Synthesis and nonlinear optical properties of novel Y-type polyurethanes with high thermal stability of dipole alignment

2,3-Di-(2¢-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and condensed with 2,4-toluenediisocyanate and 3,3¢-dimethoxy-4,4¢-biphenylenediisocyanate to yield novel Y-type polyurethanes 4–5 containing 2,3-dioxy benzylidenemalononitrile group as a nonlinear optical (NLO)-chromophore, which constituted parts of the polymer backbones. Polyurethanes 4–5 were soluble in common organic solvents such as acetone and N,N-dimethylformamide. They showed a thermal stability up to 270 C in thermogravimetric analysis thermograms and the glasstransition temperatures (Tg) obtained from differential scanning calorimetry thermograms were around 116– 135 C. The second harmonic generation (SHG) coefficients (d33) of poled polymer films at 106.4 mm–1 fundamental wavelength were around 9.07 · 10–19 C (2.72 · 10–9 esu). The dipole alignment exhibited high thermal stability up to 10 C higher than Tg, and there was no SHG decay below 145 C due to the partial main-chain character of the polymer structure, which was acceptable for nonlinear optical device applications.