning calorimetry and thermogravimetric analysis. After poling, the change in the molecular alignment in the polymer films was confirmed by atomic force microscopy. The inherent viscosity (η inh) of polyurethanes measured by Ubbelohde viscometer was in the range of 0.25–0.29 g/dl. High thermal endurance of poled dipoles was observed for all the polyurethanes. The second harmonic generation (SHG) coefficients (d33) of the poled polyurethanes determined by the Maker fringe technique were in the range of 58.66 to 89.77 pm/V at 532 nm, signifying the acceptability for nonlinear optical devices.