Studies on nonlinear optical polyurethanes containing heterocyclic chromophores

This paper presents the synthesis of highly stable nitro-substituted thiazole, benzothiazole and thiadiazole chromophores. With these, a series of second-order nonlinear optical (NLO) responsive polyurethanes were successfully synthesized from tolylene-2,4-diisocyanate (TDI) and 4,4′-methylenedi(phenyl isocyanate) (MDI). Molecular structural characterization of these polyurethanes was achieved by 1H NMR, FT-IR, GPC and analytical data. The weight-average molecular weights (Mw) of the resulting polyurethanes were determined by GPC and ranged between 22,100 and 26,700. All the polyurethanes were highly soluble in aprotic solvents such as tetrahydrofuran, cyclohexanone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulphoxide, N-methyl-2-pyrolidinone, etc. The thermal behaviour of these polyurethanes was investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Their glass transition temperatures were in the range 140–165 °C and most of the polymers showed high thermal stability. With an in situ poling and temperature ramping technique, the optimal temperatures (Topts) for corona poling were determined for the largest second-order NLO response. The second harmonic generation (SHG) coefficients (d33) of the poled polyurethane films range from 62.21 to 103.11 pm/V at 1064 nm. All the poled films showed outstanding orientational stability up to 120 °C without any measurable decay in the SHG signal. Of these, the polyurethane with nitro-substituted benzothiazole moiety (IIb) showed the best dynamical thermal stability of the poling-induced dipole alignment up to ∼150 °C.