Synthesis and characterization of thermally stable second-order nonlinear optical side-chain polyurethanes containing nitro-substituted oxadiazole and thiazole chromophores

We have newly synthesized nonlinear optical (NLO) active nitro-substituted thiazole and oxadiazole chromophores and condensed with tolylene-2,4-diisocyanate and 4,4′-methylenedi(phenyl isocyanate) to yield a series of polyurethanes. Molecular structural characterization of the resulting chromophores and polyurethanes was achieved by FTIR, UV–vis, 1H NMR and CHN elemental analyzer. The inherent viscosities (η inh) of polyurethanes measured with an Ubbelohde viscometer were in the range of 0.26–0.30 dl/g. Thermal behavior of polyurethanes was investigated using differential scanning calorimetry and thermogravimetric analysis. The glass transition temperatures (Tg) of the polyurethanes were in the range of 121–192 °C. Thin films of polyurethanes were prepared and achieved molecular orientation by inducing electric field. The change in the surface morphology of polyurethanes films before and after poling was investigated using atomic force microscopy. All the polyurethanes exhibited an excellent solubility in most of the common organic solvents, suggesting that these polyurethanes offered good processability. The second harmonic generation (SHG) coefficients (d33) of the poled polyurethanes ranged from 29.7 to 44.2 pm/V at 532 nm. High thermal endurance of the poled dipoles was observed for all the polyurethanes and this was attributed to the formation of extensive hydrogen bonds between urethane linkages. Furthermore, none of the developed polyurethanes showed SHG decay below 115 °C, and this signified their acceptability for nonlinear optical devices.