Studies on nonlinear optical polyurethanes containing heterocyclic chromophores

Abstract

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 ( M w) 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 ( T opts) for corona poling were determined for the largest second-order NLO response. The second harmonic generation (SHG) coefficients ( d 33) 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.